End Plates

The vertebral body end plates are the concave surfaces of thick cortical bone adjacent to the fibrocartilaginous intervertebral disks and a thin layer of cartilage about 1 mm thick. The end plate is strongest and most dense peripherally. The cartilaginous end plates are the superior and inferior thinner surfaces of the intervertebral disk, and they are the transition components of the intervertebral disk and the end plates. The lamina cribrosa, composed of calcium, fuses the vertebral end plate with the cartilaginous end plate. This sievelike surface permits osmotic diffusion and provides for a pathway for nutrients to reach the disk.

Uncal Process

The uncal process is a rostral projection on either side of the vertebral body that gives it a rostrally concave shape in the coronal plane. It receives the rounded caudal aspect of the adjacent rostral vertebral body to form an uncovertebral joint, and it can sometimes overlap the next level by a third of the vertebral body height. The uncovertebral joints play a role in limiting lateral translation, and they contribute to the coupling of lateral bending and rotation of the spine. From a surgical perspective, the uncovertebral joints define the lateral borders for an anterior corpectomy or diskectomy, and they also aid in defining the midline during anterior cervical plate placement.

Transverse Process

The cervical transverse processes are unique in that they contain the transverse foramina from C1 to C6. The vertebral artery is transmitted through these foramina, which are formed from the lateral surface of the pedicle, the dorsal surface of the anterior tubercle, and the ventral surface of the posterior tubercle.

In addition, a prominent nerve root groove on the rostral surface carries the exiting nerve root of the corresponding level. An important relationship to bear in mind is that this groove is dorsal to the transverse foramen.

Neural Foramen

The neural, or intervertebral, foramen transmits the exiting cervical nerve roots. Unlike the atlantooccipital and atlantoaxial levels, which have partial foramina, the subaxial cervical spine has true foramina with four distinct walls. The pedicles form the rostral and caudal walls. The ventral wall is made of the vertebral body rostrally and the uncovertebral joint overlying the disk space caudally. The dorsal wall is made of the facet joint capsule. The nerve roots exit above the like-numbered pedicle via the nerve root groove of the transverse process in close proximity to the cervical disk and uncovertebral joint. As a result of the close proximity, a degenerated uncovertebral joint or facet joint can lead to stenosis of the intervertebral foramen and compression of the nerve root.

Anterior and Posterior Tubercles

The anterior tubercle arises from the rostral vertebral body and projects laterally. It serves as the origin of the anterior scalene, longus colli capitis, longus colli cervicalis, and ventral intertransversus muscles. The posterior tubercle is the origin of the splenius cervicalis, longissimus, levator scapulae, middle scalene, posterior scalene, and iliocostalis muscles. It arises from the midportion of the lateral mass and projects ventromedially to join the anterior tubercle.

Pedicles

The pedicles are the dorsolateral projections of the vertebral body. They connect the vertebral bodies with the lateral masses. As opposed to the thoracic and lumbar spines, they are short, small, and medially oriented in the subaxial cervical spine. As a result, lateral mass screws are typically used when instrumentation is required.

Another regional difference of these pedicles is that they arise midway between the rostral and caudal vertebral body, unlike the thoracic and lumbar regions. The sagittal pedicle height increases gradually with descending levels. The transverse pedicle width decreases from the cervical to midthoracic area.

Spinal Canal

The dorsal concavity of the vertebral body forms the ventral aspect of the spinal canal, which is triangular in shape in the axial view. The lateral borders are the medial pedicles, and the dorsal borders are the ventral lamina. In the subaxial cervical spine, its anterior-posterior (AP) diameter decreases with descending levels. At C3, it measures approximately 17 mm, whereas it measures 15 mm at C7.

Lateral Mass

The lateral mass is a cylindroid, flattened, short structure dorsolateral to the pedicle that is actually the pars interarticularis, with the superior and inferior articulating surfaces of the facet joint on either end. The dorsal transverse process is anterior, the pedicle is ventromedial, and the lamina is medial. The nerve root is in very close proximity at each level. The sagittal diameter can range from 12 to 18 mm. Lateral masses can be used for instrumentation given the diminutive size of the subaxial cervical pedicles. In general, the size and volume tends to decrease with descending levels down to C7, a transitional level at which the lateral mass is actually thinner, and the pedicle is wider here than at the levels above.

Facet Joint

The facet joint is a coronally oriented synovial joint protected by a thin capsule. In the sagittal plane, facet joints are oriented at approximately 45 degrees. The facet joints are supplied by the vertebral, ascending pharyngeal, deep transverse cervical, supreme intercostal, and occipital arteries; the dorsal branches of the spinal nerves innervate the facet joints.

Compared with the intermediate orientation of the thoracic spine and the sagittal orientation of the lumbar spine, the coronal orientation of the subaxial cervical facet joints accounts for the varying magnitudes of rotation of these regions, because they do not significantly limit spinal movement in any direction or in rotation, except in extension. The cervical spine has a wide range of motion in flexion, extension, lateral bending, and rotation. Fortunately, the vertebral bodies are able to equally resist axial loading and translation instability. Thus instability can be managed adequately by applying a posterior tension band, as long as the vertebral bodies are intact.

