Surface Anatomy and Skin

An understanding of the relationship of surface landmarks to anatomic structures in the neck is useful for localizing vertebral levels. The hyoid bone lies at the level of C3, the thyroid cartilage lies at the level of C4, and the cricoid cartilage lies opposite C6.¹ Gentle but firm palpation laterally allows inspection of the transverse processes. Superiorly, the transverse process of the atlas is most prominent and is found just anterior and inferior to the mastoid process. To help differentiate it from the skull, rotation of the head shows the atlas moving independently from the skull. The anterior tubercle of the transverse process of the sixth cervical vertebra, Chassaignac tubercle, is an important palpable landmark. Posteriorly, in the midline, the first bony prominence palpated inferior to the occiput is the spinous process of the second vertebra. The next palpable spinous processes are typically from the sixth and the seventh vertebrae, with the seventh being the most prominent.

Anteriorly in the cervical spine, every attempt should be made to place the surgical incision in line with the skin creases. These incisions heal more easily and with a less noticeable scar than incisions that cross these lines. Anteriorly in the lower neck, the skin lines are transverse, but superiorly near the mandible they tend to run obliquely. The skin in the anterior neck tends to be more mobile, soft, and well vascularized, whereas the skin in the back of the neck is thicker and less mobile. As a result, the common extensile longitudinal midline skin incision used posteriorly often creates more prominent scars that tend to spread because of tension from the trapezius muscle.

Osseous Anatomy and Bony Articulation

The cervical spine comprises the first seven vertebrae in the spinal column. The bony anatomy and articulations of the upper cervical spine (occiput-C1-C2) are unique and distinct from the remaining lower five cervical vertebrae (C3-7).

The atlas, or C1, is a ringlike structure lacking a body and a spinous process. It consists of two thick lateral masses plus an anterior and posterior arch. The longus colli muscle and anterior longitudinal ligament attach to the anterior tubercle of the atlas, whereas the posterior tubercle serves as the bony attachments for the rectus minor muscle and suboccipital membrane. The superior and inferior oblique muscles attach to the large transverse processes. The vertebral artery passes through the foramen transversarium located within the transverse process and courses posteriorly within a sulcus on the superior aspect of the posterior arch of the atlas. In 15% of the population, the sulcus for the vertebral artery can be completely covered by an anomalous ossification, which has been called the ponticulus posticus and may have surgical implications when identifying anatomic landmarks for bony fixation of C1.

The axis, or C2, is characterized by an odontoid process or dens that projects upward anteriorly, articulating with the posterior aspect of the anterior arch of the atlas as a synovial joint. At its narrowest portion, at the base of the dens, the coronal and sagittal plane diameters are 8 to 10 mm and 10 to 11 mm.^2,3 Posteriorly, the axis has a large lamina and a bifid spinous process, which serve as attachments for the rectus major and inferior oblique muscles. The zone between the lamina and the lateral mass of the axis is indistinct, and posteriorly the neural arch connects to the body by large pedicles that are 8 mm wide and 10 mm long.⁴ Lying directly anterolateral to the pedicle is the vertebral artery, which runs through the foramen transversarium. The pedicle of the axis projects 30 degrees medially and 20 degrees superiorly from a posterior-to-anterior direction.³

The bony articulations of the upper cervical spine (occiput-C1-C2) are unique and warrant special attention (Fig. 17–1). The atlanto-occipital articulation is a shallow ball-and-socket joint allowing for considerable motion mostly in flexion, extension, and lateral bending. The greatest degree of flexion and extension of any cervical articulation occurs at this level (25 degrees).⁵ Lateral displacement is minimized because the lateral wall of the cup-shaped articulation of the atlas is higher than the medial wall. The superior articular surface of the atlas projects cephalad and medially, articulating with the occipital condyle, which projects caudad and laterally. Conversely, the inferior articular surface of the atlas projects caudad and medially and articulates with the laterally projecting superior facet of the axis. As a result of this bony configuration, axial loads on the atlas tend to result in horizontal displacement of the lateral masses.⁶

FIGURE 17–1 Coronal cryomicrotome section of upper cervical spine. Note articulation between occiput (O) and atlas (A). Atlantoaxial joint is identified by white arrow.

The atlantoaxial articulation provides about 50% of rotatory motion of the cervical spine.^5,7 The transverse ligament, which spans across the arch of the atlas, holds the odontoid process against the anterior arch of the atlas, creating a pivot joint with a synovial membrane and capsular ligaments anteriorly and posteriorly to the dens. This transverse ligament is the principal stabilizing structure for the atlantoaxial articulation and averages 21.9 mm in length.⁸ The transverse ligament has superior and inferior extensions, which form the cruciform ligament of the atlas, connecting it to the anterior edge of the foramen magnum and posterior aspect of the C2 body. To allow more rotatory motion, the inferior facets of the atlas are flatter and more circular than the superior facets and face inferiorly to articulate with the axis.

The lower cervical vertebrae are morphologically similar and increase in dimension as they proceed inferiorly from C3 to C6, with C7 as the transitional vertebra into the thoracic spine. The vertebral bodies are small and oval, with the mediolateral diameter greater than the anteroposterior diameter.

The inferior surface of the vertebral body is convex in the coronal plane and concave in the sagittal plane, with the anterior lip occasionally overlapping the inferior vertebra.⁸ Conversely, the superior surface of the vertebral body is convex or straight in the sagittal plane and concave in the coronal plane, creating projections on either side of the lateral superior surface, called the uncus, or hook. These processes project upward and conform to small grooves in the inferolateral border of the cephalad vertebra, forming the uncovertebral joints, or joints of Luschka. The width and depth of the vertebral surfaces average 17 mm and 15 mm from C2 to C6 and increase to about 20 mm and 17 mm at C7. Vertebral heights on the posterior wall in the mid-sagittal plane range from 11 to 13 mm.⁹

The pedicles project posterolaterally from the vertebral body and join the lamina to form the vertebral arch. From C3 to C7, the angulation of the pedicles varies from 8 degrees below to 11 degrees above the transverse plane and decreases from 45 degrees to 30 degrees in relation to the sagittal plane.⁹ The width and height of the pedicles increase slightly in size from C3 to C7, and average diameters are 5 to 6 mm and 7 mm. The lateral wall of the pedicle is thinner than the medial wall and should be taken into consideration if attempts at pedicle fixation are considered in this region.^10–12

At the junction of the pedicle and lamina, the anterior tubercle of the transverse process projects laterally and is connected to the posterior tubercle by the costotransverse lamella (bar), creating the foramen transversarium. Passing through the foramen transversarium is the vertebral artery and venous system. The transverse processes increase significantly in size at C6 and C7. The C6 anterior tubercle, also known as the carotid tubercle or Chassaignac tubercle, is a prominent surgical landmark.

