A Baker's cyst, also known as a popliteal cyst, is a pocket of joint fluid that forms a lump behind the knee. Often a Baker's cyst is painless, but occasionally this pocket of fluid can tear open and drain into the tissues of the lower leg, causing pain and swelling.

Symptoms include:

• Swelling behind the knee that may increase when a person stands.
• Discomfort behind the knee and into the upper calf, especially when the knee is completely bent or straight.

When swelling and pain increase, the best approach is to avoid activities such as squatting or kneeling. Wrapping the knee may help reduce swelling. Occasionally, fluid may be drained from the knee, and the person may be given a steroid injection. In cases of restricted mobility or persistent pain, surgery to remove the cyst may be recommended.