This bulge, also called a "popliteal cyst," forms behind the knee. It's a swollen bursa. A bursa is a fluid-filled sac that helps reduce friction between soft tissues and bones. There are several of these sacs in the knee and in other joints. A swollen bursa can be uncomfortable, or even painful.
This type of cyst is caused by a buildup of synovial fluid. That's a fluid that helps your joints move smoothly. If you have a knee injury, or if you have arthritis or inflammation in the joint, your knee may make too much synovial fluid. It can collect in a bursa behind the knee. The bursa swells and bulges, forming a Baker's cyst.
Symptoms may include swelling behind your knee. Your knee may feel stiff when you move it. The bulge may be painful. If it leaks or bursts, it may cause pain, swelling and bruising on the back of your knee and calf.
Treatment depends on your needs. Your cyst may go away without treatment. If you need treatment, you may benefit from medications. Your doctor may drain fluid from the cyst. You may benefit from physical therapy. Your healthcare provider will create a plan that's right for you.