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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
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
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
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.
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
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
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
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
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
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
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.
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
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
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:
* Compare and contrast the benefits and drawbacks of confocal laser scanning
microscopy and nanoindentation for analysis of tooth remineralization in
* Determine the remineralization capabilities of a Novamin bioactivee glass)
containing dentifrice on human enamel in vitro.
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.
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
mI / "- ..j-II enamel
I pulp cavity
n .~ momentum
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
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 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
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.
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.
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].
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
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].
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
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].
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
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.
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.
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
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].
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
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-
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.
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
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
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.
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
0 *. .
Contact Depth (nm)
Figure 4-6 Loading curve for nanoindentation on sound enamel.
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
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
MATF R[AL BE RKIUl ICI INDENTER VICKERS INDENTER
Elastic McAl lus IlarlIness [f. a.; LIas-ij. Mu ludu HaJd1e-L [GiP-1i
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
Figure 4-10 Vickers indents in control enamel @ 300X and 2000X.
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
Figure 5-1 Experimental flow diagram of specimen sectioning and mounting.
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].
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
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
buccal buccal All 4 sections are
further X-sectioned for
2 sections runthru
pH cycling in I
Figure 5-2 Flow diagram of lesion formation, pH cycling and cross sectioning.
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.
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.
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
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.
| 1.00E+07 -
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
*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
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.
3.5 A A& A A A
0 50 100 150 200 250 300
A BX (Colgate) -l-33 BY (Novamin) 033 LD (Demineralized Control)
Figure 6-13 Cross sectional nanoindentation of tooth 33.
0 20 40 60 80 100 120
--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
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
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
problem relative to nanoindentation. It appears to be too insensitive a technique to
distinguish differences in remineralization of enamel surface lesions.
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
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
(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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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.