Osseous Elements

The first and second vertebral segments attach to the occiput to form the craniocervical junction, which is a complex articular system permitting rotational and nutational movement. The first cervical segment, also known as C1 or the atlas, is composed of a ventral and dorsal arch, joined laterally by symmetrical lateral masses (Figs. 31-1 to 31-3). The superior and inferior articulating surfaces of the lateral mass are concave to enable articulation with the occipital condyles, superiorly, and the shoulders of the axis, inferiorly (Figs. 31-4 and 31-5). The ventral arch forms a short bridge between the lateral masses, with a dorsal articular surface forming a synovial joint with the odontoid process of C2 and an anterior tubercle ventrally to which the longus colli muscle attaches. The dorsomedial walls of the C1 lateral masses form a small tubercle that serves as the attachment for the transverse atlantoaxial ligament. The dorsal arch of the atlas typically forms a posterior tubercle in the midline that is a rudimentary equivalent to subaxial spinous processes. Caution should be exercised in dissection of this structure because the dorsal arch may be incomplete. The dorsal arch is round in cross-section in the midline and attenuates laterally to form a flatter surface where it attaches to the dorsal lateral masses. The superior surface of the lateral dorsal arch forms a groove known as the sulcus arteriosus on which the vertebral arteries run bilaterally; it is often thin or even dehiscent and great care must be taken when exposing the inferior surface of the C1 lateral mass. The lateral masses are bounded laterally by short transverse processes that are fenestrated by the transverse foramen, or foramen transversarium. As in the subaxial spine, the vertebral artery traverses the transverse foramen of C1 before turning 90 degrees medially to run along the superior surface of the dorsal arch, as described previously. This genu lies 1.5 to 2 cm lateral to midline¹ and dorsal dissection should seldom require exposure of this precarious region.

FIGURE 31-1 Atlas and axis osseous anatomy and landmarks.

FIGURE 31-2 Occipitocervical junction ligaments. The cruciate ligament, accessory atlanto-axial ligaments, and alar ligaments are shown.

FIGURE 31-3 Cross-section of atlantoaxial complex. The transverse atlantoaxial ligament inserts on tubercles along the posteromedial wall of the lateral masses. The synovial joint between C1 and C2 is visualized. Note foramen transversarium with traversing vertebral artery.

FIGURE 31-4 Coronal section through occipitocervical junction. Double arrowheads show inferior articular surface of C1 lateral mass on shoulders of C2 superior articular process. The odontoid process (O) and C1 lateral mass (L) are shown. Note section of transverse atlantoaxial ligament flanking the odontoid process; alar ligaments are also visible.

FIGURE 31-5 Parasagittal section showing articulations of occipital condyle (OC), C1, and C2. Note location of the vertebral artery (VA) above the posterior arch of C1 and the C2 nerve root (C2n) below.

The second vertebral segment, or axis (see Figs. 31-1 to 31-4), is a distinctive osseous structure. The odontoid process, a peglike rostral projection, forms a synovial joint with the atlas along its ventral border and allows the atlas to rotate on C2, with translational restriction created by a complex of ligamentous structures. The tip of the odontoid process serves as the attachment for the apical ligament, which connects C2 to the basion (the ventral lip of the foramen magnum). The tip is flanked by two bony prominences that serve as the attachments for the paired alar ligaments that span the divide between C2 and the occipital condyles. The transverse atlantoaxial ligament traverses the dorsal border of the odontoid process, which often carries a groove for this strong ligament. The neck of the odontoid process narrows to meet the vertebral body of C2 and is a common site for C2 fracture. The vertebral body culminates rostrally to form two bilaterally symmetrical “shoulders” that flank the odontoid process and articulate with the lateral masses of C1. A relatively long pars interarticularis spans the interval between superior and inferior articulating processes and is laterally bounded by the transverse process of C2. The disproportionate length of the C2 pars interarticularis has important clinical implications. Because the superior and inferior articulating processes of C2 are coronally offset, extension applies significant strain on the C2 pars interarticularis. Under forceful hyperextension, the pars interarticularis may fracture, giving rise to the mechanism and morphology of the so-called hangman’s fracture. As at other levels, this process is fenestrated by the transverse foramen, which lies immediately lateral to the C2 pedicle and serves as the conduit for the vertebral artery. Unlike at other levels, the C2 foramen is angulated 45 degrees laterally so that the vertebral artery is partially roofed by the superior articular process.² The inferior articular process forms an articulation with the C3 superior articular process and assumes the more typical orientation of subaxial lateral masses.

