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.

Study Design Experimental measurement and normalization of in vitro disc torsion mechanics and collagen content for several animal species used in intervertebral disc research and comparing these to the human disc. Objective To aid in the selection of appropriate animal models for disc research by measuring torsional mechanical properties and collagen content. Summary of Background Data There is lack of data and variability in testing protocols for comparing animal and human disc torsion mechanics and collagen content. Methods Intervertebral disc torsion mechanics were measured and normalized by disc height and polar moment of inertia for 11 disc types in 8 mammalian species: the calf, pig, baboon, goat, sheep, rabbit, rat, and mouse lumbar, and cow, rat, and mouse caudal. Collagen content was measured and normalized by dry weight for the same discs except the rat and mouse. Collagen fiber stretch in torsion was calculated using an analytical model. Results Measured torsion parameters varied by several orders of magnitude across the different species. After geometric normalization, only the sheep and pig discs were statistically different from human. Fiber stretch was found to be highly dependent on the assumed initial fiber angle. The collagen content of the discs was similar, especially in the outer annulus where only the calf and goat discs were statistically different from human. Disc collagen content did not correlate with torsion mechanics. Conclusion Disc torsion mechanics are comparable to human lumbar discs in 9 of 11 disc types after normalization by geometry. The normalized torsion mechanics and collagen content of the multiple animal discs presented is useful for selecting and interpreting results for animal models of the disc. Structural composition of the disc, such as initial fiber angle, may explain the differences that were noted between species after geometric normalization.

Study Design Investigation of injectable nucleus pulposus (NP) implant. Objective To assess the ability of a recently developed injectable hydrogel implant to restore non-degenerative disc mechanics through support of NP functional mechanics. Summary of Background Data While surgical intervention for low back pain is effective for some patients, treated discs undergo altered biomechanics and adjacent levels are at increased risk for accelerated degeneration. One potential treatment as an alternative to surgery for degenerated disc includes the percutaneous delivery of agents to support NP functional mechanics. The implants are delivered in a minimally invasive fashion, potentially on an outpatient basis, and do not preclude later surgical options. One of the challenges in designing such implants include the need to match key NP mechanical behavior and mimic the role of native non-degenerate NP in spinal motion. Methods The oxidized hyaluronic acid gelatin implant material was prepared. In vitro mechanical testing was performed in mature ovine bone-disc-bone units in three stages: intact, discectomy, and implantation vs. sham. Tested samples were cut axially for qualitative structural observations. Results Discectomy increased axial range of motion (ROM) significantly compared to intact. Hydrogel implantation reduced ROM 17% (p < 0.05) compared to discectomy and returned ROM to intact levels (ROM intact 0.71 mm, discectomy 0.87 mm, post-implantation 0.72 mm). While ROM for the hydrogel implant group was statistically unchanged compared to the intact disc, ROM for sham discs, which received a discectomy and no implant, was significant increase compared to intact. The compression and tension stiffness were decreased with discectomy and remained unchanged for both implant and sham groups, as expected because the annulus fibrosus was not repaired. Gross morphology images confirmed no ejection of NP implant. Conclusion An injectable implant that mimics non-degenerate NP has the potential to return motion segment ROM to normal subsequent to injury.

The intervertebral disc plays a critical role in providing structural support to the spine while permitting extensive flexibility in a number of orientations. Axial rotation is a key parameter in spine function and torsional instability is related to spinal degeneration [1]. Animal models are integral components in many in vivo disc studies, however each animal varies in availability, size, cost, and scientific criteria such as cell phenotype and biomechanics. Selection of an appropriate animal model requires knowledge of the similarities and differences in biomechanical and biochemical factors between the model and human discs. Previous studies have often compared the characteristics of a single animal model with the human disc [2, 3]. However, variations in animal models and testing protocols between groups hinder comparisons and interpretations between different studies. This is especially relevant in torsion mechanics, where the magnitude of an applied compressive load and other testing parameters significantly affect the apparent torsional stiffness of the disc [4]. The objective of this study was to measure and compare the torsion mechanical properties of the human disc and 11 disc types from 8 mammalian species.