Depth-dependent compressive properties of normal aged human femoral head articular cartilage: relationship to fixed charge density

Objectives: Determine the depth-varying confined and osmotic compression moduli of normal human articular cartilage from the femoral head, and test whether these moduli are dependent on fixed charge density. Methods and Results: Using an automated instrument to allow epifluorescence microscopy analysis during confined compression testing on cartilage samples, the equilibrium confined compression modulus (H-AO) was found to vary markedly with depth (z=0-1500 mum) from the articular surface. H-AO increased from 1.16 +/-0.20 MPa in the superficial (0-125 mum) layer to 7.75 +/-1.45 MPa in the deepest (1250-1500 mum) layer tested, and was fit by the expression, H-AO(z) [MPa]=1.44 exp(0.0012.z [mum]). Also, in successive slices of cartilage extending from the articular surface to the middle-deep regions, the bulk modulus (K-O) and fixed charge density (FCD) increased, consistent with previous findings. While H-AO, K-O and FCD each varied with depth from the articular surface, the dependence of H-AO and K-O on depth did not appear to be completely related to variations in FCD. Conclusions: The confined compression modulus of normal aged human femoral head articular cartilage increases markedly with depth from the articular surface, a trend similar to that observed for articular cartilage from other joints in animals but with an absolute amplitude that is several-fold higher. The compressive properties were not simply related to FCD at different depths from the articular surface, suggesting that other as yet undefined factors also contribute to compressive properties. (C) 2001 OsteoArthritis Research Society International.