After 82 patients underwent osteosynthesis with multiple pinning for a displaced intracapsular femoral neck fracture, 58 fractures were united, and 24 were not. These 82 patients were included in a discriminant analysis. Union and non-union groups were the dependent variables. Eleven presumably predictive factors measured on X-ray films obtained from these patients before and immediately after operation were taken as independent variables. Three radiographic factors were selected by a stepwise method according to the effect they had on union. These were: the distance of medial displacement between fragments before reduction; the varus angle after reduction; and the distance of medial displacement between fragments after reduction. The distance of medial displacement between fragments before reduction proved the most important factor in making a choice between osteosynthesis and primary arthroplasty, and it was revealed that osteosynthesis could not be expected to provide union if the distance was 20 mm or more. It was also confirmed that union was promoted, if the cranial fragment was reduced to a valgus position and the medial cortical bones were aligned as much as possible at the time of osteosynthesis.
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