Other types of synovial joints are the ball-and-socket joint in the hip and shoulder, the hinge joint and pivot joint of the elbow, the plane joint in the wrist, the condyloid (or ellipsoidal) joint in the fingers, and the saddle joint in the thumb. Also the joint of the jaw (temporomandibular joint) is a synovial joint.

All these joints allow movement and thus require a lubrication for the joints to function properly: synovial fluid.

What is synovial fluid?

The synovial fluid in the joint capsule has four important functions:

• it keeps the bones slightly apart, protecting their cartilage coverings from wear and tear
• it absorbs shocks, again protecting the cartilage
• it lubricates the joint, helping it to work freely and easily
• it acts as a filter, letting nutrients reach the cartilage, but blocking the passage of harmful cells and substances.

The most important component of synovial fluid is a substance called sodium hyaluronate. It is this substance that lets synovial fluid perform its four different functions all at the same time. Most of the joints in your body are synovial joints. Good examples, besides the knee, include the hip and the shoulder.

What Happens in Osteoarthritis

The sodium hyaluronate in synovial fluid does not stay there for a whole lifetime, but is continuously broken down and replaced. Normally, there is an exact balance between the breakdown of old sodium hyaluronate and the production of new sodium hyaluronate. In osteoarthritis, however, this balance is disturbed and breakdown happens faster than production. As a result, the synovial fluid becomes more watery and stops working properly.

Due to the change in the synovial fluid - and for other, more complex reasons - the cartilage in the joint gradually wears away. In some places, in fact, the cartilage may eventually disappear altogether. The thinning of the synovial fluid and wearing away of the cartilage lead to the symptoms of osteoarthritis, which include pain, stiffness and swelling.