What Is Synovial Fluid? Understanding Its Role in Supporting and Preserving Joints

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The human body is a marvel of engineering and biology. It can run and jump and swim and think, it can play and fight and build, it can touch and taste and dream — all thanks to a complicated array of organs and organ systems wrapped around a stack of rigid bones.

Even those bones are a wonder. They provide protection. They produce blood cells. They store minerals. They provide the structure and support that allows us to stand upright. And they come together in ingenious articulating joints that let us turn and lift and reach and do all the things we want to do.

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Most Joints Are Meant to Move

While some joints, such as skull sutures, allow for little or no movement, the first joints that come to mind when we think of joints are generally things like the shoulders, knees, hips, fingers, ankles, etc. These are all synovial joints.

We can classify synovial joints into groups according to the way they allow for movement: ball and socket, plane joint, hinge, pivot, saddle, or condyloid joint. 

Synovial joints are the most movable and most common joints in the body, but what they all have in common is a fibrous capsule that encloses the end of two bones and the space between them. The inner layer of this capsule is the synovial membrane, which is filled with cushioning synovial fluid.

What Does Synovial Fluid Do?

Synovial fluid is a viscous, clear fluid similar in consistency to egg whites. At its most basic, protein-rich synovial fluid is a lubricant that reduces friction between the articulating cartilage surfaces where two bones meet in a joint. It also helps with shock absorption. 

In addition, synovial fluid supplies oxygen and nutrients to the cartilage and removes carbon dioxide and metabolic waste. Synovial fluid plays an important role in protecting cartilage.

Why Is Cartilage Important?

Cartilage creates a smooth surface at the end of each bone in a joint, allowing the bones to more smoothly glide against one another as a joint moves. But because there is no direct blood flow to cartilage, cartilage is often unable to regenerate itself, so damage can be permanent under most conditions. 

Whether due to wear and tear (osteoarthritis) or underlying disease (rheumatoid arthritis) enough cartilage loss will eventually lead to painful bone-on-bone contact.

The Relationship Between Cartilage and Synovial Fluid

In a healthy joint, the cartilage spends its time in a sort of synovial fluid bath. Since the cartilage is porous, it actually absorbs some of this fluid, which can make up to 80 percent of the cartilage volume. So in addition to decreasing friction on the surface of the cartilage, the synovial fluid makes the cartilage into more of a cushion that helps with shock absorption. The synovial fluid is pushed out of the cartilage when pressure is placed on the joint, then reabsorbed in preparation for the next impact.

When Things Go Wrong

Arthritis, as mentioned above, is a major source of non-traumatic damage to joints. When the cartilage wears down, it does not usually repair itself,* so arthritis is thought to be irreversible. Treatments for advanced arthritis generally focus on pain relief until the pain becomes too great to bear, ultimately leading to surgical replacement with artificial joints.

An Ounce of Prevention

There was a time when osteoarthritis was considered to be the result of too much use of a joint. It was the inevitable result of years of abuse, friction, and wear and tear. Modern thinking, however, has upended that idea. Exercise actually keeps the joints healthier longer. And part of this has to do with how exercise encourages the flow of synovial fluid through the joint, keeping the cartilage pumped up. Without exercise, the synovial fluid slowly drains out of the cartilage, causing the cartilage to flatten and become more susceptible to damage.

How much exercise is needed to circulate the synovial fluid through the cartilage? There’s no need to run. Walking speeds are enough to generate the necessary hydrodynamic pressure, according to mechanical engineer David Burris. You could also try simple range of motion exercises for the knee if you are otherwise unable to walk.

Strengthening exercises for the muscles surrounding the joints are also important. A wide range of physical therapy products (such as the Home Ranger Pulley System) are available to keep your muscles, bones, and joints strong and healthy.

*Regeneration of Cartilage

As a side note, we mentioned already how damage to the cartilage is generally considered to be permanent. There are surgical treatments to regrow cartilage that show some promise, but cartilage doesn’t usually regrow on its own — except when it does. Recent studies suggest that some cartilage tissue does regenerate itself using a process similar to what allows salamanders to regrow their legs. However, this regrowth is more likely to happen further down the limb (in the ankles, for example) and much less likely in the knees and hips. And finally, non-surgical joint distraction in combination with other therapies has been proposed as a method for encouraging cartilage regeneration.