Topographic anatomy of the masseteric nerve applied to masseteric-facial transfer
Anatomia topográfica do nervo massetérico aplicada à transferência massetérico-facial

INTRODUCTION

Facial paralysis is a condition caused by the involvement of the facial nerve at any point along its path. Facial movement can be restored using several techniques. The masseteric nerve is used as a nerve transfer in facial reanimation surgery to treat facial paralysis, with good functional and aesthetic results¹, especially when the transfer is performed to reinnervate the buccal muscles.

Consequently, oral continence and facial expression are permitted, thus improving the quality of life of patients. Masseteric nerve transfer can be total or partial, the latter as a “babysitting” procedure, with the purpose of maintaining muscle viability during the axonal regeneration process of transfacial facial nerve grafts². The contralateral facial nerve used as a transfacial graft requires a period of 9 to 12 months for the axons to migrate through the graft, causing the facial muscles to be paralyzed during this time to atrophy. Using the masseteric nerve as a partial nerve transfer maintains the viability of the transfacial graft.

The innervation of the masseter muscle depends on the mandibular branch of the fifth cranial nerve (trigeminal nerve). The mandibular nerve leaves the middle cranial fossa through the foramen ovale to the infratemporal region, where it divides into an anterior trunk and a posterior trunk. The temporomandibular nerve originates from the anterior trunk, from which the masseteric nerve arises. The masseteric nerve emerges towards the masseteric region, passing through the mandibular notch, below the zygomatic arch, reaching the deep surface of the masseter muscle^3,4.

The use of the masseteric nerve as nerve transfer produces a minimal masticatory motor deficit in the masseter muscle since this muscle has proximal branches of the descending branch of the masseteric nerve. These allow the masseter muscle to maintain its masticatory function when the dominant branch is used for nerve transfer, in the same way as the synergy between the masseter muscle and the temporal muscle during mastication⁴.

Therefore, identification of the masseteric nerve is essential for performing this technique, for which an anatomotopographic study of the approach to this nerve was carried out and applied to facial reinnervation surgery.

OBJECTIVE

Study the topographic anatomy of the masseteric nerve approach, specifying the variations in its relationships with neighboring structures, as well as its path and termination, to obtain a rapid approach and dissection of the masseteric nerve.

METHOD

Fifteen masseteric regions were dissected from adult cadavers fixed in formaldehyde. Dissection material was used to approach the anatomical parts, as well as photographic equipment, a digital caliper to take measurements, and the use of Microsoft Windows Excel software for data collection and processing (Figure 1).

Figure 1 - Preauricular approach similar to the facelift approach in the plane under the superficial musculoaponeurotic system (SMAS).

Figure 1 - Preauricular approach similar to the facelift approach in the plane under the superficial musculoaponeurotic system (SMAS).

A preauricular approach similar to the in-plane facelift approach under the superficial musculoaponeurotic system (SMAS) was performed (Figure 2). The incision was made in the skin over the subzygomatic triangle between the zygomatic arch, the mandibular condyle, and the posterior border of the superficial fascicle of the masseter muscle (Figure 3). The masseteric pedicle was dissected from the inferior border of the zygomatic arch to its entry between the middle and deep fascicles of the masseter muscle (Figure 4). The masseteric nerve and masseteric vessels were identified. In addition, the length of the zygomatic arch was measured and the point of emergence of the masseter nerve in relation to it was identified, dividing it into three thirds (anterior, middle, and posterior) (Chart 1).

Chart 1 - Length of the zygomatic arch identifying the point of emergence of the masseter nerve in relation to it, dividing it into three thirds (anterior, middle and posterior).

Figure 2 - The subzygomatic triangle between the zygomatic arch, the mandibular condyle, and the posterior border of the superficial fascicle of the masseter muscle.

Figure 2 - The subzygomatic triangle between the zygomatic arch, the mandibular condyle, and the posterior border of the superficial fascicle of the masseter muscle.

Figure 3 - Masseteric pedicle dissected from the lower edge of the zygomatic arch to its entry between the middle and deep fascicles of the masseter muscle.

Figure 3 - Masseteric pedicle dissected from the lower edge of the zygomatic arch to its entry between the middle and deep fascicles of the masseter muscle.

Figure 4 - Emergence of the masseter nerve in relation to it, dividing it into three thirds (anterior, middle, and posterior).

Figure 4 - Emergence of the masseter nerve in relation to it, dividing it into three thirds (anterior, middle, and posterior).

The relationship between the masseteric nerve and the masseteric vessels was also verified. The distance of the masseteric nerve course was measured from its emergence under the zygomatic arch to its termination. Then, the distance between the mandibular condyle and the point of emergence of the masseteric nerve under the zygomatic arch was measured. Finally, the distance between the origin of the masseteric nerve and the lower border of the zygomatic arch was measured (Figure 4).

RESULTS

The length of the zygomatic arch varied between 53.3 mm and 25.8 mm, with an average length of 40.4 mm and a median of 42.0 mm.

The emergence of the masseteric nerve was below the posterior third of the zygomatic arch in 80% of cases, below the middle third in 13%, and below the anterior third in 7%.

The distance between the inferior border of the zygomatic arch and the emergence of the masseteric nerve below it ranged from 16.9 mm to 1.9 mm, with a mean of 8.9 mm and a median of 9.0 mm. The distance between the mandibular condyle and the emergence point of the masseteric nerve below the zygomatic arch ranged from 35.4 mm to 4.6 mm, with a mean of 14.7 mm and a median of 14.2 mm.

In 53% of cases, the masseteric nerve was found superficial to the masseteric vessels, while in 47%, it was found deep to the masseteric vessels.

The length of the masseteric nerve path from its origin under the zygomatic arch to its termination varied from 33.2 mm to 8.5 mm, with an average of 18.2 mm and a median of 18.6 mm.

The width of the nerve at its termination varied from 3.4 mm to 0.6 mm, with an average of 1.5 mm and a median of 1.5 mm.

DISCUSSION

To topography the masseteric nerve, two palpable anatomical references on the surface were used: the mandibular condyle and the lower border of the zygomatic arch. These present little variability, and their anatomy is not distorted during the surgical procedure. Collar et al.¹ propose the subzygomatic triangle for the topography of the masseteric nerve. This triangle is delimited by the zygomatic arch, a vertical line that passes through the mandibular condyle and the frontal branch of the facial nerve and is a questionable region for dissection of the masseteric nerve due to the presence of a structure with variable anatomy such as the frontal branch of the facial nerve.

Using the mandibular condyle, the lower border of the zygomatic arch, and the posterior border of the masseter muscle, a triangle with more precise limits can be determined to topograph the masseteric nerve, reinforcing a more accurate tool for facial reanimation surgery with nerve transfer of said nerve.

CONCLUSION

Reliable anatomical relationships have been determined for the identification of the masseteric nerve during masseteric-facial transfer surgery. It can be identified in an area between the zygomatic arch, masseter muscle, and mandibular condyle.