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In Vitro Remineralization of Human Enamel with Bioactive Glass Containing Dentifrice Using Confocal Microscopy and Nanoindentation Analysis for Early Caries Defense

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Title:
In Vitro Remineralization of Human Enamel with Bioactive Glass Containing Dentifrice Using Confocal Microscopy and Nanoindentation Analysis for Early Caries Defense
Creator:
ALAUDDIN, SAMMEL SHAHRIER ( Author, Primary )
Copyright Date:
2008

Subjects

Subjects / Keywords:
Dentifrices ( jstor )
Fluorides ( jstor )
Hardness ( jstor )
Lesions ( jstor )
Minerals ( jstor )
Nanoindentation ( jstor )
pH ( jstor )
Saliva ( jstor )
Teeth ( jstor )
Tooth enamel ( jstor )

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University of Florida
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Copyright Sammel Shahrier Alauddin. Permission granted to University of Florida to digitize and display this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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12/18/2004
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57722289 ( OCLC )

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Full Text












IN VITRO REMINERALIZATION OF HUMAN ENAMEL WITH BIOACTIVE
GLASS CONTAINING DENTIFRICE USING CONFOCAL MICROSCOPY AND
NANOINDENTATION ANALYSIS FOR EARLY CARIES DEFENSE
















By

SAMMEL SHAHRIER ALAUDDIN


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

Sammel Shahrier Alauddin




























This work was performed in dedication to those who gave their time and love to
support me and my efforts. It was the contribution of many and prayers of a select few
who deserve credit for my successes. My parents and loved ones believed in me and
respected my pursuits in life, even when we didn't see eye to eye. I must also thank
Almighty God for giving me the strength and perseverance to overcome in times of
difficulty and the humility to accept my shortcomings and still succeed as a man among
many.
In memorial, I dedicate this work to my grandfather Kazi Reazuddin Ahmed, who
taught me the value of sacrifice and achievement in this world. My successes are a
testament to his love and guidance at critical stages in my life. I only regret that he is not
here today to share this accomplishment with me.















ACKNOWLEDGMENTS

I would like to thank Ben Lee, Neel Bhatavedekar, Kevin Taylor, and Allyson

Barrett in Department of Dental Biomaterials at the University of Florida for their direct

involvement in making this work a success. For three months in the lab, we were almost

a family. I would also like to thank Guy La Torre, Kevin McKenzie, and Donald

Thibadeaux from Novamin Technologies Inc. for their help and guidance throughout the

project, especially during the initial stages. Kevin contributed so much of his time

helping me take this research to a new level.

From the Department of Materials Science and Engineering, I must thank Dr.

Christopher Batich, my committee member, and Dr. Susan B Sinnott, without whom

much of this would not have been possible. All credit to her for stimulating my interest

in Materials Science and Engineering as her undergraduate student at the University of

Kentucky. I shall always admire her devotion to science and academic integrity.

This project was also supported by the efforts of Major Analytical Instrumentation

Center staff. Dr. Amelia Dempere, Wayne Acree, Brad Willenberg, and Jerry Bourne

helped bring my hypotheses to life. Each analysis technique required their expertise and

cooperation.

Confocal microscopy analysis would not have been possible without Dr.

Marguerita Fontana, Dr. Carlos Gonzales-Cabezas, and Amir Haider. Dr. Fontana and

colleagues at Indiana University-Purdue University at Indianapolis sacrificed their time









and convenience to advise me and help perform the analytical centerpiece of this study.

They were available to answer every question and make their expertise my expertise.

Many thanks go to Dr. R.J. Hanrahan, Dr. David Reitze, Tim Vaught, and Dr.

James Wefel for their time and assistance at various points throughout the study.

I reserve special recognition for Dr. Mecholsky, Dr. Greenspan, and Dr. Anusavice

for their valued advice and encouragement. They provided the resources and opportunity

to this ambitious young researcher, allowing me to mature and develop my scientific

curiosities. I am ever grateful for their guidance through this long and arduous journey.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ............................... ............. .............. viii

LIST OF FIGURES ......... ......................... ...... ........ ............ ix

ABSTRACT ........ .............. ............. ...... .......... .......... xi

CHAPTER

1 BACKGROUND AND MOTIVATION .......................................... ...............1

2 FUNDAMENTALS OF THE TOOTH ..................................................................5

H ydroxyapatite .............................................. ..................... ......... 6
Demineralization and Remineralization Phenomena........................................9

3 REMINERALIZATION THEORY.................................................. 14

D entifrice and F luoride.............................................. ........................... ........... 14
Systemic Benefit ........................................ ................................ 14
Topical Benefit .................................... ........ ................... 15
B io g la s s ............................................................................................................. 1 8

4 M INERAL QUANTIFICATION ..................................................... ...... ......... 23

M icroradiography .......................................................... ....... ........... ............ ..23
Confocal Laser Scanning M icroscopy............................................... ...................25
Nanoindentation ............... ............... .. ............. ......... ..... .........28
M icroindentation........................................... ...... .... ............ .............. .. 30

5 EXPERIMENTAL METHODS AND MATERIALS............... .................35

Collection and Storage of Teeth ........................................... .......................... 35
Sectioning and m counting ...................................................................................... 35
L esion F orm ation ......... ...................................................................... ........ .. .. ..36
Treatm ent R egim en .............................................. ........... ......... 36









Cross-sectioning ................................... ..... .. ..... .. ............37
A n a ly sis ..............................................................................3 8

6 ANALYTICAL RESULTS AND DISCUSSION................................................39

7 C O N C L U SIO N S ............................................................................... ........ ....... 5 5

L IST O F R EFER EN CE S ........... ................. ................. ........................ ............... 57

B IO G R A PH IC A L SK E TCH ...................................................................... ..................62
















LIST OF TABLES


Table p

4-1 Comparison of hardness and Young's modulus data obtained with Vickers and
B erkovich indenters.......... ............................................................ .. .... .... ..... 32

4-2 Comparison of indentation fracture toughness obtained by different indenters. .....32

5-1 Tooth labeling scheme. Example 36BX ....................................... ............... 36
















LIST OF FIGURES


Figure page

2-1 Anatom y of the hum an tooth ........................................................ ...............7

2-2 Cycle of demineralization and remineralization in enamel................................. 11

3-1 Mineral Loss (expressed as Ca output) as a function of pH and [F] ..................17

3-2 Stages of bioactive glass surface reactions. ....................................... .................19

4-1 M icroradiography setup ........................................ .............................................24

4-2 Mineral density quantified as AZ in TMR. ................................... ............... 25

4-3 Schematic of a generic confocal microscope. .................................. ............... 26

4-4 Conventional microscope for fluorescence in epitaxial configuration...................27

4-5 Force-displacement (loading) curves on various materials.................................29

4-6 Loading curve for nanoindentation on sound enamel. ...........................................30

4-7 Vickers m icrohardness testing. ........................................ .......................... 31

4-8 Demineralized Enamel. Axis units in m. .................................... ...............33

4-9 Indents in Novamin treated enamel. Axis units in tm ...................................... 34

4-10 Vickers indents in control enamel @ 300X and 2000X................ ....................34

5-1 Experimental flow diagram of specimen sectioning and mounting.......................36

6-1 Confocal image of enamel lesion, tooth section 38LD ........................................40

6-2 Lesion area for each tooth for the demineralized control, Colgate and Novamin
treatm ent condition ................................................... .... ... .. ........ .... 41

6-3 Total Gray Value for each tooth for the demineralized control, Colgate and
Novamin treatment condition.. ............................................. 42

6-4 Confocal image of tooth section 41LD (Demineralized Control) ............................43









6-5 Confocal image of tooth section 41BX (Colgate treated) ................. ............43

6-6 Confocal image of tooth section 41BY (Novamin treated) ........................... 44

6-7 Confocal image of tooth section 47LD (Demineralized Control) ...........................45

6-8 Confocal image of tooth section 47BX (Colgate treated) ................ ..........46

6-9 Confocal image of tooth section 47BY (Novamin treated) ........................... 46

6-10 Confocal image of tooth section 48LD (Demineralized Control) ...........................47

6-11 Confocal image of tooth section 48BX(Colgate treated) ................ ..........47

6-12 Confocal image of tooth section 48BY (Novamin treated) ............... ......... 48

6-13 Cross sectional nanoindentation of tooth 33 ....................... .. ............... 51

6-14 Cross sectional nanoindentation of tooth 44. ................................ .................51















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

IN VITRO REMINERALIZATION OF HUMAN ENAMEL WITH BIOACTIVE
GLASS CONTAINING DENTIFRICE USING CONFOCAL MICROSCOPY AND
NANOINDENTATION ANALYSIS FOR EARLY CARIES DEFENSE

Sammel Shahrier Alauddin

December 2004

Chair: J. J. Mecholsky, Jr.
Major Department: Materials Science and Engineering

Remineralization of early caries lesions provides an oral health and economic

advantage to populations suffering from this bacteria-based disease. Preventive treatment

of caries in the early stages could reduce dental costs and frequency of clinical visits.

This study investigated the use of bioactive glasses in dentifrice in comparison to

commercial dentifrice (Colgate Regular) to determine if additional calcium and

phosphate release from the bioactive glass contributes positively to fluoride dentifrice

remineralization capability. Two techniques were used in vitro to quantify the

remineralization for dentifrice treatment of tooth enamel. Confocal laser scanning

microscopy (CLSM) and nanoindentation were used to evaluate mineral changes in

artificial surface lesions optically and mechanically. Eighteen human molars were

sectioned into four parts : demineralized control, sound control, Novamin bioactivee

glass) and Colgate dentifrice treatment. Upon demineralization, specified treatment

sections were placed in a pH cycling regimen for 20 days. Each tooth section was further









cross-sectioned through the treatment window down the lesion depth for analysis.

Novamin tooth sections remineralized lesions more than Colgate for two parameters,

lesion area and total gray value (fluorescence), which have been directly correlated with

mineral density. Single tailed T-test for treatment groups yielded significant difference

(p < 0.001). Novamin dentifrice treatment reduced lesion area an average of 41.9%,

while Colgate averaged 24.9%. Novamin sections also exhibited an average 70.5%

decrease in total gray value compared to 48.1% for Colgate. Two way ANOVA testing

of three tooth sections (original lesion, Novamin treatment, Colgate treatment) for

both parameters found significant difference (p < 0.0001). Duncan multiple range testing

found these three sections statistically different at significance level 0.01. Both

parameters signify extensive remineralization in vitro for surface lesions.

Nanoindentation did not significantly exhibit remineralization for either dentifrice

treatment. Considerable hardness variability was observed throughout the cross-sectional

lesion, as well as inconsistency among multiple teeth and treatment groups.

Nanoindentation is not recommended for mechanical analysis of in vitro remineralization

of human enamel. Vickers microhardness was performed on lesions as well but did not

yield credible results. Indents were difficult to optically identify and measure.














CHAPTER 1
BACKGROUND AND MOTIVATION

The field of dentistry enables modem societies to effectively and preventively

maintain oral health. Clinicians have been relatively successful within the realm of

dental care taking into consideration the frequency of patient visits. But that dependence

upon interaction between the patient and their dentist is not only costly to some, but also

contrary to human nature. Our human tendency to treat ailments responsively rather than

preventively forces the scientific community to develop better methods to counter the

more severe stages. So it is now the motivation for dental researchers and clinicians to

develop strategies and treatment methods for fighting dental diseases in their early stages.

Engineers call this practice preventive maintenance while physicians refer to it as

preventive medicine. Regardless of the terminology, science has been a marvelous tool

for enabling individuals to care for themselves. Decades of dental science have brought

society the common practice of daily tooth brushing, flossing and mouth rinse. This

concept of preventive dentistry provides the foundation for this study and its long-term

applications.

Dental caries remains the oldest and most prevalent oral disease in human history.

It is only recently that the advent of daily dental care and clinician oversight has reduced

the frequency of caries within large populations [WIN98]. Fluoridated water supplies

and habitual influences, such as diet, certainly have an effect on caries management. But

this study focuses primarily on daily treatment as the most effective caries defense and

ways to optimize it process through materials science.









Caries pathology and its early treatment are well researched. Only the details of the

caries prevention process and the best way to accomplish this task are left for dental

researchers to discover. The term "caries" which originates from Latin for "rot" or

"rotten" prompted original researchers of the past two centuries to develop methods to

counter this process of tooth decay or demineralization. The heart of caries research and

prevention lies in the opposition of these terms, thus replacement or remineralization.

Traditionally, surface or early caries treatment was centered around tooth mineral

loss and gain. The progressive increase in processed sugars and acidic foods and

beverages in the human diet provide oral bacteria greater opportunity to produce acids

that dissolve tooth mineral. Thus, the facilitation of regaining new mineral via the natural

oral process remains the ultimate goal. Tooth mineral, among other biological minerals,

is composed mainly of calcium and phosphorous. Methods for providing these

constituents of mineral or a new constituent to better facilitate remineralization of teeth

has been the backbone for this type of research.

Dentifrices have been around for more than 2000 years [BAR04, WIN98] but only

since Muhler's development of fluoride toothpastes in 1954 has such a dramatic effect

been found in caries prevention. Within a few years Colgate Palmolive Company and

Proctor & Gamble began marketing their own fluoride toothpastes. Decades later, the

daily use of fluoride toothpaste is still accepted as the best and simplest method of caries

prevention and tooth remineralization for large populations. Although a debate still

exists in the dental community concerning optimization methods of tooth

remineralization, it is clear that fluoride ion plays a significant role in the

remineralization process. Early caries treatment is especially important with dentifrices









to reduce the risk of treating advanced stages of the disease. If individuals can effectively

fight caries with daily brushing, the need for restorative treatment can be avoided.

The invention of bioactive glasses (Bioglass ) by Dr. Larry Hench in the late

1960s provided medical researchers with a new tool to enhance bone formation that

facilitates the body's own natural processes. This provided a whole new approach to

biomaterials. My experience in bio-inert materials was pushed aside by the possibility of

investigating new and more advanced uses for this "bio-interactive" technology.

Through good fortune and the support of Novamin Technologies Inc., I found an

opportunity in dental research to explore the nature of bioactive glasses in dentifrice and

its potential to provide the oral environment with the essential elements (Ca and P) to

remineralize teeth.

The novelty of "Bioglass toothpaste" for this study is evident but doesn't stop

here. Just as treatment methods for early caries progress, the need for newer and better

technologies in assessing caries treatment continues to grow as well. Previous analysis

methods for tooth remineralization do not provide the scope for detailing finer

quantification of early stage caries. Transverse microradiography (TMR) and

microindentation are still sound techniques for dental caries research, but do not provide

the scale or quantification requirements that current and future investigators need to

progress. In the case of caries, in vitro studies in the past used TMR for subsurface caries

lesion analysis. But early caries detection and treatment requires surface analysis and

less time. Confocal laser scanning microscopy (CLSM) and nanoindentation were viable

candidates to explore early caries treatment for more current research objectives. The

aims of this study are as follows:






4


* Compare and contrast the benefits and drawbacks of confocal laser scanning
microscopy and nanoindentation for analysis of tooth remineralization in
vitro.

* Determine the remineralization capabilities of a Novamin bioactivee glass)
containing dentifrice on human enamel in vitro.














CHAPTER 2
FUNDAMENTALS OF THE TOOTH

Basic tooth structure is based on layers and function. Teeth, like bone, are

comprised of soft inner layers to provide nutrients and growth function, whereas outer

layers are designed for structure and protection. This dual nature provides a research

perspective conducive to remineralization studies. In essence, if the tooth is generally

viewed as "living," the prospect of it regenerating or remineralizing becomes no different

than with any other biological tissue. On a larger scale, it is advantageous to consider the

oral cavity itself as a unique and independent ecosystem. The primary tissues involved in

caries research are dentin and enamel. Early caries studies focus almost entirely on

enamel and its surface layers.

Enamel is the visible outer layer of the tooth. It is translucent; and can vary in

color from yellowish to grayish white. The different colors of enamel may be attributed

to variations in thickness, translucent proprieties, the quality of the crystal structure, and

surface stains. Enamel (Figure 2-1) is the calcified substance that covers the entire

anatomic crown of the tooth and protects the dentin and pulp. It is the hardest tissue in the

human body and consists of approximately 97% inorganic minerals, 1.5% organic

materials, and 1.5% water [LEG88]. Calcium and phosphorus (as hydroxyapatite) are its

main inorganic components. Enamel can endure crushing pressure of approximately 700

MPa. A layering of the dentin and periodontium below produces a cushioning effect of

the tooth's different structures, enabling it to endure the pressures of mastication.

Structurally, enamel is composed of millions of rods or prisms. Each rod begins at the









dentino-enamel junction (zone between the enamel and dentin) and extends to the outer

surface of the crown. Enamel is formed by epithelial cells (ameloblasts) that lose their

functional ability when the crown of the tooth has been completed. Therefore, enamel,

after formation, has no power of further growth or repair, only mineral gain and loss.

Dentin (Figure 2-1) is the light yellow tissue beneath the enamel. It is more

radiolucent than enamel, very porous, and it constitutes the largest portion of the tooth.

The pulp chamber is located on the internal surface of the dentin walls. Dentin is harder

than bone but softer than enamel and consists of approximately 70% inorganic matter and

20% organic matter and 10% water [LEG88]. Calcium and phosphorus are its chief

inorganic components. Dentin is a living tissue and must be protected during operative

or prosthetic procedures from dehydration and thermal shock. The dentin is perforated by

tubules (similar to tiny straws) that run between the cemento-enamel junction (CEJ) and

the pulp. Cell processes from the pulp reach part way into the tubules like fingers. These

cell processes create new dentin and mineralize it. Dentin transmits pain stimuli by the

way of dentinal fibers and hydrostatic pressure within the tubules. Because dentin is a

living tissue, it has the ability for constant growth and repair that reacts to physiologic

(functional) and pathologic (disease) stimuli.

