The squeaky hinge gets the grease. In the same way, the joint that aches from osteoarthritis (OA) gets the attention. Since the knee works like a hinge joint, it makes sense that lubricating the joint might help people with knee osteoarthritis. Accordingly, a treatment called viscosupplementation has been used by doctors to restore lubrication in osteoarthritic knee joints.
What part of the knee does OA affect?
The main problem in knee OA is degeneration of the articular cartilage. 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. Small outgrowths called bone spurs, or osteophytes, may form in the joint.
What do doctors hope to achieve with this treatment?
A healthy knee contains about 20 milliliters of synovial fluid and a hyaluronic acid concentration of 2.5 to 4.0 milligrams per milliliter. Patients with knee OA have one-half to one-third less hyaluronic acid than normal.
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 viscosupplementation (injecting hyaluronic acid into the joint)is used in an effort to make the osteoarthritic synovial fluid more like healthy synovial fluid.
The idea behind hyaluronic acid injections is fairly simple. Since the synovial fluid in osteoarthritic patients is low in hyaluronic acid, the injections are intended to boost the level.
It isn’t clear how this works. The injected hyaluronic acid seems to leave the knee relatively quickly, so it’s possible doctors aren’t simply replacing missing hyaluronic acid. Research suggests that viscosupplementation may stimulate the body to create additional hyaluronic acid.
Injections of hyaluronic acid 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 chondrocytes, the cartilage cells.
Doctors suggest viscosupplementation for patients who can’t tolerate or shouldn’t take nonsteroidal anti-inflammatory drugs (NSAIDs). Patients who haven’t had success with other nonsurgical treatments, such as NSAIDs or corticosteroid injections, may also be candidates. The treatments 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.
Although this treatment is only prescribed for OA of the knee, someday it may be used to treat OA in other joints, such as the shoulder, hip, ankle, and wrist.
What might go wrong?
Studies have shown that viscosupplementation is safe. Pain, warmth, and swelling at the site of the injection are the most common complaints. These normally clear up in one to two days.
Severe inflammation is an unlikely complication, but it can occur. The joint may swell with fluid. The symptoms may mimic septic arthritis, an infection in the knee joint.
Any injection into the knee joint does carry a risk of infection. Because more than one injection is usually given, the risk of infection goes up.
What happens after treatment?
Patients are able to go about their usual activity after the procedure. Within a few days, patients may be instructed by their doctor to attend physical therapy.
When the shots work, they can provide relief for several months. Unfortunately, people generally don’t get equal relief when they go back for a second or third series of shots.
Alhough viscosupplementation appears to have a useful place in treating knee OA, it is best used along with a variety of proven strategies. Patients do best when they also
By decreasing pain and increasing joint movement in the knee, viscosupplementation may help patients maximize their ability to take care of their knee OA.