Diarthroses

Diarthroses, also known as synovial joints, are the most movable class of joints in the human body. They feature a fluid-filled joint cavity that separates the articulating ends of bones, which are covered by articular cartilage. This arrangement allows a wide range of motion while reducing friction and load on the bones.

The structural elements of a diarthrosis include a joint capsule composed of an outer fibrous layer and an inner synovial membrane. The capsule is reinforced by ligaments that stabilize the joint. The synovial membrane secretes synovial fluid into the cavity, providing lubrication and nutrients to the avascular articular cartilage. Accessory components such as menisci or articular discs, fat pads, bursae, and tendon sheaths may augment stability, shock absorption, and movement.

Movement in diarthroses is classified by the number of axes of rotation. Uniaxial joints move along one axis (for example, hinge and pivot joints). Biaxial joints move along two axes (such as condyloid and saddle joints). Triaxial joints allow movement in three perpendicular axes (ball-and-socket joints). Examples include the hinge joint of the elbow, the pivot joint between the atlas and axis, the metacarpophalangeal joints, the thumb carpometacarpal joint, and the shoulder and hip ball-and-socket joints. Plane (gliding) joints also occur within this category.

Developmentally, diarthroses form from mesenchymal tissue, with the joint capsule and synovial lining arising to create a lubricated, highly movable articulation. Clinically, diarthroses are sites of common pathology, including osteoarthritis, rheumatoid arthritis, gout, and traumatic injuries such as sprains.