In an infant with suspected developmental dysplasia of the hip, what radiographic findings should be evaluated on an X‑ray, and is X‑ray the preferred initial imaging study?

Imaging for Developmental Dysplasia of the Hip in Infants

X-ray is NOT the ideal imaging modality for infants under 4 months of age—ultrasound is strongly preferred, but after 4-6 months, X-ray becomes the preferred confirmatory imaging as ossification progresses. 1

Age-Based Imaging Algorithm

Infants Under 4 Months

• Ultrasound is the imaging modality of choice for suspected DDH in infants younger than 4 months 1, 2
• X-rays have severely limited value before 3-4 months because the femoral head ossific nucleus doesn't appear until 4-6 months (range 1.5-8 months), and this appearance is often delayed in dysplastic hips 1
• The acetabular margin is predominantly cartilaginous at this age, making radiographic assessment of acetabular morphology and femoral alignment unreliable 1
• Optimal timing for ultrasound screening is 4-6 weeks of age, as earlier imaging (before 2 weeks) leads to high false-positive rates from physiologic laxity 1, 3

Infants 4-6 Months and Older

• Pelvic radiography becomes the preferred confirmatory imaging modality once the ossific nucleus appears 1
• X-rays allow assessment of the femoral head ossific nucleus, proximal femur development, and bony acetabular morphology 1
• A normal pelvic radiograph at 4 months can reliably exclude DDH in high-risk children 1

What to Look for on X-Ray (When Age-Appropriate)

Key Radiographic Measurements

• Acetabular index (AI): The most commonly used measurement, approximately 30° in newborns and progressively decreases with growth 4, 5
• Femoral head ossific nucleus: Assess for presence, timing of appearance, and position (often delayed and eccentric in dysplastic hips) 1
• Acetabular morphology: Evaluate the bony acetabular development and depth 1
• Femoral head alignment: Assess relationship of femoral head to acetabulum 1

Critical Limitations

• Do not rely solely on radiographs at 3 months due to inherent anatomic limitations at this age 4
• The wide variability in ossific nucleus appearance (1.5-8 months) makes timing unpredictable 1
• Suboptimal patient positioning (especially if in harness) further limits radiographic utility 1

Clinical Context for Imaging Selection

High-Risk Groups Requiring Ultrasound Screening

• Female infants with breech presentation (highest risk group per AAP) 1, 3
• Male infants with breech presentation 1, 3
• Positive family history in female infants 1
• Positive Barlow test (dislocatable hip) or Ortolani test (relocatable hip) 1, 3
• Equivocal or inconclusive physical examination findings 1, 3

Physical Examination Findings by Age

• Under 3 months: Ortolani and Barlow maneuvers are primary screening tests 6, 7
• After 3 months: Limited hip abduction becomes the most important clinical finding, as capsule laxity decreases and Barlow/Ortolani may become negative despite hip pathology 1
• Ultrasound screening has superior diagnostic accuracy compared to clinical examination alone (sensitivity 77% vs 62%, positive predictive value 49% vs 24%) 1

Common Pitfalls to Avoid

• Ordering X-rays too early: Radiographs provide minimal diagnostic information before 3-4 months and should not be used for initial screening in young infants 1
• Ultrasound performed before 2 weeks: Results in high false-positive rates from normal physiologic laxity, leading to overtreatment 1, 3
• Assuming normal newborn exam excludes DDH: The disorder is not always present at birth—infants can develop dysplasia after a normal neonatal screening 6, 7
• Relying on asymmetric skin folds: This finding lacks specificity for DDH diagnosis 1
• Missing the transition point: After 4-5 months, ultrasound overdiagnoses DDH in 40% of cases compared to radiography 1

Treatment Monitoring Considerations

• During Pavlik harness treatment: Ultrasound is preferred to confirm concentric hip reduction and assess treatment response 1
• At conclusion of harness treatment: Radiographs document bony acetabular development and provide baseline for future surveillance 1
• Ultrasound can identify predictors of treatment failure including low post-reduction alpha angle and <20% femoral head coverage 1