The intervertebral disc (IVD) is a cartilaginous structure in the spinal column comprised of the inner nucleus pulposus (NP) and the outer annulus fibrosus (AF). With untreated damage to the AF, the pressure forces the nucleus out into the surrounding disc space causing pain and, eventually, degenerative disc disease (DDD). Once an IVD has fully degenerated, treatment options are limited to full disc replacement or spinal fusion, and as such, research groups have begun to develop methods for tissue engineering repair and regeneration of damaged IVDs including repair of defects in the AF. This study evaluates an injectable, photocrosslinkable collagen hydrogel for the mechanical and biological repair of annulus fibrosus defects. Collagen hydrogels for biomedical applications are often limited due their weak bulk mechanical properties. In order to increase their mechanical strength, researchers can increase the density of collagen in the final gel, crosslink the fibers of the resulting collagen gel, or both. The first aim observed the effects of collagen gels at different densities and degrees of riboflavin crosslinking on the repair of annulus fibrosus defects of different sizes in an in vitro rat caudal AF repair model. We observed improvements in the effective mechanical behavior of damaged IVDs with increasing collagen density and riboflavin concentration.
After establishing the collagen gel as a possible AF repair technology mechanically, we transitioned from an in vitro model to an in vivo, rat caudal model of annular repair to better assess biological healing. The second aim centered on the use of both uncrosslinkined and crosslinked collagen gel formulations from aim 1 in an in vivo rat tail puncture model. We observed increased disc height and NP hydration, two markers of disc degeneration, in all treated groups. The highest improvement was exhibited in rats treated with the riboflavin-crosslinked collagen gels. The third, and final aim discussed in this dissertation employed the same athymic rat tail puncture model to understand the effect of cell-seeded collagen gels on AF repair. Primary ovine AF cells were isolated and added to the crosslinked collagen gel formulations before injection into an in vivo AF defect. We observed greater disc height and NP hydration in animals treated with the cell-seeded collagen gel. Furthermore, cell-seeded gels exhibited integration with surrounding native AF tissue. These experiments establish that riboflavin-crosslinked collagen hydrogels are an excellent foundation for an annular repair therapy, and can be used to deliver cells in order to enhance repair.