The Dose Response Relationship between Intervertebral Disc Flexion-Extension Neutral Zone Metrics and Injected Genipin Concentration

Purpose Quantify changes in the flexion - extension neutral zone of the intervertebral disc with injections of increasing genipin concentration. Methods Bovine motion segments were treated with varying concentrations of genipin using bilateral injections of constant volume. After overnight static compression loading of the treated segments, anterior- posterior offset loading was used to simulate flexion - extension motion. Range of motion, neutral zone length, neutral zone stiffness, and an instability score were measured. Results Injection of the disc annulus with increasing concentrations of genipin resulted in corresponding changes in flexion-extension neutral zone. A minimum concentration of 40 mM was needed to observe a significant change. The largest changes were observed with the 400 mM injection. Netural zone stability was the most sensitive of the metrics with a percent change of 48% at 40 mM and over 200% at 400 mM. Conclusion This study establishes the efficacy of using injection delivery to affect disc joint mechanics and quantifies the dose response between injected genipin and the flexion-extension stability of the disc.