Tooth wear: intrinsic and extrinsic mechanical factors

Tooth Wear (TW) or non-carious tooth surface loss (NCTSL) ranks as the primary source of damage to dental hard tissues. Even as the occurrence of dental caries is on the decline, there appears to be a rising trend in the rate and intensity of tooth wear.¹ The language and terms used to describe this process differ worldwide, and the focus on the cause can shift within various regions. The most straightforward term to employ is ‘tooth wear,’ which covers attrition, erosion, abrasion, and the poorly characterized phenomenon called abfraction.²

Tooth wear damage is permanent and cumulative with age.³ It, therefore, adversely affects dental health for an individual's entire life. Consequently, the importance of addressing tooth wear is growing in ensuring the prolonged health of one's teeth.⁴

Tooth surface deterioration can be entirely ‘physiological’ and is a typical result of ageing. It has been estimated that the typical vertical enamel loss due to natural wear is about 20–38 µm each year.⁵ However, various elements, like erosion, abrasion, and attrition, can cause this loss to become ‘pathological’.⁶

Bartlett and O'Toole suggested that ‘the appropriateness of the term ‘pathological tooth wear’ is complex’. Some specialists refer to ‘pathological levels of wear’ when discussing cases where restorative work might be necessary. While many support this term, it's largely a matter of personal judgement, especially among dentists.⁷

Tooth wear is a multifactorial condition that has a complex aetiology. It encompasses two primary subtypes: chemical (or erosive) and mechanical wear (attrition and abrasion).⁸ Each of these categories can further be broken down into intrinsic and extrinsic origins. In total, there are four distinct subtypes:

• Intrinsic erosive wear, stemming from stomach acid;
• Extrinsic erosive wear caused by acidic foods and drinks;
• Intrinsic mechanical wear from direct tooth-to-tooth contact leading to attrition, such as in bruxism:
• Extrinsic mechanical wear resulting from external actions such as parafunctional habits including biting nails, chewing on pens, or the consumption of rough food elements and the use of overly abrasive dental products and aggressive toothbrushing leading to abrasion.⁹

Individual wear processes seldom operate independently and there is often interplay.^10-12 Clinically, it can be challenging, and sometimes even unfeasible, to pinpoint just one causative factor when a patient shows signs of tooth wear.³ The latest consensus statement regarding the treatment of severe tooth wear emphasizes the importance of early identification of significant tooth wear, determining its root cause and ensuring adequate preventive measures are being taken.¹³

The content of this article largely focuses on the intrinsic and extrinsic mechanical factors of TW.

Attrition

The Glossary of Prosthetic Dentistry (GPT-9) defines attrition as ‘the act of wearing or grinding down by friction or the mechanical wear resulting from mastication or parafunction, limited to contacting surfaces of the teeth’ (Figure 1).¹⁴

Clinical Signs

Flattened cusps or incisal edges, along with specific facets on occlusal or palatal surfaces, suggest a main cause stemming from attrition (Figure 2).⁶ Attrition-related lesions are most frequently seen on the biting surfaces of the teeth, specifically the incisal and occlusal areas. Early signs of attrition include a tiny, shiny spot on a tooth's cusp or ridge, or a minor levelling of the edge of a tooth. As this wear continues, the tooth's cusp may become shorter, and the chewing surfaces may become flatter, often exposing the underlying dentine (Figure 3).^3,15

In extreme situations, the clinical crown can be significantly reduced in height (Figure 4). In some instances, these areas can appear pointed and rough. If the wear breaks through the enamel, the attrition spots might display a yellowish-brown tint. The wear can also affect the spaces between teeth, causing them to shift closer mesially, producing broader interproximal contacts over time.¹⁶

Aetiology of attrition

The effects of chewing patterns

Some authors suggest that tooth wear can result from extended tooth-to-tooth contact for patients displaying a wide range of functional movement. A study by Lundeen et al¹⁷ demonstrated that certain patients have a notably expansive movement in their regular chewing behaviour, similar to a grinding movement, which results in attrition and dental wear.

In a clinical study on the analysis of chewing movement, Nishio et al found that the group with a grinding–chewing pattern showed multiple facets on their biting surfaces. In contrast, those in the chopping type group had wear facets primarily on the canine and first premolar.¹⁸ The same authors concluded that the chewing pattern is one of the aetiological factors related to the occlusal wear. Moreover, they suggested that the existence of non-working facets and signs of non-working contacts during chewing implies that prophylactic removal of all such contacts can significantly reduce the functional area of chewing.

