The motor nerve to the masseter muscle: an anatomic and histomorphometric study to facilitate its use in facial reanimation.

Introduction: The motor nerve to the masseter muscle is increasingly being used for facial reanimation procedures. However, many surgeons have been reluctant to use this versatile source of axons because of difficulty in locating it intraoperatively. In this study we conducted a detailed assessment of its gross and microscopic anatomy and develop a simple, reliable method for locating this nerve.

Methods: We defined the anatomy of the nerve to the masseter, in particular its relationship to common surgical landmarks such as the auricular tragus and the zygomatic arch, and determined its intramuscular anatomy. We also performed a histomorphometric analysis.

Results: The anatomy of the motor nerve to the masseter was consistent. A convenient starting point for its dissection was found 3.16 ± 0.30 cm anterior to the tragus at a level 1.08 ± 0.18 cm inferior to the zygomatic arch. The nerve was located 1.48 ± 0.19 cm deep to the superficial muscular aponeurotic system (SMAS) at this point. Relative to the zygomatic arch, the nerve formed an angle of 50 ± 7.6° as it coursed distally into the masseter muscle. The distance from the arch to the first branch of the motor nerve to the masseter was 1.33 ± 0.20 cm. The histomorphometric analysis demonstrated that the motor nerve to the masseter contained an average of 2775 ± 470 myelinated fibers.

Conclusions: Successful intraoperative location of the motor nerve to the masseter is facilitated by knowledge of its anatomy relative to standard surgical landmarks. A consistent and convenient starting point for dissection of this nerve is found 3 cm anterior to the tragus and 1 cm inferior to the zygomatic arch. The nerve contains over 2700 myelinated fibers, demonstrating its usefulness as a source of motor innervation for facial reanimation.