A 67-year-old woman with rheumatoid arthritis presented with a chief complaint of pain in the left calf. A physical examination revealed subcutaneous brushing at the inferior aspect of the left lateral malleolus (Picture 1). Her rheumatoid arthritis was stable without medication, but experienced a gradual recurrence of disease activity one month prior. The tenderness progressed, initially involving the left calf, followed by the elbows, knees, shoulders, and hands. Ultrasonography revealed no evidence of thrombosis in the examined veins, excluding deep vein thrombosis and superficial thrombophlebitis. Hypoechoic fluid collection was observed extending posteriorly along the surface of the gastrocnemius muscle (Picture 2). Magnetic resonance imaging (MRI) confirmed a fluid signal between the semimembranosus and medial gastrocnemius muscles extending to the Achilles tendon (Picture 3). The patient was diagnosed with a ruptured Baker's cyst. Baker's cysts most frequently develop between the bursa gastrocnemio-semimembranosa and knee joint (1). Common conditions related to Baker's cysts include osteoarthritis, rheumatoid arthritis, gout, spondyloarthritis, and pyrophosphate arthropathy (2). Upon rupture, synovial fluid descends down the surface of the gastrocnemius and ends at the Achilles tendon where the muscle inserts into calcaneus, resulting in the “crescent sign,” a crescent-shaped ecchymosis below the lateral malleolus (3). Clinicians should consider a ruptured Baker's cyst in patients presenting with calf pain, particularly when crescent signs are observed.
Picture 1.
Picture 2.
Picture 3.
The authors state that they have no Conflict of Interest (COI).
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