Chapter 34 Cervical Spine and Cervicothoracic Junction
An understanding of anatomy is the most basic tenet of surgery. Because both ventral and dorsal approaches are commonly used when operating on the cervical spine, it is essential that the spine surgeon be familiar with the anatomy of both the cervical and nuchal regions.1
Frick et al. have presented an overview of the anatomy of the neck with the cervical spine as the centerpiece.2 Dorsal to the cervical spine lies the nuchal musculature, which is covered superficially by two large muscles: the trapezius and the levator scapulae. Just ventral to the vertebral bodies lies the visceral space, which contains elements of the alimentary, respiratory, and endocrine systems. The visceral space is surrounded by the cervical musculature and portions of the cervical fascia. Dorsolateral to the visceral space but separated from the visceral space, as well as the cervical musculature, lie the paired neurovascular conduction pathways. Thus, in this scheme, the neck may be divided into five distinct regions: cervical spine, nuchal musculature, visceral space, cervical musculature, and neurovascular conduction pathways.
Knowledge of the surface anatomy of the neck is essential when planning cervical spine surgery. These relationships help establish the site of the skin incision and dictate which vertebral level(s) may be approached. Classically, several superficial anterior neck structures have been used to identify the approximate cervical spinal levels for the purposes of the skin incision. These include the hyoid bone (C3), thyroid cartilage (C4-5), cricoid cartilage (C6), and carotid tubercle (C6). These landmarks, however, may not be universally reliable because, depending on a patient’s body habitus, they may be difficult to palpate reliably. Therefore an understanding of the overall relationships of the surface anatomy is essential for operative planning.
The most prominent structure of the upper dorsal surface of the nuchal region is the inion, or occipital protuberance. This may be palpated in the midline and is a part of the occipital bone. The spinous processes of the cervical vertebrae may then be followed caudally to the vertebral prominence, variably corresponding to the spinous process of C6, C7 (most common), or T1.
The prominent surface structure of the ventral neck is the laryngeal prominence, which is produced by the underlying thyroid cartilage. The thyroid cartilage is composed of two broad plates that are readily palpable. This cartilage protects the vocal cords, which lie at the midpoint of the ventral surface. Rostral to the thyroid cartilage lies the horseshoe-shaped hyoid bone, which is easy to palpate with the neck extended. The hyoid bone lies in the mouth-cervical angle3 and mediates the muscular attachments of the muscles of the floor of the mouth (middle pharyngeal, hyoglossus, and genioglossus muscles), as well as those of the six hyoid muscles (stylohyoid, thyrohyoid, geniohyoid, omohyoid, mylohyoid, and sternohyoid). The hyoid bone provides some movement during swallowing. This movement is limited caudally to the fourth cervical vertebral body by the stylohyoid ligament.2 The transverse process of the atlas may be palpated at a point marked by a line between the angle of the mandible and a point 1 cm ventrocaudal to the tip of the mastoid process.3
Caudal to the thyroid cartilage lies the signet-ring–shaped cricoid cartilage. The cricoid cartilage marks the laryngotracheal transition of the respiratory system and the pharyngoesophageal transition of the gastrointestinal system. Caudal to the cricoid cartilage lies the trachea. The isthmus of the thyroid gland overlies the first few rings of the trachea, which may make palpation of these rings difficult. The trachea may be followed caudally to the jugular notch, which is the rostral depression of the manubrium. The trachea may be palpated dorsally and the sternal heads of the sternocleidomastoid muscle may be palpated laterally. The sternocleidomastoid muscle is the key landmark of the ventral neck, with respect to the traditional division of the neck into triangles.
The sternocleidomastoid muscle divides the neck into two large triangles, posterior and anterior, which are then subdivided into two and four triangles, respectively. Knowledge of these triangles includes a definition of the borders and the contents of each triangle (Fig. 34-1).
(Copyright University of New Mexico, Division of Neurosurgery, with permission.)
The borders of the posterior (dorsal) triangle are the dorsal edge of the sternocleidomastoid muscle, the ventral edge of the trapezius muscle, and the middle third of the clavicle. The deep cervical fascia covers the dorsal cervical triangle, thus forming its roof. The floor of the dorsal cervical triangle is formed by the scalenus posterior, scalenus medius, levator scapulae, and splenius capitis muscles, as well as the lateral extension of the prevertebral fascia that overlies these muscles. The dorsal belly of the omohyoid muscle partitions the dorsal cervical triangle into a large rostral occipital triangle named for the occipital artery exiting at its apex and a small caudal subclavian triangle named for the subclavian artery, which lies deep to it.
