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1. Introduction
Dental wear in is a mechanical and pathological undesirable result of human’s daily lives. It is
highly dependent on the occupation of the person, dietary preferences, and dental hygiene (Ashcroft
2010).
2. Background
3. Mechanisms of Dental Wear
There are several wear mechanisms known to and investigated by scientists and dentists
(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 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, microasperities 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 microfatigue.
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 further worsened 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.
Figure 1 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 1: Attritional wear (Ashcroft and Joiner 2010)
The observations from Figure 1 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 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 (Ascroft 2010).
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, and is made up of two phases.
In the initial phase, crushing, abrasive particles from the bolus are free to move around and
affect non-occlusal 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 2
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.
Figure 2: 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 they rely on abrasive particles to abrade away plaque and extrinsic stains (Ashcroft
2010). These stains are caused by either colored compounds becoming incorporated into the pellicle or
by chemical reactions (Ashcroft 2010). Ashcroft explains 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 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 very similar. Ashcroft highlights 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 explains that
some types of toothbrushes are more effective at capturing abrasive particles and keeping them in
contact with the substrate. The author highlights 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 3).
Figure 3: Dentine loss as a function of filament diameter
Figure 3 shows the relationship between dentine loss and filament diameter. As previously
mentioned, dentine loss increases as the filament diameter decreases (Wiegand 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 (Yun-Qi 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.
Table 1: Primary causes of dental erosion (Chu et al 2010)
Ashcroft explains 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 2010). The author elaborates 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, with advanced cases featuring hollowed-out etch-pits (Ashcroft 2010). Figure 4, reproduced
from Johansson et al. shows clear signs of tooth erosion. The images, originating from a 13 year old girl
how has 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.
Figure 4: Dental wear in 13-year-old female due to high intake of soft drinks
Johansson et al explain that there 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 (Yun-Qi et al. 2014). Recognizing this, Yun-Qi et al. carried
out a study with the objective of investigating the loss of human enamel due to the simultaneous
attrition-corrosion mechanism. Figure 5 summarizes the results gathered at the conclusion of this study.
Figure 5: 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, Yun-Qi 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
3.4 Fatigue Wear
3. Dental Wear Measurement and Classification
FEA Approach
4. Dental Wear Prevention
5. Conclusion