What is the best next test to confirm the diagnosis in a 3-month-old girl with suspected hip disorder, born at term via cesarean section (C-section) due to breech position, presenting with limited left hip abduction and skin findings after a popping sensation was felt in her left leg?

Best Next Test for Suspected DDH in a 3-Month-Old

Hip ultrasound is the best next test to confirm the diagnosis in this 3-month-old infant with suspected developmental dysplasia of the hip (DDH). 1, 2

Clinical Context and Rationale

This infant presents with classic features of DDH:

• High-risk factors: Breech position and cesarean delivery 3, 4
• Key clinical finding: Limited left hip abduction, which becomes the most important screening method after 8-12 weeks of age when Barlow and Ortolani maneuvers lose sensitivity 1, 2
• Negative instability tests: The absence of positive Ortolani/Barlow maneuvers is expected at 3 months, as capsule laxity decreases and muscle tightness increases by this age 1, 2

Why Hip Ultrasound at 3 Months

Ultrasound remains the optimal imaging modality at exactly 3 months of age because:

• The femoral head ossific nucleus typically appears between 4-6 months (range 1.5-8 months), making this a transitional age where ultrasound is still superior 1
• Radiographs have limited value before the ossific nucleus appears, as the femoral head and acetabular structures are predominantly cartilaginous 1, 5
• Ultrasound allows visualization of both bony and cartilaginous structures, evaluating acetabular morphology through the alpha angle and femoral head coverage 5
• Ultrasound screening demonstrates superior sensitivity (77%) compared to clinical examination alone (62%) for diagnosing DDH 1

Graf Classification for Treatment Planning

The ultrasound will categorize the hip using Graf classification 5:

• Type I (alpha angle >60°): Normal, no treatment needed 1
• Type IIa (alpha angle 50-59°): Immature but normally located; most resolve spontaneously but require follow-up 1, 5
• Types IIc, IId, III, IV (alpha angle <50°): Progressively abnormal hips requiring immediate orthopedic referral and treatment 1, 5

Why Not Other Options

Pelvis X-ray: Not appropriate at 3 months because there is no relevant literature supporting radiographs in children <3 months with DDH findings, primarily because the ossific nucleus usually hasn't appeared yet 1. Radiography becomes the preferred modality only after 4-6 months when the ossific nucleus is reliably present 1

Pelvis MRI: Not indicated as a first-line diagnostic test; ultrasound provides all necessary anatomical information without sedation, radiation concerns, or excessive cost 1

Galeazzi sign: This is a clinical examination finding (limb length discrepancy with hips and knees flexed), not a diagnostic test 6. It has low specificity for DDH diagnosis 1

Critical Timing Consideration

At 3 months, this patient is at the transitional age where:

• The AAP and AAOS recommend ultrasound screening before 4-5 months 1
• After 4-6 months, ultrasound may overdiagnose DDH in up to 40% of patients, making radiographs preferable once the ossific nucleus appears 1
• Therefore, ultrasound should be performed promptly at this age to guide timely treatment decisions 1, 5

Treatment Implications

If DDH is confirmed by ultrasound:

• Pavlik harness treatment is most effective when initiated before 6 weeks but can be used up to 6 months of age with 67-83% success rates 1
• Early detection and treatment prevent the need for surgical intervention and reduce long-term complications including early degenerative joint disease 2, 4
• Delayed diagnosis after 6 months often requires surgical correction 1, 4