The spinal accessory nerve leaves the deep surface of the sternocleidomastoid muscle to enter the dorsal triangle of the neck, which it crosses to innervate the trapezius muscle. The two important structures found in the dorsal cervical triangle, which arise above the spinal accessory nerve, are the occipital artery and the lesser occipital nerve. The occipital artery leaves the dorsal cervical triangle at its apex where the sternocleidomastoid and trapezius muscles approach one another on the superior nuchal line. This artery then ascends to supply the dorsal scalp. The lesser occipital nerve ascends along the dorsal surface of the sternocleidomastoid muscle before dividing into several superficial branches that supply the scalp dorsal to the ear.
Caudal to the spinal accessory nerve are many important anatomic structures. The external jugular vein, which is formed by the confluence of the posterior auricular and the posterior division of the retromandibular vein at the angle of the mandible, courses over the sternocleidomastoid muscle obliquely to enter the dorsal cervical triangle caudally, en route to joining the subclavian vein approximately 2 cm above the clavicle.3 Two branches of the thyrocervical trunk cross the dorsal cervical triangle. The suprascapular artery runs rostral to the clavicle before passing deep to the clavicle to supply the periscapular muscles. The transverse cervical artery lies 2 to 3 cm rostral to the clavicle and also runs laterally across the dorsal cervical triangle to supply the periscapular muscles.
Three superficial nerves also exit the dorsal triangle below the spinal accessory nerve. In all cases, these nerves arise from the cervical plexus, which is formed by the ventral rami of the rostral four cervical spinal nerves. The plexus lies within the lateral neurovascular conduction pathways located between the internal jugular vein and the sternocleidomastoid muscle. The superficial nerves then arise along the middle portion of the dorsal border of the sternocleidomastoid muscle to supply the skin of the neck and scalp between the mastoid process and the inion. The great auricular nerve crosses the sternocleidomastoid muscle and ascends toward the parotid gland, branching into dorsal and ventral rami that supply the skin in an area stretching from the angle of the mandible to the mastoid process and the skin of the neck. The transverse cervical nerve also crosses the sternocleidomastoid muscle to supply the skin overlying the ventral cervical triangle. The supraclavicular nerves arise from a single trunk that trifurcates into lateral, intermediate, and medial branches that innervate the skin of the neck, ventral chest, ventral shoulder, sternoclavicular joint, and acromioclavicular joint. The phrenic nerve arises, in part, from the cervical plexus and, in part, from the brachial plexus. The brachial nerve arises near the scalenus anterior muscle, where it crosses ventromedially and deep to the transverse cervical and suprascapular arteries and the prevertebral fascia, to descend through the superior thoracic aperture near the origin of the internal mammary artery. The upper, middle, and lower trunks of the brachial plexus lie deep to the floor of the posterior cervical triangle. They emerge between the scalenus medius and scalenus anterior muscles and cross deep to the transverse cervical and suprascapular arteries to descend under the clavicle to enter the axilla.
The borders of the anterior (ventral) cervical triangle are the ventral edge of the sternocleidomastoid muscle, the inferior border of the mandible, and the midline of the neck. The ventral cervical triangle may be subdivided into four smaller triangles: submental, submandibular, carotid, and muscular.
The submental triangle is bounded by the hyoid body and laterally by the ventral bellies of the right and left digastric muscles. This triangle has, as its floor, the two mylohyoid muscles that connect to each other in the midline by forming a median raphe. Within this triangle lie the submental lymph nodes that drain the ventral tongue, the floor of the oral cavity, the middle portion of the lower lip and the skin of the chin, and several small veins that ultimately converge to form the anterior jugular vein.
The boundaries of the submandibular triangle are the anterior and posterior bellies of the digastric muscle and the inferior border of the mandible. The floor of the submandibular triangle is formed by the mylohyoid, hyoglossus, and middle constrictor muscles. The submandibular gland fills a significant portion of this triangle, and its duct passes parallel to the tongue to open into the mouth. The hypoglossal nerve also passes into this triangle along with the nerve to the mylohyoid muscle, a branch of the inferior alveolar nerve, and portions of the facial artery and vein.
The carotid triangle is bounded by the ventral border of the sternocleidomastoid muscle, the rostral edge of the rostral belly of the omohyoid muscle, and the caudal edge of the dorsal belly of the digastric muscle. Within the carotid triangle lie the bifurcation of the common carotid artery, the internal jugular vein laterally, the vagus nerve dorsally, and the ansa cervicalis (see Fig. 34-1B).
