Function

The joint is surrounded by a joint capsule, which divides the joint cavity and surrounding tissue into two compartments. The joint cavity is thus isolated and bacteria and other stimuli cannot penetrate; on the other hand, the synovial fluid cannot “seep” into other tissue. This is also necessary because the quality of the synovial fluid must be maintained.

The primary function of the joint mucosa, or synovialis, is to produce the synovial fluid and ensure its quality. For this purpose, the blood serum is filtered to a certain degree and thus provides the necessary fluid. The synoviocytes then add hyaluronan and other mucus-forming substances to this liquid, thus creating an increased viscosity.

In addition, the synovia also serves to feed the metabolically slow cartilage. To ensure the quality of the synovial fluid, the joint mucosa contains cells that have a phagocytizing function. They can absorb and break down cell and tissue debris. They can also fight bacteria and, if necessary, cause an inflammatory reaction.

Synovial fluid

The synovial fluid, also called synovia or colloquially “synovial fluid”, is formed by the synovocytes and is present in joint cavities, tendon sheaths and bursae. It normally has a clear, slightly yellowish color and is of viscous consistency. In case of trauma, for example meniscus tears, it can also appear bloody.

With bacterial infections it becomes purulent and yellowish. Its primary functions are on the one hand to reduce friction in the joint or tendon sheath.Its jelly-like consistency prevents the joint surfaces from rubbing too strongly against each other and thus wear out. Its second important function is to feed the joint cartilage.

This cartilage is not permeated with vessels and is therefore cut off from the blood supply. Both nutrients and oxygen only enter the cartilage by diffusion and thus reach the cartilage-forming cells. This is only possible because cartilage has a very slow metabolism, which is also known as bradytrophic. If too much synovial fluid is formed in the joint cavity, which is not absorbed quickly enough, a joint effusion can develop, which can be quite painful and impair the function of the joint.