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To test the traditional classification system of slipped capital femoral epiphysis, we evaluated the presenting symptoms and radiographs of fifty-four patients and reclassified the slipped epiphyses as unstable or stable, rather than acute, chronic, or acute-on-chronic. Slips were considered to be unstable when the patient had such severe pain that weight-bearing was not possible even with crutches. Slips were considered to be stable when the patient could bear weight, with or without crutches.We reviewed the records on fifty-five hips in which the slip would have been classified as acute because the duration of symptoms was less than three weeks; thirty of these were unstable and twenty-five were stable. All slips were treated with internal fixation. A reduction occurred in twenty-six of the unstable hips and in two of the stable hips. Fourteen (47 per cent) of the thirty unstable hips and twenty-four (96 per cent) of the twenty-five stable hips had a satisfactory result. Avascular necrosis developed in fourteen (47 per cent) of the unstable hips and in none of the stable hips. We were not able to demonstrate an association between early reduction and the development of avascular necrosis.