• Yongren Wu, Sarah E Cisewski, Yi Sun, Brooke J Damon, Barton L Sachs, Vincent D Pellegrini, Elizabeth H Slate, and Hai Yao.
• Department of Bioengineering, Clemson University, Clemson, SC.
• Spine. 2017 Sep 1; 42 (17): E1002-E1009.

Study DesignRegional measurements of fixed charge densities (FCDs) of healthy human cartilage endplate (CEP) using a two-point electrical conductivity approach.ObjectiveThe aim of this study was to determine the FCDs at four different regions (central, lateral, anterior, and posterior) of human CEP, and correlate the FCDs with tissue biochemical composition.Summary Of Background DataThe CEP, a thin layer of hyaline cartilage on the cranial and caudal surfaces of the intervertebral disc, plays an irreplaceable role in maintaining the unique physiological mechano-electrochemical environment inside the disc. FCD, arising from the carboxyl and sulfate groups of the glycosaminoglycans (GAG) in the extracellular matrix of the disc, is a key regulator of the disc ionic and osmotic environment through physicochemical and electrokinetic effects. Although FCDs in the annulus fibrosus (AF) and nucleus pulposus (NP) have been reported, quantitative baseline FCD in healthy human CEP has not been reported.MethodsCEP specimens were regionally isolated from human lumbar spines. FCD and ion diffusivity were concurrently investigated using a two-point electrical conductivity method. Biochemical assays were used to quantify regional GAG and water content.ResultsFCD in healthy human CEP was region-dependent, with FCD lowest in the lateral region (P = 0.044). Cross-region FCD was 30% to 60% smaller than FCD in NP, but similar to the AF and articular cartilage (AC). CEP FCD (average: 0.12 ± 0.03 mEq/g wet tissue) was correlated with GAG content (average: 31.24 ± 5.06 μg/mg wet tissue) (P = 0.005). In addition, the cross-region ion diffusivity in healthy CEP (2.97 ± 1.00 × 10 cm/s) was much smaller than the AF and NP.ConclusionHealthy human CEP acts as a biomechanical interface, distributing loads between the bony vertebral body and soft disc tissues and as a gateway impeding rapid solute diffusion through the disc.Level Of EvidenceN/A.

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