Defining the Optimal Segment for Neurotization-Axonal Mapping of Masseter Nerve for Facial Reanimation

Recently, there has been renewed interest in using the motor nerve to the masseter for facial reanimation. This article aims to identify the ideal segment of the masseter nerve for facial reanimation by mapping its anatomy and studying the axonal count in its branches.

Fifteen fresh cadaveric heads with 30 masseter nerves were dissected under the microscope. The masseter muscle was exposed with a preauricular incision, the course of the nerve followed and measurements of the nerve and its branches were taken to identify the topography of the nerve. The nerve was then harvested en bloc, fixed, and axon counts of cross-sections of the nerve recorded with ImageJ (an image analysing software). The data were analyzed using Microsoft Excel.

The masseter consists of 3 discrete muscle layers, and the nerve to the masseter that entered the muscle between the middle and deep layers in all specimens was dissected. The average length of the masseter nerve from the mandibular notch to the last branch was 49.1 ± 10.5 mm. At origin, the nerve diameter was 0.80 ± 0.2 mm and had 1395 ± 447 axons. After the first major branch at a distance of 19.3 to 29.9 mm from the origin, the axon count of the main trunk ranged from 655 to 1025.

The segment of the masseter nerve which has an axon count of 600 to 800 is located after the first branch of the masseter nerve at a distance of 29.9 ± 7.2 mm from the start of its intramuscular course. Given that an axon count of 600 to 800 approximates that of the zygomatic branch of the facial nerve it is postulated that nerve coaptation at this level is able to produce a clinically satisfactory smile.