(5-10 MHz) linear transducer is the preferred transducer to scan the relatively superficial pediatric hip. The transducer should be placed in the sagittal oblique plane, parallel to the long axis of the femoral neck (Fig. 1).

In this view, the femoral head, femoral neck, capsule, and iliopsoas muscle are visualized (Fig. 2). The capsular-synovial thickness should be measured from the anterior concavity of the femoral neck to the posterior surface of the iliopsoas muscle (Fig. 3). When performing hip ultrasound, both hips should be evaluated to allow for comparison widths of the anterior synovial fluid space. The split screen function is particularly useful to compare the two hips for the presence of joint effusion (Fig. 4).

Ultrasound Findings

With correct patient positioning and probe placement, the sonographer should readily identify the sonographic landmarks of the pediatric hip. These landmarks include the femoral head and neck, joint capsule, and iliopsoas muscle (Fig. 2). The joint capsule extends anteriorly to the femoral neck, and the iliopsoas muscle is anterior to the joint capsule. Normally, a small amount of physiologic fluid is present within the joint space.⁷

The effusion fluid may have variable echogenicity, appearing either hypoechoic, anechoic, or hyperechoic, which distends the joint capsule.⁸ The presence of a hip effusion is defined as an anterior synovial space thickness greater than 5 mm, measured from the concavity of the femoral neck to the posterior surface of the iliopsoas muscle, or greater than 2 mm difference when compared to the asymptomatic contralateral hip.⁵

These criteria stress the importance of comparison views, especially in the pediatric patient. Additionally, capsular thickening may be present secondary to an inflammatory process on the affected side, or the presence of an anechoic effusion may be difficult to distinguish from the joint capsule. These reasons, again, highlight the importance of scanning the contralateral side to determine the normal anatomy and tissue architecture.

The presence of an effusion is concerning for a pathological process. However, the distinction between sterile and inflammatory effusions cannot be determined based solely on the ultrasonographic appearance.⁵ The presence of a septic hip can result in profound morbidity and should prompt either emergent orthopedic consultation or, depending on the practice setting, immediate joint aspiration by the emergency physician, followed by microscopic evaluation of the joint fluid with microscopy and gram staining.⁹ Aspiration of sterile fluid should steer the diagnosis toward transient synovitis and away from a septic joint.