Electrical conductivity of lumbar anulus fibrosis: effects of porosity and fixed charge density.

STUDY
DESIGN: Experimental investigation of the electrical conductivity of normal and trypsin-treated lumbar anulus fibrosis specimens.
OBJECTIVES: To measure the electrical conductivity of intervertebral disc tissues and to study the effects of tissue porosity (volume fraction of water) and fixed charge density on the electrical conductivity of anulus fibrosis in physiologic saline. SUMMARY OF BACKGROUND DATA: Specific electrical conductivity is one of the material properties of intervertebral discs. Their value depends on ion concentrations and ion diffusivities within the tissue, which in turn are functions of tissue composition and structure. To our knowledge, the electrical conductivity of intervertebral discs has not been studied. Investigation of the electrical conductivity of intervertebral discs and understanding of their relationship to tissue porosity and fixed charge density will provide insights into electromechanical phenomena (e.g., streaming potential) and ion transport in intervertebral discs.
METHODS: A total of 35 porcine lumbar anulus fibrosis specimens were divided into two groups: one control group (n = 10) and one trypsin-treated group (n = 25). The specimens in the control group were subjected to one-dimensional free swelling in phosphate-buffered saline (pH 7.4), and electrical conductivity and porosity (water content) were measured over a period of about 45 minutes. The specimens in the treated group were immersed in a trypsin solution (372 U/mL phosphate-buffered saline) for 45 minutes at room temperature, and the electrical conductivity and porosity were measured after treatment. The electrical conductivity was correlated to tissue porosity for the control and treated specimens. The influences of porosity and fixed charge density were studied.
RESULTS: The average value for control specimens was 5.60 +/- 0.89 mS/cm (mean +/- SD; n = 10) before swelling and 9.11 +/- 0.90 mS/cm (mean +/- SD; n = 10) after swelling. Tissue porosity increased from 0.74 +/- 0.03 (mean +/- SD; n = 10) before swelling to 0.83 +/- 0.02 (mean +/- SD; n = 10) after swelling. The trypsin treatment reduced anulus fibrosis porosity by 3.6% (P < 0.05) and conductivity by 13% (P < 0.05) compared to those for control specimens after swelling. No significant changes werefound in wet and dry tissue densities between control and treated groups. There was a significant, linear correlation between conductivity and porosity for control anulus fibrosis specimens (R2 = 0.87; 86 measurements).
CONCLUSIONS: Measured electrical conductivity was sensitive to tissue porosity, but not to fixed charged density for anulus fibrosis specimens in phosphate-buffered saline.