Tetrapolar Measurement of Electrical Conductivity and Thickness of Articular Cartilage

A tetrapolar method to measure electrical conductivity of cartilage and bone, and to estimate the thickness of articular cartilage attached to bone, was developed. We determined the electrical conductivity of humeral head bovine articular cartilage and subchondral bone from a 1- to 2-year-old steer to be $1.14±0.11S/m$$(mean±sd,$$n=11)$ and $0.306±0.034S/m,$$(mean±sd,$$n=3),$ respectively. For a 4-year-old cow, articular cartilage and subchondral bone electrical conductivity were $0.88±0.08S/m$$(mean±sd,$$n=9)$ and $0.179±0.046S/m$$(mean±sd,$$n=3),$ respectively. Measurements on slices of cartilage taken from different distances from the articular surface of the steer did not reveal significant depth-dependence of electrical conductivity. We were able to estimate the thickness of articular cartilage with reasonable precision $(<20%$ error) by injecting current from multiple electrode pairs with different inter-electrode distances. Requirements for the precision of this method to measure cartilage thickness include the presence of a distinct layer of calcified cartilage or bone with a much lower electrical conductivity than that of uncalcified articular cartilage, and the use of inter-electrode distances of the current injecting electrodes that are on the order of the cartilage thickness. These or similar methods present an attractive approach to the non-destructive determination of cartilage thickness, a parameter that is required in order to estimate functional properties of cartilage attached to bone, and evaluate the need for therapeutic interventions in arthritis.