The articular capsule is found in the space where two or more adjacent bones meet to form a synovial or movable joint, such as the shoulder or knee. Typically bounded by ligaments, the fibrous connective tissue that holds the articulating bones together, it contains cartilage to cushion the bones against each other and a synovial-fluid-filled cavity to lubricate the joint against any friction created by movement. These are housed within the interior of the articular capsule’s two layers, the stratum synoviale or synovial membrane. To the outside of this membrane is the stratum fibrosum, which is made up of fibrous tissue not unlike that which makes up a ligament.
While the tissue outside of the articular capsule is fairly dense and fibrous, as in the ligaments stabilizing the joint and the stratum fibrosum, the synovial membrane is made up of a thin, soft tissue layer that contains the contents of the cavity — namely the synovial fluid. The synovial fluid, viscous and jelly-like, is secreted by the membrane itself and fills not only the space in the cavity but also any gaps in the articular cartilage, which covers the ends of the articulating bones. During movement, it is pushed around in the joint by the moving tissues, keeping the friction created by the bones rubbing past each other to a minimum.
The cartilage in the articular capsule acts as a sort of bumper, preventing the adjacent bones from making direct contact and reducing the force of impact, as in the knee during gait movement. In some synovial joints, such as the shoulder, there is some distance between the cartilage-covered ends of the articulating bones, potentially lessening the friction in the joint and allowing for a greater range of motion. The articular cartilage receives nutrients from the synovial membrane, which unlike the cartilage is permeated by capillaries delivering nutrient-filled blood, allowing it to retain its volume and elasticity.
Inflammation of the articular capsule is a widespread pathology, better known as arthritis. There are many different kinds that afflict synovial joints and damage the cartilage within, but two forms are particularly common. Rheumatoid arthritis is characterized by several conditions that irritate the cartilage by preventing it from receiving nutrients, such as an excess of synovial fluid smothering the cartilage or an overproduction of synovial cells, which may then consume all the nutrients entering the joint from the membrane. Osteoarthritis, also known as degenerative joint disease, involves a degeneration of the cartilage as a consequence of injury, disease, age, or heredity and is the number one form of arthritis.