Masseter Atrophication after Masseteric Nerve Transfer. Is It Negligible?

Supplemental Digital Content is available in the text.

Sir:

It has been reported that surgical procedures for facial reanimation in which the nerve innervating the masseter muscle is used as a motor source result in minimal donor site morbidity.[1,2] The author considers that surgeons who have conducted such procedures understand that dissecting the nerve in the deep part of the masseter muscle does not affect masticatory function. However, the postoperative changes in masseter muscle volume that occur after such procedures have never been examined.

The author used computed tomography scans to perform quantitative assessments of masseter muscle area in 10 patients who underwent masseteric nerve transfer. The patients had suffered facial paralysis because of brain surgery, but were free from impairments of the trigeminal nerve. The mean age of the patients was 56 years. All of the masseteric-facial nerve transfers were performed by the same surgeon. In each case, the nerve exploration involved separating the masseter muscle fibers along their longest axis while causing minimal damage to the muscle. Postoperative assessments were carried out more than 1 year after the nerve transfer. Axial computed tomography sections were obtained at the level of the anterior nasal spine, which corresponds to the superficial part of the masseter muscle (level S), and at the level of the mandibular notch, which corresponds to the deep and middle parts of the masseter muscle (level D; Fig. 1). The bilateral masseter muscles were selected as regions of interest by tracing their outlines using image viewing software. The difference between the area of the masseter muscle on the affected side and that on the contralateral healthy side was assessed. The affected masseter muscles were 7% to 33% (mean: 18%) smaller than the contralateral masseter muscles at level D. In addition, they were 8% to 53% (mean, 30%) smaller than the contralateral muscles at level S. The reduction in masseter area seen at level S was significantly greater than that observed at level D (P = 0.02089; Supplemental digital content 1, ). In spite of these reductions in masseter muscle area, none of the patients has ever complained about postoperative disfigurement of the affected cheek nor has the author noticed any postoperative changes in the patients’ facial appearances.

Fig. 1.

Axial computed tomography sections. Level D crosses the mandibular notch and roughly represents the deep and middle parts of the masseter muscle. Level S crosses the anterior nasal spine and roughly represents the superficial part of the masseter muscle. Article reprinted with permission. Copyright © Kenhub (www.kenhub.com) / Illustrator: Y. Koh.

The nerve innervating the masseter muscle commonly runs anteroinferiorly across the deep part of the muscle. As was demonstrated in the present case series, the nerve innervating the masseter muscle does not always have an additional branch in the deep part of the muscle. Although an anatomical report[3] found that ≥2 nerve branches are present at the entrance of the masseter muscle in 97.2% of cases, our results suggest that dissecting the nerve that innervates the masseter muscle eventually causes partial atrophication of the muscle. Such changes seem to be negligible; however, it is important to perform muscle separation procedures carefully so as to cause minimal damage to the muscle. Furthermore, reconstructive surgeons should be aware of the likelihood of postoperative atrophication of the masseter muscle because this will influence the esthetic outcomes of facial reanimation surgery.

DISCLOSURE

The author has no financial interest to declare in relation to the content of this article. The Article Processing Charge was paid for by the author.