In the lower cervical spine, the neural foramina are bounded anteriorly by the uncinate process, the posterolateral aspect of the intervertebral disc, and the inferior portion of the vertebral body; posteriorly by the facet joint and superior articular process of the vertebral body below; and superiorly and inferiorly by adjacent pedicles. Vertebral notches located on the superior and inferior aspect of each pedicle contribute to the size of the neural foramina, which are 9 to 12 mm in height, 4 to 6 mm in width, and 4 to 6 mm in length and are aligned 45 degrees to the sagittal plane.^13,14 They can be visualized radiographically with oblique views, with the right neural foramina outlined on the left posterior oblique view and the left neural foramina outlined on the right posterior oblique view.

The spinal canal is triangular and at all levels in the cervical spine is significantly greater in the medial-to-lateral dimension than in the anterior-to-posterior dimension. The cross-sectional area of the spinal canal is largest at C2 and smallest at C7, with a sagittal diameter of about 23 mm at C1 and 20 mm at C2, decreasing to 17 to 18 mm at C3-6 and to 15 mm at C7.⁷ This is one reason that the passage of sublaminar wires is safer in the upper cervical spine than in the lower cervical spine.

The lateral mass, an important structure for posterior cervical plate-screw systems, forms at the junction of the lamina and the pedicle and gives rise to the superior and inferior articular processes. These processes project upward and downward and are angled approximately 45 degrees cephalad from the transverse plane and gradually assume a more vertical position as they descend into the thoracic region (Fig. 17–2). The articular process of the superior facet faces posteriorly, whereas the inferior facet of the upper vertebra faces anteriorly, and the facets oppose one another to form a zygapophyseal joint. The facet joints are true diarthrodial joints with articular cartilage and menisci surrounded by a fibrous capsule lined by a synovial membrane. The interfacet distances are relatively constant between levels, with individual variations ranging from 9 to 16 mm (average 13 mm).^4,15

FIGURE 17–2 Parasagittal cryomicrotome section of facet joints. Lateral mass of C7 is more elongated from superior to inferior and thinner from anterior to posterior. Facet joint angle is roughly 45 degrees from transverse plane and assumes more vertical position distally.

Posteriorly, the spinous processes project inferiorly and are bifid from C3 to C6; the C7 spinous process is large and not bifid and is often called the vertebra prominens. The junction between the spinous process and lamina, the spinolaminar line, is an important anatomic landmark during spinous process wiring. Inadvertent penetration of the wire anterior to this line may result in spinal cord impingement.

The cervicothoracic junction is a transition region, with C7 having similar anatomic characteristics at T1 and T2. The dimensions of the vertebral body and the sizes of the transverse processes and spinous processes are larger at C6 and C7. Additionally, dimensions of the spinal canal decrease at C6 and C7, representing a distinct transition to the thoracic region. The articulating facet joint between C7 and T1 resembles the thoracic facet joint, and the lateral mass of C7 is thinner than that of upper levels. Morphologic characteristics of pedicles of C7, T1, and T2 were obtained with respect to diameters, depths, and medial angulations. Inner diameters of the pedicles at C7, T1, and T2 from medial to lateral plane averaged 5.2 mm, 6.3 mm, and 5.5 mm. Medial angulations were 34 degrees, 30 degrees, and 26 degrees at C7, T1, and T2.^9,16 These morphologic characteristics should be remembered when performing transpedicular procedures in the cervicothoracic region.

Ligaments

In addition to the bony anatomy, the ligamentous attachments provide support to the cervical spine and associated articulations. In the atlanto-occipital complex, two membranous attachments, the anterior and posterior atlanto-occipital membranes, connect the anterior and posterior arch of C1 to the margins of the foramen magnum. The anterior atlanto-occipital membrane is the superior continuation of the anterior longitudinal ligament, whereas the posterior membrane is the superior continuation of the ligamentum flavum.

The transverse ligament is the major stabilizer of the atlantoaxial complex (Fig. 17–3). It attaches laterally to tubercles located on the posterior aspect of the anterior arch of C1, where it blends with the lateral mass. Secondary stabilizers include the thick alar ligament, which arises from the sides of the dens to the medial aspects of the condyles of the occipital bone, and the apical ligament, which arises from the apex of the dens to the anterior edge of the foramen magnum. In some individuals, an anterior atlantodental ligament exists connecting the base of the dens to the anterior arch of the atlas.¹⁷ The tectorial membrane, the superior continuation of the posterior longitudinal ligament, covers the dens and all the occipitoaxial ligaments and extends from the posterior body of C2 to the basilar portion of the occipital bone and the anterior aspect of the foramen magnum.

FIGURE 17–3 Midsagittal microtome section at upper cervical spine. Transverse ligament (T) acts as stabilizer of atlantoaxial joint by helping to restrain odontoid (O) from posterior translation. Spinal cord (S), ligamentum flavum (L), and posterior arch of atlas (A) are also identified.

The bodies of the lower cervical vertebrae (C3-7) are connected by two longitudinal ligaments and the intervertebral discs. The anterior longitudinal ligament is a strong band that attaches from the skull, as the anterior atlanto-occipital membrane, and continues caudad over the entire length of the spine down to the sacrum. The anterior longitudinal ligament is thinner and more closely attached at the intervertebral disc margins than at the anterior vertebral surfaces.¹⁸ The anterior longitudinal ligament also sweeps around and envelops the lateral aspect of the vertebral bodies under the longus collis muscle, and the lateral extension is continuous with the deep layer of the posterior longitudinal ligament in the region of the intervertebral foramina.