The muscular triangle is bounded by the median plane of the neck, the caudal edge of the rostral belly of the omohyoid muscle, and the medial border of the sternocleidomastoid muscle. Within this triangle lie the infrahyoid muscles and neck viscera.
An understanding of the cervical fascia aids the surgeon approaching a targeted cervical spine level by providing an avascular plane of dissection. There are three layers of the cervical fascia: investing, visceral, and prevertebral (Fig. 34-2). The investing fascia surrounds the entire neck, splitting to enclose the sternocleidomastoid and trapezius muscles and the submandibular and parotid glands. Rostrally, the investing fascia is connected to the hyoid bone, caudal border of the mandible, zygomatic arch, mastoid process, and superior nuchal line. Caudally, the investing fascia splits to attach to the ventral and dorsal surfaces of the sternum, thus forming the suprasternal space.3 The investing fascia forms the roof of both the ventral and dorsal cervical triangles.
FIGURE 34-2 Cervical fascia.
(Copyright University of New Mexico, Division of Neurosurgery, with permission.)
The visceral, or pretracheal, fascia courses deep to the infrahyoid muscles and surrounds the visceral space, including the thyroid gland, trachea, and esophagus. The visceral fascia is attached to the hyoid bone and the thyroid cartilage rostrally and extends caudally to the dorsal surface of the clavicles and sternum and into the mediastinum. Laterally, this layer blends into the carotid sheath. The thyroid vessels are located deep to this layer.
The prevertebral layer of cervical fascia surrounds the vertebral column and its musculature, including the scalene and longus groups of muscles. Ventral to the vertebral bodies, the prevertebral fascia splits into a ventral alar layer and a dorsal prevertebral layer, forming a potential space. This space is referred to as the “danger zone” because it extends from the skull base rostrally to the level of T12 caudally and communicates with the mediastinum. Within the prevertebral fascia, and in front of the longus colli muscle, lies the cervical portion of the sympathetic chain.
The cervical sympathetic chain (CSC) usually consists of three cervical ganglia that lie at the levels of the first rib, the transverse process of C6, and the atlantoaxial complex, respectively. The CSC lies directly over the longus colli muscles and beneath the prevertebral fascia.4 The chain runs in a superior and lateral direction with an average angle of 10.4 ± 3.8 degrees relative to the midline.4 The superior ganglion is typically located at C2-34 or C45 and lies more laterally on the splenius capitis. The average distance between the CSC and the medial border of the longus colli muscles at C6, however, is 10.6 ± 2.6 mm.4 Therefore the CSC is considerally more vulnerable to damage at lower levels due to its more medial location. While the longus colli diverge laterally when descending down the cervical spine, the CSCs converge medially at C6.4 The average diameter of the CSC at C6 is 2.7 ± 0.6 mm.4 Potential damage to the CSC may result during longus colli dissection off the anterior vertebral bodies or during lateral rectraction of the carotid sheath and/or longus colli.4 Fibers from the superior cervical ganglia pass to the internal carotid artery to innervate the pupil. Interruption of the sympathetic trunk in the neck results in an ipsilateral Horner syndrome.
The cervical musculature is divided into two layers: superficial and deep. The muscles of the superficial layer include the platysma, the sternocleidomastoid, and the infrahyoid group. The platysma lies just under the surface of the skin and is one of the muscles of facial expression, innervated by the cervical ramus of the seventh cranial nerve. It is draped like an apron from the mandible to the level of the second rib and laterally as far as the acromion processes. The sternocleidomastoid muscle arises from the region of the jugular notch and courses rostrolaterally to the mastoid process. It is dually innervated by the 11th cranial nerve and ventral branches of the C2-4 spinal nerves. The spinal accessory nerve enters the deep surface of the muscle at the border of the middle and rostral thirds. The two main actions of the sternocleidomastoid muscle are to turn the head to the contralateral side and to flex the head ipsilaterally. The infrahyoid group represents the rostral continuation of the rectus muscular system of the trunk.2 This group contains four muscles: sternohyoid, sternothyroid, omohyoid, and thyrohyoid. The first three members of this group are innervated by the ansa cervicalis, and the thyrohyoid receives its innervation from the C1 spinal nerve via the hypoglossal nerve. The main actions of the infrahyoid group are to assist in swallowing and mastication. This group, together with the suprahyoid group, determines the rostrocaudal location of the larynx between the hyoid bone and the rostral thoracic aperture and can help flex the cervical spine and lower the head.
