An 8-month-old boy is referred for suspected developmental dysplasia of the hip. Clinical examination shows limited hip abduction and positive Galeazzi sign. The treating physician wants to confirm the diagnosis and assess acetabular coverage. Which is the most appropriate investigation?

Explanation

## Investigation of Choice in DDH at 8 Months **Key Point:** At 8 months of age, ultrasound (Graf classification) remains the preferred investigation for DDH. The femoral head is still largely cartilaginous at this age, and ultrasound provides superior visualization of both the bony and cartilaginous acetabulum compared to plain radiographs. ### Age-Based Transition in Imaging ```mermaid flowchart TD A[Suspected DDH]:::outcome --> B{Age of infant?}:::decision B -->|< 6 months| C[Ultrasound — Graf classification]:::action B -->|6–12 months| D[Ultrasound still preferred; X-ray supplementary]:::action B -->|> 12 months| E[Plain radiograph primary]:::action C --> F[Alpha angle, beta angle, coverage ratio]:::outcome D --> F E --> G[Acetabular index + CE angle + Shenton line]:::outcome ``` ### Why Ultrasound at 8 Months? **High-Yield:** Per ESSR (European Society of Skeletal Radiology) and AAP guidelines, ultrasound is the imaging modality of choice for DDH up to **12 months** of age. At 8 months: 1. **Femoral head is still predominantly cartilaginous** — ossification begins at 4–6 months but is incomplete; the cartilaginous portions are invisible on plain radiographs but clearly seen on ultrasound. 2. **Graf classification** provides standardized morphological grading (Type I–IV) based on the alpha angle (bony roof) and beta angle (cartilaginous roof), directly assessing acetabular coverage. 3. **Dynamic assessment** — ultrasound allows real-time stress testing (Barlow/Ortolani manoeuvres under imaging), which plain radiographs cannot provide. 4. **No ionizing radiation** — important in a growing infant. 5. **Acetabular coverage assessment** — the question specifically asks about acetabular coverage, which is best assessed by the alpha angle on Graf ultrasound at this age. ### Graf Classification (Ultrasound) | Type | Alpha Angle | Interpretation | Action | |------|-------------|----------------|--------| | **I** | ≥ 60° | Normal mature hip | Reassure | | **IIa** | 50–59° | Physiologically immature (< 3 months) | Monitor | | **IIb** | 50–59° | Delayed ossification (> 3 months) | Treat | | **IIc** | 43–49° | Critical zone | Treat | | **III** | < 43° | Decentred, cartilaginous roof displaced | Treat urgently | | **IV** | < 43° | Completely dislocated | Treat urgently | **Clinical Pearl:** An **alpha angle < 60°** at 8 months is abnormal and indicates dysplasia requiring intervention. The alpha angle directly quantifies bony acetabular coverage — the primary concern in this clinical scenario. ### Why NOT Plain Radiograph at 8 Months? - The femoral head ossification centre may be present but **small and eccentric**, making CE angle measurement unreliable. - The **cartilaginous acetabular rim** — the most important structure in DDH — is **invisible** on plain radiographs. - Plain radiographs become the primary modality only after **12 months**, when ossification is sufficient for reliable measurement of acetabular index and CE angle. - The acetabular index on plain X-ray is a useful adjunct but is **not the investigation of choice** at 8 months per current guidelines (Tönnis, Graf, AAP). ### Comparison: Ultrasound vs. Plain X-ray at 8 Months | Aspect | Ultrasound (Graf) | Plain Radiograph | |--------|-------------------|------------------| | **Cartilage visualization** | Excellent | None | | **Bony acetabulum** | Good | Good | | **Dynamic assessment** | Yes | No | | **Radiation** | None | Minimal | | **Preferred age range** | 0–12 months | > 12 months | | **Acetabular coverage** | Alpha angle (direct) | CE angle (indirect, unreliable < 12 months) | | **Guideline recommendation** | **First-line at 8 months** | Supplementary at 8 months | **Reference:** Tönnis D. *Congenital Dysplasia and Dislocation of the Hip in Children and Adults.* Springer, 1987; Graf R. *Hip Sonography.* Springer, 2006; AAP Clinical Practice Guideline on DDH, 2000 (reaffirmed 2021). **Mnemonic:** **GAUD** for ultrasound advantages in infant DDH: - **G**raf classification (standardized) - **A**cetabular coverage (alpha angle) - **U**p to 12 months (age range) - **D**ynamic assessment (real-time stress testing)