A Two-Point Conductivity Approach to Measuring Fixed Charge Density in Intervertebral Disc Tissue

Low back pain, a major socio-economic concern in the United States, is believed to result from degeneration of the intervertebral disc (IVD) of the spine [1]. The IVD is characterized as a charged, hydrated soft tissue made up of a central nucleus pulposus (NP) surrounded by the layered annulus fibrosus (AF). The negatively-charged nature of the disc derives from the charged groups attached to the glycosaminoglycan (GAG) molecules found on proteoglycans (PG) in the extracellular matrix of the disc. The fixed charge density (FCD) is a measure of the number of negative charges attached to the disc matrix per unit volume. FCD is important to disc function, both mechanically (i.e., swelling pressure) and in terms of transport through the disc [2].