Instructional Course Lectures, The American Academy of Orthopaedic Surgeons - Articular Cartilage. Part I: Tissue Design and Chondrocyte-Matrix Interactions*†

In 1892, Walt Whitman observed that “the narrowest hinge in my hand puts to scorn all machinery.”78 Despite the remarkable advances in joint replacement, Whitman's observation stands unchallenged; no current prostheses come close to duplicating the function and durability of synovial joints. These complex structures, developed and progressively refined over hundreds of millions of years1, are formed by an arrangement of multiple distinct tissues, including joint capsule, ligament, meniscus, subchondral bone, synovial tissue, and hyaline articular cartilage. These tissues are self-renewing, respond to alterations in use, and provide stable movement with a level of friction less than that achieved by any prosthetic joint. The tissue that contributes the most to these extraordinary functional capacities is the hyaline articular cartilage15,19. It varies in thickness, cell density, matrix composition, and mechanical properties within the same joint, among joints, and among species2; however, in all synovial joints it consists of the same components, has the same general structure, and performs the same functions. Although it is at most only a few millimeters thick, it has surprising stiffness to compression and resilience; it also has an exceptional ability to distribute loads52,53, thereby minimizing peak stresses on subchondral bone. Perhaps most important, it has great durability; in most people, it provides normal joint function for eighty years or more. No synthetic material approaches this level of performance. Grossly and histologically, adult articular cartilage appears to be a simple inert tissue. When examined from inside a synovial joint, normal articular cartilage appears as a slick firm surface that resists deformation. Light microscopy shows that it consists primarily of extracellular matrix, with only one type of cell, the chondrocyte, and that it lacks blood vessels, lymphatic vessels, and nerves (Fig. 1). Compared with tissues …