Facet joints do not substantially support axial compressive loads unless the spine is in extension. Quantitatively, facets and the facet joint capsules can absorb approximately a fifth of the total compressive loads applied to the lumbar spine segment. Flexion-extension is distributed throughout the cervical spine a total of 60 to 75 degrees, and sagittal translation is limited to 2 to 3 mm at all cervical spine levels. This is a function of the facets, disks, and ligaments; thus small increases in translation may be harmful.

Lateral bending is a prominent movement, however. Between C2 and C5, there are 10 to 12 degrees of lateral bending per level. At C7–T1, there are only 4 to 8 degrees. As with other spinal levels, lateral bending is coupled with other motions, such as axial rotation; this leads the spinous processes of lower levels to rotate in the opposite direction.

Lamina and Spinous Processes

The lamina are thin dorsomedial structures that encase the posterior spinal canal. They often overlap the adjacent level and are continuous with the spinous processes, which are often small and bifid in the subaxial cervical spine.

Disk Space

The cartilaginous end plates of the bordering vertebral bodies are the rostral and caudal boundaries of the disk space. The anterior and PLLs are the ventral and dorsal borders, respectively. The uncal process limits the disk space laterally.

Intervertebral Disk

The intervertebral disks extend from end plate to adjacent end plate. Each disk is composed of the nucleus pulposus centrally and is surrounded by the annulus fibrosus, which is composed of collagen and elastin fibers ( Fig. 13-3 ).

Intervertebral disk. A, The nucleus pulposus is central; annulus fibers are concentric and peripheral. B, The disk lies between adjacent vertebral bodies. Each layer of annulus fibers is oriented in the same fashion, but the orientation of the fibers in adjacent layers differs by 30 degrees.

Annulus Fibrosus

The annulus fibrosus is a peripheral rim that consists of an alternating layer of collagen fibers that pass obliquely from the vertebral body above and below, arranged in a helicoid manner. There are several layers, and each layer’s fibers are oriented in the same fashion; the orientation of the fibers in adjacent layers differs by 30 degrees.

Nucleus Pulposus

The nucleus pulposus is centrally located and is made of a soft, pulplike, highly elastic mucoprotein gel with a high water content. Regional geometric variations parallel the morphologic differences between the various regions of the spine; for instance, the cross section of the disk increases from C2 to T1. Any load resisted by the vertebral body is transferred to the adjacent caudal vertebral body through the intervertebral disk. The heterogeneity of the material properties of the vertebral bodies and the disk makes the mechanism of load transfer complex, and age-related changes add to this. The first component to fail in a functional spinal unit is the end plate, not the disk.

Spinal Cord and Nerve Roots

Dentate ligaments that arise from the pia and attach to the dura suspend the spinal cord. Eight cervical nerve roots exit via neural foramina immediately rostral to the corresponding pedicle, and the pedicle enlarges with descending levels, with a maximal cross section at C6. Nerve roots occupy approximately a third of the neural foramina and are covered by a venous plexus and epidural fat.

The spinal cord also participates with the vertebral column in configurational changes as a result of changes in body positioning. The primary effects occur at the level of distraction, with large initial displacements occurring with small force levels. This is followed with the cord and then stiffening with additional stretch or distraction, requiring higher load levels.

In flexion, the spinal cord elongates within the canal and decreases in the AP diameter. In extension, the cord shortens and increases in AP diameter. Tensile forces are least at the center of the cord, and shear forces are greatest toward the center. Irreversible damage can occur with 30% spinal cord compression.

Spinal Cord Blood Supply

The primary blood supply to the subaxial cervical spine is through the vertebral artery. Other contributors include the ascending pharyngeal, occipital, and deep cervical arteries. Two vertebral artery branches must be noted: the ventral branch is transmitted across the midportion of the lateral surface of the vertebral bodies below the transverse process and below the longus colli muscles, and it contributes to the blood supply of the ventral vertebral body through the accompanying ventral vertebral body arterial plexus; the dorsal branch enters the neural foramen and gives off three additional branches, the first of which is transmitted along with the nerve roots and supplies the spinal cord, anastomosing with the anterior and posterior spinal arteries. The second branch supplies the inner surface of the lamina and ligamentum flavum. The third branch contributes to the supply of the dorsal vertebral body through the accompanying dorsal vertebral body arterial plexus, which passes beneath the PLL.

The anterior spinal artery, two posterior spinal arteries, and the segmental medullary arteries supply the subaxial cervical spinal cord. The anterior spinal artery originates from the vertebral arteries, and the posterior spinal arteries originate either from the vertebral arteries or the posterior inferior cerebellar arteries.

The venous drainage of the spinal cord includes three anterior and three posterior veins. An anterior and posterior venous plexus surrounds the spinal cord. The anterior venous plexus is most pronounced medial to the pedicles.

The Batson plexus is the internal vertebral venous plexus that extends from the coccyx to the occiput. It consists of many small, valveless veins that run ventral and dorsal to the thecal sac and merge at the neural foramen; the Batson plexus then exits the spinal canal, along with the nerve roots, and flows into the external venous plexus. The external venous plexus is represented in the cervical region by the vertebral veins, which form a veil around the vertebral artery and subsequently anastomose with the condylar, mastoid, occipital, and posterior jugular veins.