Tooth wear mechanisms

The mechanisms of tooth wear fall into two distinct types those of chemical origin (e.g. erosion) and those of physical origin (e.g. abrasion, attrition). In any individual, both chemical and physical insults to the tooth hard tissue will be present in some form or other, so tooth wear is the combined effect of these insults. Despite the clear definition of a number of distinct tooth wear mechanisms, it is uncommon to find a single wear mechanism present in the... 

The term tooth wear is commonly used to describe the loss of tooth hard tissue due to non-carious causes [1], This encompasses a variety of both chemical and mechanical causes of both intrinsic and extrinsic origin. The term tooth wear is preferred over some of the more precise definitions of individual hard tissue loss mechanisms, because it acknowledges the fact that wear is usually a multifactorial process one mechanism may dominate, but the overall wear is commonly due to the interaction between two or more wear mechanisms. In dentistry, the terms erosion, abrasion, attrition and abfraction are widely used to describe particular mechanisms of hard tissue loss. 

In the complex environment of the mouth, many potential wear events occur everyday. The overall degree of tooth wear present is, therefore, related to the many wear events a tooth is subjected to over a lifetime. It is clear, as was mentioned previously, that tooth wear is a complex multifactorial process and, in the majority of individuals, wear will be due to the interaction of many different factors and mechanisms. 

Of particular note is the importance of erosion in accelerating tooth wear. The softening of enamel and dentine surfaces by erosive attacks from acid renders the surface extremely susceptible to mechanical attacks [9]. Abfraction is also an example of interaction between wear mechanisms, where abrasion and erosion may act in combination with cyclic loading and unloading to produce an overall wear effect. 

The surfaces of the mouth are coated with a layer of salivary proteins known as the acquired pellicle, which provides a protective effect from both chemical and mechanical attacks to the tooth surface. The pellicle layer both moderates diffusion of ions away from the tooth surface, thus inhibiting the dissolution of enamel by erosion, and provides a lubricating layer to protect from mechanical attacks [10, 11], It is known that dental prophylaxis and the use of regular oral hygiene procedures are able to reduce or remove the pellicle layer. However, recent evidence has shown that the pellicle is able to re-form very rapidly and thus, maintain a protective layer over the tooth surface [10]. For this reason, tooth wear studies performed in vivo or in situ, where the mediating effects of pellicle are present, will give a much more realistic assessment of tooth wear than in vitro studies. For a more detailed discussion on the effects of pellicle, see chapter 2. 

The multifactorial nature of tooth wear makes the study of this phenomenon difficult, and it is therefore not surprising that many of the reported studies focus on a single wear mechanism, often using laboratory models. 

A topic of increasing interest is tooth wear. Chapter 4 describes recent research on the influence of dental product use, diet and other natural factors, and the inter-relationship between abrasive wear and chemical erosion. After describing the various physical and chemical mechanisms implicated in tooth wear, the author discusses the laboratory, in situ, and clinical approaches used to investigate the condition, illustrated with selected examples. 

The demand for aesthetic dental restorative materials continues to increase and may be the most important criterion for the promising future of the aesthetic polymeric composite resins. As the physical, mechanical, and wear properties of these materials improve, their use in dentistry will expand. The acid-etching of dental enamel [20] and dentin bonding procedures [21] will allow for conservative cavity preparation and the preservation of healthy tooth structure. 

A property not listed in Table 5.14, but which is of paramount importance to this application, is wear rate. Wear rate was described briefly in Section 8.2.2, and those concepts apply here as well. As it applies to tooth enamel, abrasive wear occurs by fracture or chipping of the enamel, chemical erosion, which may arise due to acidic medications or drinks, dietary oxalate, or high oral hydrogen ion concentrations as a result of disease, physical erosion, or abrasion, which arises due to idiopathic mechanisms, dentrifices, toothbrushes, or abrasive diets. For example, the wear rate of enamel has been measured at about 10 ttm/hour due to brushing with a toothbrush and toothpaste for 86,400 strokes [7],... 

Attrition is the mechanical removal of hard tissue by direct contacts between teeth (either natural or restored) with no foreign substance intervening [5]. This mechanism causes wear by tooth-tooth contacts as well as by tooth-restoration, and indeed restoration-restoration contacts. The action of mastication and bruxism are known causes of attrition. In the field of tribology, the term abrasion refers to the loss of material from a surface by sliding, rubbing or scratching. Two-body abrasion refers to abrasion caused by two contacting surfaces in relative motion, i.e. the mechanism in dentistry that is described as attrition. Three-body abrasion refers to abrasion caused by surfaces in... 

The problem with thin fluid film lubrication occurs when the relative motion of the solid surfaces either stops completely, stops at reversal in reciprocating motion or the dynamic loading of a cam on its follower, one gear tooth on another or on a journal within a bearing such that this lubrication mechanism fails and the surfaces make contact. Under boundary lubrication conditions the role of adsorbed molecular films of protective additives is crucial in protecting against wear. 

Toothed spline couplings are ubiquitous in mechanical transmission systems and have been studied experimoitally for many years with particular attention being given to wear behaviour. Examples include the experiments of Ku and co-woikers [1], Newdey [2] and Baker [3]. 

Particulate reinforced polymer matrix composites are widely used in dentistry as esthetic restorative materials. Although the normally measured mechanical and physical properties of this class of materials approach the properties of dental amalgam (1) composite restorative materials exhibit limited durability in clinical service. Compared to amalgam restorations, composites undergo loss of material through wear processes and exhibit breakdown at the interfacial region between the restoration and tooth structure. 

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