CorpusHuman Anatomy

Synovial Joint — Structure

Schematic longitudinal section

A generic synovial (diarthrotic) joint cut in the long axis: a convex bone end (top) articulates with a concave bone end (bottom). Each bone end has an outer shell of compact bone and an interior of spongy (trabecular) bone, and its articular surface is capped by avascular hyaline articular cartilage. The two bones are united by a two-layered joint capsule: the outer fibrous capsule (continuous with the periosteum) and the inner synovial membrane, which lines every intracapsular surface except the articular cartilage. The narrow joint cavity contains a thin film of synovial fluid.

Outside the capsule an extracapsular ligament limits movement, and on the other side a tendon glides over the joint on a bursa — a flattened synovial sac that reduces friction. The animation shows cyclical loading: as the bones are pressed together, the cartilage is compressed and fluid is squeezed within the joint space (squeeze-film and boundary lubrication; the pumping also nourishes the avascular cartilage).

Step-by-step

  1. Bone ends

    Two bone ends meet at the joint. Each has a shell of compact bone around spongy bone whose trabeculae are aligned with the lines of stress. Outside the joint, the bone is covered by periosteum; its fibrous layer is continuous with the fibrous capsule of the joint.

  2. Articular cartilage

    The articulating surfaces are capped by hyaline cartilage (≈2–4 mm in large joints). It has no blood vessels, nerves or perichondrium, gives an almost frictionless surface and distributes load. It is nourished by diffusion from synovial fluid, so it heals poorly once damaged (osteoarthritis).

  3. Fibrous capsule

    A sleeve of dense irregular connective tissue attached around the articular margins of both bones and continuous with their periosteum. It encloses the joint cavity, resists separation of the bones and is richly innervated (Hilton's law: the nerves supplying a joint also supply the muscles moving it and the skin over their insertions).

  4. Synovial membrane

    A thin, vascular areolar membrane with a surface layer of synoviocytes (type A macrophage-like and type B fibroblast-like cells). It lines the inner surface of the capsule and every non-articular surface inside the joint, stopping at the edge of the articular cartilage. It secretes synovial fluid and is the primary site of inflammation in rheumatoid arthritis (pannus).

  5. Synovial fluid

    A small volume (≈0.5–4 mL in the knee) of clear, viscous plasma dialysate enriched with hyaluronan and lubricin. It lubricates the joint (boundary and squeeze-film lubrication), absorbs shock and supplies nutrients and O₂ to the chondrocytes. Normal fluid has <200 white cells/µL; joint aspiration is diagnostic in septic arthritis and gout.

  6. Ligaments

    Ligaments reinforce the capsule. They may be capsular thickenings, extracapsular (as here) or intracapsular. Together with bony shape and muscle tone they determine joint stability; sprains are partial or complete tears of ligament fibres.

  7. Bursa and tendon

    A bursa is a closed, flattened sac lined by synovial membrane and containing a film of synovial fluid. It lies where a tendon, muscle or skin moves over bone or another structure, cushioning and reducing friction. Overuse or pressure causes bursitis (e.g. prepatellar, olecranon, subacromial bursitis).

Labelled parts