Biological Response of the Intervertebral Disc to Repetitive Short-Term Cyclic Torsion

Study Design. In vitro study of the biological response of the intervertebral disc (IVD) to cyclic torsion by using bovine caudal IVDs. Objective. To evaluate the biological response of the IVD to repetitive cyclic torsion of varying magnitudes at a physiological frequency. Summary of Background Data. Mechanical loading is known to be a risk factor for disc degeneration (DD) but the role of torsion in DD is controversial. It has been suggested that a small magnitude of spinal rotation decreases spinal pressure, increases spinal length, and enhances nutrition exchange in the IVD. However, athletes who participate actively in sports involving torsional movement of the spine are frequently diagnosed with DD and/or disc prolapse. Methods. Bovine caudal discs with end plates were harvested and kept in custom-made chambers for in vitro culture and mechanical stimulation. Torsion was applied to the explants for 1 hour/day over four consecutive days by using a servohydraulic testing machine. The biological response was evaluated by cell viability, metabolic activity, gene expression, glycosaminoglycan content, and histological evaluation. Results. A significantly higher cell viability was found in the inner annulus of the 2 degrees torsion group than in the static control group. A trend of decreasing metabolic activity in the nucleus pulposus with increasing torsion magnitude was observed. Apoptotic activity in the nucleus pulposus significantly increased with 5 degrees torsion. No statistical significant difference in gene expression was found between the three torsion angles. No visible change in matrix organization could be observed by histological evaluation. Conclusion. The IVD can tolerate short-term repetitive cyclic torsion, as tested in this study. A small angle of cyclic torsion can be beneficial to the IVD in organ culture, possibly by improving nutrition and waste exchange, whereas large torsion angle may cause damage to disc in the long term.