A LITERATURE REVIEW ON HUMAN DENTAL WEAR: FROM DEFINITIONS TO PREVENTION
Carlos Sarria
ME 6960: Friction Wear & Lubrication of Materials
Spring 2015
Rensselaer Polytechnic Institute-Hartford, CT
Professor Dr. Ernesto Guitierrez-Miravate
Submitted on: 16 May 2015
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Abstract
This high level literature review is intended to provide the reader with a general summary of
the definitions of dental wear mechanisms in addition to previously done research in the field.
It starts by first providing key medical terminology used throughout the document, followed by
explanations of the different mechanisms accepted by the engineering and dentistry
communities: Abrasion, erosion, abfraction and fatigue wear. Several research reports are
referenced throughout this document to provide a broader explanation and examples of the
topics being discussed. A brief sample of numerical studies using FEM follows where the author
summarizes the work of several researchers. Lastly, a summary on wear prevention measures
based on the latest laboratory observations highlight practices that patients can adopt to inhibit
wear rate.
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Table of Contents
1. Introduction ............................................................................................................................................... 4
2. Tooth Anatomy ......................................................................................................................................... 5
3. Mechanisms of Dental Wear ..................................................................................................................... 6
3.1 Abrasion .............................................................................................................................................. 6
3.1.1 Two-Body Abrasion ...................................................................................................................... 7
3.1.2 Three-Body Abrasion ................................................................................................................... 8
3.2 Erosion (Corrosion) ........................................................................................................................... 12
3.3 Abfraction ......................................................................................................................................... 15
3.4 Fatigue Wear ..................................................................................................................................... 17
4. Numerical Approximation Modeling to Dental Wear Problems ............................................................. 18
4. Dental Wear Prevention ......................................................................................................................... 21
5. Conclusion ............................................................................................................................................... 22
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1. Introduction
Dental wear in is a mechanical and pathological undesirable effect of human’s daily lives. It is
highly dependent on the occupation of the person, dietary preferences, and dental hygiene
(Ashcroft and Joiner 2010). Several authors (for example Wegehaupt et al 2011 and Amaechi
and Higham 2004) agree that due to the longer average life expectancies of the 21 st century in
addition to diets richer in acid foods, fruits and soft drinks, excessive teeth wear is becoming a
significant anatomical problem.
Dental wear, if left uncontrolled, can lead to reduction of teeth functionality, difficulty
speaking and even chronic tooth pain and sensibility (Barlett 2005), in addition to expensive
restoration procedures. There are several mechanisms of wear associated with teeth, and they
are usually categorized as: Abrasion, erosion, abfraction, and fatigue wear (Ashcroft and Joiner
2010, and d’Incau et al 2012). Some wear types, for example abrasion and abfraction, are
mechanically driven due to contact with either other teeth or foreign objects. There have been
studies (Ashcroft and Joiner 2010, and Wiegand et al 2009, for example) investigating not only
the resulting wear due to contact with food and teeth, but also the effects that teeth hygiene
have. Despite the benefits of practicing a good, regular dental hygiene, both brushes and tooth
pastes may be detrimental to enamel and dentine health.
Researchers such as Wu et al. investigated the effects that several of these mechanisms
have, since more often than not they will be acting simultaneously. For instance, erosion and
attrition tend to occur together due to the more common consumption of soft drinks, and
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tooth-on-tooth contact. Researchers, including Wegehaupt et al, have explored ways to inhibit
corrosion of teeth.
In addition to investigating the different wear mechanisms in labs, there is abundant
literature (for example Chai 2014 and Srirekha and Bashetty 2012) that have examined using
finite element modeling (FEM) to further develop a more comprehensive understanding of
these. Taking advantage of today’s superior computational capability of computers, scientists
and engineers can explore environmental conditions that would otherwise be difficult to
replicate in a lab setting.
