LAMINAR MORPHOLOGY OF THE HEADS OF THE MASSETER: IS THERE VARIATION?

Background Two- and 3-dimensional cadaveric studies have established that the masseter muscle (MM) is composed of a superficial head (SH) and a deep head (DH). The SH and DH are thought to take part in separate functions, including elevation, protrusion, retrusion, and ipsilateral excursion of the mandible. It has been proposed that the heads of the MM have a laminar arrangement. However, the precise morphology of the SH and DH has not been previously compared and studied in vivo using ultrasound. Comparison of the morphology of the SH and DH could suggest an anatomic basis for the differences in functions between the 2 heads of the MM. Objective(s) The aim of this study was to investigate and compare the in vivo morphology of the SH and DH in relaxed asymptomatic participants. We hypothesized that the SH and DH will have different laminar morphologies. Study Design Ultrasound scanning of 48 MMs in 24 participants (mean age 25.8 ± 4.1 years) involved the use of a Logiq E real-time ultrasound scanner (General Electric) equipped with 2 linear probes (L10-22 and L4-12 T). Axial and coronal images were analyzed to determine the morphology of the SH compared with that of the DH. Results It was found that the SH and the DH had different laminar arrangements. The SH consisted of 3 or 4 laminae in 28 and 20 MMs, respectively. The DH had 2 (11 MMs), 3 (30 MMs), 4 (4 MMs), or 5 (3 MMs) laminae. The laminae of both heads had distinct aponeurotic separation, but their spatial arrangement varied. Visual inspection showed that SH laminae were oriented parallel to the long axis of the mandibular ramus, whereas the DH laminae were oriented at an acute angle toward the mandibular ramus. Discussion/Conclusions The orientation of the laminae of the SH and DH suggest different functions. The laminar morphology of the SH was less varied compared with that of the DH. The more diverse laminar morphology of the DH may allow for more complex functional partitioning relative to the SH.