Failure of fixation of peritrochanteric fractures that have been treated with a fixed-angle sliding hip-screw device is frequently related to the position of the lag screw in the femoral head. A simple measurement has been developed to describe the position of the screw. This measurement, the tip-apex distance, is the sum of the distance from the tip of the lag screw to the apex of the femoral head on an anteroposterior radiograph and this distance on a lateral radiograph, after controlling for magnification. To determine the value of this measurement in the prediction of so-called cutout of the lag screw, 198 peritrochanteric fractures (193 patients) were studied. The minimum duration of follow-up was three months (average, thirteen months), during which period all of the fractures either healed or had failure of the fixation. Of the nineteen failures that were identified, sixteen were due to the device cutting out of the femoral head.
The average tip-apex distance was twenty-four millimeters (range, nine to sixty-three millimeters) for the successfully treated fractures compared with thirty-eight millimeters (range, twenty-eight to forty-eight millimeters) for those in which the screw cut out (p = 0.0001). None of the 120 screws with a tip-apex distance of twenty-five millimeters or less cut out, but there was a very strong statistical relationship between an increasing tip-apex distance and the rate of cutout, regardless of all other variables related to the fracture. An increasing age of the patient, an unstable fracture, a poor reduction, and use of a high-angle (150-degree) side-plate were also associated with a significantly increased risk of failure due to cutout. With use of a logistic regression model, the tip-apex distance was shown to be a stronger predictor of cutout (p < 0.0001) than any other variable, and the probability of the screw cutting out was calculated for any given tip-apex distance.