The deep layer of cervical musculature includes two groups: scalene and longus groups. The scalene group includes three muscles: anterior, medius, and posterior. These muscles form a roof over the cupula of the lung. As a group, these muscles arise from the transverse processes of the subaxial cervical spine and project to the first and second ribs. The scalene muscles are innervated by the ventral rami of C4-8. They help to elevate the rib cage during respiration. The longus group also includes three muscles: rectus capitis anterior, longus capitis, and longus colli (Fig. 34-3). As a group, these muscles arise from the ventral vertebral body, transverse processes, and basilar portion of the occiput. They project caudally along the ventrolateral aspects of the cervical and upper thoracic vertebral bodies. These muscles are innervated by the ventral rami of C1-6, and their main action is to flex the head and the cervical spine.
The longus colli attach to the anterior atlas, the vertebral bodies of C3-T3, and the transverse processes of C3-6.6 The distance between the medial borders of the longus colli muscles increases in a rostral to caudal direction, measuring 7.9 ± 2.2 mm at C3, 10.1 ± 3.1 mm at C4, 12.3 ± 3.1 mm at C5, and 13.8 ± 2.2 mm at C6.6 A great deal of variation exists in this musculature, so care should be taken in using it as a landmark for lateral dissection.
The cervical viscera are arranged in three layers: a deep gastrointestinal layer, containing the pharynx and esophagus; a middle respiratory layer, containing the larynx and trachea; and a superficial endocrine layer, containing the thyroid and parathyroid glands. These structures are not covered in detail here, and much of the anatomy of the larynx and trachea has already been described in other sections of this chapter. As previously noted, these structures are contained within the visceral or pretracheal fascia.
The pharynx is a fibromuscular tube that projects from the pharyngeal tubercle of the clivus to its transition into the esophagus near the level of C6. The dorsal surface of the pharynx lies on the prevertebral fascia and must be mobilized during ventral approaches to the cervical spine. The muscles of the pharynx may be divided into two groups: constrictors and internal muscles of the pharynx. The constrictor group includes three muscles whose main action is to sequentially constrict the pharynx during swallowing, propelling food caudally. All of the constrictors are innervated by the pharyngeal plexus, which receives its branches from both the glossopharyngeal and vagus nerves. The constrictors do not form a continuous tube but are open at four points, allowing certain structures to pass into the pharynx. Rostral to the superior constrictor, the ascending palatine artery, the eustachian tube, and the levator veli palatini muscles pass to enter the pharynx. Between the superior and inferior constrictors pass the glossopharyngeal nerve, the stylohyoid ligament, and the stylopharyngeus muscle. In the gap between the middle and inferior constrictors pass the internal laryngeal nerve and the superior laryngeal artery and vein. Caudal to the inferior constrictor pass the recurrent laryngeal nerve and the inferior laryngeal artery. The internal muscle groups of the pharynx have a common function of elevating the larynx and pharynx during swallowing and a common innervation by the glossopharyngeal nerve. At the level of C6, the pharynx blends into the esophagus, which passes through the superior thoracic aperture to the stomach. In the root of the neck, the esophagus is in close approximation to the thoracic duct as it empties into the left subclavian vein.
The thoracic duct is located on the left side within a triangle bounded medially by the longus colli muscles and the esophagus, laterally by the anterior scalene muscle, and inferiorly by the first rib.7,8 Although it may ascend as high as C6, it is most often found between C7 and T1, before it descends to empty into a variable termination at the jugulosubclavian junction.7,8 The rostral extension of the thoracic duct appears to vary by gender, as in patients who have a narrow thoracic inlet, as most women do, the duct may ascend as high as the level of the C6 vertebral body. Conversely, in patients who have a wide thoracic inlet, as most men do, the duct may ascend to the level of the C7-T1 disc, never truly leaving the mediastinum. Many have cited the increased possibility of injuring this structure in the left upper thorax as a reason for preferring a right-sided approach, especially to the upper thoracic vertebrae.7
The isthmus of the thyroid gland usually overlies the first two or three tracheal rings. The isthmus is the center bridge of glandular tissue that connects the right and left lobes. The entire gland is surrounded by a fibrous capsule, which should be differentiated from the pretracheal fascia. The thyroid gland is heavily vascularized and receives its blood supply from the superior and inferior thyroid arteries, which are branches of the external carotid and thyrocervical arteries, respectively. The recurrent laryngeal nerve is in close approximation to the inferior thyroid artery, and if this artery must be ligated, it is best ligated at a distance from the thyroid gland to avoid the nerve. A similar relationship exists between the superior thyroid artery and the external laryngeal nerve, again dictating arterial ligation distal from the substance of the gland. The thyroid gland is drained by the superior, middle, and inferior thyroid veins. The inferior thyroid veins may cover the ventral surface of the trachea and represent a potential source of bleeding during tracheotomy.