A pocket of fluid that develops behind the knee — also known as a popliteal cyst. In a Baker’s cyst, osteoarthritis or rheumatoid arthritis affecting the knee, a torn meniscus or other cartilage tissue, or some other other knee problem causes excess joint fluid to push into one of the small sacs behind the knee. This may cause tightness or stiffness behind the knee, swelling of the knee that gets worse when standing, and mild pain behind the knee and upper calf.
It is important for anyone with swelling in the back of the knee to see a doctor to rule out deep vein thrombosis (a blood clot), which can be dangerous and warrants immediate medical attention.
The doctor may use X-rays to detect arthritis and MRIs to visualize the cyst and detect any injury to the meniscus. In some cases, no treatment is needed, and the Baker’s cyst goes away on its own. If the cyst is painful, doctors try to treat the underlying problem, such as arthritis or a torn meniscus. He or she may also drain the cyst with a needle or inject a corticosteroid medication to reduce the swelling. In rare instances, the cyst may be surgically removed. Some people find relief from painful symptoms by resting the knee, using a cane or a crutch, and taking over-the-counter painkillers such as ibuprofen or naproxen.