Musculo-aponeurotic Architecture in the Human Masseter

Objectives: The masseter is one of the major muscles of mastication with a complex architecture. Its volume, cross-sectional area, fiber-bundle(FB) pennation and aponeuroses all have biomechanical implications, and are relevant factors in the interpretation of diffusion-tensor MRI and ultrasound images. The aim of this study was to map and quantify the internal architecture of masseter in three dimensions since this information is presently unavailable.
Methods: Eight formalin-embalmed specimens (5M/3F; average age 74.9±15.9 years), were serially dissected and digitized with a MicroScribe™ Digitizer. At each level, the FBs and surfaces of the aponeuroses were digitized at 1-2mm intervals. This process was repeated throughout the muscle volume. The data were reconstructed into 3D models (Autodesk® Maya®). Fiber-bundle lengths (FBLs), pennation angles (PAs), muscle volume (MV), and physiological cross-sectional area (PCSA) were quantified and compared.
Results: The 3D models included up to 2000 digitized FBs/muscle. The muscles had distinct superficial (SH) and deep heads (DH). The SH comprised 3-4 laminae. The most superficial lamina attached superiorly to an aponeurosis, and inferiorly to bone. The middle lamina had reversed attachment sites, superiorly to bone, and inferiorly to aponeurosis. This pattern continued throughout the SH. The superficial and deep laminae had FBLs about 7mm longer than, and MVs greater than those in the middle lamina, but similar PAs. DH had FBLs that were 14.5mm shorter than SH, and PAs of 7.4˚ greater, but their MVs were 4.7 times less than those in SH.
Conclusions: The masseter is a laminar muscle with two heads containing multiple aponeuroses organized in a sequential manner for the attachment of FBs. The architectural parameters differ between the heads and laminae, indicating well-defined compartmentalization with the potential for differential activation. These results can be used to develop targeted in vivo ultrasound protocols to study normal and pathological muscle architecture in living subjects.