Visual Diagnosis: Bilateral Posterior Swelling of the Knees in a 12-year-old Boy

Abstract

1. Moshe Shapira, MD* 2. Daniela Militianu, MD† 3. Miriam Butnariu, MD§ 1. *Maccabi Health Services, Nave-Sha'anan Clinic, Haifa, Israel. 2. †The Department of Medical Imaging, MRI Unit, Rambam Health Care Campus, Technion Rappaport Faculty of Medicine and Research Institute, Haifa, Israel. 3. §The Pediatric Orthopedic Unit, The Bnai-Zion Medical Center, Technion Rappaport Faculty of Medicine and Research Institute, Haifa, Israel. A 12-year-old boy presents with a 2-month history of mild bilateral posterior knee discomfort and fullness, especially on the left side. The swelling does not restrict walking or weight bearing. There is no history of recent fever, rash, anorexia, weight loss, bleeding disorder, or any other joint pain or swelling. The only finding of note on his past medical history is a fall 19 months ago that did not cause a sprain or fracture to the lower limbs. Physical examination shows a well-appearing boy whose vital signs are normal for age. Inspection and palpation of the posteromedial aspect of both knees reveals bilateral swelling, with the left side larger than the right (Fig. 1). However, there is no evidence of knee joint effusion or limited range of motion, lower extremity erythema, warmth, or edema. The remainder of the physical examination findings are within normal parameters. A complete blood cell count, erythrocyte sedimentation rate, C-reactive protein, and blood chemistry panel and urinalysis yield normal results. Ultrasonography of the posterior knees confirms the suspected clinical diagnosis. Figure 1. Bilateral swelling of the posterior aspect of the knees, with the left side larger than the right. Ultrasonography of the posterior aspect of the knees revealed fluid-filled formations measuring 5.3×1.6 cm on the left and 2.35×1.34 cm on the right (Fig. 2), confirming the clinical diagnosis of bilateral popliteal cysts. Figure 2. Ultrasonography demonstrating the right popliteal cyst. A popliteal synovial cyst is a fluid-filled mass in the popliteal fossa resulting from extrusion of joint fluid into the gastrocnemius-semimembranosus bursa through a weak portion of the posteromedial capsule of the knee between the medial head of gastrocnemius …