The articular cartilage of synovial joints is subject to high loads applied repetitively for many decades. Thus, the structural molecules, including collagens, proteoglycans, and other molecules of articular cartilage, must be organized into a strong, fatigueresistant, and tough, solid matrix.

This matrix must be capable of sustaining the high stresses and strains developed within the tissue from these loads. In terms of material behavior, this solid matrix is porous, permeable, and very soft. Water resides in the microscopic pores, and application of loads to the tissue forces the water through the porous-permeable solid matrix. Thus, articular cartilage is a biphasic material, composed of solid and fluid phases.