Function of masticatory muscles during the initial phase of activator treatment

The function of masticatory muscles and the development of the bite force were studied in 15 children before and during the first six months of the treatment of distal occlusion with an activator. Electromyographic recordings of the activity of the anterior and posterior portions of the temporal muscle, the masseter muscle and the anterior belly of the digastric muscle were made bilaterally in the rest position of the mandible and with the activator inserted. Recordings were also made during chewing and swallowing of apple and peanuts and during chewing of chewing gum as well as during maximal bite in the intercuspal position and on the activator. The mean voltage amplitude, the duration and the coordination of the activity were analysed. The maximum bite force was measured at the first molars and at the incisors. In most of the children, the clinical improvement was rapid as evident from a decrease in overjet and in the ANB angle as well as a change in the molar relation. The bite force measured at the incisors increased during the period of treatment. The muscle activity in the rest position was low and was the same with or without the activator. Thus, insertion of the activator did not increase the muscle activity. The activity of the posterior portion of the temporal muscle in the rest position was comparatively high at the start of treatment but decreased during the period of observation. The activity of the masseter and temporal muscles during maximal bite and chewing was influenced by the occlusal instability created during the course of treatment. The decrease of the postural activity of the posterior temporal muscle may reflect an adaptation to a new mandibular position.