Characterizing baseline fixed charge density in human cervical intervertebral discs

Abstract

This study provides quantification of fixed charge density in human cervical intervertebral discs. Fixed charge density, which occurs due to negatively charged proteoglycans in the extracellular matrix, is a key determinant of the intervertebral disc osmotic environment and swelling properties. While regional fixed charge density patterns have been characterized in lumbar discs, they remain unexplored in cervical discs. Using fresh-frozen cadaveric cervical discs from five donors, fixed charge density was measured using a two-point electrical conductivity method. Glycosaminoglycan content and porosity were also assessed. Fixed charge density (0.18 ± 0.1 mEq/g wet tissue) was highest in the cartilage endplate region and significantly greater than in that in the annulus fibrosus (p = 0.006). No significant difference in fixed charge density was observed between the nucleus pulposus and annulus fibrosus. Glycosaminoglycan content (40.3 ± 14.4 µg/mg wet tissue) showed a strong positive correlation with fixed charge density across regions (r = 0.65, p = 0.0047). Unlike lumbar discs, fixed charge density was found to be more homogeneous between the nucleus pulposus and annulus fibrosus regions. This result likely reflects adaptations for reduced tissue swelling in cervical discs to accommodate lower weight-bearing demands and increased flexibility. The elevated fixed charge density in the cervical endplates may protect the intervertebral disc-vertebral bone interface, potentially to avoid mechanical damage in a kinematically more mobile environment. These findings establish key benchmarks for understanding cervical disc electro-biomechanics and may inform other cervical disc tissue-characterization efforts.