Definition

Baker cyst, the most common cyst in the posterior knee, was first described more than a century and a half ago by Adams [1] and later by Baker [2]. It affects approximately 19% of asymptomatic adults (especially adults older than 50 years) [3] and 6.3% of children [4]. It is more common in boys and in children with arthritic knees or hypermobility syndrome [5]. Two age incidence peaks exist: 4 to 7 years and 35 to 70 years [6,7]. Three factors are key to the formation of Baker cyst: (1) communication between the knee joint and popliteal bursae, (2) one-way valve effect, and (3) unequal pressure between the joint and bursae during varying angles of knee movement [7].

Chronic irritation in the knee joint may increase production of synovial fluid, which may flow from the knee joint into the bursae under higher intra-articular pressure until the one-way valve formed by the gastrocnemius-soleus complex “closes,” trapping the fluid in one of the popliteal bursae. This bursa then distends and forms a palpable mass, more commonly in the posteromedial aspect of the popliteal fossa [8]. Anatomically, the lack of supporting structures in this area may predispose this region of the popliteal space to cyst formation [8]. Most commonly, the source of this chronic irritation is an inflammatory or degenerative joint disease, such as rheumatoid arthritis or osteoarthritis. Furthermore, conditions like infectious arthritis, polyarthritis, villonodular synovitis, connective tissue diseases, chondromalacia patellae, and persistent capsulitis are also commonly associated with Baker cysts [9,10]. In a study of 40 patients with radiographic evidence of primary osteoarthritis of the knee, 22% had Baker cyst diagnosed by ultrasonography [11]. Popliteal cysts are associated with meniscal tears in 71% to 82% of the cases, anterior cruciate ligament insufficiency in 30%, and degenerative cartilage lesions in 30% to 60% of the cases [9,10]. Noncommunicating cysts are rare in adults, often have no associated knee disease, and may be primary bursal enlargements from repeated trauma to the bursa itself related to muscle activation. Direct trauma is the most common cause of these cysts in children [4].

Symptoms

Baker cysts are often nonpainful and may be manifested as a fluctuant mass in the popliteal fossa (Fig. 64.1). Typical symptoms, if present, include swelling, pain, and stiffness exacerbated by activity such as walking. Symptoms are most readily elicited when knee flexion compresses the fluid-filled cyst, although knee extension may also cause tension on the cyst by the extended gastrocnemius-soleus muscles. The mass is often accompanied by leg swelling or diffuse calf tenderness. Numbness and tingling in the posterior aspect of the calf and plantar aspect of the foot may be present if there is neural or vascular involvement.

Physical Examination

Baker cysts are often visible or at least palpable along the medial aspect of the popliteal fossa. The cyst may be identified with the patient prone with the knee first extended and then flexed while the popliteal fossa is inspected and palpated. The round, smooth, fluctuant, and often tender cyst will be firm on palpation with knee extension and may soften or disappear with 45 degrees of knee flexion, a phenomenon known as Foucher sign [12]. The cyst can extend into the thigh or leg, or it can have multiple satellites along the calf and even into the foot. These satellite cysts may or may not be connected to the primary cyst through channels. When a joint effusion accompanies the cyst, it is worthwhile to search for the source of chronic irritation. Examine the knee’s range of motion, test for ligamentous laxity, and evaluate for potential patellofemoral pain and meniscal tears [9]. Furthermore, because of the proximity of the sciatic nerve and its branches to the popliteal region where cysts may be present, in rare cases nerve compression may be manifested as decreased sensation along the plantar aspect of the foot and muscle atrophy in the tibialis posterior, flexor digitorum longus, and flexor hallucis longus [13–15].

Functional Limitations

The degree of impairment produced by the cyst depends on its size and amount of tenderness. Baker cysts are usually painless and limit movement minimally, if at all, unless there is an underlying meniscal injury. However, larger cysts may be associated with moderate limitations in physical activity, particularly walking.

Diagnostic Studies

Plain films of the knee can be used to diagnose underlying degenerative joint disease but are rarely necessary to diagnose Baker cyst. Ultrasonography distinguishes solid from cystic masses and is therefore especially helpful in detecting Baker cysts when extensive joint deformities, such as those present with rheumatoid arthritis, obscure the cyst [16