According to the Graf ultrasound classification, what is the critical alpha angle measurement that defines the transition from Type I (normal) to Type II (immature) hip in DDH?

Explanation

## Graf Ultrasound Classification of DDH **Key Point:** The Graf alpha angle is the primary sonographic parameter for classifying hip maturity and dysplasia in infants. An alpha angle of 50–59° defines the threshold between normal (Type I) and immature (Type II) hips. ### Graf Classification System | Type | Alpha Angle | Beta Angle | Interpretation | Management | | --- | --- | --- | --- | --- | | **Type I (Normal)** | ≥ 60° | < 55° | Mature, well-ossified | No treatment | | **Type II (Immature)** | 50–59° | 55–77° | Immature, dysplastic | Abduction bracing | | **Type IIc (Critical)** | 50–59° | > 77° | Immature with poor coverage | Close follow-up | | **Type III (Dislocated)** | < 50° | > 77° | Femoral head displaced | Urgent intervention | | **Type IV (Dislocated)** | < 50° | > 77° | Femoral head displaced, echogenic labrum | Urgent intervention | ### Alpha Angle Definition - **Anatomy:** Angle between the iliac baseline and the acetabular roof line (measured on the standard coronal plane ultrasound). - **Represents:** Acetabular coverage of the femoral head; larger angle = better coverage. - **Critical threshold:** 60° is the boundary between normal and abnormal; 50° separates immature from clearly dysplastic. ### Beta Angle Definition - **Anatomy:** Angle between the iliac baseline and the labral line. - **Represents:** Position of the labrum; higher angle = more vertical labrum (worse coverage). - **Interpretation:** Used to distinguish Type IIc (immature with poor labral support) from Type II (immature with good labral support). **High-Yield:** The 50–59° range is the "gray zone" where hips are immature but may normalize with time or require bracing. Hips with alpha ≥ 60° are considered mature and normal; those < 50° are clearly dysplastic or dislocated. **Mnemonic:** **GRAFT** = **G**raf, **R**oof angle (alpha), **A**cetabular coverage, **F**emoral head position, **T**reatment based on type. **Clinical Pearl:** Graf ultrasound is the gold standard for DDH screening in infants < 6 months and is mandatory in high-risk populations (family history, breech, female, oligohydramnios). Serial ultrasounds are performed at 4–6 weeks and 12 weeks to assess maturation. **Warning:** Do NOT confuse alpha angle with the center-edge angle (Hilgenreiner-epiphyseal angle), which is used in older children on plain radiographs. Graf ultrasound is specific to infants; radiographic assessment begins after 4–6 months when ossification allows adequate visualization.