The posterior longitudinal ligament, lying within the vertebral canal on the posterior aspect of the vertebral body and intervertebral disc, is wider in the upper cervical spine than the lower cervical spine.¹⁸ Superiorly, it is continuous with the tectorial membrane, and as it descends it widens over the intervertebral discs and narrows behind each vertebral body. The posterior longitudinal ligament supplies additional strength and stability to the posteromedial fibers of the anulus. There is an area of relative weakness in the posterolateral corners of the disc, however, at the junction of the posterior longitudinal ligament and uncinate process; as a result, it is the site of most cervical disc herniations.¹⁹ According to Hayashi and colleagues,²⁰ the posterior longitudinal ligament is double-layered, and the deep layer sends fibers to the anulus fibrosus and continues laterally to the region of the intervertebral foramina. The superficial or more dorsal layer of the posterior longitudinal ligament is adjacent to the dura mater and continues as a connective tissue membrane, which envelops the dura mater, nerve roots, and vertebral artery, suggesting that this membrane may serve as a protective barrier.

The ligamentum flavum of the cervical spine attaches to the anterior surface of the lamina above and to the superior margin of the lamina below and extends laterally to the articular processes, contributing to the boundary of the intervertebral foramen. The ligamentum flavum consists primarily of elastic fibers, whose numbers lessen with aging, resulting in anterior buckling that can contribute to symptoms of spinal cord compression. A gap in the midline of the ligamentum flavum allows for the exit of veins.

The interspinous ligament of the cervical spine is thin and less well developed than in the lumbar region. It attaches in an oblique orientation from the posterosuperior aspect to the anteroinferior aspect of the spinous process. There is no separate supraspinous ligament in the cervical region. The ligamentum nuchae, a fibroelastic septum, is the superior continuation of the supraspinous ligament of the thoracolumbar spine and extends from the external occipital protuberance to C7.

Intervertebral Discs

Intervertebral discs are present between vertebrae except at the atlantoaxial level. Each intervertebral disc is an avascular structure that consists of the nucleus pulposus at the interior of the disc, the outer anulus fibrosus, and the cartilaginous endplates adjacent to the vertebral surfaces. The nucleus pulposus functions as a shock absorber, and the anulus fibrosus maintains the stability of the motion segment. With increasing age, the margin between the nucleus pulposus and anulus fibrosus becomes less distinct, and often by age 50 the nucleus pulposus has become a fibrocartilaginous mass similar to the inner zone of the anulus fibrosus.²¹

The anulus has an outer collagenous layer, in which the fibers are arranged in oblique layers of lamellae. The outermost fibers of the anulus fibrosus are contiguous with the anterior and posterior longitudinal ligaments and are firmly attached to the adjacent vertebral endplates. The fibers of the lamella run perpendicular to the fibers of the adjacent lamella. The collagen fibers in the posterior portion of the disc run more vertical than oblique, and this may account for the relative frequency of radial tears seen clinically. The discs are shaped to conform to the surface of the bodies; the superior surface of the disc is concave, and the inferior surface of the disc is correspondingly convex in the coronal plane. The discs are also slightly thicker anteriorly than posteriorly, which contributes to the lordotic posture of the cervical spine. The cervical intervertebral discs allow some translational movement in the sagittal plane, but the uncinate processes resist lateral movement. The uncinate process, located in the posterolateral aspect of the disc, also helps prevent disc herniations in this area. Degeneration of the anulus fibrosus (Fig. 17–4) in the cervical region is similar to the lumbar region in that concentric, transverse, and radial tears of the anulus occur, and the radial tear in the posterior aspect of the disc may be more clinically significant.

FIGURE 17–4 Mid-sagittal cryomicrotome section of degenerative cervical spine showing degeneration of anulus fibrosus and herniation of nucleus pulposus posteriorly with impingement of spinal cord.

The cartilaginous endplate is a layer of hyaline cartilage resting on the subchondral bone and serves as a barrier between the pressure of the nucleus pulposus and the adjacent vertebral bodies. This cartilage is a growth plate and responsible for endochondral ossification during growth (Fig. 17–5). The cartilaginous endplates also allow the insertion of the inner fibers of the anulus fibrosus and the diffusion of nutrients from the subchondral bone to the disc.

FIGURE 17–5 Mid-sagittal microtome section of cervical spine showing nucleus pulposus (n) and outer anulus fibrosus (a) of intervertebral disc. White arrows identify cartilaginous endplate.

Neural Elements

The cervical cord emerges from the foramen magnum as a continuation of the medulla oblongata. There is considerable variation in size of the spinal cord; however, in general, owing to the increased nerve supply to the upper limbs, the cervical cord enlarges from C3 and becomes maximal at C6. Maximal transverse diameters of 13 to 14 mm have been reported,²² with transverse areas ranging from 58.3 ± 6.7 mm² at C6²³ to 85.8 ± 7.2 mm² at C4-5.²⁴

The spinal cord includes the outer white matter and the inner gray matter. The white matter of the spinal cord contains nerve fibers and glia and is divided into the posterior, lateral, and anterior columns. The posterior column includes the fasciculus cuneatus laterally and fasciculus gracilis medially, mediating proprioceptive, vibratory, and tactile sensations. The lateral column contains the descending motor lateral corticospinal and lateral spinothalamic fasciculi, and the anterior funiculus contains the ascending anterior spinothalamic tract and other descending tracts. The lateral spinothalamic tracts cross through the ventral commissure to the contralateral side of the cord, conveying pain and temperature sensations. The anterior spinothalamic tract conveys the crude touch sensation.

The gray matter of the spinal cord contains cell bodies of efferent and internuncial neurons. The somatosensory neurons are located in the posterior horn, and the somatomotor neurons are found in the anterior horn of the gray matter. The visceral center of the gray matter is found in the intermediolateral horn. In the center of the spinal cord is the central ependymal canal for the passage of cerebrospinal fluid.

The spinal cord is covered by the pia mater, which is the outer lining of the cord, and transparent arachnoid membrane that contains the cerebrospinal fluid. The dura mater is the outer covering of the spinal cord and becomes the inner layer of the cranial dura at the level of the foramen magnum. The cervical cord is anchored to the dura by the dentate ligaments that project laterally from the lateral side of the cord to the arachnoid and dura at points midway between exiting spinal nerves. By suspending the spinal cord in the cerebrospinal fluid, the dentate ligaments cushion and protect the cord, while minimizing the movement of the cord during ranges of motion. The epidural space contains fat, internal vertebral venous plexus, and loose connective tissue. This venous plexus may be involved in spreading infection or neoplasm. There is a potential space between the dura and the arachnoid, and the subarachnoid space is between the arachnoid and the pia. The subarachnoid space contains the cerebrospinal fluid, spinal blood vessels, and nerve rootlets from the spinal cord.

