Examination of the masseter muscle in patients with hemifacial microsomia using high-frequency ultrasound and shear wave elastography

Background: This study aimed to prospectively investigate the reference values for masseter muscle thickness and hardness using ultrasonography and shear wave elastography, respectively, in patients with hemifacial microsomia (HFM). Methods: We enrolled 51 patients, aged 5-20 years, with HFM including 31 males and 20 females. The upper-lower, left-right, and anterior-posterior diameters of 102 masseter muscles and stiffness of 98 masseter muscles were determined by examining the unaffected and affected sides of each participant's face. Results: The upper-lower, left-right, and anterior-posterior diameters of the masseter muscle were significantly smaller at rest (4.26 +/- 0.83, 2.94 +/- 0.75, and- 0.80 +/- 0.25 cm, respectively) and during contraction (3.95 +/- 0.78, 2.71 +/- 0.78, and 0.87 +/- 0.29 cm, respectively) in the affected side than those in the healthy side (5.45 +/- 0.66, 3.87 +/- 0.49, and 0.97 +/- 0.20 cm, respectively, at rest and 4.99 +/- 0.45, 3.49 +/- 0.53, and 1.07 +/- 0.23 cm, respectively, during contraction, p < 0.05). In the resting state, the hardness of the masseter muscle on the affected side (0.77 +/- 0.66 m/s) was significantly greater than that on the healthy side (0.42 +/- 0.41 m/s; p < 0.05). The magnitude of changes in the upper-lower, left- right, and anterior-posterior diameters of the biting muscle in the occlusal state were sig- nificantly smaller on the affected side (-0.30 +/- 0.27, -0.23 +/- 0.17, and 0.08 +/- 0.08 cm, respectively) than those in the healthy side (-0.47 +/- 0.38, -0.37 +/- 0.25, and 0.10 +/- 0.12 cm, respectively, p < 0.05).<br /> Conclusions: The knowledge of these values allows for better understanding of the disease characteristics of HFM, which may be used for its diagnosis, treatment, and prognosis. Patients experiencing different severity levels exhibited significant differences in the morphology and function of the masseter muscle on the affected-side (p < 0.05).