Slipped capital femoral epiphysis — formative SAQs

SAQ 1 (10 marks) — The overweight adolescent with knee pain and a limp

Stem: A thirteen-year-old boy presents to the emergency department with a two-week history of left knee pain and a worsening limp. He is unable to bear weight on the left leg as of this morning. His body mass index is above the ninety-fifth percentile. On examination, the left leg is held in external rotation, and passive flexion of the left hip produces obligatory external rotation. A frog-lateral radiograph shows posteroinferior displacement of the capital femoral epiphysis with a Southwick angle of forty-five degrees. Outline your classification, immediate management, and definitive treatment. ^{[1] [7]}

Model answer

Classification (3 marks). This child has a slipped capital femoral epiphysis of the left hip. The Loder stability classification is the critical first determination: because he cannot bear weight even with crutches, the slip is unstable, and this classification predicts an avascular necrosis risk approaching fifty per cent — the finding from the Loder 1993 study that reported forty-seven per cent avascular necrosis in the unstable group and zero per cent in the stable group. The Southwick angle of forty-five degrees grades the severity as moderate, between the thirty-degree mild and the fifty-degree severe thresholds. The temporal pattern is acute-on-chronic, with the two-week history of knee pain and the acute worsening overnight. ^[1]

Immediate management (3 marks). The child is made strictly non-weight-bearing from the moment of diagnosis — transported by wheelchair or stretcher, never allowed to walk — because every step across the unstable physis worsens the displacement and threatens the retinacular blood supply. Intravenous analgesia is given for the acute pain. The unstable slip is an orthopaedic emergency: the child is kept fasting, bloods are drawn for the group and save, and the consent is obtained for the urgent reduction and fixation. The surgical team is informed and the theatre is arranged as an emergency case, because the window for the gentle reduction that protects the retinacular vessels narrows with every hour of delay. ^{[1] [10]}

Definitive treatment (4 marks). The unstable slip is taken to theatre for urgent management. The two options are gentle reduction under anaesthesia — achieved by traction and internal rotation, never by forceful manipulation — followed by single-screw fixation, or the modified Dunn procedure, a surgical hip dislocation with a subcapital osteotomy and capital realignment that restores the normal anatomy and directly visualises and protects the retinacular vessels. The choice depends on the severity, the timing, and the surgeon. The contralateral hip is assessed for the bilateral risk, which is between twenty and forty per cent, and the prophylactic pinning is considered if the child is young with an open triradiate cartilage. The child is followed for the avascular necrosis, which may declare radiographically over the months after the fixation, and the weight management programme is initiated as the secondary prevention. ^{[7] [10]}