Baker cyst, which is also known as popliteal cyst, is a common cause of posterior knee pain and swelling. Baker cyst of the knee is the result of an abnormal accumulation of synovial fluid in the medial aspect of the popliteal fossa most commonly between the tendons of the medial head of the gastrocnemius and the semimembranosus muscles.

Overproduction of synovial fluid from an inflamed knee joint results in the formation of a cystic sac (Fig. 152.1). This sac often communicates with the knee joint, with a one-way valve effect causing a gradual expansion of the cyst (Fig. 152.2). Often, a tear of the medial meniscus or a tendonitis of the medial hamstring tendon is the inciting factor responsible for the development of a Baker cyst. Patients who suffer from rheumatoid arthritis are especially susceptible to the development of Baker cysts, although any form of arthritis or pathology of the synovium can cause a Baker cyst (Fig. 152.3).

Patients suffering from the pain and functional disability associated with Baker cysts complain of a feeling of fullness behind the knee. Often, they notice a lump behind the knee that becomes more apparent when flexing the affected knee. The cyst may continue to enlarge and may dissect inferiorly into the calf. Patients who suffer from rheumatoid arthritis are particularly prone to the development of large Baker cysts. Often, the pain associated with dissection of a Baker cyst into the calf may be initially misdiagnosed as thrombophlebitis and inappropriately treated with anticoagulants. Occasionally, the Baker cyst may spontaneously rupture, dissecting inferiorly along the gastrocnemius muscle, usually occurring after squatting (Fig. 152.4).

On physical examination of the patient with Baker cyst, the clinician may identify a cystic swelling in the medial aspect of the popliteal fossa (Fig. 152.5). Baker cysts can become quite large, especially in patients who suffer from rheumatoid arthritis. In some patients, flexion of the knee will make the cyst smaller (a positive Foucher sign). Activity, including squatting, flexing the affected knee, or walking, makes the pain of Baker cyst worse. Rest and heat may provide a modicum of relief. The pain of Baker cyst is constant and is characterized as aching. Sleep disturbance is common. Baker cyst may spontaneously rupture, and resulting rubor and color in the calf that may mimic thrombophlebitis are frequently present. In contradistinction to thrombophlebitis, Homans sign is negative and no cords are palpable. Occasionally, tendonitis of the medial hamstring tendon may be confused with Baker cyst.

Plain radiographs are indicated in all patients who present with knee pain to aid in the diagnosis and to rule out occult bony pathology (Fig. 152.6). Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging or ultrasound imaging of the affected area may also confirm the diagnosis and help delineate the presence of other knee bursitis, internal derangement, calcific tendonitis, synovial disease, and tendinopathy.

The popliteal fossa is posterior to the knee joint. The boundaries of the popliteal fossa are the skin, superficial fascia, and the popliteal fascia and the popliteal surface of the femur, the capsule of the knee joint, the oblique popliteal ligament, and the fascia of the popliteus muscle. The fossa contains the popliteal artery and vein, the common peroneal and tibial nerves, and the semimembranosus bursa (Fig. 152.7). The knee joint capsule is lined with a synovial membrane that attaches to the articular cartilage and gives rise to a number of bursae, including the suprapatellar, prepatellar, infrapatellar, and semimembranosus bursae, which lie between the medial head of the gastrocnemius muscle, the medial femoral epicondyle, and the semimembranosus tendon. When these bursae and/or the synovial membranes become inflamed, they may overproduce synovial fluid, which can become trapped in sac-like cysts because of a one-way valve phenomenon. This occurs commonly in the medial aspect of the popliteal fossa resulting in the formation of a Baker cyst.