Changes in stomatognathic function induced by orthognathic surgery in patients with mandibular prognathism.

PURPOSE: This study was designed to investigate the changes in stomatognathic function through orthognathic treatment in patients with mandibular prognathism. PATIENTS AND METHODS: Thirty-six patients with mandibular prognathism were tested and compared with 30 healthy controls with normal occlusion. For each subject, the occlusal contact area and occlusal force were measured during maximum voluntary clenching (MVC). Activities of the masseter and temporalis muscles were recorded during MVC and voluntary gum chewing. Jaw movement was analyzed during chewing on the left and right sides. For the analyses, 2 parameters, asymmetry index (AI) and error index (EI), were established to further investigate the nature of masticatory function. AI was used to evaluate the bilateral balance of masticatory muscle activity, and EI indicates the rate of abnormal jaw movement pattern. RESULTS: In patients with mandibular prognathism, the occlusal contact area and maximum bite force decreased before surgery, and increased after surgery. The masseter and temporal muscle activities also decreased before surgery, but showed no substantial increase even after surgery. The occlusal and muscle efficiency exhibited significantly smaller values in the patient group than in the controls, irrespective of treatment stages. The AI decreased after surgery. The EI decreased significantly after surgery, but was still significantly greater in the patient group than in the controls. CONCLUSIONS: It is suggested that masticatory muscles in the patients with mandibular prognathism may adapt to the new environment achieved with surgically corrected dentofacial structure, although the activities remain at lower levels as compared with the controls.

Influence of functional appliances on masticatory muscle activity.

The purpose of the present study was to investigate the influence of an activator appliance and a spring active appliance on masticatory muscle activity by means of electromyography (EMG). Twelve adult males with good general health were recruited as subjects. Three functional appliances were used in each subject for long-period EMG recording during daytime and sleep and short-time EMG recording during voluntary biting. Following findings were obtained. (1) The activity of all muscles was greater during biting than during daytime and sleep, (2) the muscle activities tended to increase in the digastric muscle and to decrease in the temporal muscle with activators under all conditions, and (3) the temporalis-masseter ratios became lower with the biting use of appliances. Therefore, this study suggests that functional appliances should be used during sleep and during the day in combination with voluntary biting to achieve adaptation and development of the masticatory muscles.