Synovium - Synovial Fluid

The synovium (synovial fluid) is found in most joints and consists of two layers: the Subintima (outer layer) and the Intima (inner layer).

The synovium (synovial membrane) is the soft tissue found between the joint capsule (articular capsule) and the joint cavity of synovial joints. The synovium secretes a clear, viscid fluid (synovial fluid) which provides for an additional source of lubrication of the joints between the articulating bones, articular cartilage, and supportive ligaments. The synovium are located in the majority of the moving joints of the body.

The synovium has two layers
  • Subintima (outer layer), which may be composed of fatty, loosely "areolar" tissue, or fibrous tissue
  • Intima (inner layer) which consists of a sheet of cells only a few millimeters thick
When the subintima is loose, the intima sits on a pliable membrane, giving rise to the term synovial membrane. This membrane forms a capsule that contains the synovial fluid within from escaping when weight is put on the joint. Though the two membranes together are extremely thin, they are very important to the health of the joint and the protection of its soft tissues and articulating bones against friction and erosion.
The composition of the intima cells includes macrophages and fibroblasts. The macrophages are responsible for maintaining the purity of the synovial fluid by absorbing and removing debris and other contaminants. The fibroblasts produce hyaluronan, which work together with another molecule called lubricin to lubricate the joint surfaces. The hyaluronan, which is a long-chain sugar polymer, traps water in the joint space to maintain fluid levels as weights and pressures are put on the synovial joints as we use them.

The surface of the synovium may be flat or covered with villi, which allows it to change its shape to adapt to the movement of the joints. The intima is generously supplied by blood vessels, which supply nutrients to the synovium as well as the avascular cartilage. This network of blood vessels is able to supply nutrients to the synovium regardless of the angle of the joint and the shape of the joint surfaces. Other types of cartilage in certain joints remain further from the nutrient supplying blood vessels. These cartilage tissues receive nutrients from diffusion across tissues or possibly a "stirring" of synovial fluid.

Mechanics. The majority of the musculoskeletal system is composed of solid matter of and between structures including cartilage, bones, ligaments, and muscles. The synovial fluid and its paper thin synovial membrane, in contrast, is less than a millimeter in thickness. Despite its nearly microscopic size, this fluid-filled membrane has a lot of responsibility in protecting the bones and soft tissues of the joints they occupy. The jobs of the synovium and synovial membrane include:
  1. This lubricating membrane separates solid moving parts so that they may be able to move without friction against one another, and a minimum of bending. If this membrane becomes inflamed or deteriorates, the joint may freeze, and be unable to move, as a result of this loss of separation.
  2. The composition of the two layers of the synovium allows it to be pliable enough to change shape in order to continue to support the moving parts of the joint as the slide, bend, or turn against one another.
  3. The synovial membrane is very efficient, providing an excellent amount of lubrication and support of the solid components while taking up a very small amount of space in the joint. This volume is typically so small that the joint is under slight suction.
Synovium, Synovial Fluid, and back pain: We may experience back pain and back problems associated with injury or inflammation to synovial joints in the spine. Due to degenerative changes in the spine, synovial cysts may develop, which may impact the spinal cord itself or other soft tissues in the spine. Synovial cysts may develop for several reasons, though it will be more likely the result of facet joint degeneration in the lumbar spine (lower back).