Objective - To investigate the effects of injected protease solution on the mechanical advancement of disc degeneration, and to establish test protocol for future pre-clinical validation of spinal arthroplasty devices. The hypothesis that injection of a protease into a cadaveric lumbar disc will mimic advanced degeneration mechanics was the subject of this study. Summary of Background Information - Spinal disc degeneration is a universal condition that progresses in adults due to aging, disease, or injury. Stages of disc degeneration have been categorized in cadaver specimens, with each degeneration level exhibiting characteristic changes in flexibility parameters. Spinal disc tissue can be compromised through introduction of proteolytic enzymes into the collagenous fibers of the annulus fibrosus. Methods - 18 motion segments from 8 human lumbar spines were subjected to flexibility testing. Each specimen was either injected with 0.600 mL of trypsin solution in the annulus fibrosus, 0.600 mL of phosphate-buffed saline, or a fluid-less needle-stick. Motion testing followed with rotations applied in all three major spinal motions. Test sections were transected mid-disc after testing to characterize initial degeneration severity, and acquired motion data was analyzed to show flexibility traits over time. Results - Trypsin, saline, and control injections all caused changes in motion from pre-injection baselines. Saline injections were slightly more effective at mimicking the mechanics of higher grades of degeneration with more fidelity than trypsin injections. All motion parameters were altered by the study treatments, with hysteresis and neutral zone parameters experiencing changes similar to that seen in natural degeneration with greater fidelity. Lateral Bending motion showed the greatest magnitude response to injections, with Flexion-Extension tests showing the smallest change. Discussion - Unexpectedly, fluid-less control injections caused changes to hysteresis and neutral zone parameters, suggesting an alteration to viscoelastic properties due to simple needle puncture. Fluid injections (Trypsin and Saline) caused an immediate transient post-injection change to biomechanics that dissipated over time, except in Axial Rotation. Saline injections provided the highest fidelity in mimicking the motion of more advanced stages of degeneration.



College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering



Date Submitted


Document Type





spinal motion, disc degeneration, intervertebral disc mechanics, protease injection