The knee is a hinge-type of joint, and may be thought of has having three compartments; the inner, outer and front compartments. There also normally is a thin, smooth covering of cartilage over the bones in the joint that allows normal, smooth and painless motion. The lining of the knee produces a lubricating fluid, called synovial fluid. Most joints are ‘water tight’, and the lubricating fluid is contained within the joint.
Certain conditions (such as arthritis or a torn knee cartilage) may cause an increase in the amount of knee fluid, and the knee swells up causing ‘water on the knee’. If the muscles behind the knee form an opening into the back of the knee, increased fluid may be pushed into a small fluid filled sac (called a cyst) behind the knee. With motion and pressure, fluid is pushed out of the knee and into the cyst where it is unable to re-enter the knee. This acts as a type of one-way valve, and with increased fluid or pressure in the joint, more and more fluid builds up in the cyst.
Although many Baker’s cysts are painless, if they grow in size, an uncomfortable sensation of pressure felt behind the knee, especially when the knee is bent, or with squatting. Sometimes if the pressure is too high, the cyst can burst, causing the joint fluid to escape into the calf muscle causing severe pain and inflammation.
A knee examination shows a bulge or swelling and occasionally tenderness behind the knee.
Most Baker’s cysts do not require treatment. If the cyst is painfully large, treatment is directed at reducing the amount of fluid the knee produces, often by using cortisone injections into the knee joint. If inflammatory diseases such as rheumatoid arthritis or gout are causing the fluid to build up, these diseases may need to be controlled with medications. Cartilage tears may need repair. The Baker’s cyst is not injected with cortisone. If the symptoms from the Baker’s cyst are severe, and don’t respond to other measures, surgery may be necessary.