Synovial joints possess a joint cavity that contains synovial fluid. They are freely movable, diarthrotic, joints.
Synovial joints have a number of distinguishing features:
- Articular cartilage (or hyaline cartilage): covers the ends of the bones that form the joint
- Joint cavity: this cavity is more a potential space than a real one, because it is filled with lubricating synovial fluid. The joint cavity is enclosed by a double-layered 'sleeve' or capsule known as the articular capsule: The external layer of the articular capsule is known as the capsular ligament. It is a tough, flexible, fibrous connective tissue that is continuous with the periostea of the articulating bones. The internal layer, or synovial membrane, is a smooth membrane made of loose connective tissue that lines the capsule and all internal joint surfaces other than those covered in hyaline cartilage
- Synovial fluid: a slippery fluid that occupies the free spaces within the joint capsule. Synovial fluid is also found within the articular cartilage and provides a film that reduces friction between the cartilages. When a joint is compressed by movement the fluid is forced out of the cartilage; when pressure is relieved the fluid rushes back into the articular cartilage. Synovial fluid nourishes the cartilage, which is avascular (contains no blood vessels); it also contains phagocytic cells (cells that eat dead matter) that rid the joint cavity of microbes or cellular waste. The amount of synovial fluid varies in different joints, but is always sufficient to form a thin film to reduce friction. During injury to the joint extra fluid is produced and creates the characteristic swelling of the joint. The synovial membrane later reabsorbs this extra fluid
- Collateral or accessory ligaments: synovial joints are reinforced and strengthened by a number of ligaments. These ligaments are either capsular, i.e. thickened parts of the fibrous capsule itself, or independent collateral ligaments that are distinct from the capsule. Ligaments always bind bone to bone and according to their position and quantity around the joint, they will restrict movement in certain directions, and prevent unwanted movement. As a general rule, the more ligaments a joint has, the stronger it is. Bursae are fluid-filled sacs often found cushioning the joint. They are lined by synovial membrane and contain synovial fluid. They are found between tendons and bone, ligament and bone, or muscle and bone, and reduce friction by acting as a cushion.
- Tendon sheaths are also frequently found in close proximity to synovial joints. They have the same structure as a bursa, and wrap themselves around tendons subject to friction, in order to protect them. Articular discs (menisci) are present in some synovial joints. They act as shock absorbers (similar to the fibrocartilagenous disc in the pubic symphysis). For example, in the knee joint, two crescent-shaped fibrocartilage discs called the medial and lateral menisci lie between the medial and lateral condyles of the femur and the medial and lateral condyles of the tibia.
The seven types of synovial joints are:
1) Plane or Gliding: Movement occurs when two, generally flat or slightly curved, surfaces glide across one another
2) Hinge: Movement occurs around only one axis; a transverse one - as in the hinge of the lid of a box. A protrusion of one bone fits into a concave or cylindrical articular surface of another, permitting flexion and extension
3) Pivot: Movement takes place around a vertical axis, like the hinge of a gate. A more or less cylindrical articular surface of bone protrudes into and rotates within a ring formed by bone or ligament
4) Ball-and-Socket: Ball-and-socket joints consist of a 'ball' formed by the spherical or hemispherical head of one bone that rotates within the concave socket of another, allowing flexion, extension, adduction, abduction, circumduction, and rotation. Thus, they are multiaxial and allow the greatest range of movement of all joints.
5) Condyloid: Condyloid joints have a spherical articular surface that fits into a matching concavity. Condyloid joints permit flexion, extension, abduction, adduction, and circumduction. However, the disposition of surrounding ligaments and muscles prevent active rotation around a vertical axis
6) Saddle: Both articulating surfaces have convex and concave areas, and so resemble two 'saddles' that join them together by accommodating each other's convex to concave surfaces
7) Ellipsoid: Movement permitted include flexion, extension, adduction, abduction, and circumduction