Injection Technique for Baker Cyst

Indications and Clinical Considerations

Baker cyst of the knee is the result of an abnormal accumulation of synovial fluid in the medial aspect of the popliteal fossa. Overproduction of synovial fluid from the knee joint results in the formation of a cystic sac ( Figs. 166.1 and 166.2 ). This sac often communicates with the knee joint, with a 1-way valve effect causing a gradual expansion of the cyst. Often, a tear of the medial meniscus or a tendinitis of the medial hamstring tendon is the inciting factor responsible for the development of a Baker cyst ( Fig. 166.3 ). Patients with rheumatoid arthritis are especially susceptible to the development of Baker cysts ( Fig. 166.4 ).

FIG. 166.1, Proper needle position for injection of a Baker cyst. a., Artery; m., muscle; n., nerve; v., vein.

FIG. 166.2, Patients with Baker cyst often complain of a sensation of fullness or a lump behind the knee.

FIG. 166.3, Transverse ultrasound image demonstrating the relationship of a large Baker cyst to the joint space and surrounding structures.

FIG. 166.4, Arthrogram of knee in a patient with rheumatoid arthritis demonstrating communication between the synovial space and the Baker cyst (arrow).

Patients with Baker cysts report 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 with rheumatoid arthritis are prone to this phenomenon, and the pain associated with dissection into the calf may be confused with thrombophlebitis and inappropriately treated with anticoagulants ( Fig. 166.5 ). Occasionally, the Baker cyst spontaneously ruptures, usually after frequent squatting.

FIG. 166.5, Rupture of a giant Baker cyst mimicking thrombophlebitis. Note the gastrocnemius asymmetry.

On physical examination, the patient with Baker cyst has a cystic swelling in the medial aspect of the popliteal fossa ( Fig. 166.6 ). Baker cysts can become quite large, especially in patients with rheumatoid arthritis. Activity, including squatting or walking, makes the pain of Baker cyst worse; rest and heat provide some relief. The pain is constant and is characterized as aching and may interfere with sleep. Baker cyst may spontaneously rupture, and there may be rubor and color in the calf that may mimic thrombophlebitis ( Fig. 166.7 ). Homan sign is negative, and no cords are palpable. Occasionally, tendinitis of the medial hamstring tendon may be confused with Baker cyst.

FIG. 166.7, Ruptured Baker cyst (short arrow) . Fluid (long arrows) tracks deep and superficial to the medial gastrocnemius muscle.

Plain radiographs are indicated for all patients with Baker cyst. On the basis of the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging and ultrasound imaging of the knee are indicated if internal derangement or occult mass or tumor is suspected and also are useful in confirming the presence of a Baker cyst and identifying the presence of loculations (see Fig. 166.2 ; Fig. 166.8 ).

FIG. 166.8, Transverse ultrasound image demonstrating a loculated Baker cyst.

Clinically Relevant Anatomy

The popliteal fossa is posterior to the knee joint. The fossa contains the popliteal artery and vein, the common peroneal and tibial nerves, and the semimembranosus bursa ( Fig. 166.9 ). These nerves are subject to compression from Baker cysts ( Figs. 166.10 and 166.11 ). 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. When these bursae become inflamed, they may overproduce synovial fluid, which can become trapped in saclike cysts because of a 1-way valve phenomenon. This occurs commonly in the medial aspect of the popliteal fossa ( Fig. 166.12 ).