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.