The Internal Geometry of the Masseter Muscle: A 3D Map to Guide in Vivo Ultrasound

Masticatory muscle disorders (MMDs), a subcategory of temporomandibular disorders (TMDs), are often associated with acute and chronic muscle pain which limits the range of mandibular motion. The masseter muscle (MM), is a muscle of mastication associated with TMDs, and ultrasonography (US) has been used previously to examine it in patients with and without TMDs. For example, Ariji et al. (2004) found that compared to normal subjects, the MMs of patients with MMD were thicker, and showed indications of changed internal morphology. The intramuscular echogenic bands showed reduced or absent echogenicity in patients with MMD, suggesting changes in the internal fascia/tendon complex and/or the effects of intramuscular edema on the US images. Ultrasound can provide clinicians with evidence for the MM’s intramuscular organization, dynamic changes during activation, and muscle pathology, but in order to develop a muscle-specific US protocol, the normal geometry and architecture should be known. Previous anatomical studies of MM are largely descriptive, although its aponeurotic contents and distribution have been quantified via magnetic resonance imaging (Cioffi et al., 2012). No volumetric studies have mapped the MM’s musculo-aponeurotic geometry in 3D however. Therefore, the aim of this study was to quantify the spatial organization of the muscle’s musculo-aponeurotic components as a basis for developing future US protocols.

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.

1) Ariji Y., Sakuma S., Izumi M. Ultrasonographic features of the masseter muscle in female patients with temporomandibular disorder associated with myofascial pain. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2004;98:337-341

2) Cioffi I., Gallo L.M., Palla S. Macroscopic Analysis of Human Masseter Compartments Assessed by Magnetic Resonance Imaging. Cells Tissues Organs 2012;195:465-472