The dorsal sensory rootlets enter the cord through the lateral longitudinal sulcus, and the ventral motor rootlets exit the cord through the ventral lateral sulcus. The six or eight rootlets at each level leave the spinal cord laterally to lie in the lateral subarachnoid space bathed in the cerebrospinal fluid. The rootlets join to form the dorsal and ventral root, which together enter a narrow sleeve of arachnoid and pass through the dura to become a nerve root at each level. The cervical nerve roots that form from the ventral and dorsal nerve rootlet extend anterolaterally at a 45-degree angle to the coronal plane and inferiorly at about 10 degrees to the axial plane.¹⁴ The nerve roots enter the intervertebral foramina by passing directly laterally from the spinal canal adjacent to the corresponding disc and over the top of the corresponding pedicle. The anterior root lies anteroinferiorly adjacent to the uncovertebral joint, and the posterior root is close to the superior articular process. The nerve root is positioned at the tip of the superior articular process in the medial aspect of the neural foramen, and it courses more inferiorly to position over the pedicle in the lateral aspect of the neural foramen (Fig. 17–6).

FIGURE 17–6 Parasagittal cryomicrotome section of lateral aspect of neural foramen shows nerve root coursing more inferiorly lying over pedicle as it begins to exit foramen.

The roots occupy about one third of the foraminal space in the normal spine but much more in the degenerative spine. The roots are located in the inferior half of the neural foramen normally, but the nerve roots occupy a more cranial part of the foramina, and the size of the foramen is diminished if the neck is fully extended.²⁵ The upper half of the neural foramen contains fat and small veins.²⁶ The nerve root is enlarged in the distal aspect of the intervertebral foramen, and the dorsal root ganglion is located just distal to the foramen.²⁷ The dorsal root ganglion is located between the vertebral artery and a small concavity in the superior articular process. Just distal to the ganglion and outside the intervertebral foramen, the anterior and posterior roots join to form the spinal nerve. The spinal nerve divides into dorsal and ventral primary rami branches.

The gray rami from the sympathetic cervical ganglion join the ventral primary rami. There are interconnections between gray rami, the perivascular plexus around the vertebral artery, and the sympathetic trunk, all of which give contributions to the ventral nerve plexus to innervate the anterior longitudinal ligament, outer anulus fibrosus, and anterior vertebral body.^28,29 The dorsal nerve plexus receives contributions from the sinuvertebral nerves, which originate from the gray rami and perivascular plexus of the vertebral artery. The dorsal nerve plexus innervates the posterior longitudinal ligament, and the sinuvertebral nerves give branches to the posterior part of the anulus and the ventral part of the dura. The sinuvertebral nerves innervate two or more discs or motion segments.

The first cervical nerve or suboccipital nerve exits the vertebral canal above the posterior arch of the atlas and posteromedial to the lateral mass and lies between the vertebral artery and the posterior arch. The posterior primary ramus of the first cervical nerve enters the suboccipital triangle and sends motor fibers to the deep muscles. The anterior primary ramus of the first cervical nerve forms a loop with the second anterior primary ramus and sends fibers to the hypoglossal nerve. The cervical plexus receives fibers from anterior primary rami of C1-4. The cervical plexus is located opposite C1-3, ventral and lateral to the levator scapulae and middle scalene muscles. The cervical plexus has distributions to the skin and muscles, such as rectus capitis anterior and lateralis, longus capitis and cervicis, levator scapulae, and middle scalene. The cervical plexus forms loops and branches to supply the sternocleidomastoid and trapezius muscles. It has communications with the hypoglossal nerve from C1 and C2 and leaves this trunk as the superior root of the ansa cervicalis, which is a nerve loop that is formed with the inferior root from C2 and C3.

The second cervical nerve lies on the lamina of the axis posterior to the lateral mass, and the posterior primary ramus or the greater occipital nerve pierces the trapezius about 2 cm below the external occipital protuberance and 2 to 4 cm from the midline. Trauma or irritation to any of the three terminal nerves (the greater and lesser occipital nerve and the greater auricular nerve) can produce pain, headache, or hyperesthesia in their dermal distribution over the occiput and around the ear.

Cutaneous branches of the posterior primary rami of C2-5 are consistently present in the skin of the nuchal region, and the largest cutaneous nerve in this region is the greater occipital nerve. The lesser occipital nerve is a branch from the anterior cervical plexus and runs upward and lateral to the greater occipital nerve. The posterior primary ramus of C3 or the third occipital nerve pierces the trapezius more inferiorly and about 1 cm medial from the midline. The cervical nerve exits over the pedicle that bears the same number except the C8 cervical nerve lies between the C7 and T1 vertebrae. The posterior primary rami of cervical nerves send motor fibers to the deep muscles and sensory fibers to the skin, but the first cervical nerve has no cutaneous branches. The anterior primary rami of C1-4 form the cervical plexus, and the rami of C5-T1 form the brachial plexus.

Vascular Structures

The major blood supply of the cervical cord and the cervical spine is the vertebral artery. Variations of the course of the vertebral artery have been reported.³⁰ In most cases, the vertebral artery originates from the first part of the subclavian artery and begins its ascent behind the common carotid artery between the longus colli and the anterior scalene. In the lower cervical spine, the vertebral arteries are crossed by the inferior thyroid artery and on the left by the thoracic duct. The vertebral arteries course anterior to the ventral rami of the seventh and eighth cervical nerves and the C7 transverse process before entering the C6 transverse foramen, where they ascend within the transverse foramen of C6-C2.

The surgeon should remember that the vertebral artery is located lateral to the uncinate process and in line with the middle one third of the vertebral body just anterior to the nerve root. During anterior exposure of the vertebral body and intervertebral discs, too far lateral dissection on the inferior half of the vertebral body and uncovertebral joints would endanger the vertebral artery and spinal nerve around the intervertebral foramen. The vertebral artery may also be involved in patients with severe cervical spondylosis when it may be impinged by the osteophyte. At the level of the atlas, the artery winds posteromedially around the lateral mass and over the posterior arch of the atlas before passing through the posterior atlanto-occipital membrane into the foramen magnum, joining the other vertebral artery to form the basilar artery.

