Cervical disc deformation during in vivo functional movement

INTRODUCTION The cervical spine is comprised of seven vertebrae (C1-C7) and the corresponding intervertebral discs (IVD) that exist between two vertebral bodies. IVD act as a cushion to prevent the collision of adjacent bones or the compression of critical spinal nerves. Each intervertebral disc consists of two regions: the nucleus pulpous and annulus fibrosus. The nucleus is an incompressible, hydrostatic region in the center of each IVD. Surrounding the nucleus, the annulus, which consists of collagen fibers called lamellae, constrains the shape of the IVD under loaded conditions. As an individual ages or the spine supports excessive loads, degenerative processes ensue due to abnormal forces experienced by the IVD and surrounding vertebral bodies. Stokes et al. confirmed that abnormal loading is one of the primary causes of disc degeneration. Disc degeneration is not a process that can be easily halted. The mechanism by which disc degeneration occurs is a compounding series of events that ultimately lead to deterioration. Degeneration begins in the nucleus, and the effects spread to the annulus of the disc. As a result of a conglomerate of factors, including age, abnormal or excessive loading, or injury, the nucleus begins to lose water content, rendering it incapable of supporting the loads that it once did. Therefore, additional forces are now spread to the annulus. The fibers of the annulus must compensate for the dysfunction of the nucleus. The additional forces in the annulus fibers can cause the disc to degenerate, resulting in collisions of two vertebrae or stress and compression of spinal nerves; both cause acute pain. Individuals experiencing severe cervical disc degeneration undergo spinal fusion to alleviate pain. Spinal fusion consists of removing the IVD of the compromised motion segment, inserting a bone chip into the former IVD space, and fixing the vertebrae with a metal plate to facilitate the union of the two bones. Disc degeneration is the primary reason individuals undergo spinal fusion. Furthermore, a study completed in 2008 found that 413,171 individuals undergo spinal fusion annually, and of those cases, cervical disc degeneration was the second leading cause for surgery. Yet, with this high surgical rate, a method to detect in vivo disc degeneration using kinematic analysis does not exist. In vivo models of cervical IVD and the effects of dynamic motion on the disc are limited. Cadaveric studies compiled stress profiles of IVD in static flexed and extended positions under uniform loads; however this is not representative of the angles and forces experienced within the disc during functional motion. Therefore, the study presented will use motion capture technology to analyze in vivo IVD deformation during functional motion, a parameter that cannot be measured from cadaveric studies.