At what age is an ultrasound versus X-ray (x-ray) preferred for diagnosing hip abnormalities in infants?

Ultrasound vs. X-ray for Hip Imaging in Infants

Ultrasound is the preferred imaging modality for infants younger than 4 months of age, while X-ray is recommended for infants 4 months and older when evaluating for developmental dysplasia of the hip (DDH). 1

Age-Based Imaging Recommendations

Under 4 Months of Age

• Ultrasound is the imaging modality of choice for infants younger than 4 months 1, 2
• Ultrasound allows visualization of the cartilaginous components of the hip that are not visible on radiographs in young infants 2
• Earliest recommended timing for ultrasound screening is 2 weeks of age, as hip instability often resolves spontaneously by this time 1
• The American Academy of Pediatrics recommends selective ultrasound screening at 4-6 weeks of age for high-risk infants 1

4 Months and Older

• Radiographic evaluation (X-ray) becomes the preferred method at 4 months of age 1
• By 6 weeks of age, radiographic changes in the acetabulum and lateral displacement of the femoral neck and metaphysis can be recognized, but radiographs become more reliable at 4 months 1
• A radiographic screening program can be successfully implemented for infants at 4 months of age who were clinically normal at neonatal examination but are considered at risk for DDH 1

Rationale for Age-Based Approach

• Ultrasound in young infants:

  • Allows visualization of cartilaginous structures not yet ossified 2
  • Permits dynamic assessment of hip stability 1
  • Can detect subtle abnormalities earlier than radiographs 1

• X-ray in older infants:

  • By 4 months, sufficient ossification has occurred to make radiographic evaluation reliable 1
  • The ossific nucleus of the femoral head begins to appear, making radiographic assessment more accurate 1
  • Provides standardized measurements such as the acetabular index 1

Special Considerations

• High-risk infants (breech presentation, family history of DDH) should receive imaging regardless of normal physical examination findings 1, 3
• Ultrasound has high sensitivity but can lead to overdiagnosis and overtreatment if performed too early (before 2 weeks of age) 1
• Some recent evidence suggests ultrasound may be a feasible alternative to X-ray even at 6 months of age, though this is not yet standard practice 4
• Early detection and treatment of DDH is crucial for preventing long-term morbidity and improving quality of life 5

Monitoring During Treatment

• For infants undergoing treatment with abduction devices (e.g., Pavlik harness), serial imaging is recommended to monitor progress 1, 6
• Follow-up ultrasound at 3-4 weeks after treatment initiation shows better outcomes compared to 6-week follow-up in terms of treatment duration 6
• After successful treatment, transition to radiographic monitoring typically occurs at 4-6 months of age 1