Relationship between Mandibular Ramus Height and Masticatory Muscle Function in Patients with Unilateral Hemifacial Microsomia

Objective To clarify the relationship between mandibular ramus height and function of masticatory muscles in patients with hemifacial microsomia. Design Retrospective study of imaging and physiological data. Setting Images and physiological data were obtained from the records of Sapporo Medical University Hospital. Patients A total of 29 patients with hemifacial microsomia who showed Pruzansky grades I, II deformity. Main Outcome Measures Mandibular ramus height and masticatory muscle volume were evaluated with multi-detector row computed tomography. The electromyographic value was measured by the K7 Evaluation System. The hemifacial microsomia patients were classified into three groups based on the mandibular ramus height ratio of the affected and unaffected sides: group 0, > 1.00; group 1, 1.00 to 0.85; group 2, < 0.85. The Tukey-Kramer method and Games-Howell method were used to determine correlations between parameters. Results Decreased mandibular ramus height was significantly correlated with both reduced electromyographic values of the masseter muscle (P < .05) and the amount of mandibular lateral deviation at the time of maximum opening (P < .05) on the affected side. These differences were prominent in unilateral hemifacial microsomia patients classified as group 2. Conclusions Decreased mandibular ramus height may cause dysfunction of the masseter muscles but not the temporal muscle on the affected side in patients with hemifacial microsomia.