Changes in the masseter muscle after curved osteotomy of the prominent mandibular angle

Background: This study aimed to explore the changes in the masseter muscle after curved osteotomy of the prominent mandibular angle and to supply guidance for resection of the mandibular angle. Methods: Ultrasonography was used to assess changes in the thickness of the masseter muscle after curved osteotomy for 10 patients (20 hemimandibles) at the 6-month following-up assessment. The measurements were performed under both relaxing and maximal clenching positions through three cross sections of the masseter muscle (planes A, B, and C). Plane A contains the line from the mouth angle to the ipsilateral ear lobe. Planes B and C are parallel planes above and below plane A with a distance of 1 cm between them. Results: No significant difference between the preoperative and postoperative thicknesses of the masseter muscle for planes A and B (p > 0.05) was found, but there was a significant difference (p < 0.05) for plane C. The postoperative thickness of the masseter muscle in plane C was reduced by 0.244 +/- 0.121 cm in the relaxing position and by 0.244 +/- 0.142 cm in the clenching position, which were respectively 19.22% +/- 7.785% and 15.404% +/- 7.648% of its original thickness. There was no significant difference in the contraction amplitude of the masseter muscle under maximal clenching (p > 0.05) for any of the three cross sections postoperatively. Conclusions: The masseter muscle around the mandibular angle becomes atrophied but without functional defect after curved osteotomy. Patients with prominent mandibular angles can be treated simply with curved osteotomy instead of masseter excision.