Robert Sah, Professor, Bioengineering, University of California at San Diego
BioMechanics and MechanoBiology of Articular Cartilage and Synovial Joints:
Growth, Aging, and Osteoarthritis
Articular cartilage normally serves as a load-bearing, low-friction, and wear-resistant material that effectively covers and protects the ends of bones over a lifetime. My lab has studied the biomechanics and mechanobiology of this remarkable tissue, as it grows to a stable adult form, adapts to imposed mechanical demands, and when it fails in osteoarthritis.
The macroscopic load-bearing properties of cartilage depend on the molecular balance between the swelling properties of polyanionic glycosaminoglycans and the constraint of the dense collagen network. Growth of cartilage and the skeleton is driven in part by swelling stress that exceeds collagenous restraint. Homeostasis of healthy adult articular cartilage depends on a metabolic balance and maintenance of the depth-variation from the articular surface to the subchondral bone. Deterioration of articular cartilage biomechanical function with aging and osteoarthritis occurs at the surface and superficial zone in early stages, and then in the middle-deep zones at later stages.
The tribology of synovial joints involves the relative motion of apposing cartilage surfaces, sliding past each other in a synovial fluid solution. The low-friction and low-wear properties of articular cartilage are due in part to lubricating molecules in synovial fluid, particularly proteoglycan 4 and hyaluronan. The shear to which the articular cartilage and indwelling chondrocytes are subjected is largely governed both by the friction-dependent shear stress within, and the shear modulus of, cartilage in the superficial zone. Normally, chondrocytes respond distinctly to shear by increasing production of proteoglycan-4. The lubricating properties of synovial fluid are diminished after acute injury due to diminished concentration and altered structure of hyaluronan. Such alterations may disrupt the mechanobiology of the synovial joint.