Intrinsically radiopaque hydrogels for nucleus pulposus replacement

Degeneration of the intervertebral disc is the most common cause of back pain. In case of early stage degenerative disc disease or traumatic herniations, a suitable treatment may be to replace the nucleus pulposus, preserving the annulus fibrosus. Eight new hydrogel biomaterials were prepared and studied for their potential as a nucleus replacement. The hydrogels were designed according to the following criteria: (i), they should exhibit adequate radiopacity; (ii), they should be non-cytotoxic; (iii), implantation in the dry state and subsequent swelling in situ to fill the entire nucleus cavity; (iv), after swelling they should match the physical–mechanical properties of the native nucleus. The approach was to use copolymers consisting of 2-(4′-iodobenzoyl)-oxo-ethyl methacrylate (4IEMA) and a hydrophilic building block (either N-vinyl-2-pyrrolidinone (NVP) or 2-hydroxyethyl methacrylate (HEMA)); 4 copolymers of NVP/4IEMA and 4 copolymers of HEMA/4IEMA in different compositions (5, 10, 15 and 20 mol% 4IEMA). The study comprised 1H-NMR analysis of the copolymerization reaction NVP+4IEMA. Furthermore, the copolymers were studied with respect to their swelling behavior, mechanical properties, cytotoxicity in vitro and X-ray contrast. Hydrogels with 5 mol% 4IEMA appear to meet all criteria: they are non-cytotoxic, have adequate physical–mechanical properties and feature sufficient radiopacity in a realistic model. The potential implications of these new results with respect to treatment of degenerative disc disease are discussed briefly.