Hydrogels for nucleus replacement-Facing the biomechanical challenge

► In vivo ovine IDP was ∼0.75 MPa during day and ∼0.5 MPa during night. ► Measured in vivo IDPs corresponded to axial compressive forces of 130/58 N in vitro. ► An isolated annulus defect did not impair the disc's viscoelastic behaviour in vitro. ► Re-implantation of the natural nucleus caused a significant IDP reduction by 50%. ► Implantation of hydrogels did not restore the mechanical functionality of the disc.