Researchers are aware of the importance that healthy teeth have on humans’ daily lives
for both an aesthetic and a functional perspective. Consequently a vast wealth of knowledge
has been gathered throughout the years based on laboratory work and analytical calculations
that not only helps understand what causes the observed dental wear, but also why, and how it
can be prevented. Using these results, dentists can advise patients with regard to lifestyle
changes that encourage positive dental health.
2. Tooth Anatomy
Prior to discussing the different wear mechanisms associated with human teeth, it is necessary
to provide a high level review of tooth anatomy. Figure 1, obtained from WebMD, shows a
diagram with all the human teeth (only a half set shown), aft looking forward. The aft-most
teeth are called molars, followed by the premolars, canine and incisors.
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Figure 1: Tooth Anatomy (from WebMD)
The cross section on Figure 1 shows the enamel, which is the outer most, hardest
(usually white) part of the tooth and is made of calcium phosphate (WebMD). Below the
enamel, a layer made of living cells called dentine is found. The function of this layer is to
secrete minerals and other necessary substances. Beneath the dentine one can find the pulp,
which is a softer, inner living structure. Blood vessels and nerves run through the pulp of the
teeth. Next, the cementum binds the roots of the teeth firmly to the gums and jawbone. The
gum tissue (periodontal ligament), holds the teeth tightly against the jaw.
3. Mechanisms of Dental Wear
There are several wear mechanisms known to and investigated by researchers (d’Incau, et al
2012, and Ashcroft and Joiner 2010). Ashcroft and Joiner explain that all these types of wear
can act synchronously or sequentially, and/or synergistically or additively.
3.1 Abrasion
This category is defined as the loss of material by the repeated interaction of two or more
bodies. The mentioned loss of material occurs due to the asperities of each body when they’re
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in contact. Because the surfaces of the bodies are not smooth, contact occurs at small areas.
The pressure at these micro contacts is high enough to cause deformation or rupture of the
hard tissue (d’Incau, et al 2012). D’Incau et al explain that the tribology field defines two types
of abrasion: Two-body (also known as attrition) and three-body abrasion (most commonly
referred as abrasion).
3.1.1 Two-Body Abrasion
Two-body abrasion, or attrition, is caused because of the friction between the moving teeth as
they come into contact (d’Incau, et al 2012). The authors explain that if the objects have
substantially different levels of hardness, micro asperities move from the harder surface to the
softer one, creating a prow ahead of them. Eventually they move grooves that will result in
local deformations and ultimately material loss due to micro fatigue.
On the other hand, when the two bodies have similar hardness levels, the asperities
from the harder surface cut the softer surface cleanly without any plastic deformation (d’Incau,
e al 2012). The authors explain that the shape and volume of the resulting groove correspond
to the volume of the material being displaced. Under high pressure conditions, these grooves
lead to the formation of small cracks that propagate until they release some material.
Ashcroft and Joiner describe different ways in which attrition can take place. The most
common origin of this type of wear is due to mastication (chewing), which can be aggravated
when chewing hard foods (three-body abrasion). In addition, other processes such as bruxism
(grinding of teeth) and occupational factors also play a key role in the rate of this wear.
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Figure 2 is an example of the long term effects of attrition (Ashcroft and Joiner 2010).
Notice how the incisors on both the maxilla and mandible appear to be flattened out due to the
excessive loss of material.
Figure 2: Attritional wear (Ashcroft and Joiner 2010)
The observations from Figure 2 are supported by the results of studies on occlusal wear
of teeth (Ainamo 1972 for example). In his study of 154 army recruits, Ainamo observes that
more often than not, attrition occurs in anterior teeth “In particular the incisal edges of
maxillary and mandibular central incisors and canines.” In addition, he also notices that there is
a contradictory relationship between the amount of wear measured and the levels of tooth
hygiene. In other words, tooth cleaning, while preventing plaque buildup, encourages a higher
rate of wear. Ashcroft and Joiner discuss this wear and classify it as three-body abrasion (or
only abrasion).
