Baker's Cyst

A 47-year-old woman complained of pain and swelling of the right knee of 3 days' duration. Positive fullness of the popliteal area with no pulsations was discerned. Doppler ultrasound showed a 2.5 cm × 1.5 cm cystic structure in the right popliteal region; this confirmed the diagnosis of a Baker's cyst.

The semimembranosogastrocnemial bursa, commonly called a cyst, lies in the posterior medial aspect of the knee, behind the femoral condyle. In 50% of affected persons, the cyst is continuous with the knee joint.

Drs Gamil Kostandy, Maged Ghaly, Hesham Taha, Rahman Ilkhaqni, and Bruce Sosler of New York Methodist Hospital, Brooklyn, report that the cyst is best seen and palpated when the patient is standing. These lesions are frequently asymptomatic and rarely infected. Swelling of the bursa is commonly associated with other knee disorders such as rheumatoid arthritis and degenerative arthritis. The differential diagnosis for posterior knee fullness includes an aneurysm of the popliteal artery. Therefore, it is important to palpate any fullness for pulsations.

Rupture of a Baker's cyst can cause vague discomfort or feeling of fullness behind the knee, acute swelling, pain, and erythema of the calf and lower leg (pseudothrombophlebitis syndrome). A Doppler study or venography will exclude thrombophlebitis; sonography, MRI, and arthrography can visualize the cyst. Rarely, an unruptured Baker's cyst can compress deep veins and cause thrombophlebitis.

This patient's uncomplicated Baker's cyst was treated with the standard therapies, which include aspiration, corticosteroid injections, bed rest, heat, leg elevation, and nonsteroidal anti-inflammatory agents. The patient was advised to avoid weight bearing and use elastic bandages. As in this case, the response to therapy is usually excellent.