In Vivo Repair of Rat Annulus Fibrosus Defects using High-Density Collagen Gels

Abstract

Introduction Although a discectomy successfully relieves the neurological symptoms of a herniated intervertebral disc (IVD), it does not treat the underlying degenerative process; the annular defect remains unrepaired. Persistent annular defect is associated with an increased risk of recurrent herniation and progressive degenerative changes to the IVD. It may also be the primary cause of chronic low back pain following discectomy. To date, there is no established method for repairing annular defects in vivo. The presented study aims to determine whether injectable high-density collagen (HDC) gels can reduce further disc herniation and inhibit degenerative changes in a needle-punctured rat-tail model, and whether riboflavin (RF) cross-linking of injected collagen influences the repair process. Materials and Methods A total of 31 athymic rats were punctured with an 18-gauge needle in C3/4 of the caudal spine. They were divided into four groups: group 1 punctured and injected with HDC cross-linked with 0.5 mM (n = 6) or 0.75 mM (n = 7) RF; group 2 punctured and injected with noncross-linked collagen (n = 6); group 3 punctured and untreated (n = 8); group 4 punctured and injected with FITC-labeled cross-linked collagen (n = 4). Degenerative changes to punctured discs, NP size, and NP hydration were analyzed using X-ray, MRI), and histology. Functionality of repaired AF tissue was measured by mechanical tests, by comparing the hydraulic permeability of treated discs to that of healthy discs. Results After 5 weeks, untreated discs showed signs of terminal degenerative changes on MRI and histological sections ( Fig. 1 ). No NP tissue remained in the disc space. In contrast, discs treated with RF cross-linked collagen gels retained 63% of NP and 80% of disc height at 5 weeks, and showed minimal degenerative changes on histological section. Interestingly, a series of in vivo imaging showed that after 5 weeks, discs treated with cross-linked HDC demonstrated consistent improvement in both volume and hydration over time, while untreated discs and discs treated with noncross-linked HDC, lost volume, and water content. Injected collagen was seen to form a zipper-like adhesion to the host AF and connective tissue after 1 week and by 5 weeks a fibrous cap that persisted until 18 weeks. NP hydration and IVD functionality of treated discs were found to be similar to those of adjacent healthy discs at 18 weeks. [Figure: see text] Conclusion Injection of high-density collagen gel cross-linked with RF can repair annular defects, prevent the degenerative cascade, and maintain the functionality of IVDs in a rat-tail spine. Further studies will be performed to confirm the ability of HDC gels to repair annular defects in a large animal model.