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).

The classic criterion for a hip effusion in the adult patient is an anechoic fluid stripe greater than 5 mm located under the iliofemoral ligament, extending along the entire length of the femoral neck, or greater than 2 mm of asymmetry as compared to the unaffected hip.¹⁰ In children, 2 mm of asymmetrical effusion is also considered the classic predictor of an effusion, but unfortunately, recent data reveal uncommon cases of septic arthritis with smaller effusions.^3,11,12

As with most imaging modalities, sensitivity is well short of 100%, and all ultrasound images must be evaluated in conjunction with the clinical exam to reduce false-negative examinations.

Ultrasound-Guided Hip Arthrocentesis

Arthrocentesis is a sterile procedure, and the patient should be cleansed, prepped, and draped in the standard manner. Place the ultrasound probe in a sterile sheath (similar to ultrasound-guided central venous access) and position the ultrasound screen contralateral to the affected hip (Fig. 5). We recommend using a 20- to 22-gauge spinal needle attached to a 10- to 20-cc syringe. Image the hip as discussed above, making sure to find the femoral head and neck and anterior recess covered with the iliofemoral ligament. Move the probe in a caudal manner until the femoral head is seen in the left of the ultrasound screen (close to the probe marker). This allows visualization of the full course of the needle along the target track and ensures a trajectory far lateral to the femoral vessels. Infiltrate a 3- to 5-cc skin wheal of local anesthetic about 1 cm caudal to the probe. In our experience, additional pain medications have not been required. However, a small dose of intravenous analgesia may also be useful in patients with low pain thresholds.

An in-plane technique will be used for the block, with the needle visualized in its entirety as it progresses from skin surface to under the iliofemoral ligament. Stabilize the ultrasound with the nondominant hand, and enter the skin at the location of the skin wheal at a 30-degree angle, aiming at the anterior recess (Fig. 6). Flatter needle angles allow for better ultrasound visualization but may not allow the needle to reach the anterior recess. After locating the needle on the ultrasound screen, advance the needle slowly until the needle tip passes under the iliofemoral ligament, abutting the anterior recess. Gentle aspiration should yield synovial fluid.

Summary

Bedside ultrasound can be a useful adjunct in the evaluation of the patient suspected of a septic hip. Detection of a hip effusion and diagnostic arthrocentesis are both within the scope of practice of the EP and can be mastered relatively quickly. The clinical exam in conjunction with the bedside ultrasound will provide added information at the bedside in the evaluation of the patient with a painful hip.

Common Questions

What structures do I have to avoid?

We always recommend locating the femoral vessels when performing the exam. Along with allowing the novice sonographer to understand relevant anatomy, ensuring that the probe is lateral to the femoral vessels when performing the in-plane ultrasound-guided hip arthrocentsis will prevent inadvertent vascular puncture.

I cannot find the needle on the ultrasound screen. What should I do?

Novice sonographers sometimes are unable to locate the needle on the ultrasound screen. When performing in-plane ultrasound procedures, stabilization of the ultrasound probe is key, and slight adjustment of the probe in a slight lateral or medial parasagittal plane might be required.

Also, in a patient with a large body habitus, the angle of the needle must be steeper, decreasing ultrasound needle visualization. In these cases, we recommend compressing superficial subcutaneous tissue with gentle probe pressure. Complete needle visualization may not be possible, making this a difficult procedure for the novice operator.

References

1. Eich GF, et al. The painful hip: Evaluation of criteria for clinical decision-making. Eur. J. Pediatr. 1999;158(11):923-8.
2. Sultan J, Hughes PJ. Septic arthritis or transient synovitis of the hip in children: The value of clinical prediction algorithms. J. Bone Joint Surg. Br. 201;92(9):1289-93.
3. Miralles M, et al. Sonography of the painful hip in children: 500 consecutive cases. AJR Am. J. Roentgenol. 1989;152(3):579-82.
4. Fabry G, Meire E. Septic arthritis of the hip in children: Poor results after late and inadequate treatment. J. Pediatr. Orthop. 1983;3(4):461-6.
5. Smith SW. Emergency physician-performed ultrasonography-guided hip arthrocentesis. Acad. Emerg. Med. 1999;6(1):84-6.
6. Tsung JW, Blaivas M. Emergency department diagnosis of pediatric hip effusion and guided arthrocentesis using point-of-care ultrasound. J. Emerg. Med. 2008;35(4):393-9.
7. Vieira RL, Levy JA. Bedside ultrasonography to identify hip effusions in pediatric patients. Ann. Emerg. Med. 2010;55(3):284-9.
8. Freeman K, Dewitz A, Baker WE. Ultrasound-guided hip arthrocentesis in the ED. Am. J. Emerg. Med. 2007;25(1):80-6.
9. Cavalier R, et al. Ultrasound-guided aspiration of the hip in children: A new technique. Clin. Orthop. Relat. Res. 2003;(415):244-7.
10. Moss SG, et al. Hip joint fluid: Detection and distribution at MR imaging and US with cadaveric correlation. Radiology 1998;208(1):43-8.
11. Gordon JE, et al. Causes of false-negative ultrasound scans in the diagnosis of septic arthritis of the hip in children. J. Pediatr. Orthop. 2002;22(3):312-6.
12. Yabunaka K, et al. Sonographic measurement of transient synovitis in children: Diagnostic value of joint effusion. Radiol. Phys. Technol. 2012;5(1):15-9.

Dr. Bailey is an ultrasound fellow and attending physician in the Department of Emergency Medicine, Alameda County Medical Center, Highland General Hospital, Oakland, Calif. Dr. Mantuani is an ultrasound fellow and attending physician in the Department of Emergency Medicine, Alameda County Medical Center, Highland General Hospital. Dr. Nagdev is the emergency ultrasound division director in the Department of Emergency Medicine, Alameda County Medical Center, Highland General Hospital.