Articular cartilage under hydrostatic loading

In this study, we propose to determine the exact state of stress acting within articular cartilage under hydrostatic conditions. To do so, we must first provide an experimental verification of the intrinsic incompressibility conditions assumed in the KLM biphasic theory for cartilage developed by V.C. Mow and coworkers. This theory accounts for solid matrix deformation, interstitial fluid (water) flow, and frictional interaction between the porous-permeable solid matrix and the interstitial fluid during flow when cartilage is loaded. A fundamental assumption of this theory is that both phases, water and solid matrix, are intrinsically incompressible, i.e., existing by themselves the water and the solid matrix are incompressible. While experiments do show water to be incompressible at physiological pressures (< 10 MPa), the incompressibility of the solid matrix of cartilage has not been verified experimentally. This absence of data is due to the difficulty of actually testing a piece of the solid matrix in the absence of water. Thus, verification of the assumption of intrinsic incompressibility of the solid matrix must be ascertained from testing the tissues at full hydration. Hence, the two objectives of this investigation are: 1) to verify the intrinsic incompressibility of the solid matrix of cartilage and 2) to determine the exact states of stress in cartilage when loaded under a pure hydrostatic condition.