Alteration of masticatory electromyographic activity and stability of orthognathic surgery in patients with skeletal class III malocclusion.

PURPOSE: The purposes of this study were to 1) investigate longitudinal changes in electromyographic masticatory activity subsequent to orthognathic surgery (OGS) in patients with skeletal Class III malocclusion, and 2) compare masticatory muscle activity and skeletal factors in patients with stable versus relapsed mandibular positions after OGS.

MATERIALS AND METHODS: A consecutive series of patients with skeletal Class III malocclusion who underwent 2-jaw OGS (35 patients, 17 men and 18 women; age, 24.5 ± 5.0 yr) were included. Lateral cephalometric films were obtained preoperatively (T1), 1 month after OGS (T2), and at completion of orthodontic treatment (T3). Serial cephalometric tracings and analyses were obtained. Surface electromyograms of the anterior temporalis and masseter muscles were recorded at T1, T2, and T3 (6 mo after OGS). Resting tonus, maximum voluntary clench with habitual intercuspation and on cotton pads, and maximum muscle firing were evaluated. Percentage of overlapping coefficient and torque coefficient were calculated. Patients were categorized further into stable and relapse groups according to the sagittal relapse rate of mandibular setback. These surface electromyographic variables were compared between the 2 groups.

RESULTS: On average, the mandible showed a significant setback of 10.19 mm and a relapse of 1.12 mm (10.99%). Surgical relapse did not correlate with gender or genioplasty. Serial surface electromyographic data indicated a significant decrease from T1 to T2 that then recovered from T2 to T3. No significant difference between T1 and T3 was noted. Percentage of overlapping coefficient was significantly decreased after OGS. The torque coefficient did not differ significantly from T1 to T3. The relapse group (relapse, >11%; n = 15) had a greater resting tonus of the anterior temporalis muscle at T3, a larger percentage of overlapping coefficient at T1, and a greater maximum voluntary clench of the masseter muscles at all times than in the stable group (n = 20). The relapse group exhibited a greater decrease in facial height (2.18 mm) from T2 to T3 than did the stable group (0.5 mm).

CONCLUSION: A larger sagittal relapse of mandibular setback occurred in patients with greater masticatory muscle activity. Modifications in surgical design and overcorrection should be considered in patients with greater masticatory muscle activity before OGS.