Intervertebral Disc Degeneration

Intervertebral disc degeneration consequent to aging is virtually universal. It can also be induced prematurely by injury or disease. Common surgical treatments for low back pain, including both spinal fusion and total disc replacement have been shown to induce accelerated disc degeneration adjacent to the treated spinal levels.

We seek to understand the mechanical effects of disc degeneration, as well as to understand the consequences of disc degeneration on the functionality of surgical treatments for low back pain. To that end, we seek to characterize the change in mechanical behavior of the lumbar spine as the intervertebral discs degenerate. We are also developing a protease injection model for inducing specific grades of degeneration in cadaveric spines with the intent to study the altered functionality of surgical treatments after the adjacent segments have degenerated.

We injected the annulus fibrosus of bovine tail segments with trypsin at 8 points circumferentially. The mechanical response of a typical specimen before and after a 1 hr trypsin incubation are shown in the figure.  The FBS control specimens had a significantly wider neutral zone (p<0.05) and a significantly less stiff initial stiffness (p<0.05) in compression following incubation.  We are currently applying an advanced version of the same protocol to human cadaveric segments in order to characterize the incubation time required to obtain the mechanical response associated with a specific level of disc degeneration (e.g., Thompson Grade).

 

Correlation of Cadaveric FSU Quality of Motion with Graded Disc Degeneration

Although there have been excellent studies conducted previously correlating disc degeneration with the torque-rotation behavior of the lumbar spine, recent work (especially by Patwardhan, et. al.), has clearly shown that inclusion of a compressive follower load is essential to correctly characterizing the biomechanics of the spine. There are additional questions regarding the effects of body temperature and humidity on segmental mechanical response. We are currently performing tests to correlate cadaveric FSU quality of motion (torque-rotation behavior + helical axis location) with disc degeration level as obtained at body temperature and near 100% humidity.

 

Related Publications

Sessions, J., T. Bishop, and A.E. Bowden (2009). Mechanical characterization of a bovine tail analog for enzymatic disc degeneration. Proc 55th Annual Meeting of the Orthopaedic Research Society.

Showalter, B.L., M.B. Hunter, and A.E. Bowden (2009). Validation of a modified small punch test for characterizing the material response of spinal ligaments. Proc 55th Annual Meeting of the Orthopaedic Research Society.