The effect of different defects and hydrogels for nucleus replacements on the biomechanical response of the intervertebral disc

Hydrogels offer unique opportunities for regenerative strategies of the intervertebral disc. A nucleotomy necessary for the implantation of hy- drogels, however, disrupts the annulus integrity and destroys natural interfaces in the disc. To clarify whether hydrogels can restore the mechanical competence of the disc an experimental test was used. Intra- discal pressure(IDP) in an ovine disc was measured in vivo for 24 h and adapted to an axial compressive test consisting of three cycles 15-min diurnal and 30-min night load. To study the fluid mechanics, 30 motion segments in different defect conditions were used: (i) INTACT; (ii) DEF- ANN: isolated annulus defect; (iii) DEF-NUC: re-implanted nucleus; (iv) DDAHA and (v) iGG-MA: two hydrogels. DEF-ANN showed no signifi- cant difference in disc height loss or IDP compared to INTACT, while DEF-NUC reduced the IDP by ~30%(p = 0.03) and tended to increase the height loss(p = 0.2). Both DDAHA and iGG-MA better reflected the height loss of INTACT, but caused an even stronger loss in IDP than DEF-NUC(~34%). Neither the hydrogels nor the re-implanted nucleus, assumed to be the ideal implant, could restore the mechanical function- ality of the disc. Hydrogels designed to mimic the mechanical behavior of the native nucleus may fail in restoring IDP due to the destruction of natural interfaces and an inappropriate annulus closure. To regain a bio- mechanical equivalent of the natural nucleus, more attention needs to be paid to the anchoring of the substitute inside the disc.