Neurosurgery

compound movements in which there is spinal movement in more than one plane simultaneously. From a functional point of view, the cervical spine can be divided into two segments, based upon the different anatomic characteristics of each component. The atlantoaxial complex is unique in that its major articulation involves the odontoid process and transverse ligament complex. Although a minimal amount of flexion and lateral bending can occur across the atlantoaxial complex, the dominant movement is rotation. Fifty percent of cervical rotation takes place across C1 and C2.

Stability of the cervical spine motion segments is primarily dependent upon the liga-mentous structures and paraspinal musculatures. The facet joint capsules, anterior and posterior longitudinal ligaments and annulus limit the physiological range of motion of the cervical spine. The paraspinal musculature also contributes to stability, in addition to its obvious function of inducing spinal movement.

Degeneration of the cervical spine motion segments results from an accumulation of repeated movement, stress and strain on the osseoligamentous structures. Genetic and developmental factors may have an influence on the vulnerability of the cervical spine to degenerative change. Although all the mobile components of the cervical spine are susceptible to degenerative change, the subaxial cervical spine is most often (and severely) affected compared to the atlantoaxial complex. In particular, the lower cervical motion segments at C5-C6, C6-C7 and, to a lesser extent, C4-C5 typically incur maximal degenerative change. These segments are most vulnerable because they incur the widest degree of range of motion and the maximal amount of axial stress of the cervical spine motion segments. The precise pathogene-sis of cervical spondylosis is not clearly understood. Current concepts suggest that changes in the disk initiate the cascade of changes in cervical spondylosis. Degeneration of the facets and ligaments and reactive bony changes follow the primary diskogenic pathology.

Initial changes in the disk include loss of water content and changes in the relationship of the glucose aminoglycans and other polymers within the disk (Fig. 31.1). The volume of the disk is diminished and the biomechanical characteristics are altered. In a normal disk, the central component of the disk bears and transmits most of the weight across the motion segment. As the disk degenerates, the nature of the weight-bearing function of the disk changes and becomes more diffuse throughout the cross-sectional surface area of the disk. These changes put more stress and strain on the underlying peripheral endplates of the adjacent vertebrae. Loss of volume of the disk reduces tension on the annulus. The laxity of the annulus and the longitudinal ligaments and alterations of the disk change the character of movement across the interspace. Instead of pivoting across a stationary focal instantaneous axis of rotation, the axis of rotation becomes more widespread and its location varies, depending on the configuration of the motion segment.

The altered dynamics of motion across the disk are transmitted through the pedicles, to affect the relationship of the facet articulations posteriorly. The apposition of the facet joint surfaces is altered by the anterior changes and abnormal asymmetric stresses are placed upon the joint surfaces, which causes deterioration of the cartilaginous surfaces of the joints and promotes synovial tissue and bony reactive changes.

In response to the disk and facet changes, the ligaments and capsules which bridge the adjacent vertebrae are placed under abnormal stress and strain, and change in consistency and compliance. Traction spurs can result at the site of insertion of the ligaments, as another form of reaction to the biomechanical stress. Bony changes also result from the disk and facet joint changes. This takes the form of sclerosing of these bones and the accumulation of osteo-phytes and bone spurs.

In addition to the typical forms of spondylosis discussed above, two other unique forms of cervical degeneration have been observed: ossification of the posterior longitudinal ligament (OPLL) and diffuse idiopathic skeletal hypertrophy (DISH). Both of these disorders are considered forms of spondylosis, which take specific patterns. The reason for the characteristic manifestations of each is uncertain. Patients with OPLL have a preferential accumulation of calcium in the posterior longitudinal ligament (Fig. 31.2). The accumulation of calcium is not confined to the disk space, but can extend vertically for a variable distance along the posterior longitudinal ligament. The volume of calcification observed in this

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