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The relationship between specific occlusal contacts and jaw closing muscle activity during parafunctional clenching tasks in man MacDonald, James W. C.


Parafunctional clenching and grinding habits have been associated with masticatory muscle hyperactivity (18,39,167,176). Muscle tenderness to palpation, muscle fatigue, and stresses to the teeth and joints are the most common signs and symptoms of parafunction and are thought to result from prolonged periods of muscle contraction (158). The object of this work was to examine and describe the relationships between the electromyographic (muscle) activity in the jaw closing muscles and the location, size, and direction of applied effort to specific bite points, some chosen to simulate clincal conditions. The experiment included two preliminary studies which utilized multiple small acrylic occlusal bite stops. The stops caused an approximate 1 mm vertical opening of the occlusion from the intercuspal position as measured at the incisors. They were employed to control the location of the bite point and the variables caused by cuspal inclines and lateral displacement as their design incorporated both upper and lower teeth. The stops were used in both vertical and eccentric clenching tasks. A third study was performed to examine similar clenching tasks on specific tooth contacts on natural cuspal inclines. Bipolar surface electrodes recorded electromyographic activity from the anterior temporal, posterior temporal, and superficial masseter muscles bilaterally, while paired, insulated fine wire electrodes recorded activity in the left medial pterygoid muscle. Data were collected for each muscle and task and were stored in digitized form for analysis by a disc based computer system that calculated means and standard deviations over a 400 msec period about the centre of the response. Data were then normalized to each subject's peak task for each muscle which allowed comparisons for the group as a whole. Muscle patterns for the majority of subjects were consistently dependent upon the position and number of occlusal contacts and the direction of effort applied to the contacts. Vertical clenching efforts in the natural intercuspal position generally exhibited the highest activity for all the muscles recorded of all the tasks. As isolated bite points moved posteriorly along the arch from incisors to molars an increase in activity was observed in the ipsilateral temporal muscles while the ipsilateral medial pterygoid and the masseter muscles bilaterally were seen to decrease. The ipsilateral temporal and contralateral pterygoid muscles comprised the majority of activity during subjective vertical clenches on natural cuspal inclines as well as in lateral efforts on specific bite points. The temporal muscles, especially the posterior fibers, exhibited the most activity during retrusive efforts and the least during protrusive and incisal clenches which were primarily handled by the masseter and pterygoid muscles. When the size and number of contacts were increased anteriorly a generalized increase in muscle activity was witnessed while the same trends were not as consistent or significant posteriorly. Cross-arch contacts, when present, were associated with a slight significant increase in activity of the masseter muscle ipsilateral to the cross-arch contact but the majority of the activity was still provided by the ipsilateral temporal and contralateral pterygoid muscles. Proportionately, muscle activity between subjective maximal and half maximal clenches was similar and the same relationships of muscle and task could in general be described for both efforts. The findings have shown that patterns of muscle activity during parafunctional clenching behaviour vary predictably according to the location, size, and direction of effort applied to specific tooth contacts and are relevant to both local muscle soreness and joint biomechanics in mandibular dysfunction.

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