Annular Repair Using Cross-Linked High Density Collagen Gel Seeded with Fibrochondrocytes: In Vivo Outcomes of a Rodent Disc Study

Abstract

IntroductionDespite alleviating associated neurological symptoms, discectomy of herniated intervertebral discs (IVDs) fails to repair the underlying degenerative process. Persistent annular defect post discectomy is associated with increased risk of reherniation, progressive IVD degeneration, and chronic low back pain.1–7 Although many studies have previously employed cell transplantation to regenerate the nucleus pulposus (NP), few have focused on annulus fibrosus (AF) repair. Recently, we demonstrated the ability of riboflavin crosslinked high-density collagen gels (HDC) to facilitate annular repair in vivo.8,9 In this study, we assessed the in vivo efficacy of AF fibrochondrocytes seeded in a gel scaffold at enhancing the the repair of an annular defect to prevent further degenerative changes in a post-puncture rat-tail model. Material and Methods38 athymic rats, punctured with an 18-gauge needle in the tail disc, were divided into 3 groups: untreated (n = 6), injected with crosslinked HDC (n = 16), and injected with fibrochondrocyte-laden crosslinked HDC (n = 16). At 1, 2, and 5 weeks postoperatively, a series of in vivo images with X-ray and 7T MRI were conducted to determine the disc height index and nucleus pulposus (NP) size and hydration, respectively.8,10,11 Histological assessments were performed to evaluate the viability of implanted cells, degree of annular repair and secondary disc degeneration. The study was approved by and undertaken in accordance with guidelines outlined by the Hospital for Special Surgery Institutional Animal Care and Use Committee and New York State. For the analyses of continuous outcomes in disc height index, NP size, and NP hydration, we employed linear regression models with a generalized estimating equation and robust standard errors to estimate differences in mean changes from baseline controls (discectomy) across displaced and stable implantation groups. P values < 0.05 were considered statistically significant. ResultsThe untreated discs showed substantial NP herniation at two weeks and NP absence with signs of degeneration by five weeks, resulting in 40% loss of disc height. Both HDC gel groups, cellular and acellular, had significant retention of disc height and NP voxel count over the course of five weeks. Average NP voxel counts of cellular gels were higher than those of acellular gels at all time points and statistical significance was achieved at 1 week. Only the cellular group restored NP hydration relative to that of the adjacent healthy control. Further histological assessments indicate that while HDC gels influence the sealing of the defect, the addition of cells generates abundant tissue growth and extracellular matrices at the site of the annular defect, accelerating the reparative process. Disruption of endplate was observed in the puncture group, but not in the treated segments. ConclusionIn vivo studies on the cell-based annular repair are few in number. Our preliminary findings suggest that fibrochondrocytes can potentially improve HDC gel-based annular repair. A long term study with sufficient sample size are necessary to confirm these results.ReferencesBruske-Hohlfeld I, Merritt JL, Onofrio BM, et al. Incidence of lumbar disc surgery. A population-based study in Olmsted County, Minnesota, 1950-1979. Spine 1990;15(1):31–35Carragee EJ, Han MY, Suen PW, Kim D. Clinical outcomes after lumbar discectomy for sciatica: the effects of fragment type and anular competence. 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