The action of three types of functional appliances on the activity of the masticatory muscles.

Three commonly used functional appliances; namely, the Herbst, Frankel, and a simulation of the Clark twin block appliances were used to test the lateral pterygoid muscle hypothesis. This hypothesis states that postural and functional activity of the superior and inferior heads of the lateral pterygoid muscle increases after the insertion of a functional appliance. This increased activity, especially in the superior head of the lateral pterygoid muscle then acts to stimulate increased condylar growth. The electromyographic (EMG) activity of the masseter, digastric, superior, and inferior heads of the lateral pterygoid muscles was monitored with chronically inserted EMG electrodes, in nonhuman primates, to determine whether functional appliances actually produce a change in functional activity of these muscles. The involuntary swallow was used to represent functional activity. Thus swallow-related EMG activity levels were monitored longitudinally and compared before and after the insertion of each appliance type. The insertion of these three appliances was associated with a decrease in functional EMG activity of the four muscles. This decrease was statistically significant in all muscles 3 and 6 weeks after appliance insertion. This is consistent with our previous findings that functional appliances are associated with a decrease in the postural activity of the above muscles in nonhuman primates. In view of the fact that the animals showed large skeletal changes in the temporomandibular facial area, this study could not support the lateral pterygoid muscle hypothesis.