Extra-articular is an anatomical term used to refer to the joint-related structures outside of a joint. While there are several joint classifications, joints typically are made up of the adjacent surfaces of two or more articulating bones. Joints also have intra-articular spaces between them, which contain connective tissue, cartilage, a fluid-filled joint capsule, or more than one of the above. Finally, the extra-articular space of a joint is not found immediately between the bones but to the outside of the joint itself, often filled with soft tissue like ligaments that surround the joint and hold the bones together.
The body’s movable joints, like those found in the fingers, shoulders, hips, and knees, are known as synovial joints. These typically feature a membrane-lined, fluid-filled cavity in the joint’s intra-articular space called the joint capsule, which also contains cartilage to prevent the bones from making direct contact with one another. The first layer to the outside of the joint capsule is included among the extra-articular structures and is a fibrous membrane that protects the joint. This membrane is made largely of collagen and elastin fibers, which is a similar composition to ligaments.
Ligaments are also found in the extra-articular space, and they may be continuous with the fibrous membrane surrounding the joint. The function of ligaments is first and foremost to hold the bones together and provide stability to the joint. In the knee, for instance, a ginglymoid or hinge joint, the collateral ligaments run vertically along either side of the joint and help stabilize it against lateral forces on the knee, such as a football player colliding with the side of another player’s leg.
Many joints, the knee included, feature several accessory structures in their extra-articular space. One such tissue is the bursa. Like tiny pillows filled with synovial fluid instead of feathers, bursae are situated to the outside of joints and decrease friction between exterior joint structures, such as between a bone and the muscle tendon that runs over it. Another accessory joint structure is the fat pad, a cushion of adipose tissue or fat that fills the gap between adjacent bones, a gap similar to a weak point in a suit of armor. In doing so, it helps eliminate wearing on the joint cartilage as is seen at the knee joint between the kneecap and the tibia bone in the leg.
The articulating surfaces of the bones in the joint may themselves be considered extra-articular structures. These surfaces may be flat and close together to allow sliding movements, or they may fit around each other like puzzle pieces and produce movement in multiple directions. Muscle tendons are also sometimes counted as extra-articular tissues as they cross just to the outside of joints and make joint movement possible. Tendons attach muscle to bone and pull on the bone when the muscle contracts, causing the joint to move.