Mylène Jansen

10 Chapter 1 The knee joint The knee is a synovial joint: the articulating bones forming the joint are covered by a layer of hyaline articular cartilage, and united by a joint capsule that consists of an outer fibrous layer and inner synovial membrane. 1 Bone Bone is hard, calcified connective tissue, and is fully innervated and vascularized. 2 It possesses an intrinsic capacity for repair and undergoes constant regeneration and remodeling in response to loading and unloading. 3,4 Its function is not only mechanical, but metabolic as well, storing important minerals and secreting cytokines and growth factors. 5 At the endings of articulating bones, an outer cortical bone plate (or subchondral bone plate) with underneath more porous trabecular bone make up the subchondral bone, which is covered by cartilage. 6 Cartilage Articular cartilage is hyaline in nature, with exceptional resilience and tensile strength and an extremely low friction coefficient, responsible for providing a smooth surface for articulation, load distribution and resistance to compressive forces during normal and high impact loading. It consists mainly of extracellular matrix (ECM), produced and maintained by a relatively small number of chondrocytes. ECM is primarily composed of water and macromolecules such as collagen, predominantly collagen type II, which forms an extensive network in which large aggrecan molecules are entrapped, providing cartilage with mechanical integrity. Cartilage is considered to have a limited, low rate, regeneration capacity and is aneural and avascular, depending on diffusion from the surrounding synovial fluid for its nutrition. 7–9 Synovium The synovium, or synovial membrane, lines the inner surface of synovial joints. It is made up of a thick fibrous outer layer and a thin inner layer called the intema, which consists of cells called synoviocytes and is in direct contact with the synovial fluid inside the joint. The synovium is responsible for maintaining joint homeostasis through this synovial fluid, which in turn provides lubrication of cartilage surfaces and nutrition of chondrocytes. 10,11 It is the integrated activity between these 3 tissues (in addition to menisci, ligaments, muscles and tendons stabilizing the joint), all in contact with each other mechanically and biochemically, that determines the condition of a knee joint.