Anatomy of the MMN and cervical branch of the facial nerve during neck dissections
Local and regional spread of head and neck tumours occur mostly through the lymphatic system to neck nodes. Neck dissection is not only important for prognosis, but there is evidence to suggest that high lymph node yields equate to better survival even if nodes are negative3. National guidelines recommend therapeutic or elective neck dissection  based on  evidence  of nodal metastases, or where occult metastasis risk is over 20%. In head and neck cancer, this means that neck dissection should be considered in all but low stage glottic and oral mucosal malignancy. The practice of cervical lymphadenectomy has changed  such  that it  is now common to perform modified radical necks with selective dissections. The levels dissected depend on location of primary tumour and nodal status.
The marginal mandibular nerve is at risk during dissection of level Ib and IIa4 (figure 1). Manoeuvres to protect the nerve have been described such as Hayes-Martin, however, their oncological safety has been contested. This nerve emerges from the caudal border of the parotid gland on its anterior aspect. It exits the gland anterior or inferior to the mandible angle. At this position it lies deep to the parotid-masseteric fascia and the investing layer of the deep cervical fascia5. The MMN then courses forward, usually remaining above the inferior border of the mandible, superficial to the anterior facial vessels, deep to the masseteric  fascia. If the MMN courses below the level of the mandible it runs over the posterior belly of  digastric and the submandibular gland. In this case the nerve is deep to the cervical fascia and takes a curved course6. The nerve may run as inferiorly as 3cm below the inferior border3. Near the mid-body of the mandible, at its inferior border, the nerve perforates the deep cervical fascia to run underneath platysma. It crosses the mandibular border at the anterior border of the masseter6, this happens at a point where the nerve intersects with facial vessels. It usually crosses superficial to the facial vein7 approximately 3cm anterior to the masseteric tuberosity. From here it enters the buccal space to innervate the perioral musculature8.
There can be significant variation, in particular it’s branching and relationship to lower border of the mandible. There have also been disputes over the depth of the MMN, some authors stating it lies between the deep investing fascia and the platysma7. The nerve is thought to be most vulnerable after it penetrates the cervical fascia, coursing cephalad over the mandibular border, close to the facial vessels3 and anterior to the masseter (Figure 1).
The cervical nerve is at risk of damage in level II and III (figure 1). This nerve passes inferiorly from anterior aspect of the caudal half of the parotid close to the MMN. By the time the nerves reach the mandible angle their courses have diverged. The MMN runs anterior to the cervical. The cervical nerve usually exits the parotid as a single branch and passes posterior to the angle of the mandible, running deep to platysma. Studies have shown the nerve to lie an average distance of 0.83cm posterior to the gonion8 (figure 1).
The nerve usually curves anteriorly; when it reaches the posterior aspect of the submandibular gland it divides into several slender branches, forming a plexus supplying the platysma; this plexus forms anterior to the hyoid8. Branches to MMN are rare, but common to the greater auricular nerve8. Communicating branches between the transverse cervical nerves occur at the tip of the greater Cornu of the hyoid where an ascending branch of the transverse cervical nerve joins the cervical nerve. The cervical branches tend to be larger than the MMN branches especially after it has received these contributions8.
The nerve branching pattern is highly variable. The plexus forms multiple arches spreading across the deep surface of the suprahyoid platysma providing innervation. This variation and division makes the cervical branches inherently vulnerable when raising sub-platysmal flap. The greater branching means injury to a single division is less likely to give functional loss than MMN.