Synovium is also known as synovial fluid and has a high viscosity. In addition to nourishing the joint, its functions include reducing friction on the joint surfaces. In joint diseases such as osteoarthritis, the composition of synovial fluid changes.
What is the synovium?
The medical profession uses the term synovia to describe the lubricating fluid in real joints. This synovial fluid is also found in tendon gliding devices such as bursae and tendon sheaths and is formed by the membrana synovialis. This is the inner layer of the joint capsule of a joint. On the joint surfaces, the synovia forms a film that ensures smooth gliding. The term has been around since the 16th century. At that time, the physician and alchemist Paracelsus composed it from the Greek conjunction “syn” and the noun “ovia” for protein. Literally translated, synovia therefore means “together with protein”. This designation already gives an indication of the composition of the viscous fluid. Not all joints contain the same amount of synovia. The amount varies with the expected friction.
Anatomy and structure
The synovium of a healthy joint is viscous, yellowish, and clear. Ninety-four percent of the fluid is water. The pH of synovial fluid is about 7.5. Synovia is obtained from blood plasma. Its electrolyte composition was therefore very similar to that of plasma. Various plasma proteins as well as plasma enzymes and acid phosphatase are found in it. Plasma proteins include both albumins and globulins. As a relic of the membrana synovialis, the fluid also contains mucilages such as hyaluronic acid. This acid gives the synovial fluid its viscosity through pressure resistance, water-binding capacity and adhesive effects. Glucose and glycosaminoglycans ensure viscosity that varies with load.
Function and tasks
The synovium fulfills two main functions. First, it nourishes the articular cartilage with glucose. Second, fewer frictional forces occur due to the fluid. In addition, the viscous mixture fulfills a shock-absorbing effect and thus ensures that the joints do not suffer any damage. Without synovia, the joints in the human body would show signs of wear after a very short time and would thus disintegrate bit by bit. The viscosity of the synovial fluid changes with the load exerted. Hyaluronic acid is responsible for this. If, for example, the shear forces become stronger, the viscosity of the hyaluronic acid decreases and thus creates a balance. Since the acid is actually liquid, the synovial fluid remains highly molecular in shape. This high-molecular form means that the viscosity is sufficient to prevent water loss in the joint due to pressure movements. Because of chemical interactions, hyaluronic acid adheres ideally to the cartilage of a joint. Depending on the movement, the molecules in the lubricant bind together to form sphere-like structures as soon as strong compressive forces act on the joint. As spheres, they hang on the surface of the articular cartilage. This property is particularly important for movements such as jumping. During rapid movements or sudden shearing movements, the toughness of the synovial fluid is reduced. This reduction lowers the friction in the joint. In this way, synovial fluid protects each joint from wear and tear and high stresses because it adapts to the movement at hand and can change its shape within seconds. Fluid exchange and cartilage nourishment occurs through alternating stress and strain. If a joint has to be immobilized for a long time, this coordinated system of loads and unloads is disrupted. As a result, the nutrition of the articular cartilage is also disturbed. Therefore, cartilage damage occurs due to malnutrition.
Diseases
Synovium may undergo pathological changes in its composition and quantity. Such a phenomenon is present, for example, in diseases such as osteoarthritis, but also in other joint diseases. This is a degenerative joint disease. The organism reacts to all joint changes with an excessive production of synovia. This phenomenon is also known as joint hydrops and can occur, for example, as a result of an inflammatory process in the joint or after a sign of wear and tear in the joints. Due to the excess of synovial fluid, the joint henceforth appears swollen from the outside.Either an actual effusion or a swelling of the capsule is present. Simultaneously with overproduction, the synovium also changes its composition. The synovial fluid becomes more watery. Either the fluid becomes cloudy due to cellular debris or it even turns dark due to hemorrhage. If hemorrhages are present, the synovia even has an aggressive effect on the articular cartilage. Because of the lost viscosity, the synovial fluid can no longer fulfill its tasks. If there are no color changes and the synovial fluid is still clear, there is still a functional loss. The capsule of the joint can overstretch as a result. This is then referred to as an irritable effusion, which is all too common in the context of osteoarthritis. The fluid can be drained from the affected joint by puncture. Laboratory analysis of the drained fluid may reveal various changes in the composition of the synovium. In arthritis, inflammation can be detected through the synovial fluid. In gout, laboratory analysis shows evidence of a disturbance in uric acid metabolism.