Ligamentous Structures

Although the articular surfaces of the occipitocervical junction are oriented to permit constrained flexibility, a complex of intervening ligamentous structures acts in concert to restrict excessive translation and rotation (see Figs. 31-2 to 31-4). Three ligaments span the divide between the odontoid process and the occiput. In the midline, the apical ligament spans the interval between the basion and the tip of the odontoid process. This ligament, also known as the middle odontoid ligament or suspensory ligament, is of unclear biomechanical significance because it has been described as absent in 20% of specimens.³ The alar ligaments are bilaterally symmetrical structures approximately 1 cm in length spanning from the dorsolateral odontoid tip to the medial occipital condyles. Each alar ligament restricts excessive rotation to the contralateral side and excessive lateral bending to the contralateral side.⁴

The cruciate, or cruciform, ligament is the most important ligamentous structure of the craniovertebral junction. It is composed of four limbs that unite over the dorsal odontoid process. The superior limb, or ascending band, inserts on the occiput, whereas the inferior limb, or descending band, inserts on the dorsal body of C2. The transverse atlantoaxial ligament forms the transverse limbs of this complex and attaches to bony tubercles on the medial borders of the C1 lateral masses. The transverse ligament is a strong, inelastic structure composed of primarily dense collagen with a ventrolateral transition to fibrocartilaginous tissue near its insertion on the C1 lateral masses. As a consequence, this lateral transition portion is the zone most susceptible to traumatic rupture, and the transverse atlantoaxial ligament has been demonstrated to rupture under loads of 400 to 1100 N.⁵ Rupture of this ligament may be identified on MRI, particularly on gradient-echo sequences,⁶ and is associated with atlantoaxial instability. On occasion, the insertion of this ligament will fracture off the medial wall of one or both lateral masses while the ligament itself remains intact. CT and MRI characterization of these disruption patterns has important implications for the management of traumatic atlantoaxial instability.

The anterior atlanto-occipital membrane is the rostral continuation of the anterior longitudinal ligament (ALL), extending from the atlantoaxial complex to the basion. The posterior atlanto-occipital membrane is the occipitocervical homologue to the ligamentum flavum. It extends from the posterior arch of C1 to the posterior rim of the foramen magnum, or opisthion.

Osseous Structures

Each vertebral segment can be divided into a ventral portion, the vertebral body, and a dorsal portion, the dorsal or vertebral arch. The cervical vertebral body is roughly cylindrical in geometry, although the anteroposterior diameter is typically smaller than the transverse diameter. Relative to the vertebral arch, the cervical vertebral bodies are smaller than the vertebral bodies of the thoracolumbar spine, likely because they bear significantly less load. The superior and inferior surfaces of the vertebral body serve as the superior and inferior end plates, respectively. The lateral edges of the superior end plate curve sharply upward to form the uncinate processes bilaterally, a unique feature of the cervical spine. These processes articulate with complementary bevels on the lateral surfaces of the adjacent inferior end plate. Although this articulation is referred to as the “uncovertebral joint,” it contains no synovial fluid and as a consequence is not a true joint⁷ (Figs. 31-6 and 31-7).

FIGURE 31-6 Oblique sagittal section along axis of pedicles (P). Note superior articular process (S) and its relationship to the neuroforamen. Progressive caudal degenerative changes include disc desiccation, uncovertebral hypertrophy, and ventral osteophyte formation.

FIGURE 31-7 Coronal section through midcervical spine. Note prominent uncinate process and uncovertebral joints (arrows).

The most distinctive feature of the cervical spine is the fenestration of the transverse processes that flank each vertebral body. The transverse process projects ventrolaterally with a deep groove along its superior surface; this serves to carry the cervical spinal nerves. The transverse process terminates laterally with two prominences—the anterior and posterior tubercles. The anterior tubercle serves as an attachment for the ventral cervical musculature and the posterior tubercle serves as an attachment for the dorsal cervical musculature. The transverse foramen, or foramen transversarium, fenestrates the transverse process and carries the vertebral arteries bilaterally (Figs. 31-8 to 31-10).

FIGURE 31-8 Subaxial cervical spine, sagittal (A) and axial (B) views. Note relative positions of transverse processes, which are fenestrated by the foramina transversaria, and adjacency to lateral masses and cervical nerve roots.

FIGURE 31-9 Coronal section through foramina transversaria showing vertebral arteries traversing adjacent foramina. Note proximity to uncovertebral joints and tortuosity caused by uncovertebral hypertrophy.

