Synovial joint
A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. This joint unites long bones and permits free bone movement and greater mobility. The synovial cavity/joint is filled with synovial fluid. The joint capsule is made up of an outer layer, the articular capsule, which keeps the bones together structurally, and an inner layer, the synovial membrane, which seals in the synovial fluid. They are the most  common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. Structure. Synovial joints contain the following structures: Many, but not all, synovial joints also contain additional structures: The bone surrounding the joint on the proximal side is sometimes called the "plafond", especially in the talocrural joint. A damage to this occurs in a Gosselin fracture. Blood supply. The blood supply of a synovial joint is derived from the arteries sharing in the anastomosis around the joint. Types. There are  seven types of synovial joints. Some are relatively immobile, but are more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. In ascending order of mobility, they are: Function. The movements possible with synovial joints are: Clinical significance. The "joint space" equals the distance between the involved bones of the joint. A "joint space narrowing" is a sign of either (or both) osteoarthritis and inflammatory degeneration. The normal joint space is at least 2 mm in the hip (at the superior acetabulum), at least 3 mm in the knee, and 4–5 mm in  the shoulder joint. For the temporomandibular joint, a joint space of between 1.5 and 4 mm is regarded as normal. Joint space narrowing is therefore a component of several radiographic classifications of osteoarthritis. In rheumatoid arthritis, the clinical manifestations are primarily synovial inflammation and joint damage. The fibroblast-like synoviocytes, highly specialized mesenchymal cells found in the synovial membrane, have an active and prominent role in the pathogenic processes in the rheumatic joints. Therapies that target these cells are emerging as promising therapeutic tools, raising hope for future applications in rheumatoid arthritis.