3.1.2 Three-Body Abrasion
Three-body abrasion, more commonly known as abrasion, is a wear mechanism in which the
loss of dental hard tissue occurs due to the contact with a foreign object or substance (Imfeld
1996). Abrasive substances or particles include hard foods, particles due to occupational
activities (exposure to abrasive dust, for example) and even hygienic devices such as tooth
brushes and pastes (Ashcroft and Joiner 2010).
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D’Incau et al explain that there a two types of three-body abrasion mechanisms, based
on the proximity of the solid moving bodies (teeth). First, when the solid bodies are far apart,
the abrasive particles (foreign body) are free to move, acting like slurry on all surfaces. Under
this condition, only around 10% of these particles contribute to abrasion wear. On the other
hand, when the two solid bodies are close enough for contact to occur, the abrasive particles
get trapped between the bodies and are carried away by these bodies. Consequently, the third
body (or bodies) causes specific types of grooves and striations (d’Incau et al 2012). The
authors explain that there are two types of three-body abrasion: Generalized and localized.
3.1.2.1 Generalized Three-Body Abrasion
This type of abrasion takes place during mastication due to the abrasive loads of the
food bolus, and it affects all tooth surfaces. It is made up of two phases. In the initial phase,
known as crushing, abrasive particles from the bolus are free to move around and affect nonocclusal contact areas. Additionally, wear is further reinforced by the actions of the tongue and
soft tissues in areas where there is no bacterial plaque (d’Incau et al 2012).
The second abrasion phase is known as the sliding phase and it takes place as the teeth
approach each other and the food bolus is gradually shredded. The abrasive particles are
dragged and trapped between the teeth surfaces forming temporarily gouges, furrows, pits and
scratches. These features are highly dependent on masticatory cycles and masticated foods
(d’Incau et al 2012). Figure 3 shows a Scanning Electron Microscope (SEM) micrograph of the
occlusal surface of a first upper left molar in a medieval adult individual (reproduced from
d’Incau et al). The top left region (1), in the region labeled “E” (the enamel region) is
predominantly pits rich. Region 2 (top right corner) is characterized by surface scratches.
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Figure 3: SEM micrograph of occlusal surface of a first upper left molar
2.1.2.2 Localized Three-Body Abrasion
D’Incau et al explain that localized abrasion is associated with tooth brushing. In this case, the
third body is represented by the abrasive particles from the toothpaste. These particles are
then interposed between the brush and teeth. Abrasion due to commercial toothpastes is
undesirable and unavoidable, since tooth pastes rely on abrasive particles to abrade away
plaque and extrinsic stains (Ashcroft and Joiner 2010). These stains are caused by either
colored compounds becoming incorporated into the pellicle or by chemical. Ashcroft and
Joiner explain that there are several substances that contribute considerably to the staining of
teeth including tea, coffee, curry and certain fruits.
Previous studies regarding the abrasion of dental hard tissue by toothpastes lead to the
conclusion that under normal use, there is little or no abrasion of the enamel and only minor
abrasion of the dentine over the lifetime of a human being (Ashcroft and Joiner 2010). Modern
toothpastes contain abrasive particles that are softer than the enamel. The impact to the
dentine, however, is much higher since the hardness of the abrasive particles and dentine are
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very similar. Ashcroft and Joiner highlight that based on previous tests, the estimated total
amount of wear over a lifetime’s use of toothpaste is less than 1mm.
Wiegand et al carried out a study with the objective of evaluating the impact of
toothpaste slurry abrasivity and toothbrush filament diameter on eroded dentine. Based on
the results, the authors concluded that the abrasion of eroded dentine was influenced by the
abrasivity of the toothpaste and, to a lesser extent, by the toothbrush hardness (Wiegand et al
2009). They observed that toothpastes with a higher relative abrasivity of sound dentine (RDA)
caused higher wear on eroded dentine than a less abrasive toothpaste.
Tooth brushing can also increase the abrasion of enamel and dentine. Ashford and
Joiner explain that some types of toothbrushes are more effective at capturing abrasive
particles and keeping them in contact with the substrate. The authors highlight that there are
conflicting conclusions about the impacts of toothbrushes on abrasion. In their study, Wiegand
et al observed that dentine loss increased with decreasing filament diameter (Figure 4).
