Intra-articular refers to the space inside of a joint between two bones, specifically to the portion of the joint contained by the joint capsule. Meaning “inside of a joint,” intra-articular may refer to the space itself or, in the case of the body’s movable joints, to any tissues or fluid found inside of the synovial membrane, the lining of the joint capsule. Within the synovial membrane is the synovial fluid, the lubricating fluid of the joint, as well as articular cartilage, which provides a cushion between the adjoining bony surfaces. Other joint types may feature ligaments in their intra-articular space that hold the two bones together. In synovial or movable joints, these tissues are extra-articular, or outside of the joint capsule.
The body’s joints may be classified according to either their structure or their function, but it is the structural classification that highlights the characteristics of a joint type’s intra-articular space. Fibrous joints, for example, are those that lack a joint capsule and instead feature only connective tissue holding the bones together. Teeth in their sockets and the rounded, plate-like bones forming the skull are among this joint type, and the connective tissue filling the space between keeps the adjacent bones in contact with one another with little to no movement between them.
Another structural joint classification is the cartilaginous joint, in which the intra-articular space contains only cartilage. Cartilage differs from ligaments and other connective tissue, in that it forms a dense matrix that covers the end of a bone or fills the space between bones rather than link them linearly like tiny ropes. Cartilaginous joints such as those between the ribs and the breastbone allow a greater amount of motion than do fibrous joints. The intervertebral joints of the spine, for instance, permit a small degree of movement at individual joints that translates to larger motions by segments of multiple vertebrae.
A final joint classification according to structure is the synovial joint, which in turn is divided into six sub-classifications according to the shape of the bones within the joint. Regardless of bone shape, synovial or movable joints like the elbow and knee feature the same components within their intra-articular space. These include the ends of the adjoining bones, the friction-reducing synovial fluid in the space between the bones, the cartilage between the bony surfaces for cushioning and shock absorption, and the synovial membrane surrounding the cavity. With few exceptions, ligaments are found in the extra-articular space of a synovial joint.