A popliteal cyst is a fluid-filled mass that develops behind the knee joint due to injury or degenerative disease. It can cause swelling, tenderness, and limited range of motion. Treatment options include medication, aspiration, or surgery. Most cysts go away on their own, but a doctor should be consulted for an accurate diagnosis.
A popliteal cyst is a mass of fluid and inflamed tissue that develops behind the knee joint due to an acute injury or chronic degenerative disease. Most cysts are small and painless, but extensive damage to internal knee components can lead to swelling, tenderness, and limited range of motion. Doctors can usually shrink the cysts and fight symptoms by treating the underlying condition. Surgical aspiration or excision may be necessary if a popliteal cyst keeps coming back or causes significant pain.
Also called a Baker’s cyst after the doctor who first described it, a popliteal cyst is essentially a buildup of excess joint fluid. Small sacs called bursae, found in most joints in the body, are filled with synovial fluid that cushions and protects internal structures. If the bursa behind the knee becomes irritated and inflamed, synovial fluid seeps into the joint and leads to the development of a cyst. Arthritis is a leading cause of inflammation of the popliteal bursa, but even an injury from a direct blow or a bad fall can cause fluid buildup.
Most popliteal cysts form barely noticeable lumps in the knee fossa, but an untreated mass can potentially grow to about 2 inches (about 5 centimeters) in diameter or more. A large cyst typically feels soft and tender and may turn red or purple. Knee pain is common, although the pain is usually associated with the underlying arthritis or lesion rather than the popliteal cyst itself. The swelling can make it difficult to bend the knee and support weight on the leg. It is important to make an appointment with a doctor when knee swelling is prominent in order to receive an accurate diagnosis.
A doctor can usually diagnose a popliteal cyst by evaluating the physical appearance of the mass and performing a magnetic resonance imaging (MRI) scan. MRI results may reveal damage to the bones, cartilage, ligaments, or blood vessels inside the knee. Depending on the size of the cyst and the severity of the symptoms, different treatment options may be considered.
Most painless cysts that do not impair physical activity go untreated. They tend to go away on their own in one to six months without causing any lasting problems. If a cyst is causing discomfort, your doctor may choose to aspirate it with a specialized needle to relieve tension. When arthritis is determined to be the underlying cause, anti-inflammatory medications are usually prescribed to relieve symptoms. Surgery is rarely needed for a popliteal cyst, but a procedure may be needed to repair damaged ligaments or meniscus tissue.
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