In the foramen magnum region, the vertebral artery gives branches anteriorly that join together to form the single anterior spinal artery, whereas the paired posterior spinal arteries are branches from the posterior inferior cerebellar arteries. The anterior and posterior spinal arteries are the major blood supplies of the spinal cord. The posterior spinal arteries give rise to plexiform channels that are arranged transversely on the dorsum of the cord. The anterior spinal artery supplies most of the spinal cord except the posterior columns.³¹ The spinal cord also receives blood supplies from radicular arteries or medullary feeders from the vertebral arteries and ascending cervical arteries.³¹ The segmental arteries that are branches of the vertebral artery are present at each level to supply the vertebrae and surrounding tissues, but only a few segmental vessels give rise to radicular arteries or medullary feeders to the spinal cord. These vessels have a variable distribution, but medullary feeders are more commonly present at C6 and C3 from the left and C5 and T1 from the right.¹⁸

Venous blood returns from the cord through three veins posteriorly and three veins anteriorly. The venous system within the spinal canal consists of valveless sinuses in the epidural space. The venous plexus is most apparent anteriorly just medial to the pedicles over the mid-portion of the vertebral bodies and anastomoses with the veins from the opposite side and with the basivertebral sinus, which is located in the space between the posterior longitudinal ligament and the posterior aspect of the vertebral body.

Musculature

The musculature of the cervical spine can be grouped into the anterolateral and posterior muscle groups. The anterolateral muscles of the neck include platysma muscle, sternocleidomastoid muscle, hyoid muscles, strap muscles of the larynx, scalene muscles, longus colli muscle, and longus capitis muscle. The posterior musculature is subdivided into superficial, intermediate, and deep muscle groups.³²

The platysma is a thin muscle underneath the subcutaneous tissue that spans from the deltoid and upper pectoral fascia and crosses over the clavicle and passes obliquely upward and medially to insert to the mandible, muscles of the lip, and skin of the lower part of the face. The platysma depresses the lower jaw and the lip and tenses and ridges the skin of the neck.

The sternocleidomastoid originates from the sternum and the medial clavicle to the mastoid process and the lateral half of the superior nuchal line of the occipital bone. The second cervical nerve and the spinal accessory nerve innervate the sternocleidomastoid, which functions to draw the head toward the ipsilateral shoulder and rotate it and point the chin craniad toward the contralateral side. The sternocleidomastoid muscles together flex the head and raise the thorax when the head is fixed.

Muscles that attach to the hyoid bone include the digastric, stylohyoid, mylohyoid, geniohyoid, and omohyoid muscles; the strap muscles of the larynx include the sternohyoid and sternothyroid muscles. These muscles do not control the cervical spine but are important in controlling the movement of the hyoid and larynx and are important landmarks in the anterior approach to the cervical spine.

The longus colli and longus capitis are the prevertebral muscles of the neck. The longus colli spans from C1 to T3 and extends laterally to attach to the anterior tubercles of the transverse processes of C3-6. The longus capitis originates from the anterior tubercles of the transverse processes of C3-6 and attaches on the inferior surface of the basilar part of the occipital bone. Underneath the longus capitis, the rectus capitis anterior spans from the lateral mass of the atlas to the base of the occipital bone, and the rectus capitis lateralis runs laterally from the transverse process of the atlas to the inferior surface of the jugular process of the occipital bone.

The scalenus anterior originates from the anterior tubercles of the transverse processes of C3-6 and inserts on the first rib, and the scalenus medius originates from the posterior tubercles of the transverse processes of C2-7 and inserts on the first rib. A vascular impingement of the subclavian artery may occur as it runs between the scalenus anterior and scalenus medius as seen in the thoracic outlet syndrome. The scalenus posterior originates from the posterior tubercles of the transverse processes of C4-6 and inserts on the second rib.

The posterior muscles of the neck are divided into superficial, intermediate, and deep groups.³² The most superficial muscle is the trapezius, which originates from the external occipital protuberance and the medial nuchal line of C7-T12 spinous processes and inserts on the spine of the scapula, the acromion, and the lateral aspect of the clavicle. The trapezius is innervated by the 11th cranial nerve and functions to extend the head. The intermediate muscles beneath the trapezius muscle are the splenius capitis and splenius cervicis, which originate from the spinous processes of the lower cervical and upper thoracic spines and insert on the transverse processes of the upper cervical spine and the mastoid process. In the deep layer, the erector spinal muscles continue into the cervical region, which includes the iliocostalis laterally; the longissimus cervicis and longissimus capitis centrally; and the spinalis cervicis, semispinalis capitis, and semispinalis cervicis medially. Beneath the semispinalis muscles lie the multifidus from C4-7 and rotatores muscles, which cross only one segment from the transverse processes to the spinous processes.

In the upper cervical spine, suboccipital muscles attach at the occiput to the second vertebra. The rectus capitis posterior major originates from the C2 spinous process and inserts to the inferior nuchal line of the occiput, and the rectus capitis posterior minor originates from the posterior tubercle of the atlas and inserts to the occiput. The obliquus capitis inferior originates from the C2 spinous process and inserts on the transverse process of the atlas, and the obliquus capitis superior originates from the transverse process of the atlas and inserts on the occiput between the superior and inferior nuchal lines. Most posterior muscles are involved in producing extension of the neck and head, and some muscles produce rotation and lateral flexion. The posterior deep muscles are innervated by the posterior primary rami, and the blood supply is by the deep cervical vessels.

Fascial Layers

The key to understanding the anterior approach to the cervical spine lies in recognizing the fascial layers of the neck, which invest the muscles and viscera and separate them into different compartments.¹⁹ Anteriorly, the cervical fascia is divided into one superficial and four deep layers. The superficial fascia contains fat and areolar tissue, including the platysma muscle, external jugular vein, and cutaneous sensory nerves. The deep cervical fascia, including the outer investing layer of deep fascia, middle cervical fascia, and prevertebral fascia, compartmentalizes the structures deep to the superficial fascia. The superficial layer of the deep fascia extends from the trapezius muscle over the posterior triangle and splits to enclose the sternocleidomastoid muscle. The middle layers of the deep cervical fascia enclose the strap muscles and omohyoid and extend as far laterally as the scapula. The deeper middle layer is the visceral fascia that surrounds the thyroid gland, larynx, trachea, pharynx, and esophagus. The alar fascia spreads behind the esophagus and surrounds the carotid sheath structures laterally. The carotid sheath encloses the carotid artery, internal jugular vein, and vagus nerve. The deepest layer of the deep fascia is the prevertebral fascia, which covers the scaleni muscles, longus colli muscles, and anterior longitudinal ligament.

