A 13-year-old obese boy presents with a 6-week history of progressive left groin pain and limp. On examination, he has obligatory external rotation of the hip on flexion and limited internal rotation. AP hip X-ray is obtained. The line drawn along the superior cortex of the femoral neck (marked **C** in the diagram) fails to intersect the femoral head epiphysis, creating a characteristic gap. What is the most likely diagnosis and the immediate management priority?

Explanation

## Why "Slipped capital femoral epiphysis (SCFE); immediate non-weight-bearing and urgent surgical in-situ screw fixation" is right The failure of Klein's line (the line drawn along the superior cortex of the femoral neck, marked **C**) to intersect the femoral head epiphysis is the classic "crescent sign" — a sensitive radiographic sign of slipped capital femoral epiphysis. The clinical presentation (obese adolescent boy, groin/hip pain, obligatory external rotation on hip flexion, limited internal rotation) is pathognomonic for SCFE. The posterior and medial slippage of the femoral head epiphysis through the proximal femoral physis is a surgical emergency. Immediate management includes non-weight-bearing status and urgent in-situ cannulated screw fixation across the physis to prevent further slippage and allow physeal closure. This is the standard of care per Apley's Orthopedics and pediatric orthopedic guidelines. ## Why each distractor is wrong - **Developmental dysplasia of the hip (DDH)**: DDH typically presents in infants and young children (detected on newborn screening or by 6 months of age), not adolescents. The clinical presentation and Klein line finding are not consistent with DDH. Abduction bracing is appropriate for DDH in infants, not for SCFE. - **Legg-Calvé-Perthes disease**: LCP disease occurs in children aged 4–8 years (much younger than this 13-year-old), involves avascular necrosis of the femoral head epiphysis, and does not produce a positive Klein line sign. The radiographic findings and clinical context are entirely different. - **Avascular necrosis of femoral head**: While AVN is a potential *complication* of untreated or mismanaged SCFE, it is not the primary diagnosis here. The Klein line sign is diagnostic of SCFE, not primary AVN. Weight-bearing as tolerated would be contraindicated and harmful in acute SCFE. **High-Yield:** Klein line failure to intersect the femoral head epiphysis (crescent sign) = SCFE until proven otherwise; obligatory external rotation on hip flexion is the pathognomonic exam finding; SCFE is a surgical emergency requiring immediate non-weight-bearing and urgent in-situ screw fixation to prevent AVN. [cite: Apley's System of Orthopaedics and Fractures, 10th edition]