Acid erosion involves a chemical process, a dissolution of hard tissue structures, without bacterial involvement. The erosive process is not necessarily a simple one. One way to think about it is by focusing on changes happening in the saliva. When the pH drops in the saliva, it then drops in the acquired, or salivary, pellicle. After that, acidic changes occur on the tooth surface, which initiates the series of events that lead to tooth surface loss. In reality, nothing happens on the tooth until it happens first in the saliva. As such, saliva has become a primary area of interest to monitor.
Poor salivary flow impacts clearance of acids and buffering, and therefore delays a return to the resting pH. The saliva also supplies the pellicle; pellicle helps prevent and stop progressive erosion unless overwhelmed by a strong acid challenge. Synergistic wear may occur, for example, by the tongue abrading softened enamel palatally and lingually.10
An excellent technical description of dental erosion has been offered by Ganss: “Dental erosion can be defined as dissolution of tooth by acids when the surrounding aqueous phase is undersaturated with respect to tooth mineral. When the acidic challenge is acting for long enough, a clinically visible defect occurs. On smooth surfaces, the original luster of the tooth dulls. Later, the convex areas flatten or shallow concavities become present which are mostly located coronal to the enamel-cementum junction. On the occlusal surfaces, cusps become rounded or cupped and edges of restorations appear to rise above the level of the adjacent tooth surfaces. In severe cases, the whole tooth morphologically disappears and the vertical crown height can be significantly reduced. The result of continuing acid exposure, however, is not only a clinically visible defect, but also a change in the physical properties of the remaining tooth surface. It is recognized that erosive demineralization results in a significant reduction in microhardness, making the softened surface more prone to mechanical impacts. Although independent in origin, erosion is linked to other forms of wear not only because it contributes to the individual overall rate of tooth tissue loss, but also by enhancing physical wear.”17
In terms of acid erosion per se, we first have a surface softening, followed by surface loss, as depicted in Figure 3. Surface loss occurs as a result of frictional forces impacting the softened tooth mineral, followed by the initiation of a second softened layer, which is partly demineralized. The affected area of the tooth is susceptible to further frictional challenges, which leads to additional tooth surface loss.
(a) SEM showing loss of enamel and, (b) at greater magnification, the softened layer at the advancing front of the lesion.
Because of the interaction between frictional forces and acid, we have to consider tooth wear the result of a rather complicated process. In general, ETW is a multifactorial process that involves acid erosion and frictional forces of abrasion and attrition.