Musculo-Aponeurotic Architecture of the Human Masseter Muscle: An In Vivo Study of Architectural Changes during Mandibular Protrusion and Lateral Excursion

Temporomandibular joint disorders (TMD) affect 5% to 12% of the population. Architectural changes may occur in the masticatory muscles; notably the masseter muscle (MM) in TMD. Without an understanding of the normal architecture of MM, it may be difficult to understand such changes in disease. Studies investigating the in vivo multilaminar MM architecture are scarce. The aim of this study was to evaluate the in vivo musculo-aponeurotic architecture of the superficial head of MM in asymptomatic individuals with ultrasound in excursive movements of the mandible. An ultrasound protocol was developed, and twenty-four participants’ MMs were scanned bilaterally. On protrusion and contralateral excursion, superficial laminae showed significant changes compared to the relaxed state; no significant difference was found in the deep lamina. On ipsilateral excursion, superficial and deep laminae showed significant changes. These findings suggest there are differential changes to the laminae of MM during mandibular protrusion and lateral excursion.