Re-swelling Degenerate Intervertebral Discs by Percutaneous Injection:an In-vitro Investigation

INTRODUCTION Intervertebral discs (IVD) are swollen structures which support substantial static and dynamic loads. Load-carriage is achieved by hydrostatic pressurization of the fluid-filled nucleus pulposus (NP) contained by the ultra-low hydraulic permeability of the endplates and mechanically strong annulus fibrosis (AF). During the diurnal loading cycle, fluid is slowly wrung out in a process of consolidation, and the subsequent disc height recovery is achieved by a formidable swelling pressure [1]. The disc contains several highly specialized structures. Of importance to swelling is the NP, with its high concentration of proteoglycans (PG), and the surrounding fibrous tissues, with their selective permeability. The high osmotic potential of the nucleus attracts fluid through the endplates, which together with the AF act as membranes to contain the PGs. This osmotic pressure mechanism is vital to maintain the hydration and health of the disc. Although the exact mechanism of disc degeneration remains unclear, it is widely known that degenerate discs generally contain less PG and hence less fluid. It has been suggested that decompressed discs may cause nerve impingement resulting in pain , and pain relief can be effected by temporary extension of the discs. Therefore, restoration of the natural swelling capacity of the disc would appear to be a critical requirement for the effective treatment of damaged or degenerate discs. In this project , the intrinsic swelling behavior and potential degeneration processes were investigated. The concept of artificially increasing the osmotic potential of PG-depleted intervertebral discs by direct injection of a biocompatible, osmotically active gel was investigated as a means to recover both disc height and hydration.