Kim et al¹⁹ compared two groups: one with 15 individuals exhibiting a standard chopping mastication pattern and another with 15 people demonstrating a more expansive grinding–chewing motion. They observed that those with the grinding–chewing style experienced a noticeably higher degree of wear on their biting surfaces than those with the chopping style. Importantly, the participants in this study were young, between the ages of 23 and 25 years. As a result, the disparity in wear patterns between the two groups could be expected to grow even more pronounced as the individuals grew older.

Functional wear

Yadav²⁰ conducted a survey of 500 adults with equal numbers of men and women in the age group of 18–55 years who were examined for signs and symptoms of bruxism, attrition and temporomandibular disorders, and showed that as age increased, the attrition score also rose. The findings of this study indicated that functional wear increased with age, and attrition was not solely caused by bruxism or other non-standard activities, but also by regular functional actions. The author concluded that attrition as a single factor cannot be regarded as a reliable sign of bruxism.

On the other hand, some authors have proposed that there is no connection between age and the extent of attrition, and postulating that severe attrition is primarily due to bruxism and, to a smaller degree, from regular functional wear.^21–24

The role of bruxism

The American Academy of Sleep Medicine defined bruxism as the ‘repetitive jaw muscle activity characterized by the clenching or grinding of teeth and/or bracing or thrusting of the mandible.’²⁵

The understanding of bruxism has developed in recent years, moving away from the previous notion that it simply meant grinding teeth during sleep.²⁶ Bruxism can be categorized into awake bruxism (AB) and sleep bruxism (SB).²⁷ According to a recent International Consensus on Bruxism²⁸ there are three distinct definitions of bruxism:

• Possible SB/AB: when the diagnosis is determined by a confirmation from the patient or their parents/guardians;
• Probable SB/AB: when the diagnosis is grounded on observable clinical indicators, regardless of whether there is a confirming report;
• Definite SB/AB: when there is a confirmed instrumental analysis, either through polysomnography (PSG), electromyography, or ecological momentary assessment (EMA), irrespective of a corroborative report or the existence of clinical symptoms.

Bruxism is a debated topic. The way it is defined and diagnosed makes the literature on its causes challenging to decipher. Nonetheless, there's agreement on its origins being of multifactorial aetiology.^28–30

There is substantial evidence suggesting that the impact of occlusal characteristics and other structural factors on bruxism is minimal, if they play any role at all. However, there is also a strong belief that sleep bruxism is a reaction to arousal.³¹ The central dopaminergic system's disturbances also play a part in the causes of bruxism. Selective serotonin re-uptake inhibitors have been proposed to cause or worsen both awake bruxism and sleep bruxism.³²

Additionally, elements such as smoking, alcohol, certain diseases, trauma, and genetics have a role, but the previously believed significant role of stress and other mental factors is likely to have been overestimated. In essence, bruxism is mostly influenced by central factors rather than peripheral ones.²⁹

Bruxism can cause tooth wear by attrition, with wear facets mainly found on the incisal edges and also on the tips of canines and molars (Figure 5). Damage from bruxism can be minor or substantial. For numerous individuals, it is not a major concern. However, for a few, this behaviour causes considerable issues and significant tissue damage (Figure 6).^{10, 12,33–37}

In a recent attempt to validate the relationship between the severity of possible sleep and awake bruxism and attrition tooth wear facets in children and adolescents, Martins et al carried out a cross-sectional population-based study in 434 children and adolescents.³⁸ They found that there was an association between the severity of possible sleep and awake bruxism and the attrition tooth wear facets. The location of the attrition tooth wear facets differed, with anterior teeth associated with possible sleep bruxism and posterior teeth associated with possible awake bruxism. The same authors concluded that the number of attrition tooth wear facets is proportional to the severity of possible sleep and awake bruxism.

Using portable at-home EMG devices, Jonsgar et al³⁹ conducted a study to evaluate EMG activity in subjects showing signs of attrition-based tooth wear in comparison to a control group of similar age and gender with little to no such wear. They concluded that subjects with attrition-based tooth wear did not display increased EMG activity during sleep compared to their counterparts. The same authors recommended that dental professionals should not directly associate tooth wear with active sleep bruxism.