FIGURE 31-10 Axial section showing vertebral artery. Note proximity to exiting nerve root (double arrowheads) and uncovertebral joint (UVJ) in section.

Importantly, the cervical pedicle serves as the dorsomedial wall of the foramen transversarium, exposing the vertebral artery to hazard when pedicle screws breach laterally. The cervical pedicles connect the dorsal arch with the vertebral body and are angled medially between 38 and 48 degrees from the midsagittal plane.⁸ The pedicles of C3 through C7 range in outer diameter from 6.0 to 6.5 mm⁹ and in inner diameter from 2.7 to 3.1 mm.⁸ The pedicle is thinnest along its lateral wall. In aggregate, these features make safe placement of cervical pedicle screws a challenge. The medial wall is close to the thecal sac, the lateral wall is close to the vertebral artery, and the superior wall is close to the superjacent nerve root.

The superior and inferior articular processes of the cervical spine are oriented obliquely on sagittal projection, with complementary surfaces on adjacent segments. Together with the intervening bone, these articular processes combine to form lateral masses at each level that are parallelogram-shaped in sagittal cross-section. Adjacent lateral masses in the subaxial cervical spine are in close apposition so that in aggregate they form a flexible, cylindrical column of bone dorsolateral to the vertebral bodies. This pillar-like architecture affords axial load-bearing capacity to the dorsal vertebral arches. The oblique configuration of articular surfaces imparts a shingle effect to the lateral masses and allows for flexion and extension while restricting translation, affording osseous neuroprotection for the enveloped cervical spinal cord and nerve roots (see Fig. 31-6). The lateral mass serves as a common anchor point for instrumentation, but an awareness of adjacent neurovascular structures is essential for safe placement of lateral mass screws that avoid the vertebral artery and cervical nerve roots. A rostral screw trajectory is protective of the exiting nerve root, whereas a lateral trajectory protects the vertebral artery (Figs. 31-11 to 31-13).

FIGURE 31-11 Parasagittal section showing satisfactory lateral mass screw trajectories.

FIGURE 31-12 Parasagittal section showing unsatisfactory screw trajectory with violation of neural foramen, causing nerve root distortion. Failure to maintain a rostral trajectory jeopardizes the nerve root.

FIGURE 31-13 Sagittal section showing unsatisfactory screw trajectory with vertebral artery distortion. Failure to maintain a lateral trajectory jeopardizes the vertebral artery.

The dorsal arch is completed by bilateral laminae that unite in the midline to form a bifid spinous process rostrally (typically from C3 to C5) and a monofid spinous process caudally (typically from C6 and below). The laminae are narrow, with a thinner superior than inferior edge. The height of the lamina is 10 to 11 mm at C4, whereas the thickness of the lamina is about 2 mm at C5.¹⁰ The spinous processes act as insertions for the semispinalis cervicis muscle. In aggregate, the pedicles, lateral masses, laminae, and spinous processes form the dorsal vertebral arch, which circumscribes the spinal cord and affords neuroprotective function (see Fig. 31-8).

Discoligamentous Structures

The cervical disc space is created by the interval between adjacent superior and inferior vertebral end plates. The inferior end plate is typically concave, creating a ventral lip on the superjacent vertebral body and taller disc space at midbody. Together, the cervical discs constitute 20% of the total cervical height.¹¹ In a normal cervical spine the disc space height increases with consecutive levels from C3-4 to C7-T1 and the disc space depth increases from approximately 16 mm at C3 to 20 mm at C7.¹² These progressive dimensional changes must be factored into instrumentation selection for ventral cervical procedures.

The intervertebral disc is composed of two components: a central nucleus pulposus that is circumscribed in axial cross-section by the anulus fibrosus. The nucleus pulposus is an avascular structure composed of loose fibrous strands suspended in a proteoglycan gel. It has a high water content that can be readily evaluated on MRI and its hydration declines with normal aging, with 88% water content at birth declining to 64% by the seventh decade¹³ (see Fig. 31-6). The intervertebral disc receives nutrition by diffusion¹⁴ and creates an immune-isolated avascular space susceptible to infection. The anulus fibrosus is composed of concentric rings of obliquely oriented fibers spanning the disc space from one vertebral body to the other. These fibers insert on the epiphyseal ring of the end plate, where the anchoring fibers are called Sharpey fibers. These concentric fibrous rings confer multiaxial shear resistance, like the steel belt of an automobile tire (Fig. 31-14).

