Quantitative description of masseter muscle involvement in craniofacial microsomia

Masseter deficits are common in craniofacial microsomia (CFM), however studies on masseter muscle involvement are limited. The aim of this study was to describe the morphology and functional involvement of the masseter muscles quantitatively. Ninety-eight patients with CFM who underwent three-dimensional computed tomography and surface electromyography were included. The mean action potential during maximum voluntary contraction in the intercuspal position was recorded. Asymmetry of the compound muscle action potential (ACMAP) was calculated as an indicator of functional involvement. Differences between the affected and unaffected sides, the correlation between morphology and function, and the relationship between ACMAP and the OMENS-Plus classification were assessed by paired t-test, Pearson correlation analysis, and Spearman correlation analysis. The masseter muscle was absent on the affected side in 11 patients. In the remaining 87 patients, the mean volumes of the affected and unaffected masseters were 5.22±2.47cm³ and 9.62±3.30cm³, respectively, with mean action potentials of 41.40±28.52uV and 73.78±45.90uV, respectively. Both morphology and the function of the masseter showed moderate correlation with the OMENS 'M' classification. No significant correlation was found between ACMAP and the OMENS-Plus soft tissue grading. A masseter function classification is proposed: type I, ACMAP<0.2; type II, ACMAP 0.20-0.34; type III, ACMAP 0.35-0.54; type IV, ACMAP≥0.55. The masseter function classification may be a beneficial tool in patients with CFM.