Tip Apex Distance: a useful method to assess extra-capsular neck of femur fixation?

Abstract

Introduction: Fixation of trochanteric femoral neck fractures can be problematic. The tip apex distance (TAD) is a simple measurement that predicts screw cut-out in the femoral head from fractures treated with a fixed angle sliding hip screw device. By assessing whether the measurements in our centre were comparable to published results, we wanted to assess our proficiency in fixing these injuries as well as the general usefulness of this measurement in day to day practice. Methods: A retrospective review was conducted of 102 consecutively treated trochanteric fractures over a 12 month period, with 11 patients excluded. Three observers (n=3) used a standardised method to measure the TAD (from 2 orthogonal projections with a correction for magnification). The stability of the fracture patterns with the accuracy of reduction were measured according to published criteria. Results: 91 fractures were sustained in 90 patients, with one patient being treated for bilateral hip fractures. The mean tip apex distance was 19.06 mm ±0.77 mm (±95% CI). Assessing the inter-observer variability, the standard deviation between the 3 observers was 1.99mm, equivalent to 10% of the mean TAD. 12 out of 91 sliding hip screw devices (13.1%) had a TAD greater then 25mm, predictive of a higher extrusion rate. Conclusions: Measurement of the TAD can provide a reliable method to examine the adequacy of screw placement in the femoral head. Our results compare favourably to those published and we have found a similar level of inter-observer variability, confirming the usefulness of this measurement in assessing the quality of fixation and its reproducibility. By determining which patients have an increased risk of failure of fixation by screw cut-out we have been able to offer targeted surveillance rather than routinely following up all patients: bringing back those with a high TAD for x-ray follow up.