Mechanical Effectiveness of Polyvinyl Alcohol/Polyvinyl Pyrrolidone (PVA/PVP) as an Intervertebral Disc Polymer

The intervertebral disc is one of the body’s most vital structures. It provides support and enables six degree of freedom (6DOF) motions in the spine: flexion, extension, right and left lateral bending, compression, and axial rotation. When individuals suffer from degenerative disc disease, the nucleus pulposus deteriorates, causing a loss of articulation in the intervertebral disc. To address this problem, replacements for the nucleus pulposus can be used. The objective of this study was to evaluate a potential nucleus pulposus replacement consisting of a hydrogel polymer. The hydrogel was synthesized by physically cross-linking 95%-weight polyvinyl alcohol (PVA) and 5%-weight polyvinyl pyrrolidone (PVP). PVA and PVP were selected for the hydrogel implant due to the natural biocompatibility when the two are physically cross-linked. In order to evaluate the mechanical effectiveness of the hydrogel, a slider-crank mechanism was designed and constructed to create the 6DOF motions when interfaced with a Universal Mechanical Testing System. The viscoelastic properties of the polymer were obtained using a rheometer, which determined the elastic (G′) and viscous (G″) moduli of the PVA/PVP hydrogel polymer by calculating the complex shear modulus (G*) under low-frequency oscillating shear deformation. This allows for study of the viscoelastic performance of the isolated nucleus pulposus and hydrogel implant. The elastic modulus of the hydrogel was tested at parameters 5%, 10%, and 15% strain with results of 228.6 Pa, 988.8 Pa, and 1793 Pa, respectively. However, the elastic modulus tested for the natural bovine specimen at 5%, 10%, and 15% strain were 712.9 Pa, 522.1 Pa, and 363.3 Pa, respectively.