Figure 4: Dentine loss as a function of filament diameter
Figure 4 shows the relationship between dentine loss and filament diameter. As
previously mentioned, dentine loss increases as the filament diameter decreases (Wiegand
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2009). The authors make note that this is the case in all toothpaste slurry groups except for the
abrasive free control group. However, they warn that the impact of the filament diameter on
dentine loss was less evident compared to the RDA value.
The results from Wiegand et al are based on the assumption that toothpastes with
lower abrasivity only abrade the outermost aspects of the dentine and collagen matrix. The
authors’ conclusions may be explained by the increased duration and area of bristle contact to
the brushed surface due to their softness (Wiegand 2009). Moreover, as filament diameter
increase, there are less number of bristles per toothbrush, resulting in a brush less capable of
capturing abrasive particles.
3.2 Erosion (Corrosion)
Another tooth wear mechanism that has become more common in humans due to the change
in dietary practices is erosion, also known as corrosion (Wu et al 2014). Dental erosion is
defined as “Loss of dental hard tissue by a chemical process that does not involve the influence
of bacteria (Johansson et al 2011). Erosion is significantly stimulated by the consumption of
acidic foods and beverages, certain medications and occupations, and eating disorders (Chu et
al 2010). Table 1 summarizes the different intrinsic and extrinsic causes that lead to dental
erosion.
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Table 1: Primary causes of dental erosion (Chu et al 2010)
Ashcroft and Joiner explain that acid erosion results in damage to the enamel due to the
shifting of the dissociation equilibrium of hydroxyapatite, the main mineral component of hard
tissue, to shift towards dissolution (Ashcroft and Joiner 2010). The authors elaborate that the
amount of damage on the enamel is highly dependent on the quality and quantity of saliva
produced by the patient, as it contains pH buffering properties.
Initial signs of erosion are characterized by the production of a smooth, polished-looking
surface, while advanced cases feature hollowed-out etch-pits (Ashcroft and Joiner 2010). Figure
5, reproduced from Johansson et al. shows clear signs of tooth erosion. The images, originate
from a 13 year old girl with a high intake of soft drinks. The top image (a) shows occurrence of
buccal erosion and crown shortening of the maxillary front teeth (Johansson et al 2011). Note
the typical “inverted V-sign” characteristic of soft-drink-induced dental erosion. The bottom
image (b) shows severe erosive damage, with shoulder formation on the palatal surfaces of
maxillary anterior teeth.
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Figure 5: Dental wear in 13-year-old female due to high intake of soft drinks
Johansson et al explain that sometimes “cupping” occurs, which is a concavity in the
enamel, usually on a cusp tip. The authors also describe how in advanced cases of erosion, the
pulp can be visible through the remaining tooth substance.
Wear mechanisms acting upon human teeth seldom occur separately. More often than
not they may work simultaneously and symmetrically (Wu et al. 2014). Recognizing this, Wu et
al. carried out a study with the objective of investigating the loss of human enamel due to the
simultaneous attrition-corrosion mechanism. Figure 6 summarizes the results gathered at the
conclusion of this study.
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Figure 6: Attrition-corrosion wear loss by volume
The authors concluded that the degree of demineralization depends on the solution to
which the enamel was exposed to, with the acetic acid being the most corrosive, and distilled
water as the least. Furthermore, Wu et al determined that mechanism of enamel-on-enamel
wear mechanism shaved the softened layer with acidic lubricants, while delamination
dominated with distilled water. Finally, the authors highlight that a lubricant with high
corrosive potential can still be destructive for enamel.
3.3 Abfraction
Ashcroft and Joiner define Abfraction as the loss of hard tissue due to biomechanical loading
forces, resulting in lesions caused by flexure and ultimate fatigue of hard tissue away from the
point of loading. There are three types of stresses that act on teeth during the mastication
process: Compressive, shear and tensile (Romeed et al. 2012). Based on FEA studies, scientists
determined that the concentration of tensile stresses in the cervical region of teeth resulted in
cervical lesions known as abfraction. The authors also explain that occlusal forces from
mastication and contact generate a “significant” tensile stress in the enamel. This loading
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creates differential flexure resulting in weakening and formation of micro cracks (Romeed et al.
