An electromyographic investigation of the first six months of progressive mandibular advancement of the Herbst appliance in adolescents.

The effects of the progressive activation of the Herbst appliance on the activity of the masseter and temporalis elevator muscles of the mandible were monitored in a group of 14 consecutively treated 10-15-year-old subjects with an Angle Class II, division I malocclusion. A cast silver splint Herbst appliance was activated in multiple stages at a rate of 2 mm/2 mo. The functionality of the superficial masseter and anterior portion of the temporalis muscles was monitored at maximum bite force using surface electromyography (EMG). The EMG recordings were taken at an incisal edge-to-edge position and a retruded mandibular position, both at a vertical interincisal separation of 3 mm using an acrylic bite plate. Measurements of maximum voluntary isometric clenches were taken during adaptive functional changes at pretreatment (baseline) and during the first 6 months of Herbst appliance therapy. Results showed great individual and inter- and intrasessional differences in electromyographic activity of the muscles before and during treatment. At the retruded position, the masseteric activity increased by the sixth month while temporalis activity remained at the same level. Following treatment, the masseteric imbalance was reduced, but the temporalis imbalance was unchanged. At the edge-to-edge position, masseteric activity increased by the sixth month, while temporalis activity remained unchanged. The masseteric imbalance was reduced by the sixth month, while the temporalis imbalance was reduced from the fourth month into treatment. The results imply a favorable muscular response to a progressive regime of Herbst appliance activation.