Bilateral Baker Cysts

Photoclinic

A 3-year-old boy was brought in by his mother for assessment of a bump behind his right knee, which had been first noted approximately 1 month previously. The mother was concerned because she thought the bump had enlarged over the previous 2 weeks. The child had complained of some pain in that area about 2 weeks ago, but he had been asymptomatic since then.

His vital signs were stable. There was no redness or discharge from the area, he had no history of fever, and his gait did not appear to be affected. There was no history of trauma other than minor bumps and scrapes to the knees. The child had been meeting his milestones normally, and growth parameters were within normal limits for age. There were no pertinent findings in the past, family, or social histories.

Physical examination results were unremarkable except for a small, nontender, left inguinal lymph node and soft masses in both popliteal fossae. The masses were nontender, nonerythematous, and slightly mobile. They appeared to be cystic but were not compressible.

Based on the examination results, the boy received a diagnosis of Baker cysts of both popliteal fossae.

ETIOLOGY AND PATHOLOGY

Baker cysts, or popliteal cysts, are fluid-filled cysts that result from extrusion of synovial fluid from the knee joint into the popliteal bursa, usually following minor trauma or pathologic processes of the knee joint. They are more common in adults, with a reported incidence of 5% to 19%, and less common in children, with a reported incidence of approximately 6%.

They most frequently are located in the bursa beneath the medial head of the gastrocnemius muscle or beneath the semimembranosus muscle; these two bursae often coalesce, forming a gastrocnemio-semimembranosus bursa.

Primary Baker cysts usually occur in children. They usually are asymptomatic and thus are typically found incidentally in children. Primary cysts are thought to result from minor trauma that either causes a connection between a weak area in the knee joint capsule and the bursa adjacent to it, or causes inflammation and enlargement of the bursa. The trauma may be as minimal as occurs with bumping the backs of the knees while sitting on a chair and swinging the legs. These cysts do not have a persistent connection with the joint cavity and do not undergo significant enlargement.

Secondary Baker cysts, on the other hand, arise secondary to pathology in the joint cavity and thus are generally more symptomatic. They are more common in adults. Significant trauma, juvenile idiopathic arthritis, infections (especially Lyme disease), and malignancy have all been associated with secondary cyst formation. These cysts usually have a persistent connection with the knee joint; they can enlarge significantly and compress surrounding structures, most commonly the popliteal vein, which can result in thrombophlebitis. They may result in restriction of joint movement. Secondary Baker cysts may spontaneously rupture and result in significant inflammation and pain.

DIAGNOSIS AND PROGNOSIS

The differential diagnosis of a popliteal cyst in adults includes venous varix, deep vein thrombosis or thrombophlebitis, popliteal artery aneurysm, lipoma, and, rarely, malignancy including synovial sarcoma and fibrosarcoma. Magnetic resonance imaging is the gold standard for diagnosis of a popliteal mass. However, most popliteal cysts can be diagnosed using Doppler ultrasonography to show a well-defined cystic lesion that does not expand on knee movement and lacks flow through it. Any solid component in the lesion should prompt referral to an orthopedic surgeon.

Primary Baker cysts have a benign course and usually resolve spontaneously by 18 years of age. Therefore, they usually are followed conservatively until resolved and almost never require treatment. Secondary cysts, on the other hand, usually require medical or surgical management, or both. If an infectious or inflammatory intra-articular pathology is found, its treatment usually stabilizes or resolves the cyst. If the cyst does not resolve or causes problems by compressing surrounding structures, surgical management usually is indicated. In these cases, excision of the cyst or closure of the connection with the synovial knee cavity, or both, are done. A newer technique consists of debriding only the connection between the cyst and the joint cavity and has shown excellent results.

OUTCOME

Because the boy’s mother thought the cyst had been progressively enlarging, he was referred to pediatric surgery. Doppler ultrasonography showed that the cysts measured 2.5 × 1.5 × 0.8 cm on the right and 3.2 × 2.0 × 0.9 cm on the left, with no flow, no connection with the knee joints, and no compression of surrounding structures.

The patient’s family was reassured that the boy required no treatment, only careful observation. The boy returned for a follow-up visit 6 months later, at which time the right cyst had almost completely resolved, and the left one was much smaller than previously. The plan was to continue close observation of the boy’s popliteal fossae.