UC San Diego

Articular cartilage has depth-varying composition and structure that are manifest as depth-varying biomechanical properties. In addition, the subchondral bone region has a hydraulic permeability which is markedly increased in osteoarthritis. Thus, the effects of the depth-varying properties of articular cartilage and of the permeability of the deep boundary on the biomechanical behavior of cartilage were determined for oscillatory confined compression both experimentally and using a poroelastic multi-layer model. Homogeneous (a) and inhomogeneous (b) sample structures were both experimentally tested as well as simulated in two confined compression testing setups, one with an impermeable base (I) and a freely-permeable base (II) at 15% offset. The model and experimental results demonstrate that the high modulus and low permeability of the deep layer of cartilage normally provide a sealing effect, such that fluid pressurization and dynamic stiffness are maintained much more similarly for conditions Ib and IIb than for conditions Ia and IIa, with these differences between I and II diminishing at lower frequencies for both sample structures. In the simulation, strain and solid stress were more distributed throughout the sample depth and propagated deeper into the tissue from permeable boundary conditions in Ia and IIa than in Ib and IIb. In summary, the depth varying properties of the inhomogeneous sample and an impermeable deep boundary provide increased fluid pressurization capability that when combined have a cumulative effect, effectively shielding the tissue from large strain and stress levels