Abrasion of occlusal contacts
As the teeth erupt, they have prominent cusps and ridges, by which they align themselves with each other in occlusal contact until optimal contact is achieved. The prominent functional surfaces are abraded as a result of masticatory function. The degree of wear depends on:
- The strain caused by the nature of the food
- The age of the dentition
- The resistance of the enamel
- The nature of the occlusal contact
The last of these conditions makes it clear that the correct positioning of a tooth within the dentition is not merely an esthetic requirement but a functional necessity. Only when the teeth are accurately positioned and have the correct inclination within the dentition can exact, optimal occlusal contact be achieved with minimal wear. This situation can be referred to as physiologic abrasion, whereby the functional surfaces are "ground in," depending on the path of movements during chewing.
This is a process of adaptation whereby the relief of the masticatory surfaces adapts to the particular mandibular movements, which are determined by the inclination of the path followed during movement of the TMJs. The abrasion surfaces of the teeth are the "worn-in" tracks of mandibular movements. Conclusions can be drawn from these abrasion surfaces (also known as articulation surfaces) about the paths followed by the TMJs. Depending on the degree of wear, different occlusal contacts can be found in a natural dentition (Fig 5-70):
- Full-surface contact in an abraded normal occlusion with considerable friction resistance during movement contacts
- Partial-surface contact, which offers precise support and minimal friction resistance during horizontal forward movement
- Supporting cusp contact with imprecise guidance and virtually free sliding movements, which can result in nonphysiologic loading of the periodontal tissue caused by tipping and can damage the TMJ
- Punctiform occlusal contact with centric periodontal loading and the slightest of horizontal friction resistance. This occlusal contact is regarded as the physiologic contact and needs to be reconstructed when artificially restoring the masticatory surface. The contacts should, of course, be checked while lateral movements are carried out.
Once again there is a connection with the law of form and function: The course of movements of the mandible during chewing are determined by the musculature and guided by the TMJs, like a railroad car being guided by the tracks. This guidance by the joints is matched by the way in which the abrasion surfaces on the teeth are ground in, which in turn alters the joint guidance. Pathologic forms of movement, such as tooth grinding and a nervous chewing motion with the mouth empty, lead to abrasion surfaces that may produce eccentric loading of teeth during normal chewing processes. This will lead to pathologic changes to the periodontal tissue and the TMJ.
The aim of systematically reproducing the masticatory area with punctiform contacts, in which the functional surfaces of the teeth meet evenly at several points, is to rebuild pathologic abrasion surfaces in order to achieve centric transmission of forces and to reduce friction resistance during horizontal movements. This balanced sliding situation cannot be technically created because, for instance, the reconstruction of masticatory surfaces cannot be done accurately under loading exerted by masticatory pressure. Therefore, contacts of the lingual cusps should be avoided and removed when checking movements in the articulator. The buccal cusps are able to absorb enough loading from masticatory pressure.
In terms of differentiating the normal tooth position in the normal occlusion, the teeth will incline until this sliding contact is achieved during lateral movement. This is because an excessive lingual contact would force the mandibular teeth into lingual tipping. It is interesting to note in this respect that there is a pronounced lingual inclination of the masticatory surfaces on all the mandibular posterior teeth.
Another advantage of reconstructing punctiform occlusal contacts is that further abrasion can be reduced, preventing permanent damage to the periodontium and the TMJ. Frequently, dysfunctional changes may even be reversed, especially damage to the TMJ.