Hydroxyapatite

Calcium phosphates are the most important inorganic constituent of biological hard

tissues. In the form of carbonated hydroxyapatite, they are present in tendons, bone and

teeth to give these organs stability, hardness and function. The maturation of tooth

enamel and dentin involves the second major calcification process in mammals, bone

being the first. The formula for tooth mineral in enamel consists primarily of calcium

hydroxyapatite, Calo(P04)6(OH)2.










mI / "- ..j-II enamel

crown dentin
I pulp cavity
gum
neck gingivaa)

n .~ momentum

peridontal
5 membrane

root canal
root
iI bone



foramen


Figure 2-1 Anatomy of the human tooth. Note: Taken from Dentistry and Real Life
http:// users.hol.gr/-jelian/anatomy.htm with requested permission. [DRL03]

Human biological apatites can vary considerably based upon formation and

functional conditions as well as basic Ca:P molar ratios. Enamel maintains a Ca:P ratio

of nearly 1.63 as compared with the general hydroxyapatite (HA) ratio, 1.67 and bone,

1.71 [LEG88]. The similarities of biological apatite in enamel to pure HA make it

possible for researchers to develop chemical relationships to study tooth minerals. Tooth

enamel contains parallel crystals of biological apatite, which are much larger than those

of bone and dentin. The needle-like crystal rods may grow to 100 |pm in length and 50

nm wide. The enamel maturation process is vital to understand the solution dynamics

involved for this study. Calcium deficient hydroxyapatite (CDHA) is the main

constituent of developing enamel and considered to be quite carbonated. The subsequent

mineral formation is driven by the introduction of Ca2+ into the apatite and simultaneous

loss of carbonate. Featherstone describes newly mineralized bone and teeth as

carbonated hydroxyapatite (CAP), which is essentially impure HA, represented by the









formula Calo-x (Na)x (PO4)6-y (CO3)z (OH)2-u (F)u [FEAOO]. In enamel this "calcification"

produces a mineral increase from 45 wt% to 96 98 wt% and a significant rise in Ca:P

molar ratio. This ultimately results in the most highly mineralized and hardest skeletal

tissue in the body [DOR02]. Although the biological apatite of tooth enamel varies due

to crystal impurities and apatite combinations, ionic exchange at the enamel surface is

based primarily upon calcium phosphate solubilities.

At neutral pH, saliva is supersaturated with calcium and phosphate. This saturation

is necessary to counter the recurrent acid challenge of dietary cycles and residual sugars

which are used by oral bacteria to create acid via fermentation. These ions, among

others, give saliva a buffering capacity in addition to a mineralization reservoir. Below

the critical pH of human enamel, 5.2 to 5.5 [LAR03], the dissolution of enamel mineral

follows basic solubility laws for hydroxyapatite. At lower pH, the dissolution of apatite

mineral continues until the oral pH returns to normal. Along with saliva's own buffering

capacity, salivary bicarbonate in equilibrium with CO2 from the respiratory process shifts

the equilibrium to a more alkaline condition. The state of subsaturation for calcium and

phosphate at lower pH prevents remineralization under these conditions. When oral pH

again rises above "critical" status, the calcium/phosphate saturation of the saliva again

supercedes that of the enamel and mineral deposition begins. Due to the supersaturation

condition of saliva at pH 7.0, one would expect that hydroxyapatite mineral would

continue to form on the enamel surface. As in many cases within the human body

however, this micro-environment is equipped with its own checks and balances. The

supersaturation of calcium and phosphate does not over mineralize teeth because of the

protein rich film (tooth pellicle) on the enamel surface. It is thought that saliva proteins









also play a part in maintaining this balance. Still the question remains, what if the

salivary pH becomes severely alkaline? This increase in pH results in dental calculus,

essentially calcium phosphate precipitate in the plaque fluid. A broader look at the

demineralization and remineralization process is necessary, especially when topical

fluoride is introduced into the system.

Demineralization and Remineralization Phenomena

Generally, the life of dental hard tissues is well understood and research has

revealed the structures and concepts involved in natural processes of the oral

environment. The nature of these tissues and how they behave under certain conditions is

clear, but what is not clear is the degree to which these natural processes can be

influenced or even accelerated. Over the course of human life, enamel and dentin

undergo unlimited cycles of demineralization and remineralization. The debate involves

ways to measure and influence this process. Years of work have brought fluoride to the

forefront of remineralization studies and application. But the influence of fluoride on

demineralization and remineralization of enamel is yet to be agreed upon. The crux of

this argument lies in the balance of these two competing phenomena. A tip in the balance

one way or the other will either lead to stronger healthier teeth or greater susceptibility to

dental caries and other oral complications. To promote general oral healthcare, the use of

fluoride toothpastes for daily promotion of remineralization has become standard

practice. Evaluating these tissues at different stages of the oral cycle and measuring the

optical and mechanical properties are the key to determining a net increase or decrease in

mineral flux.

The consumption of simple dietary sugars (particularly monosaccharides and

disaccharides like Sucrose) provides not only nourishment for our bodies but also a food









source for oral bacteria. As bacteria making up the normal oral flora adhere to the

pellicle, a bacterial mass or film called plaque is formed [MAR99]. The plaque bacteria,

particularly Streptococcus mutans and lactobacilli, ingest sugars for glycolysis to

produce weak organic acids (such as lactic, pyruvic, acetic acid). These acids lower the

surface pH and diffuse through the plaque and into the tooth, leaching calcium and

phosphate from the enamel. At this time the plaque pH may have dropped to 4.0 4.5

[WIN98]. This mineral loss compromises the mechanical structure of the tooth and could

lead to cavitation over a long period of time. The stages of caries progression are clear

and in the interest of preventive maintenance, early carious lesions appear to be the best

opportunity for countering this destructive process. As alluded to earlier, saliva alone has

the capability to increase plaque pH with bicarbonates although typically this process

may take up to 2 hours. The susceptibility of apatite in enamel surface layers makes it

critical to control the acidity of the plaque fluid and the Ca2+ and PO43- ion concentrations

in saliva [FEAOO]. The subsequent remineralization process is nearly the reverse. When

oral pH returns to near neutral, Ca2+ and PO43- ions in saliva incorporate themselves into

the depleted mineral layers of enamel as new apatite. The demineralized zones in the

crystal lattice act as nucleation sites for new mineral deposition. In the presence of

fluoride (at high concentrations), the original CAP loses its remaining carbonate and is

replaced with a hybrid of hydroxyapatite (HAP) and fluorapatite (FAP) [FEAOO]. This

cycle is fundamentally dependent upon enamel solubility and ion gradients. Essentially,

the sudden drop in pH following meals produces an undersaturation of those essential

ions (Ca2+ and P043-) in the plaque fluid with respect to tooth mineral. This promotes the

dissolution of the enamel. At elevated pH, the ionic supersaturation of plaque shifts the









equilibrium the other way, causing a mineral deposition in the tooth. Figure 2-2 provides

an overview of these processes.



S Enamel ... Plaque Saliva


Figure 2-2 Cycle of demineralization and remineralization in enamel. Note: Taken from
Winston AE, Bhaskar SN, "Caries prevention in the 21st century." JAm Dent
Assoc 129, p 1579 1587, 1998 with requested permission.

Saliva

Saliva plays multiple roles in these oral processes. Aside from providing a constant

rinse, the value of saliva as a reservoir for calcium, phosphate and fluoride has been well

established [LAR03]. Saliva offers a myriad of other benefits, although many are not

widely known but contribute significantly to enamel remineralization.

The buffering capacity of human saliva plays a major role in countering

fluctuations in pH. Again, acidic beverages and/or sugary foods cause temporary pH

drops during which demineralization is accelerated. Stephan's work in 1944 found

patients with little to no caries activity maintained a resting salivary pH of 7.0-7.2

[STE44]. It was later understood that bicarbonates in saliva played a major role in

elevating low oral pH after meals. The relationship of bicarbonate concentrations in









saliva and blood has led other investigators to study caries incidence relative to blood

properties [BAC99, BIE04]. Other buffers present in saliva include urea proteins.

Urease enzyme in plaque fluid metabolizes urea producing ammonia and an increase in

plaque pH. Arginine rich proteins in saliva can also metabolize into alkaline substances

such as arginine and ammonia. Phosphate has also been found to contribute to buffering

capabilities [DOW99]. Various salivary components also demonstrate antibacterial

capability. Iron binding protein lactoferrin has been shown to inhibit aerobic and

facultative anaerobic bacteria (such as Streptococcus mutans) which require iron to

metabolize. Lysozyme also exhibits direct antibacterial function. This enzyme, well

known for its presence in tear and nasal secretions (discovered by Fleming in 1922),

complexes with salivary ions such as bicarbonate, iodine and fluorine, which bind to

bacterial cell walls and induce autolysis [AMA01, DOW99].

It is clear that a reduction in salivary flow or its constituents would negatively

affect our capability to fight caries [DOW99]. Xerostomia is defined as the perception of

oral dryness or hyposalivation. This is due to any number of factors including radiation,

medication and diseases such as diabetes. The most widely studied disease state affecting

salivary function is exocrinopathy or Sjogren 's Syndrome. This autoimmune disease is

characterized by inflammation of glands with lymphocyte infiltration. Secretory

components begin to deteriorate causing decreased saliva flow. Specifically, the

reduction in salivary flow has been associated with a marked increase in caries incidence

[DRE76]. Although a number of remineralizing factors are affected, the near absence of

calcium and phosphate from the oral cavity cannot be compensated. Remineralization

becomes nearly impossible without key constituents. External constituents, such as






13


fluoride from dentifrice, have been proven to positively affect remineralization. The

influence of saliva in the presence of topical fluoride to form greater levels of CaF2 (a

remineralization agent) was demonstrated by Larsen. This relationship also revealed a

decrease in caries incidence [LAR01]. But Larsen also explains that the inter-

dependence of calcium, phosphate, and fluoride enables the remineralization effect.

Therefore, absence of calcium and phosphate (such as Xerostomia) would essentially

deem the presence of fluoride irrelevant.














CHAPTER 3
REMINERALIZATION THEORY

While a great deal has been discovered about the remineralization and

demineralization process, details regarding the mechanism of mineral deposition are not

universally agreed upon. This study focuses on remineralization of early caries lesions

through dentifrice application. This is the most common form of anti-caries treatment

today and has proven quite effective in reducing caries incidence since its introduction.

Dentifrice and Fluoride

Around the turn of the century, primitive toothpastes were primarily used as

abrasive cleansing agents. However, a relative explosion in dental caries forced scientific

investigation to aid the affected populations. Bibby continued the functional

development of toothpastes in 1942 with the first clinical trials of fluoride toothpastes.

Proctor & Gamble later introduced a fluoride toothpaste in 1955 using a stannous fluoride

agent. Further development led to sodium monofluorophosphate (Na2PO4F) and sodium

fluoride (NaF) agents in dentifrice. Fluoride levels are generally regulated to 1100 ppm

in modern commercial toothpastes. This topical application of fluoride is one of two

ways it is believed to promote remineralization.

Systemic Benefit

Recently, scientific studies have shown the systemic action of fluoride is

secondary to a topical application. It is nevertheless a contributor to the overall beneficial

effect for prevention of tooth decay. Over time, fluoride ions ingested from fluoridated

water, fluoridated milk, fluoride supplements and foods processed with fluoridated water,









can become incorporated into the structure of developing teeth. It has also been

suggested that teeth which develop under the effect of systemic fluorides have shallower

pits and grooves, allowing less trapping of oral bacteria and food particles, which

contribute to tooth decay [BRU90].

Topical Benefit

On the surface of the tooth, there is a constant exchange of mineral ions between

crystals of the enamel surface and plaque fluid bathing the tooth surfaces. When topical

fluorides are regularly applied to the teeth (through toothpastes, mouth rinses, fluoridated

water, and professional applications) it is possible that even a poorly mineralized enamel

surface can be progressively improved by the natural fluoride exchange equilibrium

described above. In the case of early caries lesions, studies have shown the availability of

fluoride in relatively low concentrations to result in the arrest and even remineralization

of targeted enamel. From this information, we see that adults may benefit from fluorides

as well as children via the topical benefit of fluorides [BRU90, ISM95].

Extensive studies on the use of dentifrice to introduce fluoride have proven its anti-

caries efficacy. Fluoride has three principal topical mechanisms: 1) Free fluoride ion

combines with H+ to produce hydrogen fluoride, which migrates throughout acidified

plaque. This ionized form is lipophilic and can readily penetrate bacterial membranes.

Bacterial cytoplasm is relatively alkaline, which forces the dissociation of H+ and F-.

Fluoride ion inhibits various cellular enzymes key to sugar metabolism. Hydrogen ions

simultaneously acidify the cytoplasm, thus slowing cellular activities and inhibiting

bacterial function. [MAR99] 2) Fluoride integrated in the enamel surface (as fluorapatite,

FAP) makes enamel more resistant to demineralization than HAP during acid challenge.

3) Fluoridated saliva not only decreases critical pH, but also further inhibits









demineralization of the deposited CaF2 at the tooth surface. Fluoride at the enamel

surface has been found to attract and "bind" to calcium ions. This enhances nucleation of

new mineral at specific demineralized zones. Computer simulation has advanced this

assertion and supported previous studies involving fluoride applications on early caries or

surface lesions [FEAOO, PEP04].

The exact mechanism of fluoride in the surface lesions is still not well understood

but a few details are clear. The generalized reaction of HAP (enamel) dissolution below

critical pH:

Calo(P04)6(OH)2 + 8H+ 10Ca2+ + 6HP042- + 2H20 (3-1)

But fluoride application allows for a substitution reaction to produce FAP

Calo(P04)6(OH)2 + 2F- Caio(P04)6F2 + 20H- (3-2)

FAP is more resistant to acid challenge due to lower solubility ( HAP Ksp = 2.34x10-59

and FAP Ksp = 3.16x10-60 ) [LAR03]. In fact, even before FAP formation, ten Cate,

Nelson, Featherstone and colleagues found that topical fluoride levels in solution around

synthetic CAP (3% carbonate similar to enamel) reduced solubility significantly. The

same studies revealed that systemic fluoride incorporation did not significantly improve

apatitic acid resistance, thus arguing in favor of topical applications throughout lifetime

as the best defense against early caries [FEAOO, TEN99].

The argument for topical fluoride gained strength with a number of in vitro and in

situ studies involving dentifrice sources and inherent fluoridated enamel. Ten Cate and

Duijsters found that enamel dissolution was primarily a function of two external

properties, pH and fluoride concentration of surrounding solution. Low pH environments

were especially sensitive. Transverse microradiography (TMR) confirmed that surface









lesion depth and mineral loss were a direct function of fluoride concentrations. Figure 3-1

summarizes their findings graphically [TEN83, TEN99].


5-




4-

2-








lSl


2' PH
50

Figure 3-1 Mineral Loss (expressed as Ca output) as a function of pH and [F-]. Note:
From ten Cate JM, Duijsters PPE, "The influence of fluoride in solution on
tooth enamel demineralization I. Chemical data." Caries Res 17, p 193 199,
1983, courtesy of Karger AG, Basel with permission.

Later studies found that low concentrations of fluoride, ambient 0.06 ppm and

dentifrice 1100 ppm, significantly decreased enamel mineral loss by 5% and 9%

respectively under simple pH cycling regimen [TEN95]. Histomorphometric analysis of

lesions after remineralization with fluoride found the new enamel crystallites to be

dimensionally larger than the original sound enamel. They were also found to be

randomly oriented (lacking organization) which rendered this enamel slightly less dense.

Still the ability of fluoride to bind to free calcium and phosphate, including ions leaving

the tooth during demineralization to form FAP reduces the probability for enamel









dissolution in subsequent acid challenges. Because FAP is much less soluble and has

significantly less buffering capacity, an even lower pH will be required to force enamel

dissolution [SHE94].

Bioglass

The discovery of bioactive glasses by Hench in 1969 pushed the boundaries of

biomaterials capability and function. In an era of bio-inert materials and implantation,

Hench determined the critical steps for bioactive glass ceramic interaction with the

human body in order to bond. Bioglass is a multi-component inorganic compound

made of elements (silicon, calcium, sodium and phosphorous) naturally found in the body

[HEN96]. The development and success of bioactive glasses is due to their highly

biocompatible nature. Previous implant materials regardless of initial success, failed over

long periods of time. Numerous metals and polymers eventually succumbed to the

aggressive defense mechanisms and corrosive nature of body fluids. Hence, the

advantage of bioactive glass is not only acceptance within the body, but also its ability to

chemically bond. More than three decades of study has revealed the series of reaction

steps involved in Bioglass bonding mechanisms within the body. This study involves

the use ofbioactive glasses in powder particulate form. This form provides easy

dispersion in this dentifrice application and exploits the fact that fine glass powder

particulates resorb much faster than bulk implants. Upon implantation, Bioglass in

aqueous environment immediately begins surface reaction in three phases, leaching and

exchange of cations, network dissolution of SiO2, and precipitation of calcium and

phosphate to form an apatite layer. The 5 critical stages for glass surface reactions are

detailed below [HEN93].










STAGE
1 Rapid exchange of NaC or K+ with H+ or H30O from solution:
Si O Na* +H + OH- SI-OH+ Na+ (solution) OH"
This stage is usually controlled by diffusion and exhibits a r -12 dependence.

2 Loss of soluble silica In the form of Si(OH)4 to the solution, resulting from breaking
of SI-O-SI bonds and formation of Si-OH (sllanols) at the glass solution Interface:
SI O SI + H0 H Si OH OH SI
This stage is usually controlled by interfacial reaction and exhibits a 1'.0 dependence.