Understanding these fascial planes also helps localize the source of cervical infections. Abscesses originating from either the vertebral body or the intervertebral disc generally start in the midline, whereas abscesses that are pharyngeal in origin tend to occur lateral to the midline. This is because the prevertebral fascia and alar fascia are fused laterally over the transverse processes but not in the midline. If this infection breaks through the prevertebral fascia, it can spread inferiorly between the alar fascia and prevertebral fascia into the posterior mediastinum. With pharyngeal infections, the opposite occurs because the visceral fascia and alar fascia are fused in the midline; these abscesses tend to occur laterally, on either side of the midline.

Triangles of the Neck

The cervical region is divided into two anatomic compartments, the anterior and posterior triangles, by the sternocleidomastoid. The anterior triangle is formed by the midline anteriorly, the anterior border of the sternocleidomastoid posteriorly, and the inferior border of the mandible superiorly. The posterior triangle is bound anteriorly by the posterior border of the sternocleidomastoid, posteriorly by the anterior border of the trapezius, and inferiorly by the middle third of the clavicle. Understanding the structures within the triangles and their complex relationship helps the surgeon to learn these important landmarks during surgical approaches to the neck.⁵

The anterior triangle is subdivided further into the digastric (submandibular), carotid, and muscular triangles. The digastric triangle, so called because it is bound by the two bellies of the digastric muscle and inferior border of the mandible, contains the submandibular gland; facial artery and vein; mylohyoid artery and nerve; and, posteriorly, a portion of the parotid gland and external carotid artery. Lying deeper in the digastric triangle is the internal carotid artery, jugular vein, and glossopharyngeal and vagus nerves. The carotid and muscular triangles are separated by the superior belly of the omohyoid muscle. The carotid triangle contains the carotid artery and its bifurcation; the superior thyroid, lingual, and facial branches of the external carotid artery; and the internal jugular vein. It also contains the ansa cervicalis; portions of cranial nerves X, XI, and XII; and the larynx, pharynx, and superior laryngeal nerve.

The posterior triangle is subdivided into the occipital and supraclavicular triangles by the inferior belly of the omohyoid muscle. The posterior triangle contains the accessory nerve, the brachial plexus, the third part of the subclavian artery, the dorsal scapular nerve, the long thoracic nerve, the nerve to the subclavius, the suprascapular nerve, and the transverse cervical artery.¹⁸

The brachial plexus travels behind the inferior belly of the omohyoid, crossing between the anterior and middle scalene muscles and over the first rib and beneath the clavicle. Its location in the posterior triangle can be identified by drawing a line from the posterior margin of the sternocleidomastoid at the level of the cricoid cartilage to the midpoint of the clavicle. The accessory nerve lies on the levator scapula on the floor of the posterior triangle. Emerging from behind the posterior border of the sternocleidomastoid muscle are the lesser occipital, greater auricular, and supraclavicular nerves. The subclavian artery lies inferior to the inferior belly of the omohyoid in the subclavian triangle and courses behind the anterior scalene laterally toward the border of the first rib.

Anterior Approaches to Upper Cervical Spine

The complex anatomy of the upper neck makes adequate and safe exposure of the upper cervical spine challenging. The two main techniques are transoral and retropharyngeal exposures. If necessary, both techniques can be combined with a mandibulotomy or dislocation of the temporomandibular joint to gain additional local exposure.

The transoral approach provides anterior exposure to the atlantoaxial complex. Inferior exposure down to C3-4 can be obtained with the addition of a lip-splitting approach with mandibulotomy, whereas superior exposure up to the clivus of the occiput can be obtained by splitting the uvula, soft palate, and posterior pharyngeal wall.^33,34 If necessary, a portion of the hard palate can also be cut with a rongeur. Thoughtful placement of a self-retaining retractor system also facilitates exposure by retraction of the hard and soft palate and tongue.

Several variations to the retropharyngeal exposure have been described and can be divided into anteromedial and anterolateral approaches depending on the relationship of the dissection to the carotid sheath.^35–37 The anteromedial retropharyngeal approach uses the interval medial (anterior) to the carotid sheath, whereas the anterolateral approach uses the plane lateral (posterior) to the carotid sheath. In both cases, a thorough understanding of the local anatomy is imperative. For right-handed surgeons, the approach is typically from the patient’s right side. At this level, above C5 the recurrent laryngeal nerve has already crossed the surgical field from lateral to medial and runs safely within the tracheoesophageal groove.

Anteromedial Retropharyngeal Technique

Described by deAndrade and McNab in 1969³⁶ and later by McAfee and colleagues in 1987,³⁵ the anteromedial retropharyngeal approach is the superior extension of the anteromedial approach to the lower cervical spine as described by Southwick and Robinson.⁴² Similar to the anteromedial approach to the lower cervical spine, familiarity with the fascial planes is vital to understanding the approach. These planes include (1) the superficial fascia containing the platysma; (2) the superficial layer of the deep fascia extending from the sternocleidomastoid anteriorly and enclosing the trapezius posteriorly; (3) the middle layer of the deep fascia covering the strap muscles and omohyoid and visceral fascia surrounding the thyroid gland, larynx, trachea, pharynx, and esophagus; and (4) the deep layer of the cervical fascia, which includes the alar fascia connecting the two carotid sheaths laterally and fusing in the midline to the visceral fascia and the prevertebral fascia covering the scaleni and longus colli muscles and the anterior longitudinal ligament (Fig. 17–7).

FIGURE 17–7 Anteromedial approach to upper cervical spine. A, Dissection is done through retropharyngeal approach as extension of Southwick-Robinson approach to lower cervical spine. B, Longus colli muscle is retracted to expose anterior tubercle of atlas and body of axis.

A transverse submandibular incision is used in this approach, or, alternatively, an incision is made along the anterior aspect of the sternocleidomastoid muscle and curved toward the mastoid process (Fig. 17–8A). The platysma and the superficial layer of the deep cervical fascia are divided in line with the incision to expose the anterior border of the sternocleidomastoid (Fig. 17–8B). With the help of a nerve stimulator, the marginal mandibular branch of the facial nerve (cranial nerve VII) is isolated and protected. Because the branches of the mandibular nerve are superficial to the lateral crossing veins, ligating the retromandibular vein as it joins the internal jugular vein and keeping the dissection deep and inferior to the vein during the exposure help protect the superficial branch of the facial nerve.

