[Effects of lateral mandibular deviation on masseter muscle activity].

The investigation of the physiological response of masticatory muscle which is associated with the perception of the mandibular lateral deviation is clinically important. Therefore, the effects of various lateral mandibular deviation on the masseter muscle function were investigated on five healthy subjects. The mandibular position was deviated 0.5, 1.0, 1.5, 2.0 and 3.0 mm to the right and left from a reference position corresponding to the rest position. The mandible was passively retained in each test position by means of a bite block. Electromyographic activity (background activity) was recorded with bipolar surface electrodes applied to the right and left masseter muscles. Jaw-jerk reflex was evoked by a reflex hammer, and motoneuron activity was indirectly examined. The background activity as well as the amplitude of the jaw-jerk reflex on the mediotrusive side mainly increased in proportion to the mandibular deviation. However, most subjects showed no significant change about 3 mm from the reference position in the background activity and about 2 mm in the range of muscle activity evoked by the jaw-jerk reflex. These results suggested that the masseter jaw-jerk reflex may assist clinical examination of small mandibular deviations.