Towards a functional radiopaque hydrogel for nucleus pulposus replacement.

Patients with severe back pain, attributed to a herniation of the nucleus pulposus of the intervertebral disc, can benefit from a replacement of only the nucleus pulposus, provided the annulus fibrosus is still functional. This study investigated four intrinsically radiopaque hydrogel biomaterials, which were designed specifically to replace the herniated nucleus pulposus. The important characteristic of these hydrogels is that they can be visualized entirely with both MRI and X-rays. The materials are based on copolymers of N-vinyl-2-pyrrolidinone (NVP) or 2-hydroxyethyl methacrylate (HEMA) and a radiopacity introducing monomer, 2-(4'-iodobenzoyl)-oxo-ethyl methacrylate (4IEMA). Two of the formulations also contain the chemical crosslinker allyl methacrylate (AMA). Physical-mechanical properties like the water-uptake, biocompatibility, stiffness, and fatigue and creep behavior were studied, while keeping an eye on the intended application. All four materials were designed with 5-6 mass % of iodine to ensure sufficient X-ray visibility between two vertebrae. It was found that the materials display appropriate stiffness and biocompatibility. The crosslinked materials hold most promise as a functional nucleus prosthesis, as they combine these properties also with high water content, fatigue resistance, and recovery after loading.