3 :Condensation and repolymerizatlon of a SIO2-rich layer on the surface depleted In
alkalis and alkaline-earth cations:


I I I I
0 0 0 0
0 91 OH + HO -di 0 0 SI 0 S1 0 + HO

3-
O O O O

4 Migration of Ca2* and PO groups to the surface through the SIO2-rich layer forming a
.: Ca-P205-rlch film on top of the Si02-rlch layer, followed by growth of the amorphous
CaO-P2Os-rich film by Incorporation of soluble calcium and phosphates from solution.

5 Crystallization of the amorphous CaO-P205 film by Incorporation of OH- CO2-or F
anions from solution to form a mixed hydroxyl, carbonate, fluorapatite layer.
Figure 3-2 Stages of bioactive glass surface reactions. Note: Taken from Hench LL,
Wilson J, An Introduction to Bioceramics Singapore, World Scientific
Publishing, 1993 with requested permission.

The initial Na+ and H+/H30+ ion exchange and de-alkinization of the glass surface

layer is quite rapid, within minutes of implantation and exposure to body fluids. The net

negative charge on the surface and loss of sodium causes localized breakdown of the

silica network with the resultant formation of (silanol) Si(OH) groups, which then

repolymerize into a silica rich surface layer [GRE99]. This stage involves the base

catalyzed hydrolysis of Si-O-Si bonds of the glass structure. This mechanism is based on

previously well documented corrosion studies of alkali silicate glasses as well as infrared

spectroscopy studies that appear to show the formation of nonbridging oxygen species

following Si-O-Si bond breakage [HIL96]. The subsequent stages (4 and 5) involve the

development of silica rich and amorphous calcium phosphate layers respectively. These









stages incorporate the noticeable presence of biological moieties such as blood proteins,

growth factors and collagen. Within 3-6 h in vitro, the calcium phosphate layer will

crystallize into the carbonated hydroxyapatite (CAP) layer which is essentially the

bonding layer. Chemically and structurally, this apatite is nearly identical to bone and

tooth mineral, thus allowing the body to attach directly to it. These Bioglass surface

reactions from implantation to 100-150 |tm CAP layer formation takes 12 to 24 h

[HEN93, KON02]. Hench further investigated 6 more stages primarily focused around

bone and tissue bonding. These stages, although not completely understood, depend

significantly on the specific formulation of Bioglass involved.

The standard for Bioglass formulation is commonly known as 45S5 which has

been used extensively in research studies. It contains 45 wt% SiO2, 24.5 wt% Na20 and

CaO, and 6 wt% P205. Bioactive glasses have traditionally kept the P205 fraction

constant while varying the SiO2 content. In fact, the network breakdown of silica by OH-

was found to be time dependant upon the concentration of SiO2. It is now understood

that keeping the silica below 60 wt% and maintaining a high CaO/P205 ratio guarantees a

highly reactive surface.

Novamin, a trade name for bioactive glass, is manufactured by Novamin

Technologies Inc. (Alachua, FL). The material is reported to have a long record of safety

and efficacy as an implant material used to regenerate new bone in defects [GRE99].

When bioactive glass is incorporated into toothpaste formulations, the ions released from

the amorphous calcium phosphate layer are believed to contribute to the remineralization

process of the tooth surface. Bioactive glasses have been successfully used clinically as

bone grafting material for over 15 years and has been cleared by the FDA for use in oral









and orthopedic bone grafting for nearly 10 years [LOW96, WIL87]. Bioglass is also

marketed worldwide under the trade names PerioGlas and NovaBone. Recently, it

has been demonstrated that fine particulate bioactive glasses (<90 [tm) incorporated into

an aqueous dentifrice have the ability to clinically reduce the tooth hypersensitivity

through the occlusion of dentinal tubules by the formation of the CAP layer [LIT97].

Investigators using bioactive glass compositions have demonstrated a significant anti-

microbial effect towards caries pathogens (S. mutans, S. sanguis) upon exposure to

bioactive glass powders as well as solutions and extracts [ALL01, ST096, ST098].

Despite advances in oral care over the last 40 years in large part because of the

incorporation of fluoride into a large number of products there are still greater than 150

million cavities filled in the US every year [ADA90] at an estimated cost of $12 20

billion. A substantial number of these cavities result from inadequate saliva, without

which fluoride is of limited value [LEO01, SPA94]. This study could benefit many

individuals who experience reduced calcium, phosphate and fluoride ions caused by

hyposalivation resulting from old age, prescription drug use, Sjogren's Syndrome,

diabetes and radiation therapy. In addition, women are at increased caries risk due to

inadequate salivary calcium levels at different points in their lives including ovulation,

pregnancy and post-menopause, resulting in the same net effect as reduced saliva fluoride

efficacy. Approximately 9.7% of the general population is estimated to have insufficient

salivation or chronic xerostomia [PUJ98], representing over 20 million Americans who

consume $100-160 million annually in consumer toothpaste. Currently there are several

additional strategies for preventing, reversing or arresting the caries process including

application of fluorides, sealants, anti-microbials, salivary enhancers as well as patient









education [NHI01]. But these approaches represent a time and financial investment for

the patient. A reformulation of fluoride dentifrice containing Novamin can 1) enhance

remineralization, 2) counteract demineralization, 3) control anti-caries activity more

effectively than current fluoride toothpastes and may be a significant oral health

contribution to the general population.

The Novamin dentifrice used in this study resembles commercial toothpaste

formulations, including the Colgate Regular product also used in this study.

Novamin 4505 was added at 5 wt% in addition to the 1100 ppm fluoride (from sodium

monofluorophosphate) which is used in current Colgate products. The Novamin

4505 used in this study is similar to 45S5 glass compositionally, but is designed to be a 5

jtm diameter particle. Particle size distribution of this particulate ranges from 15 ptm to

less than 1 ptm and serves as a more highly reactive bioactive glass because of its greater

surface to volume ratio.














CHAPTER 4
MINERAL QUANTIFICATION

The quantification of apatite mineral in dental studies is an evolving process. The

past 50 years have shown us that a greater understanding of dental caries also brought

about the explosion of consumer products to treat the disease. The United States Food

and Drug Administration (FDA) is the regulatory agency for potential consumer products

to be marketed to the masses. Although widely understood that early dentifrices cleaned

the teeth well, the degree of efficacy in fighting dental caries was yet to be defined.

Traditionally, scientific study requires researchers to provide proofs and logic to support

their hypotheses. Qualification of these scientific arguments only provides proof of their

existence but quantification provides the magnitude to which those arguments can be

made and compared. Dental researchers understood the role of tooth mineral as the

exchange commodity in cariology study. To justify research efforts, the FDA and

numerous researchers have developed a number of analytical techniques to quantify the

remineralization of teeth and caries resistance.


Microradiography


Transverse microradiography (TMR) has been developed over time to become the

standard bearer for tooth remineralization studies. Theoretically it draws similarity to

other photo-X ray techniques. The ability to quantify mineral density changes in addition

to obvious visual qualification makes TMR the most powerful and trusted technique

among researchers.









Sample preparation for TMR requires thin (typically <100 [tm) sections that are

polished and glued to glass slides. A cathode tube source emits polychromatic X-rays that

are filtered (Ni) and directed toward the reference system and specimen. Figure 4-1

depicts the TMR setup in detail [MJ086].


















R S

Figure 4-1 Microradiography setup (A) Cathode Tube, (B) light-proof casing, (T) filter,
(R) reference step wedge, (S) specimen, and (F) radiographic film

Filtered monochromatic X rays reach the specimen and reference system, usually

an aluminum step wedge. Typically these steps range from 10 150 |tm. Gelhard and

Arends further developed this method running at 20 kV and 15-20 amp to produce a

wavelength of k = 0.15 nm, which in turn guaranteed 95% absorption inorganic (mineral)

[GEL84, MJ086]. Sample thickness was estimated to 1 tm accuracy on the radiographic

film. Quantification of enamel tooth mineral is made with microdensitometric tracing of

the film using specially designed software. Angmar's formula is used to calculate

relative mineral volumes with respect to cross-sectional depth with transverse

microradiography [DIJ86].










V 50.48 (ta/t,) 100% (4-1)

where ta is the step wedge thickness and ts is the specimen thickness

The volume percent mineral is then plotted against cross sectional depth from the surface

to graphically produce the Z parameter. Z is defined as the integrated area under the

densitometric tracing for each specimen in units of vol% mineral |tm. Plotting Z for

both sound (original) and demineralized enamel provides a direct quantified measurement

of mineral density flux, thus the parameter AZ [GEL84]. A decrease in AZ for a

specimen therefore signifies mineral deposition de novo. This is represented in Figure 4-

2 [DIJ86].


100-

A

Vol%


50







0 50 100 150
depth (d) pm -
Figure 4-2 Mineral density quantified as AZ in TMR.

Confocal Laser Scanning Microscopy

Confocal laser scanning microscopy (CLSM) is a powerful imaging technique

based upon optical behavior of light within specimens. Generally, confocal analysis

features excitation (such as fluorescence) although emission detection is also possible.

The fluorescence phenomenon involves the absorption of light of a given wavelength by









a fluorescent molecule followed by the emission of light at longer wavelengths.

Fluorescence detection has three major advantages over other light-based investigation

methods: high sensitivity, high speed, and safety, not only for the operator but also for the

sample because they are not affected or destroyed in this process [WEB99]. This

provides an extremely fine scale view of desired specimen planes. Although planar

imaging is common among many techniques, the ability of CLSM to analyze thick (3D)

specimens makes it so valuable in the scientific community. Conceptually, real time

imaging of a single specimen for various depths or "sections" provides researchers with a

practical advantage, especially considering biological specimens and time dependant

reactions. For this study, it is imperative that fluorescence detection and imaging be

understood. A typical confocal microscope is shown in Figure 4-3.


Scanning
Engine j^t Laser(s)



i s__ler \ llDetector(s)







Figure 4-3 Schematic of a generic confocal microscope.

The theory of CLSM originates with the laser light source being focused upon the

specimen and reflected back through a dichroic mirror to the detector. For fluorescence,

the light returning from the sample through the objective lens reaches the beam splitting

mirror and passes as excitation light. The detector processes the image based upon the










focal intensities it receives through this path [WEB99]. Figure 4-4 depicts the imaging of

a thick sample in greater detail.

Image of thick sample _


Extended light source















Thick sample
Illumination light path Collection light path


Figure 4-4 Conventional microscope for fluorescence in epitaxial configuration.

The florescence of teeth was observed by both dye-assisted imaging and auto-

fluorescence. Preliminary confocal work performed at the McKnight Brain Institute

(University of Florida) showed a significant degree of autofluorescence in demineralized

enamel through both fluorescein isothiocyanate (FITC) and Texas Red filters.

Autofluorescence of enamel has been shown to significantly correlate with TMR and

dye-assisted CLSM results [BEN89, GON99]. Similar work on nearly identical

specimens was performed at the Oral Health Research Institute at Indiana University-

Purdue University at Indianapolis (IUPUI). Both sets of imaging revealed again a high

fluorescing lesion compared to near zero for sound enamel. Fontana and Gonzales-

Cabezas [FON96][GON99] have shown a significant correlation between cross-sectional









confocal analysis in fluorescence and mineral flux. Their fluoride toothpaste studies

revealed a significant match for two parameters (lesion area and total fluorescence)

between confocal microscopy and microradiography of tooth enamel and dentin. In vitro

demineralization and remineralization of surface enamel was easily imaged and

quantified. Significant correlations were also found for half tooth (thick sections)

remineralization comparing TMR and confocal microscopy. For remineralization

studies, AZ pre and post remineralization were combined as a single parameter

representing their mathematical difference, AM. Fontana and Gonzales found Pearson

correlation coefficients (perfect correlation = 1) of 0.71 for AM vs. total lesion area and

0.70 for AM vs. total fluorescence. A previous study involving only demineralized

samples were found to correlate with TMR to the same degree. A subsequent study also

revealed nearly identical correlation in mineral changes with TMR when comparing

autofluorescence (no dye) of enamel vs. the dye. The fluorescent dye (0.1 mM

Rhodamin B) was used to enhance fluorescence imaging of lesion areas in these studies

[BEN89].

Nanoindentation

Nanoindentation is a relatively new method for characterization of material

mechanical properties on a very fine scale. Features less than 100 nm across and thin

films less than 5 nm thick can be evaluated. Test methods include indentation for

comparative and quantitative hardness measurements, and scratching for evaluation of

wear resistance and thin film adhesion. Nanoindentation is often performed in

conjunction with atomic force microscopy (AFM). The area for testing is located by

AFM imaging and indentations are imaged by AFM after testing. A three-sided, pyramid-










shaped diamond probe tip (Berkovich) is commonly used for sample indenting and

scratching. For indenting, the probe is forced into the surface at a selected rate and

maximum force. For scratching, the probe is dragged across the sample surface while

controlling the force, rate, length, and angles. Imaging is performed using the same probe

for intermittent contact via (tapping mode) AFM. A force-displacement curve based on

piezoresistive loading is generated during indentation and provides further indication of

mechanical properties [HAN03, HYS03].







-s-- sgle crystal alumrinum



NOW. .- ..' ', ... .. ... .. .













Triboindentor (Hysitron Inc., Minneapolis, MN): hardness and reduced modulus. The

load-displacement data from the unloading curve is fit to a power law relation to

determine mechanical properties.

P=A(ho-hf)" (4-2)

where m 2 (conical), P is the load, h, is the initial depth, hf isfinal depth, and A
is the area.
is the area.










The derivative of this power law with respect to the depth gives the material stiffness (S)

at the maximum load, Pmax

S = dP/dh at P,,ax (4-3)

The contact depth, he, can be calculated from:

h = hmax 0.75(Pmax/ S) (4-4)

The hardness, H, and reduced modulus, Er are then found by:

H = Pma /Ah (4-5)

E, =[S T] / [2 V(Ahc)] (4-6)

A typical loading curve for sound enamel is shown in Figure 4-6.


Control Enamel 1

1800
1600
1400
12100
1000

-o 600
200

0 *. .

Contact Depth (nm)

Figure 4-6 Loading curve for nanoindentation on sound enamel.

Microindentation

Microindentation, on the other hand, is a more established technique for measuring

the hardness of materials. For ceramics in particular, hardness is a critical mechanical

property. For engineering and characterization applications, approximately 60% of

worldwide published ceramic hardness values are obtained using Vickers diamond









indentation, with loads typically in the range of a few Newtons to 9.8 N (1 kg-f) and

occasional data for high-toughness ceramics as high as 98 N (10 kg-f). At small

indentation loads, problems arise from the load dependence of hardness and from

measurement uncertainty due to the small indentation size. At higher loads, cracking and

fracture become problems in some cases, making measurement impossible. Typical hard

ceramics have Vickers hardness in the 10-30 GPa range [QUI98]. Teeth particularly are

considered ceramic composites. Typical hardness values for enamel range from 2.9-3.9

GPa and dentin 0.6 GPa [FOR91, WIL92].









-^ "12 "__f n


Figure 4-7 Vickers microhardness testing.

The square pyramidal shape of the Vickers indenter creates a smaller deeper

impression, although more likely to crack than Knoop indentations. ASTM standard E

384, Microhardness of Materials, covers Vickers hardness; C 1327 is the new standard

for Vickers hardness of advanced ceramics and recommends a load of 9.8 N. The

universal standard [ASTOO] for this calculation is described in equation 4-7.


Vickers Hardness (HV) = 1854.4 P/d2 (4-7)

where P is the load in grams force
d is the mean diagonal length of indentation in jn,
assuming standard face angle of indenter tip is 136










To obtain GPa from these units (kgf/mm2), multiply by 0.0098. ASTM standard E

384 section 7.1 clearly states that optimum accuracy of measurement requires that the

specimen be flat with a polished or otherwise suitably prepared surface [ASTOO].

Although it was developed in the 1920s (by engineers at Vickers Ltd. in the U.K.), this

hardness testing method continues to be a significant standard, especially for hard

ceramics.

The analysis theory is quite different for these two techniques. As mentioned

earlier, nanoindentation is based upon a loading/unloading curve calculation whereas

Vickers is solely dependent upon the indent surface area. Oliver and Pharr demonstrated

(Table 4-1 and 4-2) that results from the two techniques are comparable [OLI92].

Table 4-1 Comparison of hardness and Young's modulus data obtained with Vickers and
Berkovich indenters.
MATF R[AL BE RKIUl ICI INDENTER VICKERS INDENTER
Elastic McAl lus IlarlIness [f. a.; LIas-ij. Mu ludu HaJd1e-L [GiP-1i
[GPa4L [nal
Sa,1 Iiur -13 .' 92 4% I. 14
Soda Lime Glass 78 5.24 82 6.18
Fused Silica 72 8.44 71 8.87
Silicn 169 11.13 169 12.32
Window Glass 77 5.96 78 7.14
Alumina 449 26.10 439 25.89
Niik] 218 57 218 6.73
Imriu 'ITitanate 228 11.54 219 11.92

Table 4-2 Comparison of indentation fracture toughness obtained by different indenters.
Insrumentl Indenter Tip MwLerial K MPa m'a ms!de
anjoindenler Cube Corer Silicon a-ide 3 6)3 (O 5[,
WilsoruTukon Vickers Silicon Carbide 2.18 i$) 3)
Nanomdenter Vickcre Bariur Titanale 0.66 (0.15)
Wilson"Tuln Vi:ck'-i Barium Tilia-e 2 ', (0 6 l'O 2 F,

Preliminary evaluation of the nanoindentation technique was performed and

imaged for bovine enamel with AFM in conjunction with nanoindentation, see Figures 4-

8 and 4-9.

























Figure 4-8 Demineralized Enamel. Axis units in tm.



































Figure 4-9 Indents in Novamin treated enamel. Axis units in tm.

Imaging for preliminary Vickers indents was performed with SEM for bovine

enamel also. All Vickers testing for this group was performed at 300g load with 15s

dwell time.


Figure 4-10 Vickers indents in control enamel @ 300X and 2000X.














