Ultrasound answers one question — is there an effusion — and it cannot tell you whether it is infected. Say both things.
Orient first
- The clinical problem is separating transient synovitis from septic arthritis, and IMAGING CANNOT DO THAT. Ultrasound detects the effusion; aspiration and blood tests decide its nature.
- A report implying that a small effusion means transient synovitis is the harm this entry exists to prevent.
- The hip is measured on the ANTERIOR synovial space, in a plane along the femoral neck, and compared with the other side.
- Ultrasound also does not exclude osteomyelitis, which is a separate diagnosis needing MRI.
Acquire the study
- High-frequency linear probe. Child supine, hip in neutral and slightly externally rotated — flexion and rotation change the measurement and can empty the recess.
- Scan ANTERIORLY, along the long axis of the femoral neck, so the probe is parallel to it.
- Measure from the anterior surface of the femoral neck to the posterior surface of the iliopsoas/joint capsule.
- Scan BOTH hips at identical settings and in an identical plane — the comparison is the measurement that matters.
The manoeuvre
- Obtain the anterior oblique long-axis view of the femoral neck on the symptomatic side.
- Measure the anterior synovial space (an effusion is generally over about 5 mm, or more than about 2 mm greater than the asymptomatic side).
- Repeat identically on the other hip and state both figures.
- Describe the fluid: anechoic versus echogenic or septated. Echogenic fluid raises concern but does NOT establish infection.
- Look at the capsule for convex bulging, and use Doppler for synovial hyperaemia.
- Examine the bone surface for cortical irregularity or a subperiosteal collection suggesting osteomyelitis.
- Check the adjacent soft tissues and the other joints if the history suggests a polyarticular process.
- State clearly whether an aspiration window exists if one is being considered.
What confirms it
- An anterior synovial space over about 5 mm, or asymmetrically larger than the other side, with capsular bulging, is a joint effusion.
What licenses you to exclude it
- Symmetrical, normal-depth anterior synovial spaces bilaterally make a significant hip effusion unlikely.
- ⚠️ ABSENCE OF AN EFFUSION DOES NOT EXCLUDE SEPTIC ARTHRITIS, and the PRESENCE of one does not diagnose it. Both sentences belong in the report.
- Ultrasound does not exclude osteomyelitis — recommend MRI if that is the question.
The classic misread
- Reporting an effusion as "consistent with transient synovitis".
- Measuring with the hip flexed or rotated, which empties the anterior recess.
- Not scanning the asymptomatic side.
- Stopping at the hip when the child will not weight-bear — consider the whole limb and the spine.