What is a Baker's Cyst?

A Baker’s cyst, or “popliteal cyst” is a benign swelling found behind the knee (Figure 1). This particular cyst lies posterior to the medial femoral condyle. The cyst is connected to the knee joint through a valvular opening. Knee effusion or swelling from intra-articular pathology allows the fluid to go through the valve to the cyst in one direction—behind the knee. This causes the patient to complain about the swelling behind the knee, and alerts the clinician to the possibility of having a problem inside the knee joint itself. The cyst is located between the semimembranosus and the medial gastrocnemius muscles. The cyst is usually located at or below the joint line.

Diagnosis

When diagnosing a Baker’s Cyst, you will notice that the patient usually has swelling behind the knee with pain, fullness, and tenderness. The presence of a knee effusion, which is excessive fluid inside the knee, will create fluid pressure that allows unidirectional passage of the fluid from the knee joint, through the valve, and into the cyst (Figure 2).

A Baker’s cyst is easier to see with the knee fully extended, especially when the patient is in the prone position. That diagnosis is usually confirmed by an MRI that will show the associated intra-articular pathology (Figure 3). Ultrasounds can be important, especially if the cyst is found to be outside of its typical normal position.

The two most common causes of Baker’s Cysts are knee arthritis and meniscal tears—especially the medial meniscus. Tears of the medial meniscus are three times more likely than those of the lateral meniscus. Tears of the posterior horn of the meniscus that extend to the capsule, may cause a defect, or one-way valve to develop between the knee joint and the bursa that lies between the gastrocnemius and semimembranosus muscles. If the cyst is present in an atypical location, consider a tumor as part of the differential diagnosis. A Baker’s cyst is a fluid filled cyst and not a solid tumor. The cyst should transilluminate (Figure 4)!

Treatment

Treatment of painful, large cysts may include ice, compression wrap, anti-inflammatory medications, strengthening exercises, and aspiration of the cyst. Recurrence of excised Baker’s cysts is common if the intra-articular pathology continues. The best treatment is arthroscopy and debridement of the intra-articular pathology. Due to the high rate of recurrence, the main treatment of Baker’s cysts should be directed towards treating the intra-articular pathology (usually meniscal tear or arthritis). The cyst may burst, causing calf pain and swelling. The physician will want to rule out deep vein thrombosis (DVT) or thrombophlebitis.

Popliteal cysts in children are common at the soft tissue mass at the back of the knee (Figure 5). These cysts occur more in boys, transilluminate, are asymptomatic, and are not tumors! The cyst may not be intra-articular and may not have a connection to the knee joint. Popliteal cysts in children are usually not associated with a meniscal tear. The cyst is usually treated by observation and surgery is rarely necessary. Spontaneous resolution of the cyst can occur in 10-20 months. In difficult cases, aspiration of the cyst may be indicated. The cyst may respond to an aspiration and steroid injection, because it is not connected to the knee joint.

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