Applied Surgical Anatomy of the Anterior Approach to the Cervical Spine

Applied Surgical Anatomy of the Anterior Approach to the Cervical Spine‌

 

 

Overview

 

The key to understanding the anatomy of the anterior approach to the cervical spine lies in appreciating the three fascial layers of the neck. The most superficial fascial layer is the investing layer of deep cervical fascia. The fascia surrounds the neck like a collar, but splits around the sternocleidomastoid and trapezius muscles to enclose them. Posteriorly, it joins with the ligamentum nuchae (nuchal ligament). The superficial layer is incised along the anterior border of the sternocleidomastoid muscle.

Dividing the layer of fascia allows the sternocleidomastoid to be retracted laterally and separated from the underlying strap muscles. The only structures that lie superficial to it are the platysma muscle (a remnant of the old panniculus carnosus, or muscle of the skin) and the external jugular vein, which can be divided safely if it intrudes into the operative field (Figs. 6-77 and 6-78).

The next fascial layer is the pretracheal fascia, which forms a layer between sliding surfaces. It invests the strap muscles and runs from the hyoid bone down into the chest (see Fig. 6-78). Its key relationship is with the carotid sheath, which encloses the common carotid artery, the internal jugular vein, and the vagus nerve. The pretracheal fascia is continuous with the carotid sheath (see Figs. 6-77 and 6-79). Hence, the pretracheal fascia must be divided on the medial border of the carotid sheath so that the carotid sheath can be retracted laterally and the midline structures can be retracted medially. Two sets of vessels, the superior and inferior thyroid vessels, run from the carotid sheath through the pretracheal fascia into the midline. On rare occasions, the thyroid vessels have to be divided to enlarge the exposure (see Fig. 6-80). The superior laryngeal nerve, however, which runs with the superior thyroid vessels, must be preserved.

The deepest layer of fascia is the prevertebral fascia, a firm, tough membrane that lies in front of the prevertebral muscles. On its surface runs the cervical sympathetic trunk, which lies roughly over the transverse processes of the cervical vertebrae. Beneath the prevertebral fascia are the left and right longus colli muscles (see Fig. 6-77).

 

Landmarks and Incision

Landmarks

The carotid tubercle is the enlargement of the anterior tubercle of the transverse process of C6. It is larger than all other vertebral tubercles (there is no anterior tubercle of C7) and may be palpable. The tubercle of C6 is the key surgical landmark in the anterior incision (Fig. 6-82).

 

 

Figure 6-77 Cross section at the level of C5. Note the deep cervical fascia, the pretracheal fascia, and the prevertebral fascia. Note the relationship of the pretracheal fascia to the carotid sheath.

 

The cricoid ring is easily palpable just beneath the thyroid cartilage. The only complete ring of the trachea, it is opposite the C6 vertebral body (see Figs. 6-71 and 6-78).

The sternocleidomastoid muscle runs obliquely down the side of the neck from the mastoid process and lateral superior nuchal line to the sternum and clavicle. It is enclosed in fascia, which must be divided on the medial side before the muscle can be retracted laterally. The nerve supply of the sternocleidomastoid comes from the accessory nerve, which innervates the muscle from its posterior and lateral surfaces. There is no danger of neurologic damage as long as the dissection remains on the medial or anteromedial side of the muscle. If it strays to the posterior side, however, the spinal accessory nerve, which supplies not only the sternocleidomastoid, but also the trapezius, can be damaged (see Fig. 6-78).

Incision

Ideally, the skin incision should run parallel to the cleavage lines of the skin of the neck. Inferiorly and anteriorly, these lines run transversely, making the skin crease incision advantageous. The skin on the anterior part of the neck is thinner and more mobile than is the skin on the back of the neck, because of both the loose subcutaneous tissue and the superficial fascia that remains unconnected to the investing fascia of the neck. As a result, skin retraction is easy; the skin incision can be moved to accommodate the needs of the surgery. For extensive exposures, a longitudinal, slightly oblique incision can be made parallel to the medial border of the sternocleidomastoid muscle.

 

Superficial Surgical Dissection and Its Dangers

 

The platysma muscle is split in line with its fibers. The muscle is difficult to denervate, because most of its nerve supply comes from the cervical branch of the facial nerve and begins in the region of the mandible. In any case, the muscle is not of great functional importance; sewing it carefully during closure will improve the cosmetic appearance of the scar.

 

 

Figure 6-78 The platysma and deep cervical fascia have been removed. Note that the deep cervical fascia (investing fascia) encloses the sternocleidomastoid. The deeper pretracheal fascia encloses the strap muscles and thyroid structures.

 

Dividing the fascia on the anterior border of the sternocleidomastoid muscle reveals the carotid sheath (see Fig. 6-74, cross section). The sheath contains the common carotid artery, which divides at the upper border of the thyroid cartilage into internal and external carotid arteries. It also contains the internal jugular vein and the vagus nerve (see Fig. 6-79). After the plane between the carotid sheath and the trachea and esophagus has been entered, it is easy to develop by blunt dissection. The esophagus, however, is a fragile structure that is damaged easily by injudicious retraction.

 

Deep Surgical Dissection and Its Dangers

 

The longus colli muscles lie on the anterior surface of the vertebral

column, between C1 and T3. The muscles are pointed at their ends and broad in the middle. They must be removed from the vertebral bodies to expose the vertebrae. Removal does not denervate them, because they are innervated segmentally and laterally from their posterior surfaces. Running on the anterolateral surfaces of the longus colli muscles is the cervical sympathetic trunk, with its numerous ganglia. These must be avoided (see Figs. 6-77 and 6-80).

 

Recurrent Laryngeal Nerves

 

The two recurrent laryngeal nerves are branches of the vagus nerve. The left recurrent laryngeal nerve descends into the thorax within the carotid sheath. It curves around the aortic arch and ascends back in the neck, running between the trachea and esophagus to supply the larynx. The right recurrent laryngeal nerve descends within the carotid sheath and curves around the subclavian artery before ascending into the neck at a higher level than the left recurrent laryngeal nerve. In addition, the right recurrent laryngeal nerve is, on rare occasions, aberrant, leaving the carotid sheath at a higher level and crossing the operative field at the level of the thyroid gland (see Figs. 6-81 and 6-80). Thus, left-sided approaches often are preferred. The nerves usually are safe as long as retractors are placed correctly underneath the longus colli muscles. Damage to these nerves is extremely serious and may result in a recurrent laryngeal nerve palsy with alteration in voice, hoarseness and possible breathlessness on exercise.

 

 

Figure 6-79 The sternocleidomastoid and strap muscles, and the pretracheal fascia have been resected. The carotid sheath and its contents have been exposed. The thyroid gland, cartilage, and trachea are seen. Note the course of the recurrent laryngeal nerve.

 

 

Figure 6-80 The carotid sheath and its contents have been resected. The larynx and its related structures are retracted medially. The longus colli and scalenus muscles with their overriding prevertebral fascia are seen. The sympathetic chain lies on the lateral border of the longus colli muscle. Note the position of the recurrent laryngeal nerve between the trachea and esophagus.

 

 

Figure 6-81 The longus colli, the longus capitis, and the scalenus anticus muscles have been resected to reveal the anterior portion of the vertebral bodies and transverse processes. Note the course of the vertebral artery through the transverse processes anterior to the spinal nerve. Note the course of the superior and inferior thyroid vessels.

 

 

Figure 6-82 Osteology of the cervical spine, anterior view.