Relationship between mandibular position and the coordination of masseter muscle activity during sleep in humans

Although bruxism has been regarded as having a possibly important role in t he aetiology of craniomandibular disorders, the activity of masticatory mus cles relative to mandibular position during sleep grinding as recorded by e lectromyography (EMG) has not yet been clarified. Surface EMGs of the bilat eral superficial masseter muscles were recorded simultaneously with mandibu lar position during sleep from 12 volunteers for three consecutive nights. The incidence of two mandibular positions were recorded with magnetic senso rs for both left- and right-sided mandibular grinding. One of the mandibula r positions was the canine edge-to-edge position, and the other was the mid point between the intercuspal position and the canine edge-to-edge position . The mode of the working/balancing activity ratio ranged from 1/10 to 2/10 , showing the marked predominance of balancing side masseter muscle activit y during sleep grinding. During sleep grinding, EMG bursts of masseter musc le were observed mainly with mediotrusive mandibular movement from the cani ne edge-to-edge position. From the results of the present study, it is sugg ested that muscular dynamics during sleep are unique compared to that durin g voluntary clenching, and exert a greater mechanical load to the balancing side temporomandibular joint.