FIGURE 31-14 Sagittal section through intervertebral disc showing central nucleus pulposus and peripheral anulus fibrosus merging with adjacent anterior and posterior longitudinal ligaments.

The ALL is composed of interdigitating collagen fibers running longitudinally from the anterior tubercle of C1 along the ventral vertebral bodies to the sacrum. It is a broad-based, flat structure without clear lateral boundaries that attenuates laterally where it merges with paramedian, prevertebral connective tissue. The fibers of the ALL are lamellated, with the deepest fibers spanning only adjacent levels, its more superficial layers spanning two to three levels, and its most superficial layers spanning many levels. It serves to restrict hyperextension and excessive axial traction and is adherent to the underlying vertebral bodies and intervertebral discs. Along with the anulus fibrosus, the ALL is the predominant restricter to hyperextension. The posterior longitudinal ligament runs along the dorsal vertebral bodies from C2 to the sacrum. Its rostral extension, called the tectorial membrane, inserts at the basion, along the clivus of the occiput. Like the ALL, it is a flat band of fibrous collagen tissue, although it is composed of two layers. The ventral layer is adherent to the dorsal vertebral bodies and intervertebral discs and the dorsal layer is adjacent to the thecal sac. The epidural venous plexus is sandwiched between these two layers. The posterior longitudinal ligament is three to four times thicker in the cervical spine than in the thoracolumbar spine and serves to restrict hyperflexion and axial traction.¹⁵ It is believed to provide further resistance to disc herniation and neural compression (Fig. 31-15).

FIGURE 31-15 Midsagittal section showing varying degrees of disc disruption. Note three visible membranes oriented longitudinally dorsal to the vertebral bodies. From ventral to dorsal, these membranes are the two layers of the posterior longitudinal ligament (PLL; black arrow and white arrow) with intervening epidural venous plexus visible, and the ventral dura (double arrowheads). The caudalmost disc extrusion appears to have herniated through both layers of the PLL, whereas the rostral extrusions have not. The anterior longitudinal ligament is similarly visible in close apposition to the anterior vertebral bodies. The ligamentum flavum is seen spanning adjacent laminae (single arrowhead), and the interspinous and supraspinous ligaments are seen spanning adjacent spinous processes.

The dorsal elements are spanned by numerous ligamentous structures that permit constrained flexibility in the cervical spine. The ligamentum flavum is a two-layered structure that spans the interlaminar space between adjacent segments, originating on the ventral surface of the superjacent lamina approximately halfway up the lamina and inserting on the superior edge of the subjacent lamina.¹⁶ Importantly, it also inserts laterally on the medial edge of the superior articular process. The ligamentum flavum derives its name from its high elastin content, which confers a yellow appearance to it.¹³ The ligamentum flavum restricts hyperflexion; with aging, the contractile elasticity of the ligamentum flavum diminishes and hyperextension creates redundancy in this structure, which may narrow the anteroposterior diameter of the spinal canal, potentially contributing to spinal cord compression.

Intertransverse ligaments are short bands of fibrous tissue that bridge adjoining transverse processes. These ligaments serve to restrict lateral cervical bending. The interspinous ligaments are paired midline structures that bridge adjoining spinous processes. In the cervical spine the nuchal ligament represents the rostral extension of the supraspinous ligament. It spans the interval from the occipital protuberance rostrally to the spinous process of C7 caudally. It is a thick, elastic fibrous band that serves to resist hyperflexion. These ligaments are predominant elements of the so-called posterior tension band (Fig. 31-16; see also Fig. 31-15).

FIGURE 31-16 Axial section through a herniated intervertebral disc, seen distorting the left side of the cervical spinal cord. Note one right-sided ventral rootlet (single arrowhead) and its proximity to midline. Central and paracentral disc herniations may cause ventral rootlet symptoms. Right-sided foraminal stenosis is seen as a consequence of uncovertebral hypertrophy (arrow). Note ligamentum flavum attachment to superior articular process of subjacent level (double arrowhead). Circumferential spinal cord demarcations include ventral median fissure, anterolateral and posterolateral sulci, and posterior median sulcus. Note gross distinction between central gray matter and peripheral white matter.