CHAPTER 5
EXPERIMENTAL METHODS AND MATERIALS

Collection and Storage of Teeth

A total of 48 extracted human molars were collected from dental surgical clinics at

the University of Florida. Teeth were required to have intact surfaces, no carious lesions,

and no restorations. In keeping with the Health Insurance Portability and Accountability

Act (HIPAA) regulations, we were not aware of the cause for tooth extraction, age, name,

or gender of the patients. The use of human teeth in this study was also approved by the

Institutional Review Board (IRB) at the University of Florida. A total of 18 (out of 48) of

the collected teeth were randomly segregated for use in this toothpaste study. All teeth

were stored in 0.1% Thymol (w/v) solution until sterilization [BIS03]. Teeth were

cleaned by removal of soft tissue debris and later sterilized using Co60 source for gamma

irradiation. They were subjected to a dose of 500 krad, well above the required 173 krad.

This process does not alter the tooth structure or caries susceptibility [HAN62, WHI94].

Sectioning and Mounting

Each tooth (n = 18) was sectioned into quadrants along both mesiodistal and

buccolingual planes (Figure 5-1) using a diamond tipped circular saw (Buehler Isomet

300 Low Speed). A strict labeling method was assigned to each tooth section designating

tooth number and treatment protocol (see Table 5-1). Sections were mounted

individually in epoxy mounting resin and labeled. A 3-mm diameter treatment window

was opened on each tooth section by grinding off 200 [tm of surface enamel. Removal of

the outer enamel layer was essential for standardizing the experiment.









Table 5-1 Tooth labeling scheme. Example 36BX

Tooth Section Tooth # Label Treatment
Mesiolingual 31 thru 48 LC None (Control)
Distolingual 31 thru 48 LD Demineralized Control
Mesiobuccal 31 thru 48 BX Colgate Regular
Distobuccal 31 thru 48 BY Novamin Dentifrice (5 wt%)

Fluoridation and cyclic remineralization/demineralization of the surface layers

makes each tooth surface inherently inhomogenous. In order to reduce variability, these

layers were removed to expose the virgin enamel deeper in the tissue [RIC89].

Mechanical polishing was performed with successive 120, 400, 600 and 1200 grit paper

and confirmed with a digital caliper.

18 humans molars Each sectioned into quadrants (top view) Each mounted and ground to
expose 3 mm diameter window of
9M lin enamel
LC LD



buncal buccal

Figure 5-1 Experimental flow diagram of specimen sectioning and mounting.

Lesion Formation

Tooth sections LD, BX and BY (Figure 5-2) were demineralized in stirred solution

containing 2.2 mM CaC12, 2.2 mM NaH2PO4, 0.05 M Lactic Acid, and 0.5 ppm F-

adjusted to pH 4.5 with 50% NaOH. Surface lesion formation was maintained at 370C in

order to produce a uniform lesion depth of 100-150 tm [IVA03].

Treatment Regimen

Section BX and BY were treated in a pH cycling regimen including 3:1 toothpaste

solutions of Colgate Regular and Novamin dentifrice respectively. Both dentifrices,

nearly identical in composition, contain 1100 ppm fluoride although the Novamin paste

contains 5 wt% bioactive glass particles in place of silica abrasive. Tooth sections were










immersed cyclically in stirred treatment solutions, demineralization solution (detailed

above) and Fusayama's synthetic saliva [LEU97] for 20 days. Complete pH cycling was

performed at 370C with exception to dentifrice solution treatment, which was done at

250C. The daily treatment regimen (pH cycling) included the following sequence:

To Demineralization solution 30 minutes
To + 0.5 hrs Dentifrice treatment 3 minutes / distilled H20 wash / synthetic saliva
To + 7.5 hrs Demineralization solution 30 minutes
To + 8.0 hrs Dentifrice treatment 3 minutes / distilled H20 wash / synthetic saliva
for 16 h

Cross-sectioning

All four sections of each tooth (LC, LD, BX and BY) were cross-sectioned through

the treatment window to expose the lesion depth along the cross-sectional surface. Two

halves of each tooth section were then available for optical and mechanical analysis.

Throughout the study, tooth sections were intermittently kept in refrigeration for storage

(-60C). Teeth were individually wrapped in distilled water soaked Kimwipes tissue.

lingual Lesion formation (100-150 um) on 3 sections

CONTROL
LC DEMIN




COLGATE NOVAMIN

buccal buccal All 4 sections are
further X-sectioned for
Confocal and
Nanoindentation
2 sections runthru
pH cycling in I
treatment regimen

Figure 5-2 Flow diagram of lesion formation, pH cycling and cross sectioning.






38


Analysis

CLSM was performed on 72 cross-sections (4 sections of 18 teeth) at the Oral

Health Research Institute at Indiana University-Purdue University at Indianapolis

(IUPUI) under the supervision of Dr. Marguerita Fontana. Randomly selected cross-

sections of the remaining halves were chosen for nanoindentation at the Major Analytical

Instrumentation Center (MAIC) at the University of Florida. Microindentation was then

performed (Buehler Ltd. Micromet 3 Microhardness Tester, Lake Bluff, Illinois) on the

same sections for hardness comparison of both techniques.














CHAPTER 6
RESULTS AND DISCUSSION

Cross sectional analysis of enamel lesions with CLSM were based upon digital

images taken at specified controlled conditions. Sound enamel registers near zero

fluorescence (grayscale value 0) and appears pitch black. Lesions slightly

autofluoresce but imbibition of the Rhodamine B dye (0.1 mM) allows the porous

demineralized layer to fill and appear with considerable contrast. Analysis of all samples

was conducted with a specially modified Nikon microscope fitted with Odyssey confocal

capability (Odyssey, Noran Instruments, Inc., Middleton, WI). The accompanied

software (Metamorph version 4.1.6, Universal Images Corp., West Chester, PA)

calculates image-based parameters of selected lesion zones. Using a 10X Nikon

objective, the specimens were illuminated with an argon laser at 50% intensity using a

488 nm excitation wavelength. Confocal slits were set at 25 |tm with a 515 nm long-pass

filter. A 350-[tm characteristic length was randomly chosen within a representative part

of the lesion for each sample. Figure 6-1 shows a typical cross-sectional image.


















Surface Lesion < 350 tun 100oo u















Figure 6-1 Confocal image of enamel lesion, tooth section 38LD

The complete analysis (all 4 tooth sections of 18 teeth) was performed at contrast

level 1500 and minimum threshold value 50 (grayscale). The two parameters that

correlate well with TMR are lesion area and total gray value (or total fluorescence). Data

for these two parameters was collected and tabulated to compare sections within each

tooth to determine changes in lesion properties after 20 days of dentifrice treatment. Data

could not be collected for LC (sound enamel control) naturally because of non-

fluorescence. Parameters are specifically defined as:

Lesion Area -direct summation offluoresced (above threshold) pixels in
characteristic length of lesion.

Total Gray Value direct summation of gray values (0 255) for pixels in i/hin
characteristic length of lesion.










Lesion areas for 3 sections of all 18 teeth were plotted to compare the effect of

Novamin and Colgate dentifrices in Figure 6-2.


40000

35000

30000

25000

S20000
U)
-1 15000

10000

5000


31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Tooth #

MLD (Demin Control) BX(Colgate) E BY(Novamin D3 4505 5%)


Figure 6-2 Lesion area for each tooth for the demineralized control, Colgate and
Novamin treatment condition.

The Novamin dentifrice reduced the lesion area more than Colgate for 16 out of

18 teeth. Single tailed T-testing (assuming unequal variances) found these two groups to

be significantly different (p < 0.001). For nearly all samples, the 20 day dentifrice

treatment under pH cycling halted and reversed the caries process as evident in the

reduction of the lesion area. Colgate reduced the lesion area by an average of 24.9%

from the original lesion LD. Novamin decreased the lesion area by 41.9%.

Total gray value was also plotted (Figure 6-3) to determine the fluorescence levels

within the lesions. This parameter is commonly reported as total fluorescence because

greater fluorescence corresponds to higher gray values. Thus, smaller values are

indicative of less porosity and dye penetration, or more mineral.











1.60E+07

1.40E+07 -

1.20E+07 -

| 1.00E+07 -

S8.00E+06 -

6.00E+06 -
I-
4.00E+06 -

2.00E+06 -

O.OOE+00
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Tooth #

*LD (Demin Control) BX(Colgate) E] BY(Novamin D3 4505 5%)


Figure 6-3 Total Gray Value for each tooth for the demineralized control, Colgate and
Novamin treatment condition.

The Novamin dentifrice reduced the total gray value more than Colgate for the

same 16 out of 18 teeth. Treatment groups were again found to be significantly different

(p < 0.001). Colgate reduced the fluorescence (total gray value) by an average of

48.1%, Novamin 70.5%.

Statistical review (SAS System software) using two-way analysis of variance

(ANOVA) of all three groups (LD, BX and BY) yielded statistical significance (p <

0.0001) for both confocal parameters, lesion area and total fluorescence. Duncan

multiple range testing for both parameters found all three groups to be statistically

different in means at a significance level of a = 0.01.

Cross sectional images for these teeth sections show remineralization "bands"

revealing the depth at which most remineralization occurred.


































image of tooth section 41LD (Demineralized Contr


Figure 6-5 Confocal image of tooth section 41BX (Colgate treated)

































Figure 6-6 Confocal image of tooth section 41BY (Novamin treated)

The remineralization "bands" are clearly visible signifying mineral deposition,

similar to those reported in previous toothpaste studies. The depth of these bands could

be due to several variables. A combination of diffusion rates for fluoride, calcium, and

phosphates into the enamel surface in addition to twice daily acid challenge may

influence band depth. Fractional variations in composition from one tooth to another

probably play a role as well. Slight differences in impurity element levels in the tooth

structure cannot be ignored. Although the natural surface enamel was removed, the

maturation history of these specimens could vary considerably. It is likely that

fluoridated water sources and dietary habits of the original patients play some role in the

demineralization and remineralization capability of these teeth. Ultimately, these

variables also determine the size and shape of these bands. The most influential factor,









however, is the daily acid challenges and acid diffusion capability. The noticeable

(bright) demineralization layer above the remineralization band is unusual considering

this layer always receives the first and heaviest dose during remineralization treatment.

Logically, the remineralization band should begin at the surface and fade with depth. But

this would be true if only remineralization was taking place within the lesion. The fact

that pH cycling includes daily acid challenges keeps the outer surface demineralized and

in a state of constant flux. The depth of that demineralized band is most likely controlled

by acid diffusion, possibly an Arrhenius relationship. Novamin samples clearly show a

larger remineralization band for nearly all samples and a noticeably darker lesion.


Figure 6-7 Confocal image of tooth section 47LD (Demineralized Control)

































Figure 6-8 Confocal image of tooth section 47BX(Colgate treated)


Figure 6-9 Confocal image of tooth section 47BY (Novamin treated)
































Figure 6-10 Confocal image of tooth section 48LD (Demineralized Control)


Figure 6-11 Confocal image of tooth section 48BX (Colgate treated)

































Figure 6-12 Confocal image of tooth section 48BY (Novamin treated)

The nature and degree of interaction between fluoride and Novamin is clearer. It

appears that in vitro, these two agents positively interact and do not inhibit the

remineralization function of the other. Although the techniques used in this study cannot

confirm or deny this hypothesis, ultimately the lack of oral bacteria and plaque leave

room for question. An in vivo study with plaque is necessary to better determine the

degree to which these two agents contribute to the remineralization process at the tooth

interface.

CLSM has proven to be not only a powerful technique to assess remineralization,

but also an efficient one. The samples, considered large by past experimental methods,

required no special post-treatment preparation for analysis other than cross-sectioning

and 24 h dye soaking. This technique reduces time and cost as well as improved operator

safety compared to with TMR. The most valuable asset may be yet to come however. If









autofluorescence of surface lesions could be correlated with TMR to the degree of dye-

assisted fluorescence analysis, timed studies could be performed at intervals during the

treatment of same tooth sections rather than different sections of the same tooth. This

provides a more accurate account of the remineralization process because it eliminates

the slight variability among different sections of the same tooth.

Cross-sectional CLSM analysis proved to be the optimal approach to surface

remineralization study in vitro. From cross-sectional imaging it was clear that most

remineralization took place somewhat below the surface during exposure to fluoride

containing toothpaste. This is consistent with earlier studies [GON99]. Therefore data

from the treatment surface alone would be inaccurate, misleading, and incomplete.

Reducing variability is extremely important for remineralization studies because

teeth are inherently inhomogeneous, especially surface layers. To obtain meaningful

data, in vitro analysis requires the investigator to eliminate as much experimental

variability as possible. Relative success of this study should be credited to meticulous

standardization in each procedural step. Sample preparations, solutions, and processes

were all subject to strict uniformity control for all tooth sections. This leaves only

inherent specimen variability unaccounted.

It was also confirmed that in vitro remineralization/demineralization studies are

predominantly controlled by basic chemistry fundamentals such as pH, ion

concentrations and (enamel) solubility. This study did not include organic components

such as oral bacteria or plaque. Therefore anti-microbial effects were not active. The key

factors for remineralization here were ions (Ca2+ and P043-) and the pH at the tooth

surface. The release of these ions from Novamin provides a solubility gradient in favor









of mineral deposition. This explains why the samples treated with Novamin dentifrice

experienced greater remineralization, the environment was more conducive to this

process. This presents a stronger case for Novamin dentifrice in Xerostomic

conditions. Surface pH appeared to be influential because Novamin dentifrice solution

maintain a significantly higher pH than Colgate solution. Dentifrice treatment solutions

were changed daily and pH differences within that period were recorded for each

solution. Average pH for freshly made Novamin solution was measured at 9.8. After

24 h, the same solutions averaged pH 10.4. Fresh Colgate solution pH averaged 7.2

while 24 h aged solution averaged 7.3. It is likely the Novamin dentifrice solution

neutralized the preceding acid challenge significantly better than its counterpart. While

half of the cross-sectioned sample was sent to IUPUI for CLSM, the remaining half was

tested with nanoindentation for mechanical property changes.

Nanoindentation analysis of enamel cross-sections was performed throughout the

depth of the lesion and into the sound enamel for two selected teeth. A graphical plot of

hardness versus the cross sectional depth from the surface confirmed the demineralized

nature of the lesion within the first 100 [tm of these teeth. The hardness within the lesion

was consistently low for all sections with the exception of those areas previously

identified as remineralization bands. These bands, typically halfway through the lesion

depth, exhibited a noticeable hardness spike for both dentifrice treated sample groups.

Figures 6-13 and 6-14 illustrate this point.











5.5

4.5

3.5 A A& A A A



1.5
2.5 A





-0.5
-0.5 J---------------------------------
0 50 100 150 200 250 300
Depth [urn]
A BX (Colgate) -l-33 BY (Novamin) 033 LD (Demineralized Control)
Figure 6-13 Cross sectional nanoindentation of tooth 33.





I









0 20 40 60 80 100 120
Depth [um]
--LC (Sound Enamel Control) --LD (Demineralized Control) -A- BX(Colgate) -- BY (Novamin)
Figure 6-14 Cross sectional nanoindentation of tooth 44.

At greater depths, the hardness increases to the expected range of sound enamel.

For tooth 33, the hardness spike appears at 40-50 [tm depth while tooth 44 shows a

similar spike between 60-80 tm. Both dentifrices (BX and BY) exhibited hardness

fluctuations throughout the lesion depth without statistically significant difference (p >









0.05). Both groups had markedly larger hardness values than the original lesion section

(LD) within the lesion (below 100 [tm). However, such variability within sample groups

is not advantageous to quantitative analysis. Five more teeth were plotted similarly and

showed no significant difference between either test group. In fact, a number of tooth

sections exhibited highly irregular hardness behavior. Some displayed greater hardness

values for the demineralized section compared to sound and treated counterparts.

Multiple tooth sections also exhibited gross hardness fluctuations of 10 GPa or more.

Nanoindentation did not graphically or statistically provide significant evidence for

remineralization of lesions for the treatment groups. Select teeth exhibited positive

trends but quantitative analysis of enamel remineralization with nanoindentation is not

recommended. Cross-sectional indentation does however have some qualitative value

although inconsistency is clearly an issue. Any number of factors could influence these

results.

Primarily, nanoindentation is heavily dependent upon sample preparation.

Variation in polishing or sample tilt could overshadow any differences in mineral

content. These samples were particularly re-mounted in epoxy and again fine polished

(with 0.05 [tm gamma alumina) in order to reduce surface roughness for indentation.

Multiple polishing effects or residual epoxy resin deposition could possibly influence

mechanical properties of the porous lesion. A degree of operator skill is also relevant.

Initial placement of the indents is key to determining the hardness relationship to depth.

A number of samples appeared as though initial indents were started in the bordering

epoxy region, thus introducing an inherent offset for subsequent indents that are related to

the former by position.









Similar to microindentation, soft porous surfaces are extremely difficult to measure

for hardness. The lesion zones may have presented pore sizes similar to that of the

indenter tip. Features such as inherent crystal growth inhomogeneity and mineral

deposition also introduce influential variables on a scale within range of the indenter tip.

Crystal diameters within lesions of 0.1 am [EKS88] could affect hardness measurements

for this study considering the indentation depths of 0.5 2.0 [m at 5000 kN load. In

sound enamel, crystal orientation (growth angles) and enamel mineral anisotropy have

proven to affect hardness significantly [HAB01], thus demineralization and subsequent

mineral deposition could introduce additional variation. Rather than representative

hardness, nanoindentation may be too sensitive a technique to determine mineral changes

over large lesion areas. It appears that ultra-fine features tend to dominate hardness data

in enamel, preventing broad range determination of remineralization effect within enamel

lesions.

