Which finding best discriminates between developmental dysplasia of the hip (DDH) and slipped capital femoral epiphysis (SCFE) in a child?

Explanation

## Discriminating DDH from SCFE ### Clinical and Radiological Distinction **Key Point:** DDH and SCFE are distinct hip pathologies with different ages of presentation, mechanisms, and directions of femoral head displacement. Age and displacement direction are the most reliable discriminators. ### Comparative Table | Feature | DDH | SCFE | | --- | --- | --- | | **Age at presentation** | Birth to 18 months (peak 0–6 mo) | 10–16 years (adolescence) | | **Mechanism** | Abnormal acetabular development; femoral head subluxes/dislocates | Shear force through growth plate; femoral head slips medially/posteriorly | | **Direction of displacement** | **Posterolateral** (head moves out of socket) | **Posteromedial** (head slips relative to neck) | | **Physis appearance** | Normal physis | Widened, irregular physis | | **Ortolani/Barlow sign** | Positive (reducible hip) | Negative (fixed deformity) | | **Femoral anteversion** | Often increased | Normal or decreased | | **X-ray finding** | Shallow acetabulum, small epiphysis | Metaphyseal blanch sign, posterior displacement | ### Why Age and Displacement Direction Distinguish Them 1. **Age is pathognomonic**: DDH presents in infancy (0–18 months); SCFE is an adolescent pathology (10–16 years). This alone narrows the differential dramatically. 2. **Displacement direction differs**: - **DDH**: Femoral head moves **posterolaterally** out of the acetabulum (subluxation/dislocation). - **SCFE**: Femoral head slips **posteromedially** relative to the femoral neck (epiphyseal displacement on metaphysis). 3. **Mechanism is distinct**: DDH is a developmental abnormality of the socket; SCFE is a biomechanical failure of the growth plate under shear stress. **High-Yield:** **DDH = infant + posterolateral displacement; SCFE = adolescent + posteromedial displacement**. These two facts alone solve most exam questions. ### Clinical Pearl In DDH, the Ortolani sign (click when hip is abducted and lifted) indicates a **reducible** hip — the femoral head can be relocated into the socket. In SCFE, there is no click; the deformity is **fixed** because the epiphysis has already slipped and remodeled. This clinical distinction reinforces the anatomical difference.