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Since the knee works like a hinge joint, it makes sense that lubricating the joint might help people with knee osteoarthritis.  Hyaluronic acid injections are used by surgeons to restore lubrication in osteoarthritic knee joints.   It is becoming a common form of treatment for patients with knee osteoarthritis.

Articular cartilage is the smooth lining that covers the ends of the leg bones where they meet to form the knee joint. The cartilage gives the joint freedom of movement by decreasing friction.  The articular cartilage is kept slippery by joint fluid made by the joint lining (the synovial membrane). The fluid, called synovial fluid, is contained in a soft-tissue enclosure around the knee joint called the joint capsule. An important substance present in articular cartilage and synovial fluid is called hyaluronic acid. Hyaluronic acid helps joints collect and hold water, improving lubrication and reducing friction. It also acts by allowing cells to move and work within the joint.  Osteoarthritis results in less hyaluronic acid in the synovial fluid. As a result, the joint surfaces of the knee don't get lubricated and are more likely to get injured from daily stresses and strain on the joint.  When the articular cartilage degenerates, or wears away, the bone underneath is uncovered and rubs against bone.

The missing hyaluronic acid changes the viscosity (the stickiness) and the elasticity of the synovial fluid. That's a problem because the viscosity of the synovial fluid is thought to help maintain normal joint lubrication and to protect the joint from shock and strain. When there are reduced levels of hyaluronic acid the joint may be more susceptible to injury. So injecting hyaluronic acid into the joint is used in an effort to make the osteoarthritic synovial fluid more like healthy synovial fluid.  

Research suggests that hyaluronic acid injections may stimulate the body to create additional hyaluronic acid and reduce the chemicals that cause inflammation within the synovial fluid of patients with arthritis. These anti-inflammatory properties may explain why some patients report pain relief. The injections are also thought to potentially protect or repair the cartilage cells.

Hyaluronic acid that is used in medicine is extracted from rooster coombs or purified from bacterial fermentation.  

Hyaluronic acid injections are helpful for patients with mild knee osteoarthritis who need better knee function and for patients with advanced knee arthritis who hope to prolong the time before needing a total joint replacement.  Some people have had good results for up to eight months after getting a hyaluronic acid injection.