A conductivity approach to measuring fixed charge density in intervertebral disc tissue.

A new method for measuring the fixed charge density (FCD) in intervertebral disc (IVD) tissues employing a two-point electrical conductivity approach was developed. In this technique, the tissue is first confined and equilibrated in a potassium chloride (KCl) solution, and the tissue conductivity is then measured. This is then repeated with a second concentration of KCl solution. The FCD can be determined from the conductivity measurements. Using this method, the FCD values of bovine annulus fibrosus (AF) and nucleus pulposus (NP) tissues were determined to be 0.060 +/- 0.027 mEq/g wet tissue and 0.19 +/- 0.039 mEq/g wet tissue, respectively. The FCD of AF was significantly lower than that of NP tissue, similar to results in the literature for human IVD tissues. In order to verify the accuracy of the new method, the glycosaminoglycan (GAG) contents of the tissues were measured and used to estimate the tissue FCD. A strong correlation (R (2) = 0.84-0.87) was found to exist between FCD values measured and those estimated from GAG contents, indicating that the conductivity approach is a reliable technique for measuring the FCD of IVD tissues.