2012). The lesions occur once the theoretical yield stress of the enamel is exceeded. The
authors also suggest that erosive acids may also contribute to these lesions by weakening the
structure of enamel and dentine in the buccocervical area, making the area more susceptible to
fracture.
Features characteristic of abfraction include wedge-shaped lesions to the cervical region
of the dentition with sharp internal and external angles (Sarode and Sarode, 2013). Figure 7
shows these features. The authors explain that the shape and size of the lesion are dictated by
the direction, magnitude, frequency, duration and location of forces due to tooth contact.
Figure 7: Abfraction showing various degrees of severity
Sarode and Sarode also highlight that the lesion should be located in the region of
greatest tensile stress concentration, and display similar characteristics as shown in Figure 7.
Furthermore, they mention that previous studies propose that the direction of the lateral
forces acting on a tooth determines the location of the lesion. Additionally, these types of
lesions are thought to be responsible for the chronic sensitivity of the teeth to cold foods and
liquids.
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3.4 Fatigue Wear
Dental fatigue wear takes place when enamel surfaces are subjected to occlusal contact,
leading to considerable pressure during mastication (d’Incau et al 2012). The authors explain
that despite enamel’s hardness compared with dentine, it is a very brittle material because of
its high modulus of elasticity and low tensile strength. Micro crack propagation is controlled
due to enamel’s prismatic organization. D’Incau et al. elaborate that once a micro crack
initiates, it causes delamination in the enamel but eventually arrests because of the enameldentine junction which disperses stresses.
D’Incau et al. reference the observations from some authors who noticed the formation
of sub-vertical grooves initiated by micro cracks. Figure 8 shows the micrograph of sub-vertical
grooves (white arrows) on the distal facet of a first upper left premolar belonging to the
neandertal dental remains, obtained from d’Incau et al.
Figure 8: ESEM micrograph of sub-vertical grooves
The creation of these grooves highly depend on the strength and direction of the
associated loads in addition to the acidity of the environment (d’Incau et al 2012). On the other
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hand, the authors indicate that there are others who argue that the frequency of these grooves
showing little occlusal wear and no micro-cracks invalidates the previous ideas.
4. Numerical Approximation Modeling to Dental Wear Problems
Due the level of complexity to estimate the impact of wear on human teeth, there have been
numerous FEM studies done on the field. Of particular interest, the reader can refer to the one
done by Chai. This study aims at assessing crown failure by comparing analytical predictions to
numerical approximations (Chai 2014). Figure 9 is a schematic of the problem being
considered. The crown is represented by a ball of radius r, an enamel surface of thickness d
inclined at an angle θ to the horizontal.
Figure 9: Conical bilayer model used for predicting crown failure.
The author assumed an axis-symmetric 2-D ANSYS model simulating real model teeth. A
specified load is transmitted uniformly to the enamel surface to make the analysis fully axissymmetric. Frictionless contact is assumed between all parts. A sensitivity study was done to
verify the impact of the ball radius (representing the crown), enamel thickness and inclination
angle (Chai 2014).
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Figure 10: Axisymmetric model results
Figure 10 summarizes the results obtained from this study. The stress plots on the left
(a) are radial (σP) and hoop (σz) stresses, the latter being responsible for ring and radial
cracking. The results were obtained by assuming a thickness d=1.4mm, ball radius r=1mm,
angle θ=27⁰ and a load P=920N. The plats on the right (b) show the FEM predictions for the
load needed to initiate ring (open circles) and radial (solid circles) cracks vs. θ for the conditions
specified. The solid lines represent the analytical predictions (Chai 2014).
Chai concludes that the main forms of damage under the test assumptions are median
radial and cylindrical cracks originating from a contact spot. These types of damage may have
an undesirable effect on the enamel coat. The loads required to propagate the cracks along the
enamel wall may serve as a viable indicator of failure.
