Eight (5M/3F) formalin-embalmed specimens (average age 74.9±15.9 years) were used. The skin and fascia overlying MM were removed to expose the fiber bundles (FBs) and aponeuroses of the superficial surface of the muscle. Each FB was delineated and digitized at 1-2mm intervals using a MicroScribe™ Digitizer. The aponeuroses and bony attachment sites were also digitized. Following digitization, the aponeuroses and FBs were removed to reveal the next layer. This process of dissection and digitization was repeated throughout the muscle volume. The data were reconstructed into 3D models using Autodesk® Maya®. The 3D model of each specimen was used to determine the relationship of the FBs and aponeuroses. The digitized data were used to quantify muscle architectural parameters: fiber-bundle length (FBL), pennation angle (PA), muscle volume (MV), and physiological cross-sectional area (PCSA). The muscle geometry and architectural parameters were compared between muscles.
Each of the 3D models of MM included up to 2000 digitized FBs/muscle. Each MM consisted of well-defined multi-laminar superficial (SH) and deep heads (DH), with FBs usually spanning between superior and inferior aponeuroses. In comparison to DH, SH had FBs that were on average 12mm longer, and as a whole the volume was 4x greater. DH had a PCSA that was about one third of SH, with a PA that was on average 7° greater than SH. In 7/8 specimens, both SH and DH had a laminar structure with alternating superior and inferior aponeuroses. On average SH had 3 laminae and 5 alternating aponeuroses, while DH had 2 laminae with 2 alternating aponeuroses.
The masseter is a multi-laminar muscle with two distinct heads containing alternating aponeuroses organized in a sequential manner for the attachment of FBs. Each head has distinct architectural parameters, indicating a well-defined compartmentalization with the potential for differential activation. The results of this study can aid in the development of targeted in vivo ultrasound protocols for studying normal and pathologic muscle architecture. It also provides data with which detailed finite element models can be constructed for simulating functional biomechanics in the muscle.
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