Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

INTRODUCTION:This study addresses lumbar disc herniation, a common cause of radicular pain in middle-aged individuals. While biomaterial-based treatments have shown promising preclinical results, the current gold-standard remains discectomy, which provides symptomatic relief but does not prevent re-herniation. Our approach combines (i) annulus fibrosus (AF) repair using a mechanically interlocked patch (iPatch) comprising polyethylene terephthalate fibers,¹ and (ii) nucleus pulposus (NP) augmentation with hyaluronic acid-tyramine hydrogel (HA-Tyr). The study aimed to test the impact of the HA-Tyr viscosity on the mechanical function of the repaired disc after experimental herniation.

METHODS:Bovine caudal intervertebral disc (IVD) segments (N=23) underwent an induced injury through annulotomy and partial nucleotomy and were divided into three treatment groups (n=6) and one control group (n=5). The treated groups were repaired with iPatch and HA-Tyr injection with different hydrogen peroxide quantities (0.34 mM for low viscosity, 0.68 mM for medium viscosity, and 1.02 mM for high viscosity). Control group segments remained unrepaired to simulate discectomy. The protocol first assessed biomechanical parameters under compressive-tensile cycles for intact, injured, and repaired IVD conditions. Second, it determined load capacity until failure, defined as either NP herniation (control) or hydrogel extrusion (treatment groups). This test had a maximum limit of 10 MPa to prevent setup or endplate failures. Range of motion (ROM) and compressive stiffness were measured. Matched mixed-effects model analysis with Tukey’s post-hoc was used to statistically compare the groups. Significance level was α=0.05.

RESULTS:ROM for injury was significantly increased in all groups compared to intact disc segments (p<0.001)(Fig.1). ROM values in the high viscosity group were closest to the intact state (mean diff. to injury = 13.1%; p<0.001), followed by the medium viscosity group (mean diff. to injury=7.3%; p=0.01), while the low viscosity group remained mechanically similar to the injured disc (mean diff. to injury=3.4%; p=0.48)(Fig.1). Improvements from the injured condition were observed in all three treatment groups although none fully recovered compressive stiffness.

During the failure strength test, the low viscosity group had a favorably lower herniation rate (33%) compared to the control group (80%)(Fig.2). The medium viscosity group had intermediate herniation frequency (50%), while high viscosity (83%) resembled discectomy (Fig.2). Both the low viscosity and medium viscosity groups reported either herniation over the human physiological stress level or no herniation (Fig.3). All treated groups showed higher failure strength compared to control group, with highest difference in low viscosity (mean diff.=4.1 MPa; p=0.256)(Fig.3).

DISCUSSION:This study demonstrates that IVD mechanics can be partially restored using a mechanically interlocked patch and hydrogel injection. Samples combining an iPatch repair with either low or medium viscosity hydrogel successfully withstood physiological stresses (2.3 MPa) and could meaningfully restore partial NP biomechanical parameters. While this performance seems to overcome a long-standing challenge to the field, this study is limited in that it only approximates in vivo mechanical functions, and excludes lateral bending, flexion, or extension. Nonetheless, we conclude that combining a HA-Tyr hydrogel and iPatch holds potential for further research, with the hydrogel serving as viable carrier for cells or drugs while offering load protection.