The evaluation of the pediatric patient with a limp or refusal to walk or the adult patient with hip pain requires a consideration of septic joint. The clinical exam is not adequately sensitive to rule out this diagnosis, even when coupled with serum biomarkers for inflammation.^1,2 Plain films are insensitive for effusion (26%-55%), and comprehensive diagnostic imaging with ultrasound or MRI in the radiology department is often unavailable after business hours.^1,3 Even in cases of high clinical suspicion without definitive radiologic imaging, hip arthrocentesis is often performed by consultative services (orthopedic surgery and/or interventional radiology), often delaying time to diagnosis. With the integration of bedside ultrasound into clinical emergency medicine practice, the diagnosis of a hip effusion and subsequent arthrocentesis can be performed at the bedside, reducing diagnostic delay and in turn patient morbidity.^2,4

Ultrasound-guided hip arthrocentesis was first described in the emergency medicine literature in 1999.⁵ Since then several articles (adult and pediatric) have detailed cases of EPs accurately identifying hip effusions and safely performing ultrasound-guided arthrocentesis.6-8 Though the reported series are small, these reports have led a growing number of clinicians to incorporate bedside ultrasound into the evaluation of the painful hip.^1,9 Familiarity with basic ultrasound skills and comfort with in-plane needle technique is needed to allow the EP to confidently diagnose and aspirate effusions of the hip.

Procedure

Hip Effusion Identification by Bedside Ultrasound

For adults, we recommend using a large-footprint curved-array transducer (5-2 MHz). In the pediatric patient, a high-frequency linear-array transducer (13-6 MHz) will be ideal secondary to the shallow depth of the hip structures. The patient should be supine with the knee slightly flexed and hip mildly internally rotated to encourage any fluid collection to move anteriorly. We recommend starting with the nonaffected hip to allow for comparative studies.

Palpate the femoral artery, and place the transducer (probe marker facing to the patient’s right) in a parallel plane to the inguinal ligament, so that the vessels are visualized (Fig. 1). EPs who perform bedside ultrasound for the detection of deep vein thrombosis or ultrasound-guided femoral vein cannulation should be familiar with the ultrasound anatomy at the level of the femoral vessels. Given the increased field of view associated with the low-frequency curvilinear transducer, the operator should reduce the depth on the ultrasound screen to better visualize the femoral vessels. Slowly slide the ultrasound probe laterally until the hyperechoic femoral neck is noted – in thin patients, the probe may have to be moved only a few centimeters. Rotate the ultrasound probe in a clockwise manner until the probe marker points to the umbilicus. The probe should be positioned in the same plane as the femoral neck (parasagittal), with the operator stabilizing the probe with the dominant hand (Fig. 2). The important landmarks to visualize include the femoral head and neck, acetabulum, iliofemoral ligament, and anterior synovial recess as shown (Fig. 3). An effusion is noted by the presence of an anechoic/hypoechoic fluid collection under the iliofemoral ligament, in the anterior synovial recess (Fig. 4).