Depth- and strain-dependent mechanical and electromechanical properties of fullthickness bovine articular cartilage in confined compression

Compression tests have often been performed to assess the biomechanical properties of fullthickness articular cartilage. We tested whether the apparent homogeneous strain-dependent properties, deduced from such tests, reflect both strain- and depth-dependent material properties. Full-thickness bovine articular cartilage was tested by oscillatory confined compression superimposed on a static offset up to 45%, and the data fit to estimate modulus, permeability, and electrokinetic coefficient assuming homogeneity. Additional tests on partialthickness cartilage were then performed to assess depth- and strain-dependent properties in an inhomogeneous model, assuming three discrete layers (i=1 starting from the articular surface, to i=3 up to the subchondral bone). Estimates of the zero-strain equilibrium confined compression modulus (HA0), the zero-strain permeability (kp0) and deformation dependence constant (M), and the deformation-dependent electrokinetic coefficient (ke) differed among individual layers of cartilage and full-thickness cartilage. HA0i increased from layer 1 to 3 (0.27 to 0.71 MPa), and bracketed the apparent homogeneous value (0.47 MPa). kp0i decreased from layer 1 to 3 (4.6×10-15 to 0.50×10-15 m2/Pa s) and was less than the homogeneous value (7.3×10-15 m2/Pa s), while Mi increased from layer 1 to 3 (5.5 to 7.4) and became similar to the homogeneous value (8.4). The amplitude of kei increased markedly with compressive strain, as did the homogeneous value; at low strain, it was lowest near the articular surface and increased to a peak in the middle-deep region. These results help to interpret the biomechanical assessment of full-thickness articular cartilage.