Vickers microhardness was performed on the same samples analyzed with

nanoindentation (seven total). A 100 g load at 15 s dwell time was used with a standard

Vickers diamond tip. It was impossible to visually identify indents in the porous lesion

for most samples. Hardness for the few samples in which indents were optically visible

was still difficult to quantify. Indent edges were blurred and only marginally in focus,

thus measuring diagonals was reduced to a subjective judgment. Generally,

microhardness data collection was unsuccessful. If these indents were visible, it is still

unlikely that microhardness data could indicate a mineralization difference. The size of

the indents nearly matched that of the lesion, thus providing only representative hardness

on much a much larger scale than nanoindentation. Microhardness presents the reverse






54


problem relative to nanoindentation. It appears to be too insensitive a technique to

distinguish differences in remineralization of enamel surface lesions.














CHAPTER 7
CONCLUSIONS

A number of conclusions can be made from the results we observed. First, in vitro

study using confocal laser scanning microscopy (CLSM) has shown that Novamin

dentifrice exhibits a greater degree of remineralization than Colgate dentifrice of early

caries lesions in human enamel. Statistical analysis (T-test) found significance (p <

0.001) among treatment groups. For both CLSM parameters, ANOVA (p < 0.0001) and

Duncan multiple range testing (a = 0.01) also yielded significant difference among

groups. CLSM has also proven to be an efficient analysis technique for cross-sectional

study. Post-treatment tooth preparation was minimal and non-destructive.

It appears that remineralization of surface lesions was successful independent of

any anti-microbial effect. There were no organic components, such as plaque or bacteria,

involved in this study. Therefore, surface variables including pH and Ca2+/P043- most

likely influenced the conditions at which remineralization of the porous lesions took

place.

Reducing variability throughout the study notably enabled relevant data collection.

Inherent tooth inhomogeneity was beyond experimental control, therefore strict

uniformity in laboratory technique enhanced the statistical significance of the two

treatment groups. Statistical analysis appears to confirm this.

Hardness testing for in vitro remineralization of enamel surface lesions was

statistically insignificant and provided minimal quantitative value. Nanoindentation

hardness testing was dependant on too many variables. Sample preparation and porosity






56


(relative to the tip size) makes it apparent that nanoindentation could be too sensitive a

technique to determine remineralization effect of dentifrice on demineralized enamel.

Conversely, Vickers microhardness is too insensitive a technique to distinguish

differences in the remineralization effect on the desired scale. Thus, neither

microhardness nor nanonindentation techniques are recommended for remineralization

studies. However, confocal laser scanning microscopy is potentially a very good

technique for studying remineralization.















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103, p 362, 1995

[TEN99] ten Cate JM, Van Laveren C, "Fluoride mechanisms." Cariology 43 (4), p
713 742, 1999

[WEB99] Webb R, "Theoretical basis of confocal microscopy." Methods of
Enzymology 307, p 3 26, 1999

[WHI94] White JM, Goodis HE, Marshall SJ, Marshall GW, "Sterilization of teeth
by gamma radiation." JDentRes 73 (9), p 1560 1567, 1994

[WIL87] Wilson J, Low S, Fetner A, Hench LL, "Bioactive materials for
periodontal treatment: a comparative study." In: Pizzoferrato A, Machetti
PG, Ravaglioli A & Lee, A.J.C. (eds) : Biomaterials and Clinica
Applications Amsterdam: Elsevier, p 223 228, 1987

[WIL92] Willems, G, Lambrechts, P, Braem, M, Celis, JP, Vanherle, G, "A
classification of dental composites according to their morphological and
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DentAssoc 129, p 1579 1587, 1998















BIOGRAPHICAL SKETCH

Sammel Alauddin was born in Dhaka, Bangladesh, and immigrated to the United

States at the age of two. Raised in Frankfort, Kentucky, he graduated high school at the

age of 16 to pursue higher study at the University of Kentucky in Lexington. In 2002, he

completed both a B.A. in Organic Chemistry and a B.S. in Materials Science

Engineering. A chance meeting with a former professor at a conference led to her

suggestion that he pursue graduate studies and consider her new institution, the

University of Florida. At UF, he expanded his interest in biomaterials research and

improving the health of future generations. With these goals in mind, he strives to make

his contribution to science.




Full Text

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IN VITRO REMINERALIZATION OF HUMAN ENAMEL WITH BIOACTIVE GLASS CONTAINING DENTIFRICE USING CONFOCAL MICROSCOPY AND NANOINDENTATION ANALYSIS FOR EARLY CARIES DEFENSE By SAMMEL SHAHRIER ALAUDDIN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by Sammel Shahrier Alauddin

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This work was performed in dedication to those who gave their time and love to support me and my efforts. It was the contribution of many and pray ers of a select few who deserve credit for my successes. My pa rents and loved ones believed in me and respected my pursuits in life, even when we didnt see eye to eye. I must also thank Almighty God for giving me the strength a nd perseverance to overcome in times of difficulty and the humility to accept my shortcomings and still succeed as a man among many. In memorial, I dedicate this work to my grandfather Kazi Reazuddin Ahmed, who taught me the value of sacrifice and achiev ement in this world. My successes are a testament to his love and guidance at critical stages in my life. I only regret that he is not here today to share this accomplishment with me.

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ACKNOWLEDGMENTS I would like to thank Ben Lee, Neel Bhatavedekar, Kevin Taylor, and Allyson Barrett in Department of Dental Biomaterials at the University of Florida for their direct involvement in making this work a success. For three months in the lab, we were almost a family. I would also like to thank Guy La Torre, Kevin McKenzie, and Donald Thibadeaux from Novamin Technologies Inc. for their help and guidance throughout the project, especially during the initial stages. Kevin contributed so much of his time helping me take this research to a new level. From the Department of Materials Science and Engineering, I must thank Dr. Christopher Batich, my committee member, and Dr. Susan B Sinnott, without whom much of this would not have been possible. All credit to her for stimulating my interest in Materials Science and Engineering as her undergraduate student at the University of Kentucky. I shall always admire her devotion to science and academic integrity. This project was also supported by the efforts of Major Analytical Instrumentation Center staff. Dr. Amelia Dempere, Wayne Acree, Brad Willenberg, and Jerry Bourne helped bring my hypotheses to life. Each analysis technique required their expertise and cooperation. Confocal microscopy analysis would not have been possible without Dr. Marguerita Fontana, Dr. Carlos Gonzales-Cabezas, and Amir Haider. Dr. Fontana and colleagues at Indiana University-Purdue University at Indianapolis sacrificed their time iv

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and convenience to advise me and help perform the analytical centerpiece of this study. They were available to answer every question and make their expertise my expertise. Many thanks go to Dr. R.J. Hanrahan, Dr. David Reitze, Tim Vaught, and Dr. James Wefel for their time and assistance at various points thoughout the study. I reserve special recognition for Dr. Mecholsky, Dr. Greenspan, and Dr. Anusavice for their valued advice and encouragement. They provided the resources and opportunity to this ambitious young researcher, allowing me to mature and develop my scientific curiosities. I am ever grateful for their guidance through this long and arduous journey. v

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TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT.......................................................................................................................xi CHAPTER 1 BACKGROUND AND MOTIVATION......................................................................1 2 FUNDAMENTALS OF THE TOOTH........................................................................5 Hydroxyapatite.............................................................................................................6 Demineralization and Remineralization Phenomena....................................................9 3 REMINERALIZATION THEORY............................................................................14 Dentifrice and Fluoride...............................................................................................14 Systemic Benefit..................................................................................................14 Topical Benefit....................................................................................................15 Bioglass...................................................................................................................18 4 MINERAL QUANTIFICATION...............................................................................23 Microradiography.......................................................................................................23 Confocal Laser Scanning Microscopy........................................................................25 Nanoindentation..........................................................................................................28 Microindentation.........................................................................................................30 5 EXPERIMENTAL METHODS AND MATERIALS................................................35 Collection and Storage of Teeth.................................................................................35 Sectioning and mounting............................................................................................35 Lesion Formation........................................................................................................36 Treatment Regimen....................................................................................................36 vi

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Cross-sectioning.........................................................................................................37 Analysis......................................................................................................................38 6 ANALYTICAL RESULTS AND DISCUSSION......................................................39 7 CONCLUSIONS........................................................................................................55 LIST OF REFERENCES...................................................................................................57 BIOGRAPHICAL SKETCH.............................................................................................62 vii

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LIST OF TABLES Table page 4-1 Comparison of hardness and Youngs modulus data obtained with Vickers and Berkovich indenters..................................................................................................32 4-2 Comparison of indentation fracture toughness obtained by different indenters......32 5-1 Tooth labeling scheme. Example 36BX..................................................................36 viii

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LIST OF FIGURES Figure page 2-1 Anatomy of the human tooth......................................................................................7 2-2 Cycle of demineralization and remineralization in enamel......................................11 3-1 Mineral Loss (expressed as Ca output) as a function of pH and [F ].......................17 3-2 Stages of bioactive glass surface reactions..............................................................19 4-1 Microradiography setup...........................................................................................24 4-2 Mineral density quantified as Z in TMR...............................................................25 4-3 Schematic of a generic confocal microscope...........................................................26 4-4 Conventional microscope for fluorescence in epitaxial configuration.....................27 4-5 Force-displacement (loading) curves on various materials......................................29 4-6 Loading curve for nanoindentation on sound enamel..............................................30 4-7 Vickers microhardness testing.................................................................................31 4-8 Demineralized Enamel. Axis units in m...............................................................33 4-9 Indents in Novamin treated enamel. Axis units in m..........................................34 4-10 Vickers indents in control enamel @ 300X and 2000X...........................................34 5-1 Experimental flow diagram of specimen sectioning and mounting.........................36 6-1 Confocal image of enamel lesion, tooth section 38LD............................................40 6-2 Lesion area for each tooth for the demineralized control, Colgate and Novamin treatment condition...................................................................................................41 6-3 Total Gray Value for each tooth for the demineralized control, Colgate and Novamin treatment condition................................................................................42 6-4 Confocal image of tooth section 41LD (Demineralized Control)............................43 ix

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6-5 Confocal image of tooth section 41BX (Colgate treated).....................................43 6-6 Confocal image of tooth section 41BY (Novamin treated)...................................44 6-7 Confocal image of tooth section 47LD (Demineralized Control)............................45 6-8 Confocal image of tooth section 47BX (Colgate treated).....................................46 6-9 Confocal image of tooth section 47BY (Novamin treated)...................................46 6-10 Confocal image of tooth section 48LD (Demineralized Control)............................47 6-11 Confocal image of tooth section 48BX (Colgate treated).....................................47 6-12 Confocal image of tooth section 48BY (Novamin treated)...................................48 6-13 Cross sectional nanoindentation of tooth 33............................................................51 6-14 Cross sectional nanoindentation of tooth 44............................................................51 x

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science IN VITRO REMINERALIZATION OF HUMAN ENAMEL WITH BIOACTIVE GLASS CONTAINING DENTIFRICE USING CONFOCAL MICROSCOPY AND NANOINDENTATION ANALYSIS FOR EARLY CARIES DEFENSE Sammel Shahrier Alauddin December 2004 Chair: J. J. Mecholsky, Jr. Major Department: Materials Science and Engineering Remineralization of early caries lesions provides an oral health and economic advantage to populations suffering from this bacteria-based disease. Preventive treatment of caries in the early stages could reduce dental costs and frequency of clinical visits. This study investigated the use of bioactive glasses in dentifrice in comparison to commercial dentifrice (Colgate Regular) to determine if additional calcium and phosphate release from the bioactive glass contributes positively to fluoride dentifrice remineralization capability. Two techniques were used in vitro to quantify the remineralization for dentifrice treatment of tooth enamel. Confocal laser scanning microscopy (CLSM) and nanoindentation were used to evaluate mineral changes in artificial surface lesions optically and mechanically. Eighteen human molars were sectioned into four parts : demineralized control, sound control, Novamin (bioactive glass) and Colgate dentifrice treatment. Upon demineralization, specified treatment sections were placed in a pH cycling regimen for 20 days. Each tooth section was further xi

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cross-sectioned through the treatment window down the lesion depth for analysis. Novamin tooth sections remineralized lesions more than Colgate for two parameters, lesion area and total gray value (fluorescence), which have been directly correlated with mineral density. Single tailed T-test for treatment groups yielded significant difference (p < 0.001). Novamin dentifrice treatment reduced lesion area an average of 41.9%, while Colgate averaged 24.9%. Novamin sections also exhibited an average 70.5% decrease in total gray value compared to 48.1% for Colgate. Two way ANOVA testing of three tooth sections (original lesion, Novamin treatment, Colgate treatment) for both parameters found significant difference (p < 0.0001). Duncan multiple range testing found these three sections statistically different at significance level 0.01. Both parameters signify extensive remineralization in vitro for surface lesions. Nanoindentation did not significantly exhibit remineralization for either dentifrice treatment. Considerable hardness variability was observed throughout the cross-sectional lesion, as well as inconsistency among multiple teeth and treatment groups. Nanoindentation is not recommended for mechanical analysis of in vitro remineralization of human enamel. Vickers microhardness was performed on lesions as well but did not yield credible results. Indents were difficult to optically identify and measure. xii

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CHAPTER 1 BACKGROUND AND MOTIVATION The field of dentistry enables modern societies to effectively and preventively maintain oral health. Clinicians have been relatively successful within the realm of dental care taking into consideration the frequency of patient visits. But that dependence upon interaction between the patient and their dentist is not only costly to some, but also contrary to human nature. Our human tendency to treat ailments responsively rather than preventively forces the scientific community to develop better methods to counter the more severe stages. So it is now the motivation for dental researchers and clinicians to develop strategies and treatment methods for fighting dental diseases in their early stages. Engineers call this practice preventive maintenance while physicians refer to it as preventive medicine. Regardless of the terminology, science has been a marvelous tool for enabling individuals to care for themselves. Decades of dental science have brought society the common practice of daily tooth brushing, flossing and mouth rinse. This concept of preventive dentistry provides the foundation for this study and its long-term applications. Dental caries remains the oldest and most prevalent oral disease in human history. It is only recently that the advent of daily dental care and clinician oversight has reduced the frequency of caries within large populations [WIN98]. Fluoridated water supplies and habitual influences, such as diet, certainly have an effect on caries management. But this study focuses primarily on daily treatment as the most effective caries defense and ways to optimize it process through materials science. 1

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2 Caries pathology and its early treatment are well researched. Only the details of the caries prevention process and the best way to accomplish this task are left for dental researchers to discover. The term caries which originates from Latin for rot or rotten prompted original researchers of the past two centuries to develop methods to counter this process of tooth decay or demineralization. The heart of caries research and prevention lies in the opposition of these terms, thus replacement or remineralization. Traditionally, surface or early caries treatment was centered around tooth mineral loss and gain. The progressive increase in processed sugars and acidic foods and beverages in the human diet provide oral bacteria greater opportunity to produce acids that dissolve tooth mineral. Thus, the facilitation of regaining new mineral via the natural oral process remains the ultimate goal. Tooth mineral, among other biological minerals, is composed mainly of calcium and phosphorous. Methods for providing these constituents of mineral or a new constituent to better facilitate remineralization of teeth has been the backbone for this type of research. Dentifrices have been around for more than 2000 years [BAR04, WIN98] but only since Muhlers development of fluoride toothpastes in 1954 has such a dramatic effect been found in caries prevention. Within a few years Colgate Palmolive Company and Proctor & Gamble began marketing their own fluoride toothpastes. Decades later, the daily use of fluoride toothpaste is still accepted as the best and simplest method of caries prevention and tooth remineralization for large populations. Although a debate still exists in the dental community concerning optimization methods of tooth remineralization, it is clear that fluoride ion plays a significant role in the remineralization process. Early caries treatment is especially important with dentifrices

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3 to reduce the risk of treating advanced stages of the disease. If individuals can effectively fight caries with daily brushing, the need for restorative treatment can be avoided. The invention of bioactive glasses (Bioglass ) by Dr. Larry Hench in the late 1960s provided medical researchers with a new tool to enhance bone formation that facilitates the bodys own natural processes. This provided a whole new approach to biomaterials. My experience in bio-inert materials was pushed aside by the possibility of investigating new and more advanced uses for this bio-interactive technology. Through good fortune and the support of Novamin Technologies Inc., I found an opportunity in dental research to explore the nature of bioactive glasses in dentifrice and its potential to provide the oral environment with the essential elements (Ca and P) to remineralize teeth. The novelty of Bioglass toothpaste for this study is evident but doesnt stop here. Just as treatment methods for early caries progress, the need for newer and better technologies in assessing caries treatment continues to grow as well. Previous analysis methods for tooth remineralization do not provide the scope for detailing finer quantification of early stage caries. Transverse microradiography (TMR) and microindentation are still sound techniques for dental caries research, but do not provide the scale or quantification requirements that current and future investigators need to progress. In the case of caries, in vitro studies in the past used TMR for subsurface caries lesion analysis. But early caries detection and treatment requires surface analysis and less time. Confocal laser scanning microscopy (CLSM) and nanoindentation were viable candidates to explore early caries treatment for more current research objectives. The aims of this study are as follows:

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4 Compare and contrast the benefits and drawbacks of confocal laser scanning microscopy and nanoindentation for analysis of tooth remineralization in vitro. Determine the remineralization capabilities of a Novamin (bioactive glass) containing dentifrice on human enamel in vitro.

