After the facial nerve divides into temporofacial and cervicofacial divisions, the marginal mandibular branch takes origin and extends anteriorly and inferiorly within the substance of the parotid gland. The marginal mandibular branch or branches, which supply motor fibers to the facial muscles in the lower lip and chin, represent the most important anatomic hazard when performing the submandibular approach to the mandible. Studies have shown that the nerve passes below the inferior border of the mandible in a significant minority of cases (see Fig. 9-1). In Dingman and Grabb's classic dissection of 100 fascial halves, the marginal mandibular branch was as much as 1 cm below the inferior border in 19% of cases (1). Anterior to the point where the nerve crossed the facial artery, all dissections displayed the nerve above the inferior border of the mandible.
Ziarah and Atkinson (2) found an even higher number of cases in which the marginal mandibular branch passed below the inferior border. In 53% of 76 facial halves, they found the marginal mandibular branch below the inferior border reaching the facial vessels, and in 6%, the nerve continued for a farther distance of as much as 1,5 cm before turning upward and crossing the mandible. The farthest distance between a marginal mandibular branch and the inferior border of the mandible was 1,2 cm. In view of these findings, most surgeons recommend that the incision and deeper dissection be at least 1,5 cm below the inferior border of the mandible.
Another important finding in the study by Dingman and Grabb (1) was that only 21% of cases had a single marginal mandibular branch between the angle of the mandible and the facial vessels (Fig. 9-2); 67% had two branches (see Fig. 9-1), 9% had three branches, and 3% had four.
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