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Anisotropic Ion Diffusivity in Intervertebral Disc: An Electrical Conductivity Approach

Study Design. Investigation of the transport behavior of ions in intervertebral disc using an electrical conductivity method. Objectives. To determine the electrical conductivity and ion diffusivity of nucleus pulposus and anulus fibrosus in 3 major directions (axial, circumferential, and radial). Summary of Background Data. Knowledge of diffusivity of small molecules is important for understanding nutrition supply in intervertebral disc and disc degeneration. However, little is known on the anisotropic behaviors of ion diffusivity and of electrical conductivity in intervertebral disc. Methods. Electrical conductivity measurement was performed on 24 axial, circumferential, and radial anulus fibrosus specimens and 24 axial nucleus pulposus specimens from bovine coccygeal discs. The diffusivity of Na+ and Cl− were estimated by the analysis of conductivity data. Results. The electrical conductivity (mean ± standard deviation; n = 24) of the bovine anulus fibrosus was 4.70 ± 1.08 mS/cm in the axial, 2.86 ± 0.83 mS/cm in the radial, and 4.38 ± 1.25 mS/cm in the circumferential direction. For nucleus pulposus, the electrical conductivity (mean ± standard deviation; n = 24) was 8.95 ± 0.89 mS/cm. The mean value for nucleus pulposus was significantly higher than that of anulus fibrosus (t test, P < 0.05). For anulus fibrosus, the conductivity in the radial direction was significantly lower than in axial or circumferential directions. Similar trends were found for both Na+ and Cl− diffusivities. Both electrical conductivity and ion diffusivity were highly sensitive to water content. Conclusions. Electrical conductivity and ion diffusivity of anulus fibrosus are anisotropic.