This young boy presented complaining of right hip pain and refusing to walk.
There is a right hip effusion shown by the bulging of the joint capsule anterior to the femoral neck.
Ultrasound can easily confirm or exclude a hip effusion in the pediatric patient. However, if present, it does not answer the (usual) question of whether the effusion is due to septic arthritis or reactive ("irritable hip"). If there is doubt, then aspiration under ultrasound guidance can be performed.
The distance from the femoral neck to the posterior edge of the iliopsoas muscle is measured. A distance greater than 5mm or more than 2mm greater than the other side is considered abnormal.