FINITE ELEMENT PARAMETRIC STUDY OF A HYBRID HYDROGEL-FOAM PROSTHETIC NUCLEUS PULPOSUS REVEALS A FOAM DENSIFICATION MECHANISM INCREASING EXTRUSION RESISTANCE

An ideal prosthetic nucleus pulposus would combine flexibility with resistance to extrusion through annular defects. Prior unconfined compression and confined extrusion experiments have shown promising results with a hybrid composed of open-cell flexible polymer foam saturated with hydrogel. This study is a finite element parametric analysis to investigate mechanisms underlying the experimental results. In unconfined compression, decreasing the foam stiffness or increasing hydrogel permeability decreased the overall hybrid material stiffness as expected. However, in the confined extrusion configuration, decreasing the foam stiffness instead increased the hybrid stiffness due to densification of the foam as it was forced into the defect by the hydrogel. This localized densification mechanism provided by low stiffness foams suggests a useful, self-healing process in the presence of a defect, such as an annular tear.