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Osteonecrosis is a serious complication of the treatment of slipped capital femoral epiphysis. The purpose of the present study was to identify factors influencing the development of osteonecrosis.Two hundred and forty patients who had been treated for slipped capital femoral epiphysis between 1965 and 1999 were retrospectively evaluated. Treatment included stabilization with a spica cast or fixation with one to four pins or screws. Radiographs that had been made at the time of presentation, before and after the operation, and at consecutive follow-up examinations were reviewed. Osteonecrosis was defined retrospectively on the basis of radiographic evidence of sclerosis and collapse of the femoral head. The risk of development of osteonecrosis was correlated with various clinical and radiographic parameters.All twenty-one patients in whom osteonecrosis developed had presented with an unstable slipped capital femoral epiphysis. None of the 204 patients who had presented with a stable slipped capital femoral epiphysis, regardless of grade, had development of osteonecrosis. In the group of patients who had presented with an unstable slipped capital femoral epiphysis, the risk of development of osteonecrosis increased with the severity (grade) of the slip. Osteonecrosis was more likely to develop in patients who had been treated with multiple pins than in those who had been treated with a single cannulated screw.Patients who have a stable slipped capital femoral epiphysis are not at risk for the development of osteonecrosis when treated with pinning in situ. Patients who have an unstable slipped capital femoral epiphysis have a decreased risk of osteonecrosis when treated with pinning in situ. Complete or partial reduction of an unstable slipped capital femoral epiphysis increases the risk of development of osteonecrosis. Pinning in situ without reduction with a single cannulated screw is the method of choice for the treatment of a slipped capital femoral epiphysis.Therapeutic study, Level III-2 (retrospective cohort study). See Instructions to Authors for a complete description of levels of evidence.