173 - Pathogenesis and pathology of osteoarthritis

Understanding of joint dysfunction requires some knowledge of normality and the comparison of the diseased state to the physiologic situation (Fig. 173.2). The most important requirements for normal joint function are the freedom of the opposed articular surfaces to move pain free—and largely frictionlessly—over each other within the required range of motion as well as the correct distribution of load across joint tissues. Thus, pathology in many cases might be caused by acute or chronic mechanical overloading, systematic mal-loading, or habitual underloading, leading to disuse atrophy. The correct functioning of the joint is largely dependent on its design. Joints are highly specialized organs whose properties are provided by the articular cartilage and its extracellular matrix that, under physiologic conditions, is capable of sustaining high cyclic loading. Articular cartilage covers the joint surfaces and is mainly responsible for the unique biomechanical properties of the joints. Joints are, however, complex composites of different types of connective tissue, including subchondral bone, cartilage surfaces, ligaments, and the joint capsule. The bony backbone defines the shape of the joint and the articulating surfaces—in combination with the articular cartilage— determine the absorption properties during movement. Also, the synovial capsule and, in particular, the synovial membrane (i.e., the synovial lining cell layer) vastly contribute to the physiologic functioning of the articulating joints. It is the synovial capsule together with the ligaments that provides the mechanical stability of the joints and ultimately determines their flexibility and range of motion. The synovial membrane, containing high metabolically active surface cells, the synoviocytes, plays a crucial role in nourishing the chondrocytes as well as maintaining the normal metabolic milieu within the joints by removing metabolites and matrix degradation products from the synovial space. Also, synoviocytes produce large amounts of important mediators (cytokines and growth factors), matrix degrading enzymes, as well as hyaluronic acid and other factors such as lubricin/superficial zone protein, which provide the joint surfaces with its lubrication capacity. All these factors enable maintaining the local milieu of the synovioarticular joint organ. Together all different tissues with their own functional capacities permit correct functioning and integrity of the joints.