The effect of relative needle diameter in puncture and sham injection animal models of degeneration.

STUDY DESIGN: Biomechanical study and literature review. OBJECTIVES: To quantify the acute effect of needle diameter on the in vitro mechanical properties of cadaver lumbar discs in the rat and sheep. To review published in vivo animal studies and evaluate disc changes with respect to the relative needle size. SUMMARY OF BACKGROUND DATA: There are many cases where a disc needle puncture or injection is applied to animal models: puncture injuries to induce degeneration, chemonucleolysis to induce degeneration, and delivery of disc therapies. It is not clear what role the size of the needle may have in the outcome. METHODS: Mechanics were measured after sham phosphate buffered saline injection with a 27 G or 33 G needle in the rat and with a 27 G needle in the sheep. A literature review was performed to evaluate studies in which animal discs were treated with a needle puncture or a sham injection. For each study, the ratio of the needle diameter to disc height (needle:height) was calculated. RESULTS: When the rat was injected with a 27 G needle (52% of disc height), the compression, tension, and neutral zone stiffnesses were 20% to 60% below preinjected values and the neutral zone length was 130% higher; when injected with a 33 G needle (26% of disc height), the only affected property was the neutral zone length, which was only 20% greater. When the sheep was injected with a 27 G needle (10% of disc height), none of the axial properties were different from intact, the torsion stiffness was not different, and the torque range was 15% smaller. Twenty-three in vivo studies in the rat, rabbit, dog, or sheep were reviewed. The disc changes depended on the ratio of needle diameter to disc height as follows: significant changes were not observed for needle:height less than 40%, although between 25% and 40% results were variable and some minor nonsignificant effects were observed, disc changes were universal for needle:height over 40%. CONCLUSION: A needle puncture may directly alter mechanical properties via nucleus pulposus depressurization and/or anulus fibrosus damage, depending on the relative needle size. As more basic science research is aimed at treating disc degeneration via injection of therapeutic factors, these findings provide guidance in design of animal studies. Such studies should consider the relative needle size and include sham control groups to account for the potential effects of the needle injection.

Nucleus pulposus glycosaminoglycan content is correlated with axial mechanics in rat lumbar motion segments.

The unique biochemical composition and structure of the intervertebral disc allow it to support load, permit motion, and dissipate energy. With degeneration, both the biochemical composition and mechanical behavior of the disc are drastically altered, yet quantitative relationships between the biochemical changes and overall motion segment mechanics are lacking. The objective of this study was to determine the contribution of nucleus pulposus glycosaminoglycan content, which decreases with degeneration, to mechanical function of a rat lumbar spine motion segment in axial loading. Motion segments were treated with varying doses of Chondroitinase-ABC (to degrade glycosaminoglycans) and loaded in axial cyclic compression-tension, followed by compressive creep. Nucleus glycosaminoglycan content was significantly correlated (p < 0.05) with neutral zone mechanical behavior, which occurs in low load transition between tension and compression (stiffness: r = 0.59; displacement: r = -0.59), and with creep behavior (viscous parameter eta(1): r = 0.34; short time constant tau(1): r = 0.46). These results indicate that moderate decreases in nucleus glycosaminoglycan content consistent with early human degeneration affect overall mechanical function of the disc. These decreases may expose the disc to altered internal stress and strain patterns, thus contributing through mechanical or biological mechanisms to the degenerative cascade.