Optimized hydrogel viscoelasticity and interlocking patch repair enhance compressive range of motion in injured discs and prevent re-herniation under physiological load in an Ex Vivo model.

PURPOSE: Evaluate how variations in hydrogels viscoelasticity for nucleus pulposus (NP) replacement affect biomechanical function and re-herniation resistance when combined with a mechanically competent annulus fibrosus (AF) repair technique in an intervertebral disc explant model.

METHODS

Bovine caudal discs (n = 48) were divided into three repair groups (each n = 12) and one no-repair group (n = 12). Repair groups were treated with hyaluronic acid-tyramine hydrogel (HA-Tyr) of varying viscosities (low, medium, high), and AF was repaired using a mechanically interlocked patch (iPatch) made of polyethylene terephthalate fibers. Disc biomechanics were assessed under rotational and compressive-tensile cycles for each specimen (A) in intact state, (B) after experimental herniation/discectomy, and (C) after treatment. Afterwards, load capacity until NP herniation or hydrogel extrusion was determined.

RESULTS

Discectomy significantly altered axial and torsional parameters compared to intact discs. Combined iPatch/hydrogel treatment improved axial parameters by reducing range of motion (ROM), neutral zone length, and restoring compressive stiffness, whereas tensile stiffness remained unaffected. Significant differences in ROM were found between unrepaired discs and medium or high viscoelasticity hydrogels. Torsional parameters improved likewise. Low viscoelasticity hydrogel showed the highest load resistance (8.5 MPa) and lowest herniation probability. Under physiological load, only medium viscoelasticity hydrogels did not herniate.

CONCLUSION

Higher viscoelastic hydrogels enhance compressive ROM and axial compressive stiffness after discectomy but increase herniation risk. Medium viscoelastic hydrogels resist physiological loads without herniating while still enhancing ROM. This study highlights the need to optimize NP replacement and suggests a combination that opens promising therapeutic avenues for future applications.