Hip joint fluid in the presence of the vacuum phenomenon.

A 3-year old boy with an acute onset of hip pain and limitation of motion had 4 mm of crescent shaped intra-articular gas produced by manual traction radiography. Hip aspiration produced more than 2 ml of fluid from the hip. Manual traction radiography producing the vacuum phenomenon with release of crescent shaped intra-articular gas did not exclude joint fluid in this patient.