The intervertebral disk is a complex structure that has been the focus of much attention in clinical practice. The prevalence of low back and neck pain, which are thought to be associated with degenerative changes in the disk, represents a major epidemiologic problem. In the United States, back pain is the second leading symptom that prompts visits to physicians. As many as 80% of adults in the United States experience at least one episode of low back pain during their lifetime, and 5% experience chronic problems.  An understanding of degenerative disk disease is important for managing these patients. (See the images below.)
The spine is composed of seven cervical vertebrae, 12 thoracic vertebrae, five lumbar vertebrae, and a fused set of sacral and vestigial coccygeal vertebrae. Spine stability is the result of three columns in one, as described by Dennis. Fracture or loss of two columns results in instability.
The anterior column consists of the anterior longitudinal ligament and the anterior portion of the vertebral body. The middle column consists of the posterior wall of the vertebral body and the posterior longitudinal ligament. The posterior column is formed by the posterior bony arch; this consists of transverse processes, facets, laminae, and spinous processes.
Intervertebral disks form one quarter of the total length of the spinal column. Each vertebra has the potential for 6° of freedom, translation in all thre axes of movement, and rotation around each axis. Not all vertebrae are created equal; the cervical vertebrae have the greatest freedom of flexion, extension, lateral rotation, and lateral flexion. This is because they are larger, they have concave lower and convex upper vertebral body surfaces, and they have transversely aligned facet joints.
Thoracic vertebrae have restricted flexion, extension, and rotation but freer lateral flexion because they are attached to the rib cage, are smaller, have flatter vertebral surfaces, have frontally aligned facet joints, and have larger overlapped spinous processes. The lumbar spine has good flexion and extension and free lateral flexion because its disks are large, the spinous processes are posteriorly directed, and the facet joints are sagittally directed. Lateral lumbar rotation is limited because of facet alignment.
The sensory of intervertebral discs is complex and varies according to their location within the spinal column. In the cervical spine, studies by Bogduk  and Mendel  demonstrated the presence of both nerve fibers and mechanoreceptors within the anulus fibrosus. Impulses from these structures are transmitted via the sinuvertebral nerves and branches of the vertebral nerves. A another study by Bogduk  found that the sensory innervation of the lumbar intervertebral disks, like that of the cervical disks, is derived from the sinuvertebral nerves but also from branches of the ventral primary rami and rami communicantes.
Of all connective tissues, the intervertebral disk undergoes the most serious age-related changes. By the third decade of life, the nucleus pulposus becomes replaced with fibrocartilage, and the distinction between the nucleus and the annulus becomes blurred. The proteoglycan, water, and noncollagenous protein concentrations decrease, while the collagen concentration increases. The increase in collagen concentration is more pronounced in the nucleus and in the posterior quadrants of the disk. It is more pronounced with age and moving caudally in the lumbar spine (similar to the Wolff law).
Biochemically, aging increases the ratio of keratin sulfate to chondroitin sulfate, and it also changes the proportion of chondroitin-4-sulfate to chondroitin-6-sulfate, with a parallel decrease in water content. Proteoglycan synthesis decreases, which decreases the osmotic swelling and the traffic of oxygen and nutrients to the disk. Because of this decreased traffic, breakdown products of link and noncollagenous proteins stagnate in the disk. Nonenzymatic glycosylation of these breakdown products accounts for the brown discoloration of the aging connective tissues.
Differentiating aging from degeneration is difficult. According to Pearce et al, "Aging and degeneration may represent successive stages within a single process that occurs in all individuals but at markedly different rates."  Aging and degeneration have in common decreased water and proteoglycan content in the disks, combined with increased collagen.
Whereas sagittal alignment, facet joint arthritis, and genetics potentially play a role in intervertebral disk degeneration, the results of one study suggest that the rate of degeneration may be associated with age. Those of African ethnicity also showed a faster rate of degeneration when compared with whites; sex did not show a significant effect on degeneration. 
One study demonstrated that the presence of juvenile disk degeneration was strongly associated with overweight and obesity, low back pain, increased low back pain intensity, and diminished physical and social functioning. An elevated body mass index was significantly associated with increased severity of disk degeneration. 
Another study found metabolic syndrome to be four times more prevalent in patients with radiographic evidence of severe degenerative disk disease as defined by degenerative spondylolisthesis or cervical or lumbar stenosis causing neurologic symptoms  .
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