Spinal Cord Perfusion

The spinal cord is perfused by three principal arteries: the anterior spinal artery (ASA) and the paired posterior spinal arteries (PSAs; Fig. 31-19). None of these vessels represent single vessels that traverse the length of the spinal cord; rather, each represents a complex of anastomotic, interrupted, longitudinal vessels that together run the length of the cord.²² These interrupted vessels draw supply from numerous segmental vessels along the length of the spine.^23,24 Moreover, the intrinsic perfusion of the spinal cord parenchyma is achieved by two independent vascular systems: the centripetal system runs along the perimeter of the spinal cord in a perimedullary plexus that gives rise to arteries that penetrate radially into the spinal cord parenchyma; the centrifugal system originates deep within the anterior median fissure from branches of the ASA.²⁵ In general, the centripetal system perfuses the white matter of the spinal cord and the centrifugal system perfuses the gray matter.

FIGURE 31-19 Arterial supply of cervical spine and spinal cord. A, General spinal arterial vasculature (left, anterior view; right, posterior view). B, Arteries of cervical cord exposed from the rear (top); arteries of spinal cord diagrammatically shown in horizontal section (bottom).

(Frank Netter illustrations from www.netterimages.com. © Elsevier, Inc. All rights reserved.)

The ASA originates from descending branches of bilateral intracranial vertebral arteries and enters the midline ventral median fissure. It supplies the ventral two thirds of the spinal cord; ASA infarct typically causes dysfunction of the anterior columns, spinothalamic tracts, and corticospinal tracts. The PSAs arise from the intracranial vertebral arteries or posterior inferior cerebellar arteries.¹¹ The paired PSAs run in the dorsolateral sulci and form a peripheral anastomotic plexus perfusing the dorsal one third of the spinal cord.

The cervical segmental vessels are paired and arise from the vertebral artery and branches of the subclavian arteries. They provide perfusion to the spinal roots and dorsal root ganglia before entering the intervertebral foramen and giving rise to three branches: the dural, radicular, and medullary branches. The dural branch perfuses the spinal dura and nerve root sleeve, the radicular branch penetrates the dura and perfuses the ventral and dorsal nerve roots, and the medullary branch anastomoses with the ASA within one or two levels of its origin.

Vertebral Artery Anatomy

The vertebral arteries are paired vessels that lie close to the cervical spine. The vertebral artery arises from the subclavian artery, from the innominate artery, or directly from the aorta. The course of the vertebral artery is divided into four segments. The V1 segment represents the segment from origin until the artery enters its first foramen transversarium. In 87.5% of cases, this is at the C6 level; in 5.4% it is at C7; in 6.6% it is at C5, and in 0.4% it is at C4.²⁶ The V2 segment is the portion that traverses the foramina transversaria and terminates after the C2 foramen transversarium. This is the most vulnerable segment of the vertebral artery in most common cervical spine surgeries. The V2 segment may have a tortuous course, and awareness of these anomalies must be achieved radiographically before performing cervical surgery (Fig. 31-20). Moreover, the foramen transversarium is closer to the uncovertebral joint at more rostral levels, warranting more cautious uncovertebral drilling. The V3 segment continues until the vertebral artery penetrates the dura. The V4 segment is the intradural portion of the vessel, ending at the vertebrobasilar junction (Fig. 31-21).

FIGURE 31-20 Axial CT image showing anomalous foramina transversaria. The right-sided vertebral artery is especially vulnerable to iatrogenic injury if this anomaly is not appreciated preoperatively.

(Reprinted with permission from Felton DL, Shetty AN, editors: Netter’s atlas of neuroscience, ed 2, Philadelphia, 2009, Saunders. All rights reserved.)

FIGURE 31-21 Vertebral artery anatomy. BA, basilar artery; PICA, posterior inferior cerebellar artery; SA, subclavian artery.

(Used with permission from Dickman CA, Fehlings MG, Gokaslan ZL, editors: Spinal cord and spinal column tumors, New York, 2006, Thieme.)

Numerous muscular and osseous branches arise from the vertebral artery at each level. At each level in the subaxial spine, anterior and posterior central arteries contribute to an epidural plexus that provides perfusion to the vertebral bodies. Anterior and posterior ascending arteries arise from the vertebral arteries at the C2 level and anastomose to perfuse the atlantoaxial complex. Collateral arterial supply is recruited from the thyrocervical and costocervical trunks in the lower cervical spine and the ascending pharyngeal and occipital arteries in the upper cervical spine. The odontoid process is perfused by an arcade of arteries, with the base of the odontoid supplied by the ascending branches of the vertebral artery and the tip supplied by the apical artery of the odontoid process, a branch of the hypoglossal artery.²⁷