Changes in length and torque of the masticatory muscles produced by the activator appliance. A cephalometric study

The purpose of this study was to measure cephalometrically the change in muscle length and torque of the masticatory muscles produced by an activator appliance. The study was based on 10 orthodontic cases with distoclusion. Muscle length and force arm (leverage) were measured on two lateral cephalometric roentgenograms of each subject, one with the teeth in centric occlusion and one with an activator between the teeth. The construction bite of the activator was taken with the mandible repositioned 2-3 mm below and 5-6 mm ahead of resting position. It was found that the temporal masseter and medial pterygoid muscles increased 5-15% in length whereas the lateral pterygoid decreased 25% in length with the activator in place. The effect on the force arm on the temporal muscle was small, a decrease of 3-6%, but that on the masseter was much more dramatic, a decrease of 21-22%. Thus, as far as the torque of the temporal muscle is concerned, the small decrease in force arm is well compensated for by the increased muscle tension due to the stretching by the activator. The clinical significance of the findings for obtaining the best construction bite of the activator is discussed.