FIGURE 17–8 A, Right-sided submandibular transverse incision. B, Anterior border of sternocleidomastoid muscle is mobilized, and digastric tendon is divided. Submandibular salivary gland and jugular digastric lymph nodes are resected. Hypoglossal nerve is identified and mobilized. C, Carotid sheath is opened, and arterial and venous branches are ligated. D, Superior laryngeal nerve is identified and protected.

Next, the superficial layer of the deep cervical fascia is incised anterior to the sternocleidomastoid, and the carotid sheath is identified by palpating for the carotid pulse. The digastric lymph nodes and salivary gland are resected, and the salivary duct is sutured to prevent a fistula. The stylohyoid and digastric muscle are identified and ligated to help mobilize the hyoid bone to improve exposure. Injury to the facial nerve may occur with excessive superior traction of the stylohyoid muscle. The nerve stimulator is used to identify and completely mobilize the hypoglossal nerve, which is retracted superiorly.

The retropharyngeal space is entered by using blunt dissection to develop the plane between the carotid sheath laterally and the visceral fascia containing the larynx and pharynx medially. Exposure can be improved by sequentially ligating tethering branches of the carotid artery and jugular vein, which may include the superior thyroid artery and vein, lingual artery and vein, ascending pharyngeal artery and vein, and facial artery and vein (Fig. 17–8C). The superior laryngeal nerve is identified, protected, and mobilized as it travels from its origin near the nodose ganglion into the larynx.

The prevertebral fascia overlying the vertebral body, intervertebral disc, and longus colli are now visible (Fig. 17–8D). The two longus colli converge in the midline on the anterior tubercle of the atlas. Because the hypoglossal, glossopharyngeal, vagus, and accessory nerves and the internal carotid artery and jugular vein are tethered to the occiput as they exit their respective foramina, they can be injured with vigorous retraction or greater than 2 cm lateral dissection from the midline. Additionally, excess anterior retraction of the pharynx can result in injury to the pharyngeal and laryngeal branches of the vagus nerve. At this point, a midline incision over the basiocciput, atlas, and axis can be performed, and the anterior longitudinal ligament and longus colli muscle can be dissected subperiosteally to obtain lateral exposure to the cervical spine.

Anterolateral Retropharyngeal Technique

Described by Whitesides and Kelly,⁴³ the anterolateral retropharyngeal approach provides exposure of the upper cervical spine by partially transecting the sternocleidomastoid and proceeding laterally and posterior to the carotid sheath (Fig. 17–9). As a result, the major branches of the external carotid and laryngeal nerves are not disturbed. Although this exposure allows for distal extension to include T1, its superior extension is limited to the ring of the atlas. Because the internal carotid artery; jugular vein; and vagus, accessory, and hypoglossal nerves are tethered to the skull, adequate retraction necessary to expose the basiocciput would result in injury to these structures.

FIGURE 17–9 Anterolateral retropharyngeal approach. A, Skin incision is made from mastoid along anterior aspect of sternocleidomastoid. B, This approach involves dissection anterior to sternocleidomastoid but posterior to carotid sheath. C, Neurovascular structures that are encountered in this approach include carotid contents and branches, superior laryngeal nerves, hypoglossal nerve, and ansa cervicalis.

A longitudinal skin incision is made from the mastoid extending distally and anteriorly along the anterior aspect of the sternocleidomastoid muscle. The external jugular vein is identified and ligated, and the greater auricular nerve running parallel to the external jugular vein is spared if possible. The sternocleidomastoid now is prominent; if only a limited exposure (C1-2) is required, consideration can be given to preserving the sternocleidomastoid. In most cases, the sternocleidomastoid and splenius capitis muscles are detached from the mastoid, leaving a fascial edge for later repair. The spinal accessory nerve enters the sternocleidomastoid approximately 3 cm distal to the mastoid tip and should be identified and protected.⁴³

Next, one can proceed laterally and posterior to the carotid sheath and dissect it free from the sternocleidomastoid. The carotid contents are retracted along with the hypoglossal nerve anteriorly and the sternocleidomastoid muscle and accessory nerve posteriorly. The plane between the alar and prevertebral fascia is developed with blunt dissection to expose the transverse processes and anterior aspect of C1-3. The most pronounced bony prominence laterally is the transverse process of C1. Although the basiocciput, clivus, and sphenoid may be palpated through this approach, they are poorly visualized.

When the appropriate level is identified, a midline longitudinal incision is made in the middle of the vertebral body, and the ligament and overlying muscles are dissected subperiosteally and laterally. Alternatively, if more lateral exposure is needed, the longus colli and capitis muscles can be separated from their bony insertion on the transverse process and retracted anteriorly. This provides direct exposure to the nerve roots, transverse processes, and vertebral artery but disturbs the sympathetic rami communicantes and may cause Horner syndrome.

Complications

Complications common to the anterolateral and the anteromedial retropharyngeal approaches include airway obstruction, hemorrhage, and nerve injury. Airway obstruction and difficulty swallowing secondary to hematoma or edema of the pharynx and larynx can be an immediate life-threatening complication. Typically, nasotracheal intubation is adequate; however, a tracheostomy can be considered either preoperatively or postoperatively for airway management if this complication is expected or encountered. Hemorrhage from the carotid artery, jugular vein, or their branches can occur and can be difficult to control.

Laryngeal and pharyngeal dysfunction can result from retraction of the laryngeal nerves. Patients should be advised preoperatively to expect difficulty with phonation and swallowing, especially in the early postoperative course. Problems can persist if the external branch of the superior laryngeal nerve is sacrificed or is transected. In three of five cases reported by deAndrade and McNab,³⁶ persistent postoperative hoarseness, laryngeal fatigue, and inability to produce high tones persisted. Nerve injury to the spinal accessory nerve can occur particularly with the anterolateral retropharyngeal approach. Care should be taken to identify and protect this nerve intraoperatively because weakness to the sternocleidomastoid and trapezius muscle can result.