Another FEM study by Srirekha and Bashetty aims at evaluating the mechanical behavior
of various restorative materials in abfraction lesion in the presence and absence of occlusal
restoration. The authors created a 3-D FEM with abfraction lesions modeled. Then different
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loads were applied at a 45⁰ angle and then the authors verified the von Mises stresses of the
model (Srirekha and Bashetty 2012).
Figure 11: FEM of a tooth
Figure 11 shows the FE model created using ANSYS, with the abrasion lesion modeled
(a). The image on the right, (b), shows the direction and location of the applied load. Srirekha
and Bashetty observed that the location of peak stresses occurred at the cervical margin of
restoration, independent of the material assumed.
Based on the results obtained, the authors concluded that restorative materials with
low modulus of elasticity are successful in abfraction lesions at moderate tensile stresses. On
the other hand, materials with high elastic modulus and mechanical properties can support
higher loads and resist more wear.
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4. Dental Wear Prevention
Excessive dental wear may result not only in undesirable aesthetic consequences but also in
discomfort, pain and reduced mastication efficiency (Barlett 2005). Consequently, it is
important to identify ways to mitigate, if not eliminate teeth wear rate. Due to the different
wear mechanisms, prior to developing a prevention plan dentists need to determine the cause
of wear in the patient (Barlett 2005). For instance, the use of fluoride is widely accepted to
protect teeth from abrasion and erosion (Barlett, 2005).
Barlett suggests that fluoride may help harden the tooth surface and increase resistance
to acid damage rather than encouraging remineralisation. Additionally, the use of resin based
sealants or bonding agents may reduce the progression of erosion, but not necessarily any
other wear mechanism. Similarly, the use of a Michigan Splint or night guard might protect
teeth from abrasion, but not erosion.
Dietary supplements can also help reduce the wear rate, especially erosion. Wegehaupt
et al carried out a study in 2011 to measure the effects that diet have on teeth after they have
been exposed to orange juice. At the completion of the study the authors were able to
conclude that the erosive an abrasive damage induced by orange juice and brushing could be
significantly reduced by using free available dietary supplements. In this study, the authors
tested bovine enamel samples by exposing those to different substances (water and orange
juice included) for a specific duration. Then the samples were brushed a certain amount of
times and the wear was measured using surface profilometry.
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Another study carried out by Amaechi and Higham lead to the conclusion that
immediately after exposure to an acidic environment (from food, medicine or occupation), a
remineralising agent such as fluoride mouth rinses, tablets, or dairy milk should be
administered. Thus, the enhancement of rapid remineralisation of the softened tooth surface is
allowed. Furthermore, the authors suggest treating the underlying medical disorders or disease
as part of the wear prevention plan. Examples of such disorders include bruxism (teeth
grinding), anorexia and bulimia.
5. Conclusion
Tooth wear is an undesirable, and ultimately unavoidable condition that can have drastic
consequences if left uncontrolled, from aesthetic reasons to chronic pain and inability to talk
and chew properly. It is very important to understand the different types of wear mechanisms
not only from a biological point of view, but also from a micro and macro mechanics
perspective. This high level literature review was intended to provide a general understanding
of abrasion, erosion, abfraction and fatigue. Additionally, it attempted to provide some insight
as to the type of research that has been done related to teeth wear. The author recognizes that
each wear mechanism is so complex by itself, that it would have been virtually impossible to
cover all details from each in a high level review such as this one. The FEM studies referenced
in the report were samples of the type of topics that can be investigated using numerical
methods.
Researchers are aware of the importance that healthy teeth have on humans’ daily lives
for both an aesthetic and a functional perspective. Consequently a vast wealth of knowledge
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has been gathered throughout the years based on laboratory work and analytical calculations
that not only helps understand what causes the observed dental wear, but also why, and how it
can be prevented. Using these results, dentists can advise patients with regard to lifestyle
changes that encourage positive dental health.
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