Ultrasonography in the study of prevalence and clinical evolution of popliteal cysts in children with knee effusions

The prevalence and clinical evolution of popliteal cysts in children with knee arthritis is not well known. Using ultrasonography, we studied 44 children with clinically detectable knee effusions secondary to juvenile rheumatoid arthritis (n = 35), spondyloarthritis (n = 3) and psoriatic (n = 2), septic (n = 2) and lupus (n = 2) associated arthritis. Popliteal cysts, defined as anechoic or hypoechoic masses measuring at least 1 cm in 2 of 3 dimensions, were identified in 27 children (61%). Of the 30 children with bilateral arthritis, 11 (37%) had bilateral cysts. The size of the cysts ranged from 1 to 40 cm³ (median 3.0 cm³). There was a significant correlation between the presence of a cyst and popliteal pain and the size of the suprapatellar effusion (p < 0.001) but not the child's age or underlying diagnosis (p > 0.05). A cohort of 25/27 children with cysts were followed prospectively with serial sonograms for 18-24 months. The resolution of the cyst followed that of the suprapatellar effusion in those children whose arthritis improved or resolved. Two children (8%) had rupture of the popliteal cysts. Popliteal cysts are readily documented in children with knee effusions using ultrasonography, and their presence and evolution correlates with the size of the suprapatellar effusion.