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CHAPTER 2 FUNDAMENTALS OF THE TOOTH Basic tooth structure is based on layers and function. Teeth, like bone, are comprised of soft inner layers to provide nutrients and growth function, whereas outer layers are designed for structure and protection. This dual nature provides a research perspective conducive to remineralization studies. In essence, if the tooth is generally viewed as living, the prospect of it regenerating or remineralizing becomes no different than with any other biological tissue. On a larger scale, it is advantageous to consider the oral cavity itself as a unique and independent ecosystem. The primary tissues involved in caries research are dentin and enamel. Early caries studies focus almost entirely on enamel and its surface layers. Enamel is the visible outer layer of the tooth. It is translucent; and can vary in color from yellowish to grayish white. The different colors of enamel may be attributed to variations in thickness, translucent proprieties, the quality of the crystal structure, and surface stains. Enamel (Figure 2-1) is the calcified substance that covers the entire anatomic crown of the tooth and protects the dentin and pulp. It is the hardest tissue in the human body and consists of approximately 97% inorganic minerals, 1.5% organic materials, and 1.5% water [LEG88]. Calcium and phosphorus (as hydroxyapatite) are its main inorganic components. Enamel can endure crushing pressure of approximately 700 MPa. A layering of the dentin and periodontium below produces a cushioning effect of the tooth's different structures, enabling it to endure the pressures of mastication. Structurally, enamel is composed of millions of rods or prisms. Each rod begins at the 5

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6 dentino-enamel junction (zone between the enamel and dentin) and extends to the outer surface of the crown. Enamel is formed by epithelial cells (ameloblasts) that lose their functional ability when the crown of the tooth has been completed. Therefore, enamel, after formation, has no power of further growth or repair, only mineral gain and loss. Dentin (Figure 2-1) is the light yellow tissue beneath the enamel. It is more radiolucent than enamel, very porous, and it constitutes the largest portion of the tooth. The pulp chamber is located on the internal surface of the dentin walls. Dentin is harder than bone but softer than enamel and consists of approximately 70% inorganic matter and 20% organic matter and 10% water [LEG88]. Calcium and phosphorus are its chief inorganic components. Dentin is a living tissue and must be protected during operative or prosthetic procedures from dehydration and thermal shock. The dentin is perforated by tubules (similar to tiny straws) that run between the cemento-enamel junction (CEJ) and the pulp. Cell processes from the pulp reach part way into the tubules like fingers. These cell processes create new dentin and mineralize it. Dentin transmits pain stimuli by the way of dentinal fibers and hydrostatic pressure within the tubules. Because dentin is a living tissue, it has the ability for constant growth and repair that reacts to physiologic (functional) and pathologic (disease) stimuli. Hydroxyapatite Calcium phosphates are the most important inorganic constituent of biological hard tissues. In the form of carbonated hydroxyapatite, they are present in tendons, bone and teeth to give these organs stability, hardness and function. The maturation of tooth enamel and dentin involves the second major calcification process in mammals, bone being the first. The formula for tooth mineral in enamel consists primarily of calcium hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2

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7 Figure 2-1 Anatomy of the human tooth. Note: Taken from Dentistry and Real Life http://users.hol.gr/~jelian/anatomy.htm with requested permission. [DRL03] Human biological apatites can vary considerably based upon formation and functional conditions as well as basic Ca:P molar ratios. Enamel maintains a Ca:P ratio of nearly 1.63 as compared with the general hydroxyapatite (HA) ratio, 1.67 and bone, 1.71 [LEG88]. The similarities of biological apatite in enamel to pure HA make it possible for researchers to develop chemical relationships to study tooth minerals. Tooth enamel contains parallel crystals of biological apatite, which are much larger than those of bone and dentin. The needle-like crystal rods may grow to 100 m in length and 50 nm wide. The enamel maturation process is vital to understand the solution dynamics involved for this study. Calcium deficient hydroxyapatite (CDHA) is the main constituent of developing enamel and considered to be quite carbonated. The subsequent mineral formation is driven by the introduction of Ca 2+ into the apatite and simultaneous loss of carbonate. Featherstone describes newly mineralized bone and teeth as carbonated hydroxyapatite (CAP), which is essentially impure HA, represented by the

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8 formula Ca 10-x (Na) x (PO 4 ) 6-y (CO 3 ) z (OH) 2-u (F) u [FEA00]. In enamel this calcification produces a mineral increase from 45 wt% to 96 98 wt% and a significant rise in Ca:P molar ratio. This ultimately results in the most highly mineralized and hardest skeletal tissue in the body [DOR02]. Although the biological apatite of tooth enamel varies due to crystal impurities and apatite combinations, ionic exchange at the enamel surface is based primarily upon calcium phosphate solubilities. At neutral pH, saliva is supersaturated with calcium and phosphate. This saturation is necessary to counter the recurrent acid challenge of dietary cycles and residual sugars which are used by oral bacteria to create acid via fermentation. These ions, among others, give saliva a buffering capacity in addition to a mineralization reservoir. Below the critical pH of human enamel, 5.2 to 5.5 [LAR03], the dissolution of enamel mineral follows basic solubility laws for hydroxyapatite. At lower pH, the dissolution of apatite mineral continues until the oral pH returns to normal. Along with salivas own buffering capacity, salivary bicarbonate in equilibrium with CO 2 from the respiratory process shifts the equilibrium to a more alkaline condition. The state of subsaturation for calcium and phosphate at lower pH prevents remineralization under these conditions. When oral pH again rises above critical status, the calcium/phosphate saturation of the saliva again supercedes that of the enamel and mineral deposition begins. Due to the supersaturation condition of saliva at pH 7.0, one would expect that hydroxyapatite mineral would continue to form on the enamel surface. As in many cases within the human body however, this micro-environment is equipped with its own checks and balances. The supersaturation of calcium and phosphate does not over mineralize teeth because of the protein rich film (tooth pellicle) on the enamel surface. It is thought that saliva proteins

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9 also play a part in maintaining this balance. Still the question remains, what if the salivary pH becomes severely alkaline? This increase in pH results in dental calculus, essentially calcium phosphate precipitate in the plaque fluid. A broader look at the demineralization and remineralization process is necessary, especially when topical fluoride is introduced into the system. Demineralization and Remineralization Phenomena Generally, the life of dental hard tissues is well understood and research has revealed the structures and concepts involved in natural processes of the oral environment. The nature of these tissues and how they behave under certain conditions is clear, but what is not clear is the degree to which these natural processes can be influenced or even accelerated. Over the course of human life, enamel and dentin undergo unlimited cycles of demineralization and remineralization. The debate involves ways to measure and influence this process. Years of work have brought fluoride to the forefront of remineralization studies and application. But the influence of fluoride on demineralization and remineralization of enamel is yet to be agreed upon. The crux of this argument lies in the balance of these two competing phenomena. A tip in the balance one way or the other will either lead to stronger healthier teeth or greater susceptibility to dental caries and other oral complications. To promote general oral healthcare, the use of fluoride toothpastes for daily promotion of remineralization has become standard practice. Evaluating these tissues at different stages of the oral cycle and measuring the optical and mechanical properties are the key to determining a net increase or decrease in mineral flux. The consumption of simple dietary sugars (particularly monosaccharides and disaccharides like Sucrose) provides not only nourishment for our bodies but also a food

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10 source for oral bacteria. As bacteria making up the normal oral flora adhere to the pellicle, a bacterial mass or film called plaque is formed [MAR99]. The plaque bacteria, particularly Streptococcus mutans and lactobacilli, ingest sugars for glycolysis to produce weak organic acids (such as lactic, pyruvic, acetic acid). These acids lower the surface pH and diffuse through the plaque and into the tooth, leaching calcium and phosphate from the enamel. At this time the plaque pH may have dropped to 4.0 4.5 [WIN98]. This mineral loss compromises the mechanical structure of the tooth and could lead to cavitation over a long period of time. The stages of caries progression are clear and in the interest of preventive maintenance, early carious lesions appear to be the best opportunity for countering this destructive process. As alluded to earlier, saliva alone has the capability to increase plaque pH with bicarbonates although typically this process may take up to 2 hours. The susceptibility of apatite in enamel surface layers makes it critical to control the acidity of the plaque fluid and the Ca 2+ and PO 4 3ion concentrations in saliva [FEA00]. The subsequent remineralization process is nearly the reverse. When oral pH returns to near neutral, Ca 2+ and PO 4 3ions in saliva incorporate themselves into the depleted mineral layers of enamel as new apatite. The demineralized zones in the crystal lattice act as nucleation sites for new mineral deposition. In the presence of fluoride (at high concentrations), the original CAP loses its remaining carbonate and is replaced with a hybrid of hydroxyapatite (HAP) and fluorapatite (FAP) [FEA00]. This cycle is fundamentally dependent upon enamel solubility and ion gradients. Essentially, the sudden drop in pH following meals produces an undersaturation of those essential ions (Ca 2+ and PO 4 3) in the plaque fluid with respect to tooth mineral. This promotes the dissolution of the enamel. At elevated pH, the ionic supersaturation of plaque shifts the

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11 equilibrium the other way, causing a mineral deposition in the tooth. Figure 2-2 provides an overview of these processes. Figure 2-2 Cycle of demineralization and remineralization in enamel. Note: Taken from Winston AE, Bhaskar SN, Caries prevention in the 21 st century. J Am Dent Assoc 129, p 1579 1587, 1998 with requested permission. Saliva Saliva plays multiple roles in these oral processes. Aside from providing a constant rinse, the value of saliva as a reservoir for calcium, phosphate and fluoride has been well established [LAR03]. Saliva offers a myriad of other benefits, although many are not widely known but contribute significantly to enamel remineralization. The buffering capacity of human saliva plays a major role in countering fluctuations in pH. Again, acidic beverages and/or sugary foods cause temporary pH drops during which demineralization is accelerated. Stephans work in 1944 found patients with little to no caries activity maintained a resting salivary pH of 7.0-7.2 [STE44]. It was later understood that bicarbonates in saliva played a major role in elevating low oral pH after meals. The relationship of bicarbonate concentrations in

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12 saliva and blood has led other investigators to study caries incidence relative to blood properties [BAC99, BIE04]. Other buffers present in saliva include urea proteins. Urease enzyme in plaque fluid metabolizes urea producing ammonia and an increase in plaque pH. Arginine rich proteins in saliva can also metabolize into alkaline substances such as arginine and ammonia. Phosphate has also been found to contribute to buffering capabilities [DOW99]. Various salivary components also demonstrate antibacterial capability. Iron binding protein lactoferrin has been shown to inhibit aerobic and facultative anaerobic bacteria (such as Streptococcus mutans) which require iron to metabolize. Lysozyme also exhibits direct antibacterial function. This enzyme, well known for its presence in tear and nasal secretions (discovered by Fleming in 1922), complexes with salivary ions such as bicarbonate, iodine and fluorine, which bind to bacterial cell walls and induce autolysis [AMA01, DOW99]. It is clear that a reduction in salivary flow or its constituents would negatively affect our capability to fight caries [DOW99]. Xerostomia is defined as the perception of oral dryness or hyposalivation. This is due to any number of factors including radiation, medication and diseases such as diabetes. The most widely studied disease state affecting salivary function is exocrinopathy or Sjgrens Syndrome. This autoimmune disease is characterized by inflammation of glands with lymphocyte infiltration. Secretory components begin to deteriorate causing decreased saliva flow. Specifically, the reduction in salivary flow has been associated with a marked increase in caries incidence [DRE76]. Although a number of remineralizing factors are affected, the near absence of calcium and phosphate from the oral cavity cannot be compensated. Remineralization becomes nearly impossible without key constituents. External constituents, such as

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13 fluoride from dentifrice, have been proven to positively affect remineralization. The influence of saliva in the presence of topical fluoride to form greater levels of CaF 2 (a remineralization agent) was demonstrated by Larsen. This relationship also revealed a decrease in caries incidence [LAR01]. But Larsen also explains that the inter-dependence of calcium, phosphate, and fluoride enables the remineralization effect. Therefore, absence of calcium and phosphate (such as Xerostomia) would essentially deem the presence of fluoride irrelevant.

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CHAPTER 3 REMINERALIZATION THEORY While a great deal has been discovered about the remineralization and demineralization process, details regarding the mechanism of mineral deposition are not universally agreed upon. This study focuses on remineralization of early caries lesions through dentifrice application. This is the most common form of anti-caries treatment today and has proven quite effective in reducing caries incidence since its introduction. Dentifrice and Fluoride Around the turn of the century, primitive toothpastes were primarily used as abrasive cleansing agents. However, a relative explosion in dental caries forced scientific investigation to aid the affected populations. Bibby continued the functional development of toothpastes in 1942 with the first clinical trials of fluoride toothpastes. Proctor & Gamble later introduced a fluoride toothpaste in 1955 using a stannous fluoride agent. Further development led to sodium monofluorophosphate (Na 2 PO 4 F) and sodium fluoride (NaF) agents in dentifrice. Fluoride levels are generally regulated to 1100 ppm in modern commercial toothpastes. This topical application of fluoride is one of two ways it is believed to promote remineralization. Systemic Benefit Recently, scientific studies have shown the systemic action of fluoride is secondary to a topical application. It is nevertheless a contributor to the overall beneficial effect for prevention of tooth decay. Over time, fluoride ions ingested from fluoridated water, fluoridated milk, fluoride supplements and foods processed with fluoridated water, 14

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15 can become incorporated into the structure of developing teeth. It has also been suggested that teeth which develop under the effect of systemic fluorides have shallower pits and grooves, allowing less trapping of oral bacteria and food particles, which contribute to tooth decay [BRU90]. Topical Benefit On the surface of the tooth, there is a constant exchange of mineral ions between crystals of the enamel surface and plaque fluid bathing the tooth surfaces. When topical fluorides are regularly applied to the teeth (through toothpastes, mouth rinses, fluoridated water, and professional applications) it is possible that even a poorly mineralized enamel surface can be progressively improved by the natural fluoride exchange equilibrium described above. In the case of early caries lesions, studies have shown the availability of fluoride in relatively low concentrations to result in the arrest and even remineralization of targeted enamel. From this information, we see that adults may benefit from fluorides as well as children via the topical benefit of fluorides [BRU90, ISM95]. Extensive studies on the use of dentifrice to introduce fluoride have proven its anti-caries efficacy. Fluoride has three principal topical mechanisms: 1) Free fluoride ion combines with H + to produce hydrogen fluoride, which migrates throughout acidified plaque. This ionized form is lipophilic and can readily penetrate bacterial membranes. Bacterial cytoplasm is relatively alkaline, which forces the dissociation of H + and F Fluoride ion inhibits various cellular enzymes key to sugar metabolism. Hydrogen ions simultaneously acidify the cytoplasm, thus slowing cellular activities and inhibiting bacterial function. [MAR99] 2) Fluoride integrated in the enamel surface (as fluorapatite, FAP) makes enamel more resistant to demineralization than HAP during acid challenge. 3) Fluoridated saliva not only decreases critical pH, but also further inhibits

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16 demineralization of the deposited CaF2 at the tooth surface. Fluoride at the enamel surface has been found to attract and bind to calcium ions. This enhances nucleation of new mineral at specific demineralized zones. Computer simulation has advanced this assertion and supported previous studies involving fluoride applications on early caries or surface lesions [FEA00, PEP04]. The exact mechanism of fluoride in the surface lesions is still not well understood but a few details are clear. The generalized reaction of HAP (enamel) dissolution below critical pH: Ca 10 (PO 4 ) 6 (OH) 2 + 8H + 10Ca 2+ + 6HPO 4 2+ 2H 2 O (3-1) But fluoride application allows for a substitution reaction to produce FAP Ca 10 (PO 4 ) 6 (OH) 2 + 2F Ca 10 (PO 4 ) 6 F 2 + 2OH (3-2) FAP is more resistant to acid challenge due to lower solubility ( HAP Ksp = 2.34x10 -59 and FAP Ksp = 3.16x10 -60 ) [LAR03]. In fact, even before FAP formation, ten Cate, Nelson, Featherstone and colleagues found that topical fluoride levels in solution around synthetic CAP (3% carbonate similar to enamel) reduced solubility significantly. The same studies revealed that systemic fluoride incorporation did not significantly improve apatitic acid resistance, thus arguing in favor of topical applications throughout lifetime as the best defense against early caries [FEA00, TEN99]. The argument for topical fluoride gained strength with a number of in vitro and in situ studies involving dentifrice sources and inherent fluoridated enamel. Ten Cate and Duijsters found that enamel dissolution was primarily a function of two external properties, pH and fluoride concentration of surrounding solution. Low pH environments were especially sensitive. Transverse microradiography (TMR) confirmed that surface

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17 lesion depth and mineral loss were a direct function of fluoride concentrations. Figure 3-1 summarizes their findings graphically [TEN83, TEN99]. Figure 3-1 Mineral Loss (expressed as Ca output) as a function of pH and [F ]. Note: From ten Cate JM, Duijsters PPE, The influence of fluoride in solution on tooth enamel demineralization I. Chemical data. Caries Res 17, p 193 199, 1983, courtesy of Karger AG, Basel with permission. Later studies found that low concentrations of fluoride, ambient 0.06 ppm and dentifrice 1100 ppm, significantly decreased enamel mineral loss by 5% and 9% respectively under simple pH cycling regimen [TEN95]. Histomorphometric analysis of lesions after remineralization with fluoride found the new enamel crystallites to be dimensionally larger than the original sound enamel. They were also found to be randomly oriented (lacking organization) which rendered this enamel slightly less dense. Still the ability of fluoride to bind to free calcium and phosphate, including ions leaving the tooth during demineralization to form FAP reduces the probability for enamel

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18 dissolution in subsequent acid challenges. Because FAP is much less soluble and has significantly less buffering capacity, an even lower pH will be required to force enamel dissolution [SHE94]. Bioglass The discovery of bioactive glasses by Hench in 1969 pushed the boundaries of biomaterials capability and function. In an era of bio-inert materials and implantation, Hench determined the critical steps for bioactive glass ceramic interaction with the human body in order to bond. Bioglass is a multi-component inorganic compound made of elements (silicon, calcium, sodium and phosphorous) naturally found in the body [HEN96]. The development and success of bioactive glasses is due to their highly biocompatible nature. Previous implant materials regardless of initial success, failed over long periods of time. Numerous metals and polymers eventually succumbed to the aggressive defense mechanisms and corrosive nature of body fluids. Hence, the advantage of bioactive glass is not only acceptance within the body, but also its ability to chemically bond. More than three decades of study has revealed the series of reaction steps involved in Bioglass bonding mechanisms within the body. This study involves the use of bioactive glasses in powder particulate form. This form provides easy dispersion in this dentifrice application and exploits the fact that fine glass powder particulates resorb much faster than bulk implants. Upon implantation, Bioglass in aqueous environment immediately begins surface reaction in three phases, leaching and exchange of cations, network dissolution of SiO 2 and precipitation of calcium and phosphate to form an apatite layer. The 5 critical stages for glass surface reactions are detailed below [HEN93].

