The influence on masticatory performance of jaw movements, chewing side preference, occlusal contacf area, muscle activity and jaw tremor

Abstract

The primary function of the jaws and teeth in mammals is chewing and swallowing. In man there are additional functions of speech, non-verbal communication and cosmetic appeal. Chewing is a complex operation requiring both adequate skeletal structures, and a well co-ordinated muscle system. There is considerable variation in both these components of chewing within which adequate function appears to be possible, at least for a modern refined diet. For example, the dental arches may not conform to the modal arrangement and teeth may be missing, yet adequate function remains (Slagter et al 1993). There are unfortunately no baseline requirements for an adequate dentition nor the minimal chewing performance necessary to avoid indigestion. A common rule of thumb when replacing missing posterior teeth is that the extent of the prosthesis can be reduced to the premolars without seriously affecting chewing (Kayser, 1984). This arbitrary estimation has not been defined by a minimum area for functioning posterior occlusal surfaces. The same lack of quantifiable measurement is a feature of assessing orthodontic treatment goals and outcomes (Omar, McEwen and Ogston 1987). The clinical rules for correcting malocclusions, usually, have more to do with the restoration of modal tooth, arch and skeletal relationships, than with the restoration of function; if restoration of function is a concern of treatment, it is not measurable in the same way that tooth positions can be assessed on plaster casts or angles measured on a radiograph. Muscle tenderness and limited movement are both features of temporomandibular dysfunction. The boundary between normal subjects, who may have some signs of dysfunction and patients, who may not have distinctly more severe signs cannot always be made (Widmar 1992). By some definitions based on the morphology of the joint structures, even symptomless individuals could be categorised as abnormal. One of the difficulties in assessing functional incapacity of a patient with muscle pain is the absence of the same baseline data needed to assess malocclusion, or the handicap due to reduced occlusal area. It is encouraging to find that a simple test, such as measuring maximum opening, is a useful indicator of treatment progress in temporomandibular dysfunction. This sign, although simple and of limited diagnostic use, reflects the poverty of useful tests for masticatory function.