Morphological and biomechanical effects of annulus fibrosus injury and repair in an ovine cervical model

Abstract

Tissue engineering repair of annulus fibrosus (AF) defects has the potential to prevent disability and pain from intervertebral disc (IVD) herniation and its progression to degeneration. Clinical translation of AF repair methods requires assessment in long‐term large animal models. An ovine AF injury model was developed using cervical spinal levels and a biopsy‐type AF defect to assess composite tissue engineering repair in 1‐month and 12‐month studies. The repair used a fibrin hydrogel crosslinked with genipin (FibGen) to seal defects, poly(trimethylene carbonate) (PTMC) scaffolds to replace lost AF tissue, and polyurethane membranes to prevent herniation. In the 1‐month study, PTMC scaffolds sealed with FibGen herniated with polyurethane membranes. When applied alone, FibGen integrated with the surrounding AF tissue without herniation, showing promise for long‐term studies. The 12‐month long‐term study used only FibGen which showed fibrous healing, biomaterial resorption and no obvious hydrogel‐related complications. However, the 2 mm biopsy punch injury condition also exhibited fibrotic healing at 12 months. Both untreated and FibGen treated groups showed equivalency with no detectable differences in histological grades of proteoglycans, cellular morphology, IVD structure and blood vessel formation, biomechanical properties including torque range and axial range of motion, Pfirrmann grade, IVD height, and quantitative scores of vertebral body changes from clinical computed tomography. The biopsy‐type injury caused endplate defects with a high prevalence of osteophytes in all groups and no nucleus herniation, indicating that the biopsy‐type injury requires further refinement, such as reduction to a slit‐type defect that could penetrate the full depth of the AF without damaging the endplate. Results demonstrate translational feasibility of FibGen for AF repair to seal AF defects, although future study with a more refined injury model is required to validate the efficacy of FibGen before translation.

Mechanical and biological characterization of a composite annulus fibrosus repair strategy in an endplate delamination model

“…The cytotoxicity findings were supported by studies done in vitro. 40,42 On the other hand, it is in contradiction with some previously published work which showed no effect on cell viability in ex vivo organ cultures 21 and in vivo, 57 although the exact recipes for the FibGen hydrogel vary between the studies, as well as the geometry of the injury and the repair. shown to promote cell infiltration and effectively seal AF defects in small animal models in vivo and restore some aspects of biomechanics.…”

“…If, however, free genipin molecules are an issue, in vivo environment would provide more continuous motion and fluid exchange, which could potentially further reduce the effect of the genipin. This assumption is supported by in vivo study by Long et al which showed no long-term cytotoxic or inflammatory effect from genipin in ovine IVDs 57. In the latter more likely case, the reinforcing effect would stand in direct contradiction with cell survival.…”

“…Long et al 24 who have attempted repair strategies on large animal models in vivo did not record a significant improvement in disc height on repair.…”

“…The use of hydrogels made up of collagen [17][18][19][20] and fibrin-genipin (FibGen) 21 as fillers for AF defects have shown to limit biomechanical deterioration. The fillers have also been enhanced with a retention scaffold on the IVD surface [22][23][24] to further reinforce the injury site. The second class of strategies applies tissue-engineering principles to replace the injured AF with cell-laden scaffolds [25][26][27][28] or cell sheets.…”

Advanced Strategies for the Regeneration of Lumbar Disc Annulus Fibrosus

“…Despite intense research interest, attempts to regenerate the IVD have failed so far and no effective strategy has translated into a successful clinical outcome [ 24 , 25 ]. Of particular significance, the failure of strategies to repair the AF has been a major drawback in the regeneration of IVD and nucleus replacement [ 26 ]. Since IVD is exposed to significant load and large deformation during daily activities, it is unlikely to secure regenerative mediators (cells, genes, and biomolecules) and artificial nucleus materials after injection into a herniated IVD.…”