Marginal mandibular branch of the facial nerve (Fig. 10.12)

Define the inferior margin of the surgical field.

Identification of the EJV on the skin, before skin incision, is useful for better identification and preservation of the anatomical structures. The external jugular vein emerges in the parotid loggia from the confluence of the posterior auricular vein and the posterior division of retromandibular vein and then penetrates into the SCM region on the level with the angle of the mandible and crosses the lateral face of the muscle askew. During the dissection, it may be necessary to ligate it.

The GAN emerges from the posterior border of the SCM, at Erb’s point. It runs parallel and superior to the EJV on the superficial face of the muscle. It gives sensory branches to the skin of parotid and auricular regions. During the dissection, the surgeon usually finds the EJV first and then, about 1 cm laterally and cranially to it, the GAN.

The identification of the submandibular is an important step. This salivary gland is a fundamental landmark to recognize a safe plane where preserving the marginal mandibular branch of the facial nerve. The SG lies into the SLDCF division, while the nerve stays below the platysma and above this fascia.

The FV lies above the digastric muscle. To carry out a safe dissection, the FV has to be identified, ligated, and retracted cranially (Hayes-Martin maneuver). This trick allows protecting the marginal mandibular nerve since in more than 90 % of cases, it passes above the FV, and, in the 10 % left, it is medial to the vein (Fig. 10.13a, b and Scheme 10.3).

The surgeon must remain aware of the location of the marginal mandibular branches, which run in the plane of the fascia of the submandibular gland. The nerve can be identified as it dips just below the lower margin of the mandible. In young patients, the nerve runs on the level with the inferior margin of the mandible; however, particularly in older patients, a ptotic submandibular gland can displace the nerve inferiorly. As the flap is elevated, dissection must be kept as close as possible to the platysma muscle. Once identified, it can be transposed cranially (Fig. 10.12).

Thyrolingual-facial venous trunk (TLF) (Fig. 10.14b)

Carotid sheath or neurovascular bundle

We find the posterior belly of digastric muscle, which runs obliquely forward and downward and goes to the hyoid bone, where it is fixed with its intermediate tendon to the body and the great horn; then it continues with its anterior belly. It is known as “the resident’s friend” because once it is identified, below it is possible to identify important structures like the neurovascular bundle and the TLF.

The TLF trunk passes deeply to the SCM and above the digastric muscle to empty into the internal jugular vein. The TLF trunk is formed by the thyroid, pharyngeal, lingual, and facial veins which merge together before opening as a unique vein into the IJV. Only occasionally these veins reach the IJV independently. Under and just before the angle of the mandible, the facial vein receives the blood from the retromandibular vein, before reaching the IJV. The ligation of the TLF trunk should be done near the confluence with the IJV; alternatively the facial vein only can be ligated (Fig. 10.14b).

It is formed by the internal jugular vein laterally, by the carotid artery medially, and, in the dihedral angle formed by the two vessels, by the vagus nerve posteriorly. The first vascular structure found during the dissection is the IJV, which is more lateral: it can be identified going just under the digastric muscle.

This nerve usually runs below, from the posterior belly of the digastric muscle before piercing the deep surface of the SCM muscle. It passes through this muscle and supplies it and finally emerges from the midpoint of the posterior border of the SCM.

The hyoid bone is a fundamental landmark in neck surgical anatomy. Using this approach, below the hyoid, we have to find STA and SLN. The region above shows a high complexity, with several structures that are described in the following paragraphs.

This muscle, which is innervated by the facial nerve, arises from the styloid process of the temporal bone and inserts onto the hyoid bone. This muscle is on the same plane of the digastric muscle. When the approach is extended from C2 to CVJ, it is necessary to dissect it.

The HN emerges from the hypoglossal canal just medially to the jugular foramen, passes laterally between the internal jugular vein and the internal carotid artery, and goes superficial to the internal carotid artery to reach the posterior edge of the stylopharyngeus muscle. The nerve curves below the lingual artery and disappears beneath the posterior edge of hyoglossus muscle. The triangular space designed by the digastric muscle superiorly, the internal jugular vein laterally, and the TLF inferiorly is useful to identify the HN.

