Cervical Motion and the Spinal Cord

Many studies have focused on the effect of neck motion on the spinal cord. These concepts become particularly important in the setting of cervical kyphosis as the spinal cord is stretched over the area of deformity. Cadaveric studies have demonstrated that neck motion may result in injury to the spinal cord when the tensile load on the spinal cord exceeds the strength of spinal cord fibers to bear this load. The mechanism of injury has been identified as loss of axonal integrity secondary to stretch-induced injury. This form of injury has significant consequences on the conduction of electrical signals through white matter. These conduction deficits are as follows¹: transient ionic imbalances from altered axonal membrane permeability, conduction loss from myelin damage, and irreversible conduction loss in the setting of profound damage to axons. Furthermore, ischemic damage may result from impingement of transverse arterioles caused by anterior compression of the spinal cord.² Cervical kyphosis can aggravate this compression, placing the patient at risk of significant spinal cord damage with the potential for serious functional consequences. Prevention of further neurological injury is of primary consideration in the management of patients with cervical kyphosis.

Degenerative Processes in the Cervical Spine

The evolutionary function of the cervical spine is to allow for support and orientation of the head. This places the head in an ideal position for use of all sensory modalities including vision, hearing, taste, and smell. The cervical spine also plays a crucial role in the protection of the spinal cord and nerve roots. In their review, Yoganandan and colleagues summarize how the various bones, muscles, tendons, ligaments, and connective tissue structures serve to maintain proper alignment throughout the various movements.³ However, each of the different tissues contributes differently because of its unique structure and composition. Though an in-depth review of the soft tissue biomechanics is beyond the scope of this discussion, there are certain factors which are pertinent in the diskussion of cervical kyphosis.

Ligamentous structures are composed of collagen fibers and serve to resist tensile or distractive forces at extremes of motion. Each of the ligaments of the cervical spine resists different external forces due to its particular orientation and the components to which it is attached. Resistance to distraction is most effective when the force is applied along the direction of the fibers. As an example, interspinous ligaments are crucial for resisting excessive movement during flexion. Likewise, the anterior longitudinal ligament is most important for resisting motion during extension of the neck. With age, the composition of ligaments changes to contain proportionately more elastin than collagen.⁴ As a result, the ligaments are not as resistant to distractive forces causing the spine to become less stable both at rest and during motion. With a kyphotic deformity, age and ligamentous laxity may contribute to progression of the deformity and cause abnormal stress on the posterior elements and disk spaces, leading to facet joint hypertrophy and osteophyte formation.

While ligaments serve to resist distractive forces, the structure of the intervertebral disk allows it to resist forces in several directions. The structure of the cervical disk is unique compared to the thoracic or lumbar spine, in that the annulus is not a circumferential structure.⁵ In the cervical spine, the annulus is thickest at the anterior portion of the disk and tapers laterally toward the uncovertebral joints. Posteriorly, there is a small remnant of the annulus in the midline position (Figure 26-2). The primary function of the intervertebral disk in the cervical spine is to resist compressive forces. Through the aging process, the normal physiology of the intervertebral disk changes such that the equilibrium of matrix synthesis and degradation is altered. Moreover, the structure of the matrix becomes increasingly disorganized, water is lost, and there is narrowing of the intervertebral disk.⁶ Progression of these degenerative changes is influenced by age-related changes to the vertebral body end plates which prohibit the passage of nutrients to the intervertebral disk. With the collapse of the disk comes concomitant loss of lordosis (Figure 26-3). Over time, these changes lead to wedging of the vertebral body and cervical kyphosis.

FIGURE 26-2 Cervical disk. Note the presence of the annulus fibrosus at the anterior rim and the median position posteriorly. The remainder of the cervical disk does not have an annulus fibrosus.

(Adapted from Bogduk N, Mercer S: Biomechanics of the cervical spine. Part 1: normal kinematics, Clin Biomech 15:633-648, 2000.)

FIGURE 26-3 Progression of cervical kyphosis occurs through axial loading and bending along a moment arm in the setting of degenerative disease.

(Adapted from Steinmetz MP, et al: Cervical deformity correction, Neurosurgery Suppl 60(1):S1-90-97, 2007.)

Clinical Case Example

A 45-year-old right-handed man with Klippel-Feil syndrome presented with complaints of neck pain since the age of 12. He had also developed concomitant pain and numbness in the left upper extremity over the course of several years, which was exacerbated by extension of his neck. On clinical examination, the patient was noted to have significantly reduced range of motion in his neck on flexion and extension. Head rotation was limited to 45 degrees on either side of midline. Extension of his neck provoked a feeling of numbness which radiated into the fourth and fifth digits of his left hand. On motor examination, the patient had normal strength in all major muscle groups of the upper and lower extremities bilaterally. Tone was increased in all four limbs. A Hoffman reflex was present in the right hand and plantar responses were down-going bilaterally. He was also noted to have increased reflexes in the upper and lower extremities. Finger-to-nose and heel-to-shin testing were both normal. MR imaging demonstrated segmental kyphosis from C2 to C6 with spinal cord compression secondary to degenerative changes in the cervical spine (Figure 26-1).

FIGURE 26-1 MR imaging of the cervical spine. A, T1W sagittal. B, T2W sagittal. C, T2W axial MR imaging demonstrating segmental kyphosis from C3-C6 with foraminal and canal stenosis.

Although muscles, ligaments, and intervertebral disks all play vital roles in maintaining proper alignment of the cervical spine, the importance of the bony structures cannot be ignored. The vertebral bodies are composed of cortical and cancellous bone, the latter being most responsible for resisting compressive forces.⁷ The vascularity of cancellous bone allows for alterations in bone composition consistent with systemic metabolic changes. Osteoporosis is one such degenerative disease which affects cancellous bone in the context of hormonal changes, lack of calcium and vitamin D, and reduced mobility. These factors may induce cellular changes to cancellous bone, causing osteoporosis and predisposing the patient to fractures of the vertebral bodies. Susceptibility to fractures is directly related to the structural changes that occur with osteoporosis.⁸ In normal bone, the trabeculae are organized in horizontal and vertical planes, which reinforce the strength of the bone. In osteoporotic bone, there is loss of horizontal trabeculae,⁸ which compromises the strength of bone and predisposes the patient to fractures in the setting of minor force loads. As degenerative changes in the ligaments and intervertebral disks progress, many of the forces are transferred to the vertebral bodies. With osteoporotic changes, the vertebral bodies are unable to provide the same strength against axial loading and wedging or fractures may result. Pathological fractures and wedging of the vertebral bodies are significant contributors to the development and progression of cervical kyphosis.

In summary, despite being subjected to the same physiological axial loads, the aged spine cannot withstand the same compressive or tensile forces as the juvenile spine. Numerous degenerative processes are at play that may contribute to the development of cervical deformity. Ligamentous laxity limits resistance to distractive forces and leads to abnormal forces on the vertebral bodies, intervertebral disks, and posterior elements. Collapse of the disk space causes axial loads to become concentrated on the anterior vertebral bodies, and concomitant changes to bone density result in vertebral wedging. The end result is cervical kyphosis accelerating further degenerative change through abnormal shear forces. Ultimately, alterations in the functional anatomy of the cervical spine lead to symptoms of axial neck pain, radiculopathy, and eventually myelopathy.

Cervical Kyphosis and Inflammatory Arthritides