Changes in masseter muscle morphology after surgical-orthodontic treatment in skeletal Class III patients with mandibular asymmetry: the automatic masseter muscle segmentation model

Introduction: This study evaluated the masseter muscle changes after surgical-orthodontic treatment in skeletal Class III patients using automatic segmentation.
Methods: Images of 120 skeletal Class III malocclusion patients were obtained and reconstructed at T0 (pre-treatment), T1 (preoperative), and T2 (6–12 months postoperative). The patients were divided into symmetrical and asymmetrical groups. The volume, major axis length, maximum cross-sectional area (CSAM), horizontal cross-sectional area 5 mm above the mandibular foramen (CSAF), and orientation were calculated automatically.
Results: In the asymmetrical group, the volume and major axis length on the deviated side were lesser than on the non-deviated side at T0, T1, and T2 (p < 0.05). There were no significant differences in CSAM and CSAF bilaterally. The orientation was coronally more vertical and sagittally more forward on the deviated side (both p < 0.001). In the symmetrical group, there were no significant bilateral differences at T0, T1, and T2. The volume, major axis length, and CSAF decreased, and the coronal orientation was more vertical on the non-deviated side at T2 compared to T0 in both groups (p < 0.05). The coronal plane orientation was more inclined on the deviated side at T2 compared to T0 in the asymmetrical group (p < 0.05).
Conclusions: The smaller volume on the deviated side at T2 indicates the need for myofunctional training after surgery. The masseter muscle volume and cross-sectional area did not recover to the pre-orthodontic levels. Studies with longer follow-up durations are needed to confirm these findings.

引言：本研究采用自动分割技术（automatic segmentation），评估骨性Ⅲ类错𬌗（skeletal Class III malocclusion）患者接受正畸-正颌联合治疗后咬肌（masseter muscle）的变化情况。
方法：本研究纳入120例骨性Ⅲ类错𬌗（skeletal Class III malocclusion）患者，分别于T0（治疗前）、T1（术前）及T2（术后6~12个月）获取影像并进行重建。将患者分为对称组与不对称组，自动计算其咬肌的体积、长轴长度、最大横截面积（maximum cross-sectional area，CSAM）、下颌孔（mandibular foramen）上方5mm处的水平横截面积（horizontal cross-sectional area 5 mm above the mandibular foramen，CSAF）以及走行方向。
结果：在不对称组中，患侧咬肌的体积与长轴长度在T0、T1及T2三个时点均显著低于健侧（p<0.05）；双侧的最大横截面积（CSAM）与下颌孔上方5mm处水平横截面积（CSAF）无显著组间差异。患侧的走行方向在冠状位更趋于垂直，矢状位更靠前（均p<0.001）。对称组在T0、T1及T2三个时点的双侧各项指标均无显著差异。两组在T2时点与T0时点相比，患侧咬肌体积、长轴长度及CSAF均有所降低，且健侧冠状位走行方向更趋于垂直（p<0.05）；不对称组在T2时点与T0时点相比，患侧冠状位走行方向更倾斜（p<0.05）。
结论：T2时点患侧咬肌体积较小，提示术后需开展肌功能训练（myofunctional training）。咬肌体积与横截面积未恢复至正畸治疗前水平，需开展更长随访周期的研究以验证本研究结果。