Imaging description

Hip pain in a young child can be due to toxic or transient synovitis, which is a benign entity that will resolve spontaneously with minimal to no treatment, or due to septic arthritis, which is a serious condition that may require emergency surgery to prevent rapid total joint destruction.

The initial evaluation of a limping child with hip pain typically involves radiography of the pelvis and hips. The best indicator of hip effusion is widening of the joint space, but obliteration of para-articular fat pads may also occur [1]. Joint space widening can appear as increased distance between the ossified proximal femoral metaphysis and the radiographic teardrop when compared to the non-affected contralateral hip joint (Figure 98.1A). With joint inflammation there is para-articular soft tissue edema which obliterates the adjacent fat pads including the obturator and gluteus minimus fat pads, with the obturator fat pad being the more important of the two [1].

Ultrasound is more sensitive than radiography for joint effusions. For evaluation of the hip, the patient lies supine with the hip in extension and slight abduction. The transducer is oriented anteriorly along the axis of the femoral neck. Hip effusions are seen as fluid displacing the joint capsule away from the echogenic cortex of the femoral neck (Figure 98.1B). In subtle cases, a difference in joint distention of greater than or equal to 2mm between the symptomatic and asymptomatic hip has been reported as significant [2]. A normal ultrasound excludes septic arthritis.