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19 Figure 3-2 Stages of bioactive glass surface reactions. Note: Taken from Hench LL, Wilson J, An Introduction to Bioceramics Singapore, World Scientific Publishing, 1993 with requested permission. The initial Na + and H + /H 3 O + ion exchange and de-alkinization of the glass surface layer is quite rapid, within minutes of implantation and exposure to body fluids. The net negative charge on the surface and loss of sodium causes localized breakdown of the silica network with the resultant formation of (silanol) Si(OH) groups, which then repolymerize into a silica rich surface layer [GRE99]. This stage involves the base catalyzed hydrolysis of Si-O-Si bonds of the glass structure. This mechanism is based on previously well documented corrosion studies of alkali silicate glasses as well as infrared spectroscopy studies that appear to show the formation of nonbridging oxygen species following Si-O-Si bond breakage [HIL96]. The subsequent stages (4 and 5) involve the development of silica rich and amorphous calcium phosphate layers respectively. These

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20 stages incorporate the noticeable presence of biological moieties such as blood proteins, growth factors and collagen. Within 3-6 h in vitro, the calcium phosphate layer will crystallize into the carbonated hydroxyapatite (CAP) layer which is essentially the bonding layer. Chemically and structurally, this apatite is nearly identical to bone and tooth mineral, thus allowing the body to attach directly to it. These Bioglass surface reactions from implantation to 100-150 m CAP layer formation takes 12 to 24 h [HEN93, KON02]. Hench further investigated 6 more stages primarily focused around bone and tissue bonding. These stages, although not completely understood, depend significantly on the specific formulation of Bioglass involved. The standard for Bioglass formulation is commonly known as 45S5 which has been used extensively in research studies. It contains 45 wt% SiO 2 24.5 wt% Na 2 O and CaO, and 6 wt% P 2 O 5 Bioactive glasses have traditionally kept the P 2 O 5 fraction constant while varying the SiO 2 content. In fact, the network breakdown of silica by OH was found to be time dependant upon the concentration of SiO 2 It is now understood that keeping the silica below 60 wt% and maintaining a high CaO/P 2 O 5 ratio guarantees a highly reactive surface. Novamin, a trade name for bioactive glass, is manufactured by Novamin Technologies Inc. (Alachua, FL). The material is reported to have a long record of safety and efficacy as an implant material used to regenerate new bone in defects [GRE99]. When bioactive glass is incorporated into toothpaste formulations, the ions released from the amorphous calcium phosphate layer are believed to contribute to the remineralization process of the tooth surface. Bioactive glasses have been successfully used clinically as bone grafting material for over 15 years and has been cleared by the FDA for use in oral

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21 and orthopedic bone grafting for nearly 10 years [LOW96, WIL87]. Bioglass is also marketed worldwide under the trade names PerioGlas and NovaBone. Recently, it has been demonstrated that fine particulate bioactive glasses (<90 m) incorporated into an aqueous dentifrice have the ability to clinically reduce the tooth hypersensitivity through the occlusion of dentinal tubules by the formation of the CAP layer [LIT97]. Investigators using bioactive glass compositions have demonstrated a significant anti-microbial effect towards caries pathogens (S. mutans, S. sanguis) upon exposure to bioactive glass powders as well as solutions and extracts [ALL01, STO96, STO98]. Despite advances in oral care over the last 40 years in large part because of the incorporation of fluoride into a large number of products there are still greater than 150 million cavities filled in the US every year [ADA90] at an estimated cost of $12 20 billion. A substantial number of these cavities result from inadequate saliva, without which fluoride is of limited value [LEO01, SPA94]. This study could benefit many individuals who experience reduced calcium, phosphate and fluoride ions caused by hyposalivation resulting from old age, prescription drug use, Sjgrens Syndrome, diabetes and radiation therapy. In addition, women are at increased caries risk due to inadequate salivary calcium levels at different points in their lives including ovulation, pregnancy and post-menopause, resulting in the same net effect as reduced saliva fluoride efficacy. Approximately 9.7% of the general population is estimated to have insufficient salivation or chronic xerostomia [PUJ98], representing over 20 million Americans who consume $100-160 million annually in consumer toothpaste. Currently there are several additional strategies for preventing, reversing or arresting the caries process including application of fluorides, sealants, anti-microbials, salivary enhancers as well as patient

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22 education [NHI01]. But these approaches represent a time and financial investment for the patient. A reformulation of fluoride dentifrice containing Novamin can 1) enhance remineralization, 2) counteract demineralization, 3) control anti-caries activity more effectively than current fluoride toothpastes and may be a significant oral health contribution to the general population. The Novamin dentifrice used in this study resembles commercial toothpaste formulations, including the Colgate Regular product also used in this study. Novamin 4505 was added at 5 wt% in addition to the 1100 ppm fluoride (from sodium monofluorophosphate) which is used in current Colgate products. The Novamin 4505 used in this study is similar to 45S5 glass compositionally, but is designed to be a 5 m diameter particle. Particle size distribution of this particulate ranges from 15 m to less than 1 m and serves as a more highly reactive bioactive glass because of its greater surface to volume ratio.

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CHAPTER 4 MINERAL QUANTIFICATION The quantification of apatite mineral in dental studies is an evolving process. The past 50 years have shown us that a greater understanding of dental caries also brought about the explosion of consumer products to treat the disease. The United States Food and Drug Administration (FDA) is the regulatory agency for potential consumer products to be marketed to the masses. Although widely understood that early dentifrices cleaned the teeth well, the degree of efficacy in fighting dental caries was yet to be defined. Traditionally, scientific study requires researchers to provide proofs and logic to support their hypotheses. Qualification of these scientific arguments only provides proof of their existence but quantification provides the magnitude to which those arguments can be made and compared. Dental researchers understood the role of tooth mineral as the exchange commodity in cariology study. To justify research efforts, the FDA and numerous researchers have developed a number of analytical techniques to quantify the remineralization of teeth and caries resistance. Microradiography Transverse microradiography (TMR) has been developed over time to become the standard bearer for tooth remineralization studies. Theoretically it draws similarity to other photo-X ray techniques. The ability to quantify mineral density changes in addition to obvious visual qualification makes TMR the most powerful and trusted technique among researchers. 23

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24 Sample preparation for TMR requires thin (typically <100 m) sections that are polished and glued to glass slides. A cathode tube source emits polychromatic X-rays that are filtered (Ni) and directed toward the reference system and specimen. Figure 4-1 depicts the TMR setup in detail [MJO86]. Figure 4-1 Microradiography setup (A) Cathode Tube, (B) light-proof casing, (T) filter, (R) reference step wedge, (S) specimen, and (F) radiographic film Filtered monochromatic X rays reach the specimen and reference system, usually an aluminum step wedge. Typically these steps range from 10 150 m. Gelhard and Arends further developed this method running at 20 kV and 15-20 amp to produce a wavelength of = 0.15 nm, which in turn guaranteed 95% absorption inorganic (mineral) [GEL84, MJO86]. Sample thickness was estimated to 1 m accuracy on the radiographic film. Quantification of enamel tooth mineral is made with microdensitometric tracing of the film using specially designed software. Angmars formula is used to calculate relative mineral volumes with respect to cross-sectional depth with transverse microradiography [DIJ86].

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25 V = 50.48 (t a / t s ) 100% (4-1) where t a is the step wedge thickness and t s is the specimen thickness The volume percent mineral is then plotted against cross sectional depth from the surface to graphically produce the Z parameter. Z is defined as the integrated area under the densitometric tracing for each specimen in units of vol% mineral m. Plotting Z for both sound (original) and demineralized enamel provides a direct quantified measurement of mineral density flux, thus the parameter Z [GEL84]. A decrease in Z for a specimen therefore signifies mineral deposition de novo. This is represented in Figure 4-2 [DIJ86]. Figure 4-2 Mineral density quantified as Z in TMR. Confocal Laser Scanning Microscopy Confocal laser scanning microscopy (CLSM) is a powerful imaging technique based upon optical behavior of light within specimens. Generally, confocal analysis features excitation (such as fluorescence) although emission detection is also possible. The fluorescence phenomenon involves the absorption of light of a given wavelength by

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26 a fluorescent molecule followed by the emission of light at longer wavelengths. Fluorescence detection has three major advantages over other light-based investigation methods: high sensitivity, high speed, and safety, not only for the operator but also for the sample because they are not affected or destroyed in this process [WEB99]. This provides an extremely fine scale view of desired specimen planes. Although planar imaging is common among many techniques, the ability of CLSM to analyze thick (3D) specimens makes it so valuable in the scientific community. Conceptually, real time imaging of a single specimen for various depths or sections provides researchers with a practical advantage, especially considering biological specimens and time dependant reactions. For this study, it is imperative that fluorescence detection and imaging be understood. A typical confocal microscope is shown in Figure 4-3. Figure 4-3 Schematic of a generic confocal microscope. The theory of CLSM originates with the laser light source being focused upon the specimen and reflected back through a dichroic mirror to the detector. For fluorescence, the light returning from the sample through the objective lens reaches the beam splitting mirror and passes as excitation light. The detector processes the image based upon the

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27 focal intensities it receives through this path [WEB99]. Figure 4-4 depicts the imaging of a thick sample in greater detail. Figure 4-4 Conventional microscope for fluorescence in epitaxial configuration. The florescence of teeth was observed by both dye-assisted imaging and auto-fluorescence. Preliminary confocal work performed at the McKnight Brain Institute (University of Florida) showed a significant degree of autofluorescence in demineralized enamel through both fluorescein isothiocyanate (FITC) and Texas Red filters. Autofluorescence of enamel has been shown to significantly correlate with TMR and dye-assisted CLSM results [BEN89, GON99]. Similar work on nearly identical specimens was performed at the Oral Health Research Institute at Indiana University-Purdue University at Indianapolis (IUPUI). Both sets of imaging revealed again a high fluorescing lesion compared to near zero for sound enamel. Fontana and Gonzales-Cabezas [FON96][GON99] have shown a significant correlation between cross-sectional

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28 confocal analysis in fluorescence and mineral flux. Their fluoride toothpaste studies revealed a significant match for two parameters (lesion area and total fluorescence) between confocal microscopy and microradiography of tooth enamel and dentin. In vitro demineralization and remineralization of surface enamel was easily imaged and quantified. Significant correlations were also found for half tooth (thick sections) remineralization comparing TMR and confocal microscopy. For remineralization studies, Z pre and post remineralization were combined as a single parameter representing their mathematical difference, M. Fontana and Gonzales found Pearson correlation coefficients (perfect correlation = 1) of 0.71 for M vs. total lesion area and 0.70 for M vs. total fluorescence. A previous study involving only demineralized samples were found to correlate with TMR to the same degree. A subsequent study also revealed nearly identical correlation in mineral changes with TMR when comparing autofluorescence (no dye) of enamel vs. the dye. The fluorescent dye (0.1 mM Rhodamin B) was used to enhance fluorescence imaging of lesion areas in these studies [BEN89]. Nanoindentation Nanoindentation is a relatively new method for characterization of material mechanical properties on a very fine scale. Features less than 100 nm across and thin films less than 5 nm thick can be evaluated. Test methods include indentation for comparative and quantitative hardness measurements, and scratching for evaluation of wear resistance and thin film adhesion. Nanoindentation is often performed in conjunction with atomic force microscopy (AFM). The area for testing is located by AFM imaging and indentations are imaged by AFM after testing. A three-sided, pyramid

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29 shaped diamond probe tip (Berkovich) is commonly used for sample indenting and scratching. For indenting, the probe is forced into the surface at a selected rate and maximum force. For scratching, the probe is dragged across the sample surface while controlling the force, rate, length, and angles. Imaging is performed using the same probe for intermittent contact via (tapping mode) AFM. A force-displacement curve based on piezoresistive loading is generated during indentation and provides further indication of mechanical properties [HAN03, HYS03]. Figure 4-5 Force-displacement (loading) curves on various materials. This study considers two main mechanical properties obtained through the Triboindentor (Hysitron Inc., Minneapolis, MN): hardness and reduced modulus. The load-displacement data from the unloading curve is fit to a power law relation to determine mechanical properties. P=A(h 0 -h f ) m (4-2) where m = 2 (conical), P is the load h o is the initial depth, h f is final depth, and A is the area.

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30 The derivative of this power law with respect to the depth gives the material stiffness (S) at the maximum load, P max S = dP/dh at P max (4-3) The contact depth, h c can be calculated from: h c = h max 0.75(P max / S) (4-4) The hardness, H, and reduced modulus, E r are then found by: H = P max / Ah c (4-5) E r =[S] / [2(Ah c )] (4-6) A typical loading curve for sound enamel is shown in Figure 4-6. Control Enamel 1-200020040060080010001200140016001800-1000100200300400500Contact Depth (nm)Load (u N Figure 4-6 Loading curve for nanoindentation on sound enamel. Microindentation Microindentation, on the other hand, is a more established technique for measuring the hardness of materials. For ceramics in particular, hardness is a critical mechanical property. For engineering and characterization applications, approximately 60% of worldwide published ceramic hardness values are obtained using Vickers diamond

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31 indentation, with loads typically in the range of a few Newtons to 9.8 N (1 kg-f) and occasional data for high-toughness ceramics as high as 98 N (10 kg-f). At small indentation loads, problems arise from the load dependence of hardness and from measurement uncertainty due to the small indentation size. At higher loads, cracking and fracture become problems in some cases, making measurement impossible. Typical hard ceramics have Vickers hardness in the 10-30 GPa range [QUI98]. Teeth particularly are considered ceramic composites. Typical hardness values for enamel range from 2.9-3.9 GPa and dentin 0.6 GPa [FOR91, WIL92]. Figure 4-7 Vickers microhardness testing. The square pyramidal shape of the Vickers indenter creates a smaller deeper impression, although more likely to crack than Knoop indentations. ASTM standard E 384, Microhardness of Materials, covers Vickers hardness; C 1327 is the new standard for Vickers hardness of advanced ceramics and recommends a load of 9.8 N. The universal standard [AST00] for this calculation is described in equation 4-7. Vickers Hardness (HV) = 1854.4 P/d 2 (4-7) where P is the load in grams force d is the mean diagonal length of indentation in m, assuming standard face angle of indenter tip is 136

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32 To obtain GPa from these units (kgf/mm 2 ), multiply by 0.0098. ASTM standard E 384 section 7.1 clearly states that optimum accuracy of measurement requires that the specimen be flat with a polished or otherwise suitably prepared surface [AST00]. Although it was developed in the 1920s (by engineers at Vickers Ltd. in the U.K.), this hardness testing method continues to be a significant standard, especially for hard ceramics. The analysis theory is quite different for these two techniques. As mentioned earlier, nanoindentation is based upon a loading/unloading curve calculation whereas Vickers is solely dependent upon the indent surface area. Oliver and Pharr demonstrated (Table 4-1 and 4-2) that results from the two techniques are comparable [OLI92]. Table 4-1 Comparison of hardness and Youngs modulus data obtained with Vickers and Berkovich indenters. Table 4-2 Comparison of indentation fracture toughness obtained by different indenters. Preliminary evaluation of the nanoindentation technique was performed and imaged for bovine enamel with AFM in conjunction with nanoindentation, see Figures 4-8 and 4-9.

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33 Figure 4-8 Demineralized Enamel. Axis units in m.

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34 Figure 4-9 Indents in Novamin treated enamel. Axis units in m. Imaging for preliminary Vickers indents was performed with SEM for bovine enamel also. All Vickers testing for this group was performed at 300g load with 15s dwell time. Figure 4-10 Vickers indents in control enamel @ 300X and 2000X.

