August 26, 2020
Samuel H. F. Lam, MD, MPH, FACEP
I. Introduction and Indications
- Hip pain/limping are common complaints in children presenting to the ED.
- Differential diagnosis includes septic arthritis, transient synovitis (toxic synovitis), osteomyelitis, neoplasm, Legg-Calve-Perthes disease, avascular necrosis of femoral head, fracture, muscle strain, and juvenile rheumatoid arthritis among others.
- Along with X-ray and blood tests, ultrasound is a valuable tool to narrow down differential diagnosis.1,2
- Presence of a hip effusion may raise suspicion for septic arthritis, osteomyelitis, and other time sensitive diagnoses; while absence of effusion may decrease need for further testing
- Ultrasound can detect effusion as small as 1mL, with sensitivity of 90-100%.3,4
- In a prospective study of 28 patients (55 hips), Vieira and Levy found that emergency physician performed point-of-care ultrasound (POCUS) was 80% sensitive and 98% specific in detecting hip effusions compared to the gold standard of radiology department ultrasound.5
- A retrospective study by the same group on 516 patients (926 hips) reported POCUS sensitivity of 85% and specificity of 98%, with overall accuracy of 93%.6
- Additional case series have also been published of emergency physicians using POCUS to facilitate evaluation of pediatric hip pain.7-10
- Hip joint is where the head of femur articulates with the acetabulum of the pelvis
- Surrounded by strong fibrous capsule
- Iliopsoas muscle lies anterior/superficial to the joint capsule
Scanning Technique, Normal Findings and Common Variants
- Typically a high frequency (5-10MHz), linear transducer is used due to superficial location of structures. Lower frequency, curvilinear transducer may be necessary in patients with thicker soft tissue layers.
- Lie patient supine. Hips in slight external rotation and flexion (“frog-leg” position), if possible.
- Place transducer on anterior hip, along the long axis of the femoral neck, slightly below and perpendicular to inguinal ligament and lateral to the femoral vessels.
- Figure 1. Placement of ultrasound transducer (a) and X-ray of the hip (b)
- Indicator towards the patient’s head
- Locate the femur with its characteristic hyperechoic periosteum and posterior acoustic shadowing.
- Use femoral head and the slanting neck as landmark. Identify the hyperechoic fibrous joint capsule.
- Figure 2. Anatomy of hip joint on ultrasound (C: Cartilage/physis on femoral head, FH: Femoral Head, FN: Femoral Neck, JC: Joint Capsule, IP: Iliopsoas muscle)
- Measure thickness of joint (area from periosteum to outer joint capsule just deep to iliopsoas muscle) at the femoral neck.
- Figure 3. Measurement of hip joint fluid thickness
- A capsular-synovial thickness >5 mm, or >2 mm difference compared to the asymptomatic contralateral side indicates hip joint effusion in children
- Figure 4. Capsular-synovial thickness measurement >5mm in hip effusion (right)
- Joint capsule also appears convex instead of concave in case of effusion
- Figure 5. Hip without effusion (left) has a concave shaped joint capsule and hip with joint effusion (right) has a convex shaped joint capsule.
- POCUS can also be used to guide diagnostic arthrocentesis
- Septic arthritis may progress to destruction of cartilage and bone if untreated, leading to permanent disability
- Treatment is intravenous antibiotics +/- open joint irrigation
- However, 46-86% of patients with transient synovitis of the hip also have effusion present.11
V. Pearls and Pitfalls
- Effusion can sometimes be heterogeneous or even hyperechoic, especially with purulent material.
- Presence of hip effusion cannot distinguish between a benign condition (eg, transient synovitis) and a more serious condition (eg, septic joint, osteomyelitis). Hence clinical context and other diagnostic tests would be necessary to distinguish one from another.
- Too much pressure from the transducer may compress effusion leading to false negative result.
- Cartilage over femoral head may be mistaken as effusion as it is hypoechoic. True effusion crosses the femoral head and extends along the femoral neck while cartilage does not.
- Figure 6. Normal femoral head with hypoechoic cartilage
- Dorr U, Zerger M, Hauke H. Ultrasonography of the painful hip. Prospective studies in 204 patients. Pediatr Radiol. 1988;19(1):36-40.
- Nimityongskul P, McBryde AM Jr, Anderson LD, et al. Ultrasonography in the management of painful hips in children. Am J Orthop. 1996;25(6):411-4.
- Siegel MJ, ed. Pediatric Sonography 4th Ed. Philadelphia: Lippincott Williams & Wilkins, 2011.
- Miralles M, Gonzalez G, Pulpeiro JR, et al. Sonography of the painful hip in children: 500 consecutive cases. Am J Roentgenol. 1989;152(3):579-82.
- Vieira RL, Levy JA. Bedside ultrasonography to identify hips effusions in pediatric patients. Ann Emerg Med 2010;55(3):284-9.
- Cruz CI, Vieira RL, Mannix RC, et al. Point-of-care hip ultrasound in a pediatric emergency department. Am J Emerg Med 2018; 36(7):1174-77.
- Garrison J, Nguyen M, Marin JR. Emergency department point-of-care ultrasound hip ultrasound and its role in the diagnosis of septic hip arthritis: a case report. Pediatr Emerg Care. 2016;32(8):555-7.
- Deanehan J, Gallaher R, Vieira R, et al. Beside hip ultrasonography in the pediatric emergency department: a tool to guide management in patients presenting with limp. Pediatr Emerg Care. 2014;30(4):285-7.
- 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.
- Shavit I, Eidelman M, Galbraith R. Sonography of the hip-joint by the emergency physician: its role in the evaluation of children presenting with acute limp. Pediatr Emerg Care. 2006;22(8):570-3.
- Plumb J, Mallin M, Bolte RG. The role of ultrasound in the emergency department evaluation of the acutely painful pediatric hip. Ped Emerg Care. 2015;31(1):54-8.