The vertebral or spinal column is a flexible assembly of vertebrae stacked on top of each other extending from the skull to the pelvic bone which acts to support the axial skeleton and to protect the spinal cord and nerves. The vertebrae are anatomically organized into four generalized body regions identified as cervical, thoracic, lumbar, and sacral; the cervical region including the top of the spine beginning in the skull, the thoracic region spanning the torso, the lumbar region spanning the second back, and the sacral region including the base of the spine ending with connection to the pelvic bone. With the exception of the first two cervical vertebrae, cushion-like intervertebral discs separate adjacent vertebrae.
The stability of the vertebral column during compression and movement is maintained by the intervertebral discs. Each disc includes a gel-like center surrounded by a fibrous ring. The gel-like center, or nucleus pulposus, provides strength and shock absorption, whereby the disc can absorb and distribute external loads. The nucleus pulposus contains a mixture of type II collagen dispersed in a proteoglycan matrix. The fibrous ring, or annulus fibrosus, provides stability during motion, and contains laminated rings of type-I collagen. Thus, the annulus fibrosis and the nucleus pulposus are interdependent, as the annulus fibrosis keeps the nucleus pulposus in place, and the nucleus pulposus aligns the annulus fibrosus to accept/distribute external loads. The integrity of the composition and structure of the intervertebral disc is necessary to maintain normal functioning of the intervertebral disc.
Many factors can adversely alter the composition and structure of the intevertebral disc, such as normal physiological aging, mechanical injury or trauma, or disease, resulting in impairment or loss of disc function. For example, the content of proteoglycan in the nucleus pulposus declines with age, thus, it follows that the ability of the nucleus pulposus to absorb water concurrently declines. Therefore, in normal aging the disc progressively dehydrates, resulting in a decrease in disc height and possible de-lamination of the annulus fibrosus. Mechanical injury can tear the annulus fibrosis allowing the gel-like material of the nucleus pulposus to extrude into the spinal canal and compress neural elements. Growth of a spinal tumor can impinge upon the vertebrae or disc, potentially compressing nerves. Regardless of the cause, many of these disc pathologies become severe enough to require surgical intervention.
In most cases, surgical intervention is a partial or complete removal of the damaged intervertebral disc material, and is termed a discectomy. While the discectomy may eliminate the problems associated with the damaged disc, it creates a void in the intervertebral space that, if left empty, can destabilize and possibly collapse the vertebrae, with adverse consequences. A device may be placed within the intervertebral space to increase stability, and to reduce the possibilities, for example, of disc collapse or the displacement of intervertebral tissue.
The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such.