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CHAPTER 5 EXPERIMENTAL METHODS AND MATERIALS Collection and Storage of Teeth A total of 48 extracted human molars were collected from dental surgical clinics at the University of Florida. Teeth were required to have intact surfaces, no carious lesions, and no restorations. In keeping with the Health Insurance Portability and Accountability Act (HIPAA) regulations, we were not aware of the cause for tooth extraction, age, name, or gender of the patients. The use of human teeth in this study was also approved by the Institutional Review Board (IRB) at the University of Florida. A total of 18 (out of 48) of the collected teeth were randomly segregated for use in this toothpaste study. All teeth were stored in 0.1% Thymol (w/v) solution until sterilization [BIS03]. Teeth were cleaned by removal of soft tissue debris and later sterilized using Co 60 source for gamma irradiation. They were subjected to a dose of 500 krad, well above the required 173 krad. This process does not alter the tooth structure or caries susceptibility [HAN62, WHI94]. Sectioning and Mounting Each tooth (n = 18) was sectioned into quadrants along both mesiodistal and buccolingual planes (Figure 5-1) using a diamond tipped circular saw (Buehler Isomet 300 Low Speed). A strict labeling method was assigned to each tooth section designating tooth number and treatment protocol (see Table 5-1). Sections were mounted individually in epoxy mounting resin and labeled. A 3-mm diameter treatment window was opened on each tooth section by grinding off 200 m of surface enamel. Removal of the outer enamel layer was essential for standardizing the experiment. 35

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36 Table 5-1 Tooth labeling scheme. Example 36BX Tooth Section Tooth # Label Treatment Mesiolingual 31 thru 48 LC None (Control) Distolingual 31 thru 48 LD Demineralized Control Mesiobuccal 31 thru 48 BX Colgate Regular Distobuccal 31 thru 48 BY Novamin Dentifrice (5 wt%) Fluoridation and cyclic remineralization/demineralization of the surface layers makes each tooth surface inherently inhomogenous. In order to reduce variability, these layers were removed to expose the virgin enamel deeper in the tissue [RIC89]. Mechanical polishing was performed with successive 120, 400, 600 and 1200 grit paper and confirmed with a digital caliper. Figure 5-1 Experimental flow diagram of specimen sectioning and mounting. Lesion Formation Tooth sections LD, BX and BY (Figure 5-2) were demineralized in stirred solution containing 2.2 mM CaCl 2, 2.2 mM NaH 2 PO 4 0.05 M Lactic Acid, and 0.5 ppm F adjusted to pH 4.5 with 50% NaOH. Surface lesion formation was maintained at 37C in order to produce a uniform lesion depth of 100-150 m [IVA03]. Treatment Regimen Section BX and BY were treated in a pH cycling regimen including 3:1 toothpaste solutions of Colgate Regular and Novamin dentifrice respectively. Both dentifrices, nearly identical in composition, contain 1100 ppm fluoride although the Novamin paste contains 5 wt% bioactive glass particles in place of silica abrasive. Tooth sections were

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37 immersed cyclically in stirred treatment solutions, demineralization solution (detailed above) and Fusayamas synthetic saliva [LEU97] for 20 days. Complete pH cycling was performed at 37C with exception to dentifrice solution treatment, which was done at 25C. The daily treatment regimen (pH cycling) included the following sequence: T 0 Demineralization solution 30 minutes T 0 + 0.5 hrs Dentifrice treatment 3 minutes / distilled H 2 O wash / synthetic saliva T 0 + 7.5 hrs Demineralization solution 30 minutes T 0 + 8.0 hrs Dentifrice treatment 3 minutes / distilled H 2 O wash / synthetic saliva for 16 h Cross-sectioning All four sections of each tooth (LC, LD, BX and BY) were cross-sectioned through the treatment window to expose the lesion depth along the cross-sectional surface. Two halves of each tooth section were then available for optical and mechanical analysis. Throughout the study, tooth sections were intermittently kept in refrigeration for storage (~6C). Teeth were individually wrapped in distilled water soaked Kimwipes tissue. Figure 5-2 Flow diagram of lesion formation, pH cycling and cross sectioning.

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38 Analysis CLSM was performed on 72 cross-sections (4 sections of 18 teeth) at the Oral Health Research Institute at Indiana University-Purdue University at Indianapolis (IUPUI) under the supervision of Dr. Marguerita Fontana. Randomly selected cross-sections of the remaining halves were chosen for nanoindentation at the Major Analytical Instrumentation Center (MAIC) at the University of Florida. Microindentation was then performed (Buehler Ltd. Micromet 3 Microhardness Tester, Lake Bluff, Illinois) on the same sections for hardness comparison of both techniques.

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CHAPTER 6 RESULTS AND DISCUSSION Cross sectional analysis of enamel lesions with CLSM were based upon digital images taken at specified controlled conditions. Sound enamel registers near zero fluorescence (grayscale value ~ 0) and appears pitch black. Lesions slightly autofluoresce but imbibition of the Rhodamine B dye (0.1 mM) allows the porous demineralized layer to fill and appear with considerable contrast. Analysis of all samples was conducted with a specially modified Nikon microscope fitted with Odyssey confocal capability (Odyssey, Noran Instruments, Inc., Middleton, WI). The accompanied software (Metamorph version 4.1.6, Universal Images Corp., West Chester, PA) calculates image-based parameters of selected lesion zones. Using a 10X Nikon objective, the specimens were illuminated with an argon laser at 50% intensity using a 488 nm excitation wavelength. Confocal slits were set at 25 m with a 515 nm long-pass filter. A 350-m characteristic length was randomly chosen within a representative part of the lesion for each sample. Figure 6-1 shows a typical cross-sectional image. 39

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40 Figure 6-1 Confocal image of enamel lesion, tooth section 38LD The complete analysis (all 4 tooth sections of 18 teeth) was performed at contrast level 1500 and minimum threshold value 50 (grayscale). The two parameters that correlate well with TMR are lesion area and total gray value (or total fluorescence). Data for these two parameters was collected and tabulated to compare sections within each tooth to determine changes in lesion properties after 20 days of dentifrice treatment. Data could not be collected for LC (sound enamel control) naturally because of non-fluorescence. Parameters are specifically defined as: Lesion Area direct summation of fluoresced (above threshold) pixels in characteristic length of lesion. Total Gray Value direct summation of gray values (0 255) for pixels within characteristic length of lesion.

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41 Lesion areas for 3 sections of all 18 teeth were plotted to compare the effect of Novamin and Colgate dentifrices in Figure 6-2. 0500010000150002000025000300003500040000313233343536373839404142434445464748Tooth #Lesion Are a LD (Demin Control) BX (Colgate) BY (Novamin D3 4505 5%) Figure 6-2 Lesion area for each tooth for the demineralized control, Colgate and Novamin treatment condition. The Novamin dentifrice reduced the lesion area more than Colgate for 16 out of 18 teeth. Single tailed T-testing (assuming unequal variances) found these two groups to be significantly different (p < 0.001). For nearly all samples, the 20 day dentifrice treatment under pH cycling halted and reversed the caries process as evident in the reduction of the lesion area. Colgate reduced the lesion area by an average of 24.9% from the original lesion LD. Novamin decreased the lesion area by 41.9%. Total gray value was also plotted (Figure 6-3) to determine the fluorescence levels within the lesions. This parameter is commonly reported as total fluorescence because greater fluorescence corresponds to higher gray values. Thus, smaller values are indicative of less porosity and dye penetration, or more mineral.

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42 0.00E+002.00E+064.00E+066.00E+068.00E+061.00E+071.20E+071.40E+071.60E+07313233343536373839404142434445464748Tooth #Total Gray Valu e LD (Demin Control) BX (Colgate) BY (Novamin D3 4505 5%) Figure 6-3 Total Gray Value for each tooth for the demineralized control, Colgate and Novamin treatment condition. The Novamin dentifrice reduced the total gray value more than Colgate for the same 16 out of 18 teeth. Treatment groups were again found to be significantly different (p < 0.001). Colgate reduced the fluorescence (total gray value) by an average of 48.1%, Novamin 70.5%. Statistical review (SAS System software) using two-way analysis of variance (ANOVA) of all three groups (LD, BX and BY) yielded statistical significance (p < 0.0001) for both confocal parameters, lesion area and total fluorescence. Duncan multiple range testing for both parameters found all three groups to be statistically different in means at a significance level of = 0.01. Cross sectional images for these teeth sections show remineralization bands revealing the depth at which most remineralization occurred.

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43 Figure 6-4 Confocal image of tooth section 41LD (Demineralized Control) Figure 6-5 Confocal image of tooth section 41BX (Colgate treated)

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44 Figure 6-6 Confocal image of tooth section 41BY (Novamin treated) The remineralization bands are clearly visible signifying mineral deposition, similar to those reported in previous toothpaste studies. The depth of these bands could be due to several variables. A combination of diffusion rates for fluoride, calcium, and phosphates into the enamel surface in addition to twice daily acid challenge may influence band depth. Fractional variations in composition from one tooth to another probably play a role as well. Slight differences in impurity element levels in the tooth structure cannot be ignored. Although the natural surface enamel was removed, the maturation history of these specimens could vary considerably. It is likely that fluoridated water sources and dietary habits of the original patients play some role in the demineralization and remineralization capability of these teeth. Ultimately, these variables also determine the size and shape of these bands. The most influential factor,

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45 however, is the daily acid challenges and acid diffusion capability. The noticeable (bright) demineralization layer above the remineralization band is unusual considering this layer always receives the first and heaviest dose during remineralization treatment. Logically, the remineralization band should begin at the surface and fade with depth. But this would be true if only remineralization was taking place within the lesion. The fact that pH cycling includes daily acid challenges keeps the outer surface demineralized and in a state of constant flux. The depth of that demineralized band is most likely controlled by acid diffusion, possibly an Arrhenius relationship. Novamin samples clearly show a larger remineralization band for nearly all samples and a noticeably darker lesion. Figure 6-7 Confocal image of tooth section 47LD (Demineralized Control)

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46 Figure 6-8 Confocal image of tooth section 47BX (Colgate treated) Figure 6-9 Confocal image of tooth section 47BY (Novamin treated)

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47 Figure 6-10 Confocal image of tooth section 48LD (Demineralized Control) Figure 6-11 Confocal image of tooth section 48BX (Colgate treated)

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48 Figure 6-12 Confocal image of tooth section 48BY (Novamin treated) The nature and degree of interaction between fluoride and Novamin is clearer. It appears that in vitro, these two agents positively interact and do not inhibit the remineralization function of the other. Although the techniques used in this study cannot confirm or deny this hypothesis, ultimately the lack of oral bacteria and plaque leave room for question. An in vivo study with plaque is necessary to better determine the degree to which these two agents contribute to the remineralization process at the tooth interface. CLSM has proven to be not only a powerful technique to assess remineralization, but also an efficient one. The samples, considered large by past experimental methods, required no special post-treatment preparation for analysis other than cross-sectioning and 24 h dye soaking. This technique reduces time and cost as well as improved operator safety compared to with TMR. The most valuable asset may be yet to come however. If

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49 autofluorescence of surface lesions could be correlated with TMR to the degree of dye-assisted fluorescence analysis, timed studies could be performed at intervals during the treatment of same tooth sections rather than different sections of the same tooth. This provides a more accurate account of the remineralization process because it eliminates the slight variability among different sections of the same tooth. Cross-sectional CLSM analysis proved to be the optimal approach to surface remineralization study in vitro. From cross-sectional imaging it was clear that most remineralization took place somewhat below the surface during exposure to fluoride containing toothpaste. This is consistent with earlier studies [GON99]. Therefore data from the treatment surface alone would be inaccurate, misleading, and incomplete. Reducing variability is extremely important for remineralization studies because teeth are inherently inhomogeneous, especially surface layers. To obtain meaningful data, in vitro analysis requires the investigator to eliminate as much experimental variability as possible. Relative success of this study should be credited to meticulous standardization in each procedural step. Sample preparations, solutions, and processes were all subject to strict uniformity control for all tooth sections. This leaves only inherent specimen variability unaccounted. It was also confirmed that in vitro remineralization/demineralization studies are predominantly controlled by basic chemistry fundamentals such as pH, ion concentrations and (enamel) solubility. This study did not include organic components such as oral bacteria or plaque. Therefore anti-microbial effects were not active. The key factors for remineralization here were ions (Ca2+ and PO43-) and the pH at the tooth surface. The release of these ions from Novamin provides a solubility gradient in favor

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50 of mineral deposition. This explains why the samples treated with Novamin dentifrice experienced greater remineralization, the environment was more conducive to this process. This presents a stronger case for Novamin dentifrice in Xerostomic conditions. Surface pH appeared to be influential because Novamin dentifrice solution maintain a significantly higher pH than Colgate solution. Dentifrice treatment solutions were changed daily and pH differences within that period were recorded for each solution. Average pH for freshly made Novamin solution was measured at 9.8. After 24 h, the same solutions averaged pH 10.4. Fresh Colgate solution pH averaged 7.2 while 24 h aged solution averaged 7.3. It is likely the Novamin dentifrice solution neutralized the preceding acid challenge significantly better than its counterpart. While half of the cross-sectioned sample was sent to IUPUI for CLSM, the remaining half was tested with nanoindentation for mechanical property changes. Nanoindentation analysis of enamel cross-sections was performed throughout the depth of the lesion and into the sound enamel for two selected teeth. A graphical plot of hardness versus the cross sectional depth from the surface confirmed the demineralized nature of the lesion within the first 100 m of these teeth. The hardness within the lesion was consistently low for all sections with the exception of those areas previously identified as remineralization bands. These bands, typically half way through the lesion depth, exhibited a noticeable hardness spike for both dentifrice treated sample groups. Figures 6-13 and 6-14 illustrate this point.

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51 -0.50.51.52.53.54.55.5050100150200250300Depth [um] BX (Colgate) 33 BY (Novamin) 33 LD (Demineralized Control) Figure 6-13 Cross sectional nanoindentation of tooth 33. 012345678910020406080100120Depth [um] LC (Sound Enamel Control) LD (Demineralized Control) BX (Colgate) BY (Novamin) Figure 6-14 Cross sectional nanoindentation of tooth 44. At greater depths, the hardness increases to the expected range of sound enamel. For tooth 33, the hardness spike appears at 40-50 m depth while tooth 44 shows a similar spike between 60-80 m. Both dentifrices (BX and BY) exhibited hardness fluctuations throughout the lesion depth without statistically significant difference (p >

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52 0.05). Both groups had markedly larger hardness values than the original lesion section (LD) within the lesion (below 100 m). However, such variability within sample groups is not advantageous to quantitative analysis. Five more teeth were plotted similarly and showed no significant difference between either test group. In fact, a number of tooth sections exhibited highly irregular hardness behavior. Some displayed greater hardness values for the demineralized section compared to sound and treated counterparts. Multiple tooth sections also exhibited gross hardness fluctuations of 10 GPa or more. Nanoindentation did not graphically or statistically provide significant evidence for remineralization of lesions for the treatment groups. Select teeth exhibited positive trends but quantitative analysis of enamel remineralization with nanoindentation is not recommended. Cross-sectional indentation does however have some qualitative value although inconsistency is clearly an issue. Any number of factors could influence these results. Primarily, nanoindentation is heavily dependent upon sample preparation. Variation in polishing or sample tilt could overshadow any differences in mineral content. These samples were particularly re-mounted in epoxy and again fine polished (with 0.05 m gamma alumina) in order to reduce surface roughness for indentation. Multiple polishing effects or residual epoxy resin deposition could possibly influence mechanical properties of the porous lesion. A degree of operator skill is also relevant. Initial placement of the indents is key to determining the hardness relationship to depth. A number of samples appeared as though initial indents were started in the bordering epoxy region, thus introducing an inherent offset for subsequent indents that are related to the former by position.

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53 Similar to microindentation, soft porous surfaces are extremely difficult to measure for hardness. The lesion zones may have presented pore sizes similar to that of the indenter tip. Features such as inherent crystal growth inhomogeneity and mineral deposition also introduce influential variables on a scale within range of the indenter tip. Crystal diameters within lesions of 0.1 m [EKS88] could affect hardness measurements for this study considering the indentation depths of 0.5 2.0 m at 5000 N load. In sound enamel, crystal orientation (growth angles) and enamel mineral anisotropy have proven to affect hardness significantly [HAB01], thus demineralization and subsequent mineral deposition could introduce additional variation. Rather than representative hardness, nanoindentation may be too sensitive a technique to determine mineral changes over large lesion areas. It appears that ultra-fine features tend to dominate hardness data in enamel, preventing broad range determination of remineralization effect within enamel lesions. Vickers microhardness was performed on the same samples analyzed with nanoindentation (seven total). A 100 g load at 15 s dwell time was used with a standard Vickers diamond tip. It was impossible to visually identify indents in the porous lesion for most samples. Hardness for the few samples in which indents were optically visible was still difficult to quantify. Indent edges were blurred and only marginally in focus, thus measuring diagonals was reduced to a subjective judgment. Generally, microhardness data collection was unsuccessful. If these indents were visible, it is still unlikely that microhardness data could indicate a mineralization difference. The size of the indents nearly matched that of the lesion, thus providing only representative hardness on much a much larger scale than nanoindentation. Microhardness presents the reverse

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54 problem relative to nanoindentation. It appears to be too insensitive a technique to distinguish differences in remineralization of enamel surface lesions.

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CHAPTER 7 CONCLUSIONS A number of conclusions can be made from the results we observed. First, in vitro study using confocal laser scanning microscopy (CLSM) has shown that Novamin dentifrice exhibits a greater degree of remineralization than Colgate dentifrice of early caries lesions in human enamel. Statistical analysis (T-test) found significance (p < 0.001) among treatment groups. For both CLSM parameters, ANOVA (p < 0.0001) and Duncan multiple range testing ( = 0.01) also yielded significant difference among groups. CLSM has also proven to be an efficient analysis technique for cross-sectional study. Post-treatment tooth preparation was minimal and non-destructive. It appears that remineralization of surface lesions was successful independent of any anti-microbial effect. There were no organic components, such as plaque or bacteria, involved in this study. Therefore, surface variables including pH and Ca 2+ /PO 4 3most likely influenced the conditions at which remineralization of the porous lesions took place. Reducing variability throughout the study notably enabled relevant data collection. Inherent tooth inhomogeneity was beyond experimental control, therefore strict uniformity in laboratory technique enhanced the statistical significance of the two treatment groups. Statistical analysis appears to confirm this. Hardness testing for in vitro remineralization of enamel surface lesions was statistically insignificant and provided minimal quantitative value. Nanoindentation hardness testing was dependant on too many variables. Sample preparation and porosity 55

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56 (relative to the tip size) makes it apparent that nanoindentation could be too sensitive a technique to determine remineralization effect of dentifrice on demineralized enamel. Conversely, Vickers microhardness is too insensitive a technique to distinguish differences in the remineralization effect on the desired scale. Thus, neither microhardness nor nanonindentation techniques are recommended for remineralization studies. However, confocal laser scanning microscopy is potentially a very good technique for studying remineralization.

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BIOGRAPHICAL SKETCH Sammel Alauddin was born in Dhaka, Bangladesh, and immigrated to the United States at the age of two. Raised in Frankfort, Kentucky, he graduated high school at the age of 16 to pursue higher study at the University of Kentucky in Lexington. In 2002, he completed both a B.A. in Organic Chemistry and a B.S. in Materials Science Engineering. A chance meeting with a former professor at a conference led to her suggestion that he pursue graduate studies and consider her new institution, the University of Florida. At UF, he expanded his interest in biomaterials research and improving the health of future generations. With these goals in mind, he strives to make his contribution to science. 62