Synovial fluid

Composition and functions of synovial fluid

The synovial fluid is a clear fluid, not very stringy and viscous, which thanks to its lubricating action protects the articular surfaces from diarthrosis from wear and wear.

Diarthroses are the most common joints in the human body. Also called synovial, the diarthrosis joints enjoy a high degree of joint mobility, allowing movements in one or more directions of space. As shown in the figure, in diarthroses the articular surfaces are covered by a sheath of fibrous connective tissue, called the joint capsule, coated on the inside by the synovial membrane. Among the bony heads that form the joint, and the aforementioned joint capsule, there exists a more or less wide virtual space, filled with a thin film of synovial fluid, which in the knee joint, the largest in the body, does not exceed i 3-4 ml. This thin veil of fluid is placed to protect the cartilaginous structures; in addition to its precious lubricating action, synovial fluid also has nutritional properties for the cartilage itself.

The physical characteristics of plasticity, elasticity and viscosity of the synovial fluid are guaranteed by its particular composition, in which lubricin and hyaluronic acid (glocosaminoglycan formed by N-acetylglycosamine and glycuronic acid) abound. Being a blood plasma dialysate, the synovial fluid also contains electrolytes, glucose, immunoglobulins (antibodies) and proteins of haematic origin. Some of these components are produced at the level of the synovial membrane, consisting of very loose connective tissue, in which blood and lymphatic vessels run; the former are important for the production of synovial fluid, while the latter facilitate the reabsorption of any intra-articular effusions.

Certain components of the synovial fluid, as anticipated, are produced by specialized cells present on the synovial membrane, called synoviocytes. Some of these cells (type A) are used to gobble up any cellular or other debris, while the synthesis activity itself is the responsibility of type B synoviocytes.

The synovial fluid is also contained within the so-called mucous bags, small pockets interposed at the points of greatest friction between tightly attached articular structures.

Examination of synovial fluid

The changes in volume and composition of synovial fluid are closely related to the various joint pathologies. Consequently, by taking small samples of liquid through thin needles connected to syringes (arthrocentesis), doctors can study their composition, identifying specific cytochemical markers of joint damage (arthritis, cartilage degeneration, gout etc.). Also the evaluation of color, volume, viscosity and transparency of synovial fluid can provide valuable diagnostic elements.