Anterior Exposure of Lower Cervical Spine

Similar to approaches to the upper cervical spine, anterior exposures to the lower cervical spine can be divided into anterolateral and anteromedial approaches based on their relationship to the carotid sheath. First described by Southwick and Robinson,⁴² the anteromedial approach employs the interval between the sternocleidomastoid laterally and the strap muscles and tracheoesophageal complex medially and is used in most cases. In special circumstances, the anterolateral approach described by Henry⁴⁴ and Hogson⁴⁵ may be used. Hogson⁴⁵ described an approach to the lower cervical spine in which dissection was done posterior to the carotid sheath to expose the anterior and lateral aspects. Verbiest⁴⁶ described a modification of the original approach for the exposure of the vertebral artery. Dissection anterior to the carotid sheath, as in the anteromedial Smith-Robinson technique, provides more lateral exposure to the cervical spine and may be better in cases in which the lesion is localized more laterally or if the vertebral artery must be exposed. The spinal nerve can also be identified posterior to the vertebral artery (Fig. 17–10).

FIGURE 17–10 Verbiest’s approach. Sternocleidomastoid and carotid sheath are identified and retracted laterally, and visceral structures are retracted medially. Anterior tubercle of transverse process is identified by palpation. Muscular insertions of longus colli, longus capitis, and anterior scalene are dissected sharply to bone, and anterior tubercle is cleared of soft tissues. Costotransverse lamellae can be resected to provide exposure to vertebral artery and spinal nerve lying posteriorly.

To minimize injury to the recurrent laryngeal nerve, the cervical spine is often approached from the left, particularly at the C6-T1 region. Although a right-handed surgeon may prefer the right-sided approach, the recurrent laryngeal nerve is at greater risk of injury because it may leave the carotid sheath at a higher level on the right side. The hyoid bone overlies the third vertebra, the thyroid cartilage overlies the C4-5 intervertebral disc space, and the cricoid ring is at the C6 vertebra (Fig. 17–11).⁵ In many cases, when the neck is in a significantly extended position, these landmarks may be displaced inferior in relationship to the vertebral bodies, and moving the incision slightly higher can help accommodate for the shift. A horizontal incision is used in most cases, but a vertical incision anterior to the sternocleidomastoid may be necessary in cases in which multiple levels need to be exposed.

FIGURE 17–11 Surface anatomy can help identify approximate level of vertebral bodies in cervical spine. Hyoid bone overlies C3, thyroid cartilage overlies C5, cricoid ring is at C6, and supraclavicular level is in C7-T1 region.

Anterior Approach to Cervicothoracic Junction

Anterior approaches to the cervicothoracic junction are challenging because of the proximity of the great vessels and overlying sternum and clavicle (Fig. 17–12). Three main approaches have been described to address access in this region: the modified anterior approach, the sternal-splitting approach, and the transthoracic approach.^48–50 Each approach has its own advantages and disadvantages and should be chosen accordingly. Theoretically, the modified anterior approach can provide visualization and access to the anterior spinal structures from C4 to T4 but requires resection of the medial clavicle and sternoclavicular joint. Similarly, the sternal-splitting approach when combined with the anteromedial approach to the neck offers access from C4 to T4 through retraction of the great vessels. Although the transthoracic approach provides adequate exposure to the upper thoracic spine, access to the cervical spine is limited to C7 at best. This approach generally provides limited access to the cervical spine.

FIGURE 17–12 Anterior access to cervicothoracic junction is complicated by proximity of great vessels and associated neural structures.

Posterior Approaches

Posterior exposures to the cervical spine are among the safest and most used exposures for management of cervical spine disorders, allowing direct access to the posterior elements from the occiput to the thoracic spine.^5,51 The particular anatomy of the upper cervical spine and the transitional anatomy of the cervicothoracic junction should also be understood when approaching these regions posteriorly.

Posterior Approach to Lower Cervical Spine

A reverse Trendelenburg position minimizes venous bleeding and reduces cerebrospinal fluid pressure (Fig. 17–13). The posterior approach uses a longitudinal midline incision that extends above and below the segments required for the procedure. This extension of the skin and subcutaneous tissues is necessary because the skin of the posterior neck is less mobile and thicker for retraction. The skin is incised sharply, and electrocautery is used to incise the ligamentum nuchae in the midline. With a wide flat periosteal elevator such as a Cobb, the dissection is carried subperiosteally down the spinous processes. Inadvertent penetration of instruments into the spinal canal can be minimized by examining preoperative films for evidence of spina bifida and other bony defects and by realizing that, in the cervical spine, the laminae do not override each other as much as in the thoracic spine, resulting in wider interlaminar spaces. Care should be taken to stay subperiosteal because the bifid nature of the spinous processes may result in a bulbous expanse, and the dissection may err into the paraspinal musculature. A superficial plexus of veins may be encountered and should be cauterized as needed.

FIGURE 17–13 Standard prone positioning for posterior cervical procedures. Reverse Trendelenburg position minimizes venous bleeding and reduces cerebrospinal fluid pressure.

Subperiosteal dissection of muscles is performed to expose the spinous processes, lamina, lateral mass, and facet joints. The capsules of the facet joints should be left intact except for fusion cases. Extreme caution is needed during the exposure of the lamina and the interlaminar space to prevent dural tear and cerebrospinal fluid leakage. Care should be taken at the lateral edge of the joint because the nerve root and vertebral artery lie anterior to the spinolamellar membrane of the adjoining transverse processes. Vigorous decortication or stripping may damage the thin bone and subsequently the nerve root and vertebral artery. The segmental artery at the lateral edge of the facet joints may be cauterized as it exits between the transverse processes. Various retractors may be used to facilitate exposure. The capsule is a stabilizing structure that can be damaged easily during dissection and should be preserved except in cases of fusion. For fusion cases, one should expose only the levels to be fused because creeping fusion extension is common. Supplementation of the fusion with posterior lateral mass plating may obviate the need for a halo vest postoperatively.

First popularized by Roy-Camille and colleagues,⁶² placement of posterior cervical screws requires a thorough understanding of the lateral mass anatomy to minimize injury to associated neurovascular structures. Different entry points and screw orientations have been recommended. In the original description by Roy-Camille and colleagues,⁶² the entry point was the center of the lateral mass with the screw angled 10 degrees laterally (Fig. 17–14), whereas Magerl recommended the drilling angle to be 25 degrees laterally and 45 degrees superiorly. An and colleagues⁴ found that by orienting the screw 15 degrees cephalad and 30 degrees laterally with an entry point 1 mm medial to the anatomic center of the lateral mass, the facet joint and nerve root are avoided.

FIGURE 17–14 Roy-Camille technique for lateral mass screws. Entry point is at or near anatomic center of lateral mass and directed 10 degrees laterally.