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.