slipped capital femoral epiphysis

**Definition/Pathophysiology:** A Salter-Harris type I fracture through the hypertrophic zone of the physeal cartilage. The metaphysis (femoral neck) displaces anteriorly and superiorly (externally rotates), while the epiphysis remains in the acetabulum, so the head appears to slip posteriorly and inferiorly. Mechanical shear forces across a weakened physis are the cause. Weakening results from obesity (increased load, plus a more retroverted femoral head), rapid growth during puberty, and hormonal factors (growth hormone and sex steroids affect physeal strength).

**Epidemiology/Risk factors:** Peak age is 10-16 years, and boys are affected more than girls. Obesity is the strongest risk factor. African American and Pacific Islander ethnicity also raises risk. Bilateral disease occurs in about 20-40% of cases. A young age at presentation (<10) or an older age (>16) should prompt a search for an underlying endocrine or metabolic disorder: hypothyroidism, growth hormone deficiency/supplementation, hypogonadism, panhypopituitarism, renal osteodystrophy (CKD), or prior radiation.

**Clinical presentation:** - Hip, groin, thigh, or knee pain (referred via the obturator nerve). Knee pain with a normal knee exam should prompt a hip exam. - Limp (antalgic or Trendelenburg) or inability to bear weight. - On exam: limited internal rotation, abduction, and flexion. The affected leg is held externally rotated and may appear shortened. - Drehmann sign: obligatory external rotation of the hip with passive flexion. - Stable SCFE: the patient can bear weight (with or without crutches). Unstable SCFE: the patient cannot bear weight even with crutches, and it carries a high risk of avascular necrosis (AVN).

**Diagnosis:** - Plain radiographs: AP pelvis and frog-leg lateral (or cross-table lateral) views. - Klein line: a line drawn along the superior border of the femoral neck should intersect the femoral head. In SCFE it fails to intersect, or the head is asymmetric compared with the other side. - Early findings: physeal widening or irregularity. - Classic description: "ice cream scoop slipping off the cone" (the head slips off the neck). - MRI is useful for a pre-slip when X-rays are normal.

**Management:** - Immediate non-weight-bearing (crutches or wheelchair) and urgent orthopedic referral. - Surgery: in-situ percutaneous pinning with a single cannulated screw across the physis. Do not forcefully reduce the slip, because this raises the risk of AVN. - Prophylactic pinning of the contralateral hip is considered in high-risk patients (endocrinopathy, young age). - Evaluate for endocrine disorders in atypical patients.

**Complications:** - Avascular necrosis of the femoral head (most feared, especially with unstable slips or forced reduction). - Chondrolysis (acute cartilage necrosis and joint space narrowing). - Femoroacetabular impingement. - Early osteoarthritis. - Leg length discrepancy. - Contralateral slip.

**Differentials (high-yield comparisons):** - Legg-Calvé-Perthes disease: idiopathic AVN of the femoral head, ages 4-8, in a short, thin child with a normal or delayed bone age. X-ray shows a crescent sign and a flattened femoral head. - Transient synovitis: ages 3-10, follows a viral illness, and the child is afebrile or has a low-grade fever and normal or mildly raised inflammatory markers. - Septic arthritis: fever, inability to bear weight, and elevated WBC/ESR/CRP (Kocher criteria). It requires joint aspiration and urgent surgical drainage. - Developmental dysplasia of the hip: infants (Ortolani and Barlow signs).

**Board pearl:** An obese 12-year-old boy with knee pain and a limp has SCFE until proven otherwise. Order a hip X-ray, make him non-weight-bearing, and send him for surgical pinning.