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

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