I85PREP
⌘ K
RANZCR Anatomy study materials
RANZCR Anatomy study resources

The limping child with a normal radiograph

RANZCR Anatomy · Put your knowledge to work in context.

All case studies

The limping child with a normal radiograph

A 6-year-old boy is brought to the emergency department with a 3-day history of right hip pain and refusal to weight-bear. He is afebrile (37.1°C), and there is no history of trauma. On examination, he holds the right hip in flexion, abduction, and external rotation, and any movement is painful. White cell count is 9.2 × 10⁹/L, C-reactive protein is 18 mg/L, and erythrocyte sedimentation rate is 22 mm/h. An anteroposterior radiograph of the pelvis and a lateral frog-leg view of the right hip are reported as normal by the emergency physician. You are asked to review the images and advise on the next step.

1. What is your differential diagnosis, and how does the anatomy of the paediatric hip joint explain the clinical presentation?

Read the worked answer

Differential diagnosis

  1. Transient synovitis - most common cause in this age group; usually self-limiting; mild inflammatory markers.
  2. Septic arthritis - must be excluded; the hip is a closed compartment, and infection can rapidly destroy the joint.
  3. Legg–Calvé–Perthes disease - avascular necrosis of the femoral head; typically insidious onset, ages 4–8.
  4. Slipped capital femoral epiphysis (SCFE) - usually older (10–16 years) and often obese; less likely here.
  5. Juvenile idiopathic arthritis - chronic, polyarticular.

Anatomical explanation

  • The hip joint is a ball-and-socket synovial joint. The femoral head sits within the acetabulum, and the joint capsule attaches to the intertrochanteric line anteriorly and the femoral neck posteriorly.
  • Effusion distends the capsule, increasing intracapsular pressure and causing pain on movement. The child adopts flexion, abduction, and external rotation to maximise capsular volume and reduce pressure.
  • The blood supply to the femoral head (medial circumflex femoral artery via retinacular vessels) is at risk from raised intracapsular pressure, which is why septic arthritis and Perthes can cause avascular necrosis.

Key point

  • A normal radiograph does not exclude septic arthritis or Perthes; early Perthes may show only subtle sclerosis or a widened joint space.

2. How would you use ultrasound and MRI to distinguish transient synovitis from septic arthritis, and what are the limitations of each?

Read the worked answer

Ultrasound

  • A linear high-frequency probe is placed parallel to the femoral neck. A joint effusion appears as an anechoic or hypoechoic collection lifting the anterior capsule.
  • An effusion >5 mm or a difference of >2 mm between sides is significant.
  • Limitations: ultrasound detects fluid but cannot reliably distinguish sterile from purulent effusion. It cannot assess the femoral head or marrow. Operator dependence is significant.
  • Ultrasound is useful to guide aspiration, which is the definitive test: Gram stain, culture, and cell count.

MRI

  • Non-contrast MRI is the most sensitive test for early Perthes and marrow oedema.
  • Post-contrast MRI can show synovial enhancement and perfusion defects in the femoral head.
  • Limitations: requires sedation in young children, is time-consuming, and may delay aspiration in suspected septic arthritis.

Practical algorithm

  • If septic arthritis is suspected (fever, inability to weight-bear, raised inflammatory markers), joint aspiration should not be delayed for MRI.
  • In this child, the mild markers and low-grade temperature favour transient synovitis, but clinical follow-up and repeat examination are essential. If symptoms persist beyond 48 hours or worsen, aspiration and MRI should be reconsidered.