The anatomy of the marginal mandibular branch of the facial nerve crosses from beneath the platysma muscle in the neck up across the side of the chin to innervate multiple lower lip and chin muscles. It causes the action of the lip depressor muscles (labii inferiors and angle oris) as well as that of mentalis muscle which raises and protrudes the lower lip.
The marginal mandibular branch of the facial nerve courses through territories where numerous types of aesthetic plastic surgery are performed. It is commonly known to be aware of this nerve when performing neck liposuction and facelift type procedures. But there are several types of procedures where this nerve is potentially at even greater risk of injury…and that is when a submental incision is used for direct access to the chin bone and side of the jawline.
As this nerve branch courses across the border of the jawline at the side of the chin, it lies just superficial to the facial artery. When doing dissection when placing a chin implant, and in particular jawline implants, it is common to see the facial artery crossing amongst the tenaciously attached osteocutaneous ligaments at the far reaches of the incision. Knowing that the marginal mandibular nerve branch is right above it, it is prudent to continue the subperiosteal dissection with instruments only at the bone level. This allows one to largely navigate safely under the nerve with continued elevation of the tissues including the artery off of the bone. Getting off the bone up into the soft tissues places the nerve at risk of injury. Even of the artery gets disrupted and bleeds, cautery is to be avoided to prevent potential thermal injury to the nerve. Packing and time will usually allow the artery to constrict and clot.
In some cases, despite complete and gentle subperiosteal tissue dissection off the bone, the stretch of the tissue elevation can cause a nerve injury and some postoperative muscle weakness. With an intact nerve with a stretch injury recovery is very likely but it will take many months to do successfully do so.
While the external submental skin incision access to the chin is one potential source of nerve injury, it can also occur from intraoral incisions as well. While it can also occur from intraoral access for chin/jawline implants and sliding genioplasties, it is less likely since dissection along the inferior border is not usually performed from this approach.
Dr. Barry Eppley