Cartilage

Abstract

Articular or hyaline cartilage forms the bearing surfaces of the movable joints of the body. Hyaline cartilage also exists in tissues of the larynx, tracheal tube rings, rib and costral cartilage, nasal septum and in the growth plates of long bones. As a bearing surface, this tough, resilient tissue displays exceptional mechanical and tribologic properties due exclusively to the unique interaction of the constituents of the tissue extracellular matrix. Usually, the phenotypic cells (chondrocytes) of cartilage make up less than 10% of the total volume of the tissue and have not been considered to contribute to the mechanical properties of the tissue. The extracellular matrix consists of a tight collagen fiber network which contains and constrains a highly hydrophilic gel of aggregated proteo-glycan macromolecules. Collagen accounts for approximately 50% of the dry weight of the tissue, the remainder being proteoglycans and cellular material. In the fully hydrated state, water contributes 60% to 80% of the wet weight of the tissue. Mechanically, intact normal articular cartilage behaves as a linear viscoelastic solid. This behavior is the result of viscous drag of fluid through the tissue in concert with the intrinsic properties of the extracellular matrix. Further, fluid exudation across the cartilage surface in response to physiologic loading is thought to play a significant role in the lubrication of joints. The importance of articular cartilage as a bearing surface has led to extensive mechanical and tribologic studies of this tissue.