The STA arises from the external carotid artery at the point of the hyoid bone and, after providing the upper laryngeal artery, goes down to the upper pole of the thyroid gland. The STA is the first branch of the external carotid artery. To allow the access to the prevertebral space, it is mandatory to bind and dissect this artery. The STA is always under the hyoid bone. To safely bind the artery, we should stay near its origin from the external carotid artery. This maneuver reduces the risk of superior laryngeal nerve injury. During this approach, the STA may not be properly in the surgical field, but without its ligation, it would be difficult and dangerous to obtain a good exposure of the vertebral plane. The surgeons should keep in mind, the possible anatomical variations of the origin of the STA like from thyrolingual (2 %) or thyrolinguofacial (1 %) trunck (Figs. 10.19 and 10.20).

The SLN runs medially to the carotid sheath, forming an oblique angle with it. The nerve moves medially toward the thyrohyoid membrane. After the ligation of the STA, it is possible to identify this nerve.

The lingual artery is the second branch of the external carotid artery and its first branch above the hyoid bone. Almost immediately after its origin, accompanying the middle constrictor of the pharynx, it meets the posterior margin of the hypoglossal muscle, which takes a horizontal, parallel route to the greater horn of the hyoid bone, approximately half a centimeter above it. Piragoff’s triangle is formed by the intermediate tendon of the digastric muscle, the hypoglossal nerve, and the posterior margin of the mylohyoid muscle: this triangle allows surgeons to find the artery, which is usually bound in the space between the digastric superiorly and the HN inferiorly; alternatively, it can also be bound near its origin (Figs. 10.21, 10.22, and 10.23).

It’s the third branch of the external carotid artery. It emerges behind the posterior belly of the digastric muscle, posteriorly skimming the submandibular gland; running backward to forward, and upward to downward, it becomes superficial to surround the inferior margin of the mandible, immediately anterior to the facial vein. Ligation of the FA should be carried out at its origin. Section of DM should be necessary to approach UCS and to correctly binding of FA (Figs. 10.24 and 10.25).

The internal carotid artery passes deep to the posterior suspensory muscles of the pharynx to terminate medial to the styloid process. The ICA continues posteromedially to enter the carotid canal at the base of the skull not giving out any branches.

The ICA leaves the carotid sheath passing anterosuperiorly within the carotid triangle. It is useful to keep in mind that the external carotid artery divides the posterior suspensory complex of the larynx by passing between the digastric and stylohyoid muscles laterally and the styloglossus and stylopharyngeus muscles medially. It emits eight branches: five below the posterior belly of the digastric within the carotid triangle and three above it. Surgeons have to follow this artery in order to identify the right plane to approach cervical spine, which lies medially to the vascular structures and laterally to visceral ones (Fig. 10.26).

Retropharyngeal space

It’s a virtual space that lies between the buccopharyngeal fascia anteriorly, which surrounds the pharyngeal muscles, and the alar fascia posteriorly. This space is the upper part of the retrovisceral space and continues down with the retroesophageal space. The whole retrovisceral space extends from the skull base to the mediastinum. The caudal limit is between the level of sixth cervical vertebra and the fourth thoracic vertebra. This space contains the retropharyngeal lymph nodes. Normally, the dissection of this space is carried out in an easy way.

Prevertebral muscles

Vertebral bodies and intervertebral disks

The “longus capitis” is located more laterally than the longus colli and originates from the transverse process of C3-C6 and has its insertion at the basilar portion of the occipital bone. Longus colli is loceted on the lateral side of the vertebral bodies, from the atlas to T3. If th diseases does not extend to the lateral portion of vetrabrali bodies, the surgeon has to identify only the longus colli. The cervical midline is defined between right and left longus colli muscles and lies lateral to vertebral bodies, which are bounded across the midline by the prevertebral layer of deep cervical fascia. Prevertebral muscles are dissected in a subperiosteal plane, the contralateral longus colli is retracted medially with the visceral column, and the ipsilateral is retracted laterally with the carotid sheath. During dissection of the long muscles of the neck, the cervical sympathetic chain is easily recognizable compared to the vagus because it has a lower mobility and it also presents ganglion formations.