The articular capsule is a sleeve-like capsule that surrounds the whole joint, ensuring that the space between the bones remains a sealed environment. The capsule also serves to keep the two bones connected to one another. The capsule is composed of two layers. The outer layer (the fibrous capsule) s made of very dense connective tissue. This tissue attaches to the periosteum of the bones. The fibrous capsule is very flexible to allow relatively unrestricted movement, yet incredibly strong to offer sufficient support to prevent dislocation. The inner part of the articular capsule is known as the synovial membrane and is made up of softer connective tissue.
It is worth just having a brief description of cartilage as understanding its basic (and somewhat simplistic) structure will help a little later on. It is composed of several factors. The first components are its cells. These are called chondrocytes. These cells are surrounded by an extracellular matrix that holds them in place. This matrix is composed of water and key proteins such as collagen and glycosaminoglycans. The matrix is flexible and gives the articular cartilage the ability to compress and bear the load as the joint moves.
The synovial membrane (inner part of the articular capsule) secretes a fluid called synovial fluid. This viscous pale fluid is said to resemble the white of an egg. It is made up of hyaluronic acid and interstitial fluid and serves to offer lubrication and reduce friction within the joints. Synovial fluid also contains phagocytes that remove debris and metabolic waste that is a normal consequence of wear and tear within the joint.