Synovial Fluid Structure and Composition in Joints

Every bone is connected to another one. The bones in the skull do not move but the rest make a coordinated movement using the muscles. During this movement, every joint created by two or more bones will get eroded causing immense pain. This is why the synovial joints are filled with synovial fluid. It is the secretion of the membrane present inside the joints. In this article, we will analyze this fluid and find out its different functions.

What is Synovial Fluid?

The fluid present inside the joints of two or more bones protecting the connecting surfaces of the bones is called synovial fluid. It is also called a synovia and is produced by the synovial membrane or synovium. This membrane is the soft tissue lining present inside covering the internal spaces, bursae, and tendon sheaths.

This liquid is viscous and non-Newtonian in nature. The thickness of this fluid enables the joints to avoid friction and to provide smooth functioning of the associated bones. Another function of this fluid is to protect the bone heads and cartilages from wearing out and to repair them in time.

Structure of the Synovial Fluid

As mentioned earlier, the synovial fluid is found in the joints trapped by the synovium. It makes an occupied space between the bones and does not let the cartilages touch to avoid friction.

This fluid also acts as an ultra-filtration medium for the plasma of blood and contains the protein derivatives manufactured by the cells present in the joint tissues. It contains hyaluronan secreted by cells that look like fibroblasts. These cells are present in the morphology of the synovial membrane.

The prime lubricant present in this membrane is proteoglycan 4. It is produced by the interstitial fluid availed by filtering the blood plasma and the surface of the chondrocytes in the articular cartilage.

This interstitial fluid accumulates to form a layer around the cartilage inside the joint of size 50 μm. It acts as the fluid reserve for the synovial fluid present inside the joint. Its prime function is to fill the air cavities and irregularities of the articular cartilage surface. When the joints move, the synovial fluid is squeezed outside the cartilages due to mechanical pressure in order to maintain the layer inside the cartilage surface.

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As per the synovial fluid analysis, its chemical composition is made of two types of cells, Type A and B. These connective tissue cells do not have basement membranes and are derived from the blood monocytes. The prime function of these cells is to remove the debris generated in the fluid due to the movement of the joints. Type B is the one that produces hyaluronic acid, lubricin, collagenases, and other proteolytic enzymes.

The synovial fluid analysis also suggests that it is non-Newtonian in nature. It means that the viscosity of the fluid is not constant and it changes without showing any linearity when force or trauma is applied. It is a brilliant example of rheopexy characteristic where continuous stress leads to the escalation of fluid thickness and viscosity. This is one reason behind how the synovial fluid function of reducing friction can be explained.

Its chemical structure reveals that it is made of hyaluronic acid, it is a disaccharide polymer and is composed of beta-1,3 and beta-1, 4 glycosidic bonds with D-glucuronic acid and D-N-acetylglucosamine. This polymer can achieve a size ranging from 5000 to 20,000,000 Dalton.

As mentioned earlier, it is synthesized by the synovial membrane and then passed to the joint cavity in order to increase the thickness or viscosity of the fluid. It also aids in increasing the elasticity of the connected articular cartilages. The prime synovial fluid function of lubricating the joints is performed by hyaluronan or hyaluronic acid.

Functions of Synovial Fluid

• The prime function of the synovial fluid is to provide lubrication to the articulating joints to avoid friction and wearing of cartilages and subsequently the heads of the bones.

• Its rheopectic nature makes it a dilatant fluid between bones resulting in a more viscous fluid to protect the joints from injuries. The movement stress is removed, the viscosity returns to normal and the lubricating function is resumed.

• The synovial fluid knee and in other joints also provide a complete connection between the connective tissues providing nutrition. It is also the medium for exchanging gases for the tissues present in the joints.

• It also acts as the medium for eliminating metabolic wastes from the adjacent tissues constructing the bone joints in the skeletal system.

• It also performs the action of a molecular sieve in producing hyaluronan and forcing it to migrate outside or inside the joints. The rate depends on the molecular weight of the polymeric hyaluronan.

Deficiency of Synovial Fluid

The synovial fluid deficiency can cause friction in the joints resulting in pain and erosion of the cartilages protecting the bone heads. The inflammation in the tissues can be reduced using anti-inflammatory drugs and antirheumatic drugs. The treatment plan depends on the cause of this deficiency.

After a comprehensive set of tests, the synovial fluid cytology report suggests the reason behind its deficiency or the inflammation in the joints. It enables the doctors to diagnose and prescribe the right medicines.

This is all about the synovial fluid structure, chemical composition, function, and deficiency. Understand the importance of this fluid in the bone joints and learn how it performs all its functions in collaboration with the adjacent membrane tissues.