The synovial membrane (or synovium) is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints. The word "synovium" is related to the word "synovia" (synovial fluid), which is the clear, viscid, lubricating fluid secreted by synovial membranes.
The synovium is variable but often has two layers:
- The outer layer, or subintima, can be of almost any type: fibrous, fatty or loosely "areolar."
- The inner layer, or intima, consists of a sheet of cells thinner than a piece of paper.
Where the underlying subintima is loose, the intima sits on a pliable membrane, giving rise to the term synovial membrane. This membrane, together with the cells of the intima, provides something like an inner tube, sealing the synovial fluid from the surrounding tissue (effectively stopping the joints from being squeezed dry when subject to impact, such as running).
The intimal cells are of two types, fibroblasts and macrophages, both of which are different in certain respects from similar cells in other tissues.
- The fibroblasts manufacture a long-chain sugar polymer called hyaluronan, which makes the synovial fluid "ropy"-like egg-white, together with a molecule called lubricin, which lubricates the joint surfaces. The water of synovial fluid is not secreted as such but is effectively trapped in the joint space by the hyaluronan.
- The macrophages are responsible for the removal of undesirable substances from the synovial fluid.
The surface of the synovium may be flat or may be covered with finger-like projections or villi, which, it is presumed, help to allow the soft tissue to change shape as the joint surfaces move one on another. Just beneath the intima, most synovium has a dense net of small blood vessels that provide nutrients not only for synovium but also for the avascular cartilage. In any one position, much of the cartilage is close enough to get nutrition direct from synovium. Some areas of cartilage have to obtain nutrients indirectly and may do so either from diffusion through cartilage or possibly by "stirring" of synovial fluid.
Although a biological joint can resemble a man-made joint in being a hinge or a ball and socket, the engineering problems that nature must solve are very different because the joint works within an almost completely solid structure, with no wheels or nuts and bolts. In general, the bearing surfaces of manmade joints interlock, as in a hinge. This is rare for biological joints.
More often the surfaces are held together by cord-like ligaments. Virtually all the space between muscles, ligaments, bones, and cartilage is filled with pliable solid tissue. The fluid-filled gap is at most only a twentieth of a millimetre thick. This means that synovium has certain rather unexpected jobs to do. These may include:
- Providing a plane of separation, or disconnection, between solid tissues so that movement can occur with minimum bending of solid components. If this separation is lost, as in a "frozen shoulder," the joint cannot move.
- Providing a packing that can change shape in whatever way is needed to allow the bearing surfaces to move on each other.
- Controlling the volume of fluid in the cavity so that it is just enough to allow the solid components to move over each other freely. This volume is normally so small that the joint is under slight suction.
Synovium can become irritated and thickened in conditions such as rheumatoid arthritis. When this happens, the synovium can become a danger to the bearing surface structure in a variety of ways. Excess synovial fluid weeping from inflamed synovium can provide a barrier to diffusion of nutrients to cartilage. The synovial cells may also use up nutrients so that the glucose level in the tissue is almost zero. These factors may lead to starvation and death of cartilage cells. Synovial cells may also produce enzymes that can digest the cartilage surface, although it is not clear that these will damage cartilage with healthy cells.