A 3-month-old female infant born in rural India presents with asymmetric skin folds on the thigh and limited hip abduction on the left side. The Barlow and Ortolani tests are positive. Which is the investigation of choice to confirm developmental dysplasia of the hip?

Explanation

## Investigation of Choice in DDH by Age **Key Point:** The choice of imaging modality for DDH depends critically on the infant's age and skeletal maturity. ### Age-Based Imaging Strategy | Age Group | Investigation of Choice | Rationale | |-----------|-------------------------|----------| | < 4–6 months | Ultrasound | Cartilaginous femoral head is radiolucent; ossification centre absent; best soft-tissue visualization | | > 4–6 months | Plain radiograph (AP pelvis) | Femoral head ossification begins; X-ray becomes reliable; cheaper, widely available | | > 12 months | Plain radiograph ± MRI | Ossification complete; radiograph diagnostic; MRI for complex cases | **High-Yield:** In infants **< 4–6 months**, ultrasound is the gold standard because: - The femoral head is entirely cartilaginous and invisible on plain X-ray - Ultrasound directly visualizes the cartilaginous femoral head, labrum, and acetabular coverage - It is non-ionizing (no radiation exposure) - It allows dynamic assessment (stress views, Barlow/Ortolani maneuvers under ultrasound guidance) **Clinical Pearl:** Graf's classification (Types I–IV) is used to grade hip maturity and dysplasia severity on ultrasound. A Type I hip is normal; Type IV indicates severe dysplasia requiring urgent intervention. ### Why Ultrasound Here This 3-month-old infant is well within the window where ultrasound is superior: - Femoral head ossification has not yet begun (ossifies at 4–6 months) - Barlow/Ortolani positivity suggests instability — ultrasound can confirm - No radiation risk (important in pediatrics) --- ## Comparison of Other Modalities | Modality | Pros | Cons | When Used | |----------|------|------|----------| | **Ultrasound** | Non-ionizing; excellent soft-tissue detail; dynamic assessment; early detection | Operator-dependent; limited by ossification | < 4–6 months (gold standard) | | **Plain X-ray** | Cheap; widely available; standardized; good for follow-up | Femoral head not visible < 4–6 months; radiation | > 6 months; follow-up imaging | | **MRI** | Excellent soft-tissue detail; no radiation; 3D reconstruction | Expensive; time-consuming; requires sedation in infants | Complex cases; surgical planning; > 12 months | | **CT** | High-resolution bony anatomy | High radiation dose; not first-line in pediatrics | Rarely used; complex deformity assessment | **Warning:** Plain radiographs in infants < 4–6 months are unreliable because the femoral head is entirely cartilaginous and will not be visible. Falsely reassuring normal X-rays have delayed diagnosis of DDH.