High-Density Collagen Gel Can Repair Annular Defects and Restore Biomechanical Function to Needle-Punctured Intervertebral Discs

Abstract

Introduction Unrepaired annular defects potentially increase the reherniation rate after lumbar discectomies and have been shown to accelerate degenerative changes after discographies. This is the first study to test an injectable biological substance to repair annular defects in vivo. We used high-density collagen gel in a needle-punctured rat-tail model. Needle puncturing leads to extrusion of NP tissue with subsequent degenerative changes (4). Restoring annular integrity will help retain the NP material and therefore inhibit these changes. Materials and Methods We punctured the S3/S4 intervertebral disc (IVD) of 30 athymic rats using an 18-gauge needle. Subsequently high-density collagen (HDC) gel was injected to seal the defect. Riboflavin (RF) was added to increase the stiffness of the collagen gel by inducing chemical cross-link formation. Animals were subdivided into four groups. The first group was injected with uncross-linked HDC gel (n = 6), the second with cross-linked collagen using 0.5 mM (n = 8) and the third 0.75 mM (n = 8) concentration of RF. The fourth group (n = 8) served as control and was left untreated after needle puncture. The animals were followed up at weeks 1, 2, 5, 12, and 18 with X-ray measurements to assess the disc heights and MR imaging to evaluate degenerative changes according to a modified Pffirmann grading system. We developed an algorithm based on T2-relaxation time measurements to assess the size of the nucleus by the number of NP voxels that compose it (Fig. 1). Animals were sacrificed at 1, 2, 5 (n = 10), and 18 weeks (n = 20). Histological analysis of the collected tails was performed to study the fate of the collagen using Safranin O and fluorescent stains: FITC for collagen and DAPI for host cells. Disc degeneration was assessed histologically according to the established Han grading system and NP size according to cross-sectional area measurements. Half of the tails from the 18-week time point underwent mechanical testing. We performed stress relaxation tests to measure the stiffness of the discs and their ability to pressurize. Damping quality was measured using frequency sweep tests. Explanted segments were exposed to sinusoidal strains at variable frequencies ranging from 0.01 to 0.3 Hz at amplitudes of ± 10% strain. Results Disc Degeneration Over 18 weeks RF cross-linked groups retained significantly more NP tissue in the disc space than the uncross-linked and the control groups according to NP voxel count and histological NP cross-section measurements (p < 0.05) (Fig. 1). There was no significant difference in NP size between 0.5 and 0.75 mM groups (p > 0.05). Both groups retained approximately 70% of NP size when compared with healthy discs and maintained a disc height of over 80% compared with the prepuncture state. Both cross-linked groups showed no significant histological degenerative changes (Fig. 1). The NP showed regular cellularity and matrix morphology. The AF maintained its organization and lamellar structure. Both groups showed similar damping qual...

Putting the Pieces in Place: Mobilizing Cellular Players to Improve Annulus Fibrosus Repair

“…70 In a bovine ex vivo herniation model, AF repair with FibGen with or without NP replacement (using carboxymethylcellulose-methylcellulose) demonstrated failure at supraphysiologic stresses, suggesting that the biomaterials could improve the mechanical performance of the IVD compared to discectomy alone. 54 In vivo; small animal 10-to 12-week-old athymic rat 40,41,47,48 In vivo; large animal 4-to 7-year-old sheep morphological and biomechanical outcomes observed compared to untreated groups 12 months postinjury (Fig. 2b).…”