Risk factors: Female sex · Breech presentation · Family history · Firstborn · Oligohydramnios · Torticollis or foot deformity (associated musculoskeletal conditions)
Ultrasound — Graf Method (Alpha Angle)
The Graf alpha angle measures the bony acetabular roof inclination on a standardized coronal ultrasound image through the mid-acetabulum.
| Alpha Angle | Classification | Interpretation |
|---|---|---|
| >60° | Type I — Normal | Normal mature hip; no treatment |
| 50–60° | Type IIa (age <3 months) / IIb (age ≥3 months) | Mildly dysplastic / physiologically immature; follow-up |
| 43–50° | Type IIc / D | Critically dysplastic; at risk for dislocation |
| <43° | Type III / IV | Severely dysplastic / dislocated; treatment required |
Screening timing: Ultrasound at 4–6 weeks of age — before this, physiologic laxity may over-diagnose dysplasia. The femoral head begins to ossify at 4–6 months, at which point X-ray becomes the primary modality. Do not perform hip ultrasound after 6 months — ossified femoral head blocks the acoustic window.
Radiograph Assessment (After 4–6 Months)
Used when the femoral head begins to ossify. Key lines drawn on AP pelvis:
| Line / Measurement | How to Draw | Abnormal Finding |
|---|---|---|
| Hilgenreiner's line | Horizontal line through the triradiate cartilages (Y-cartilage) of both acetabula | Baseline reference for all other measurements |
| Perkin's line | Vertical line through the lateral edge of each acetabulum, perpendicular to Hilgenreiner's | Normal: femoral head ossification center lies in the inferomedial quadrant; lateral or superior position = dislocation |
| Shenton's line | Smooth arc drawn along the inferior femoral neck and superior obturator foramen | Disrupted arc = femoral head displacement (subluxation or dislocation) |
| Acetabular index | Angle between Hilgenreiner's line and a line from the triradiate cartilage to the lateral acetabular edge | Normal: <30° at birth, decreases with age; >30° = dysplastic acetabulum |
Shenton's line disruption is the most reliable sign of femoral head displacement on plain film. Draw it on every pediatric pelvis radiograph — a broken arc indicates the femoral head is not seated in the acetabulum.