Degenerative Conditions of the Cervical and Thoracolumbar Spine

Cause and Epidemiology

Axial neck pain affects a significant portion of the US population. Its incidence increases with age, and multiple studies have shown a linear progression of its prevalence between the ages of 20 and 60. By the age of 65, as many as 95% of US adults suffer from some form of neck pain; in up to 10% of patients, the neck pain is severe.

Numerous biomechanical and biochemical factors appear to contribute to the complex interplay that results in the sensation of axial neck pain. Examples include cumulative trauma, repetitive stress, and repetitive injury. Radiographic evidence of degenerative cervical spinal disease appears to contribute to neck pain, but the presence of such degeneration on imaging does not necessarily guarantee that a given patient will complain of neck pain.

Cervical Motion Segment

The cervical motion segment is the basic functional unit of the cervical spine. It is composed of two adjacent cervical vertebral bodies and the intervertebral disc between them. Several soft tissue structures stabilize the cervical motion segment, including the anterior longitudinal ligament (ALL), posterior longitudinal ligament (PLL), interspinous ligament, ligamentum flavum, and the synovial capsules of the facet joints that flank the posterolateral aspect of the spinal canal bilaterally.

The ALL is a strong band of the ligamentous tissue that runs along the anterior aspect of the vertebral bodies and intervertebral discs from the base of the skull to the sacrum. It functions as a primary stabilizer of the spine. The PLL runs from the body of the C2 vertebra to the posterior surface of the sacrum. Its fibers coalesce with the ligamentum flavum anteriorly and the supraspinous ligament posteriorly. The multiple cervical motion segments work together to allow complex movement.

Intervertebral Disks

The intervertebral discs are crucial to the normal function of the spinal unit. They are composed of a soft central nucleus pulposus, a tough, fibrous circumferential annulus fibrosus, and the two adjacent vertebral end plates on the proximal and distal aspects of the structure. Thus the disc unit provides a cushion between the two vertebrae it separates and facilitates motion between them. There are 6 cervical, 12 thoracic, and 5 lumbar intervertebral discs. The nucleus pulposus is composed of randomly oriented collagen fibrils and the proteoglycans that they bind. These molecules are hydrophilic, and the structure is composed primarily of water. Deforming forces on the structure cause it to disperse outward in various directions. The annulus fibrosus is a tough, fibrous, circumferential structure. It is composed of layered collagen fibers organized in alternating directions that buffer the spinal column from complex multidirectional motions. The annulus prevents extrusion of the nucleus from between the vertebral end plates when compressive forces are applied. When prevention fails, such extrusion leads to the clinical phenomenon of a herniated intervertebral disc. The vertebral end plates are composed of hyaline cartilage and form a physical buffer between the intervertebral discs and vertebral bodies in the spinal column. Together the components of the intervertebral disc allow a complex host of movements, including axial compression, distraction, axial rotation, and lateral flexion.

Facets

The spinal facet joints, otherwise known as apophyseal or zygapophyseal joints, are formed by the articulation between the superior articular facet of the inferior vertebra with the inferior articular facet of the superior vertebra; they are surrounded by robust synovial capsules. Each spinal motion segment contains two facet joints located between the pedicle and lamina of each respective vertebrae. With their corresponding intervertebral discs, the facet joints serve a crucial role in the posterior and rotatory stability of the spinal column. As dictated by the three-column model of spinal stability, the spine is composed of three interrelated columns that form the overall structure. The anterior column is composed of the ALL and anterior two-thirds of the intervertebral discs. The middle column is composed of the posterior third of the vertebral body and disc and the PLL. The posterior column is composed of all structures posterior to the PLL. Facet joint orientation gradually transitions from coronal to sagittal alignment with progression towards the lumbar spine. Cervical facet joints are oriented at 45-degree angles in relation to the coronal plane and allow flexion, extension, lateral flexion, and rotation. Thoracic facet joints are situated at 60 degrees from the coronal plane and allow only lateral flexion and rotation. Lumbar facet joints are situated perpendicular to the coronal plane and thus allow only flexion and extension.