Pediatric Septic Arthritis of Hip Etiology: infection leading to pus in hip joint Radiograph: widened hip joint space, not sensitive for hip effusion US: hip effusion DDX: transient synovitis of hip is indistinguishable from septic arthritis of hip on US Radiology Cases of Septic Arthritis of Hip AP radiograph of the pelvis shows symmetrical ossification of the femoral epiphyses and normal acetabulae bilaterally. The right femoral epiphysis was felt to be displaced laterally in comparison to the left. A hip ultrasound showed a right hip joint effusion, which when tapped under ultrasound guidance returned pus. AP radiograph of the pelvis was unremarkable. Sagittal US of the hips shows a large anechoic fluid collection in the anterior recess of the left femur, which is not present in the right hip. This left hip effusion, when tapped under ultrasound guidance, returned green pus.