Annular repair using high-density collagen gel: a rat-tail in vivo model.

Repair of annular defects could significantly improve treatment of degenerative spinal diseases.1 Open defects compromise the ability of the annulus fibrosus (AF) to contain nuclear tissue in the disc space, thereby increasing the likelihood of reherniation and progressive degeneration after discectomies.2–6 Furthermore, there has been concern that annular puncture for therapeutic or diagnostic procedures accelerates the progression of degenerative disc disease and promotes nuclear tissue extrusion.7 Successful treatment of puncture defects could inhibit these degenerative changes. Annular defects persist because of the very limited intrinsic healing capability of the AF, which does not significantly improve upon simple mechanical closure.1,8–11 As a result, several research groups have investigated using biological materials for annular repair. Rigid implants have been studied by Vadala et al. using tissue-engineered AF constructs in vitro12 and by Ledet EH using small intestinal submucosa in vivo.13 Implanted submucosa tissue reduced degenerative changes after annulotomy in sheep spine. Schek et al. have studied injectable biomaterials with genipin cross-linked fibrin hydrogels. Fibrin integrated with sections of human AF tissue, showing promising biomechanical and cell seeding properties in vitro.14 Our group tested injectable high-density collagen (HDC) gels and found that HDC can partially restore mechanical function to a needle-punctured rat-tail AF in vitro.15 However, no studies reported use of injectable biomaterials to treat annular defects in vivo. Annular defects induced by needle puncture lead to predictable patterns of disc degeneration in the rat-tail spine.16,17 In such models, degeneration is initiated by extrusion of nucleus pulposus (NP) tissue through the puncture defect. Although the needle puncture model has been frequently used to test biological materials for IVD regeneration,18–21 no study yet has used this model to investigate repair of induced defects to prevent disc degeneration. The goal of our study was to evaluate the ability of HDC to repair a needle-puncture AF defect in the rat-tail spine. Specifically, we wanted to test whether injected HDC gel can prevent nuclear tissue extrusion and consequent IVD degeneration, as determined by histological and radiological outcomes. In addition, we assessed whether cross-linking of injected collagen influences the repair process. For that purpose, riboflavin (RF), a photoactive initiator of collagen cross-linking, was added in different concentrations. In the presented study, the rat-tail model was used to screen these various compositions of collagen gels.