Cartilage and diarthrodial joints as paradigms for hierarchical materials and structures

The anatomic forms of diarthrodial joints are important structural features which provide and limit the motions required for the joint. Typically, the length scale of topographic variation of anatomic forms ranges from 0.5 to 15 cm. Articular cartilage is the thin layer of hydrated soft tissue (0.5-5.0 mm thick) covering the articulating bony ends in diarthrodial joints. This tissue has a set of unique mechanical and physicochemical properties which are responsible for its load-carrying capabilities and near-frictionless qualities. The mechanical properties of articular cartilage are determined at the tissue-scale level and these properties depend on the composition of the tissue, mainly collagen and proteoglycan, and their molecular and ultrastructural organization (ultra-scale: 10(-8)-10(-6) m). Because proteoglycans possess a high density of fixed negative charges, articular cartilage exhibits a significant Donnan osmotic pressure effect. This physicochemically derived osmotic pressure is an important component of the total swelling pressure; the other component of the total swelling pressure stems from the charge-to-charge repulsive force exerted by the closely spaced (1-1.5 nm) negative charge groups along the proteoglycan molecules. Thus these interactions take place at a nano-scale level: 10(-10)-10(-9) m. Finally, cartilage biochemistry and organization are maintained by the chondrocytes which exist at a micro-scale level (10(-7)-10(-6) m). Significant mechanoelectrochemical transduction occurs within the extracellular matrix at the micro-scale level which affects and modulates cellular anabolic and catabolic activities. At present, the exact details of these transduction mechanisms are unknown. In this review, we present a summary of the hierarchical features for articular cartilage and diarthrodial joints and tables of known material properties for cartilage. Also we summarize how the multi-scale interactions in articular cartilage provide for its unique material properties and tribological characteristics.