AN IN VIVO STUDY OF THE MUSCULOAPONEUROTIC ARCHITECTURE OF MASSETER MUSCLE

Ethical Review Approval for this study was received from the Health Sciences Research Ethics Board, University of Toronto (Toronto, ON, Canada). Background The masseter muscle (MM) is affected in conditions with chronic and acute facial pain. It has been suggested that architectural changes occur in the contractile and connective tissue elements of MM in these patients. 1 Muscle architecture is defined as the arrangement of the contractile and connective tissue elements in the muscle volume and quantifiable parameters, including fiber bundle length (FBL) and pennation angle (PA). A cadaveric study of detailed 3-dimensional, laminar MM architecture was conducted in our laboratory. 2 Cadaveric studies have been used as a basis to develop ultrasound (US) protocols used in in vivo investigations of the architecture of supraspinatus. 3,4 US studies investigating MM architecture are scarce, and such parameters as FBL and PA have not been quantified in vivo. To understand the pathologic changes in MM, it is necessary to elucidate the normal musculoaponeurotic architecture. Objective(s) To investigate the in vivo musculoaponeurotic architecture throughout the volume of MM in asymptomatic participants (ages 20–40 years) with US in the relaxed and maximally contracted states. We hypothesize that there will be significant differences in the architectural parameters of each MM lamina in the relaxed and contracted states. Materials and Methods The MM of 24 participants were scanned by using a Logiq E real-time ultrasound scanner (General Electric, Chicago, IL) with 2 linear probes (L10-22 and L4-12t). A microelectromyography (micro-EMG) surface electromyography unit (OT Bioelectronica, Reno, NV), was used to monitor MM activity, throughout scanning, on the contralateral masseter. Axial and coronal scans of the superficial and deep heads of MM were acquired throughout the volume of the masseter muscle to quantify architectural parameters, such as FBL and PA. Results Connective and contractile tissue elements of musculoaponeurotic architecture have been quantified in vivo, and differences have been observed between the relaxed and maximally contracted states. A further study will involve investigating musculoaponeurotic architectural parameters in pathologic MM.