Tooth wear assessment

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. 

A third possible approach to measuring tooth wear is to use an in situ study design. Here, volunteers wear test specimens that are either inserted into a denture or appliance, or attached directly to the teeth. Following the test phase, the test specimens are removed and the wear is assessed using laboratory procedures. An advantage of this approach is that the test specimens are exposed to the multiple interactions of the oral environment, but the sensitivity of laboratory analysis is maintained. This increased sensitivity allows for much shorter duration studies compared to a full in vivo clinical trial and it is, therefore, possible to apply some control to variables such as diet and volunteer illness, if required. 

Assessing tooth wear using an in situ approach falls somewhere between in vitro and in vivo approaches. Essentially, this approach involves human volunteers wearing test specimens in the mouth for a period of time, during which they may be exposed to one or more test treatments. Ideally, an in situ methodology should expose the test specimens to the interactions of the oral environment, whilst maintaining the sensitive measurement techniques which may be applied to laboratory analysis. 

The taking of tooth impressions to assess wear in vivo has become popular in recent years, as silicone impression materials are available with excellent detail reproduction and good dimensional stability. Several authors [35-38] have reported the use of these materials to capture the dimensions of selected teeth (or teeth surfaces) over a period of time. The silicone impressions are scanned using a profilometry system, typically a laser profilometer, to build an electronic image of the tooth surface. Several scans of the same tooth or surface, taken over a period of time, can then be electronically overlaid and subtracted, to calculate the change in surface contour, or the volume of material removed. This technique can only work if the two scans are precisely overlaid. To avoid... 

Several studies report the use of replica techniques to assess abrasion in situ [45, 48], This methodology has also been successfully applied in vivo (see previous section). In this case, the wear is assessed using optical or interference microscopy. A drawback of this approach is that the production of replicas and positive copies inevitably provides a source of error and reduces the ability to measure small changes in tooth surface profile. 

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