Abrasion

Abrasion refers to the pathological wear of teeth caused by the mechanical rubbing of external substances against the tooth's surface. Prevalence data of 28–62% can be seen in the literature.^11,40

The cervical area is most frequently subject to abrasion, and the resulting lesions are best described as non-carious cervical lesions. Observed clinically on numerous teeth, V-shaped indentations are present in the cervical area. These notches have smooth surfaces and distinct, sharp edges, often accompanied by gingival recession (Figure 7).⁴⁰

While abrasion can happen on any tooth surface, including between teeth owing to improper use of dental floss, it can be challenging to differentiate pure abrasion from a combination of erosion and abrasion.⁴¹

Food abrasion

The most prevalent type of abrasion, often unnoticed by many dentists, is the wear caused by food.⁴² In contemporary societies, even though wear from food is relatively slight, it frequently occurs alongside erosion, leading to a heightened rate of tooth wear. Sometimes, regularly consumed hard foods, such as nuts and seeds, can lead to abrasion on particular areas of a tooth.¹⁵

Occupational habits

Work-related habits can lead to wear on certain teeth, for example when hairdressers hold bobby pins or hairpins between their incisors. Environments with a lot of dust, such as those in iron factories, mines, and quarries, can also contribute to heightened tooth abrasion.¹⁵ Several studies, predominantly from Scandinavian sources, have highlighted occupational abrasion. Workers in Tanzanian cement factories, Swedish iron ore mines, and Danish granite industries have all been identified as experiencing significant abrasive tooth wear. Some authors have identified increased tooth wear among olivine miners.^{41, 43–46}

Toothbrushes and toothpastes

There are varying views on the role of toothbrushes and toothpaste in causing abrasion lesions (Figure 8).⁴⁷ Although it is widely acknowledged that tooth brushing can cause abrasion in vitro when abrasives are involved, minimal damage happens with the use of a toothbrush alone.^48–50

Clinical studies indicate that using both a toothbrush and toothpaste results in minimal dentine wear over a lifetime, and almost no enamel damage. Tooth wear from toothpaste abrasion becomes concerning primarily in instances of excessive use, or when combined with acid erosion (Figure 9).^51,52

Over-aggressive brushing with toothpaste is the second primary source of abrasive tooth wear and can frequently be observed alongside other mentioned causes. When this kind of misuse occurs, the detailed anatomy of the impacted areas appears dulled, resembling a sandblasted look. The distinct wear pattern, in terms of amount and location, indicates that the outer surfaces of the lower canines and premolars are the most affected. As the enamel erodes down to the dentine, depressions or hollows start to appear.⁴² Patients with gingival recession that reveals cementum and dentine are more susceptible to abrasion (Figure 10).⁵³

Some authors highlight the need to differentiate between ‘toothbrush abrasion’ and ‘toothpaste abuse,’ because understanding this distinction can change perceptions and deepen insights regarding the pathological dynamics at the dento-gingivo-cervical junction. While a non-rounded bristle toothbrush, often firm, can lead to gingival recession, toothpaste can cause damage to teeth resulting in a non-carious tooth surface, irrespective of the brush type used. It is vital to recognize that the negative impacts of both toothbrushes and toothpaste are distinct matters.⁴² Whitening toothpastes can be more abrasive than regular toothpastes. Charcoal toothpastes, a current trend, may be highly abrasive.^53,54

Other contributing factors

Abrasion can also result from specific practices, including pipe smoking, incorrect dental floss or toothpick usage, consuming chewing tobacco, or habits such as biting on pencils, pens, and fingernails (Figures 11 and 12).^55,56

Some authors demonstrated that patients consuming ecstasy show increased instances of tooth wear into the dentine, with 60% affected compared to 11% in those who did not use the drug. This group also indicated high occurrences of teeth grinding (89%) and dry mouth (>90%). Additionally, these users mentioned drinking up to three cans of fizzy drinks when on the drug. These findings underscore the multifaceted causes of tooth wear.^53,57

Substantial abrasion can happen when natural teeth come into contact with rough or unglazed porcelain restorations. This is especially true for individuals with irregular oral habits or those who frequently consume acidic beverages.^53,58,59

Conclusion

In conclusion, numerous studies and research papers considering intrinsic and extrinsic mechanical factors of tooth wear shed light on the complex and multifaceted nature of this dental concern. Intrinsic factors, such as bruxism, play a pivotal role in initiating and exacerbating tooth wear, often leading to structural damage and discomfort. On the other hand, extrinsic factors, such as diet, abrasive behaviors, and oral hygiene practices, contribute significantly to the progression of wear over time.

Understanding the interplay between these intrinsic and extrinsic factors is crucial for both dental professionals and individuals seeking to maintain optimal oral health.