Dichotomous location of 160 atypical femoral fractures

BACKGROUND AND PURPOSE: The risk of atypical fracture of the femur is associated with bisphosphonate use. While characterizing atypical fractures from a previous nationwide study in radiographic detail, we had the impression that the fractures were located either in the subtrochanteric region or in the shaft. We determined whether there is a dichotomy in this respect. PATIENTS AND METHODS: The distance between the atypical fractures and the lesser trochanter was measured on plain radiographs from 129 of 160 patients with atypical fractures, taken from 2008 through 2010. Analysis of the distances measured showed 2 clusters, which were then analyzed with regard to bisphosphonate use and age. RESULTS: The distribution of the distances would be best described as 2 clusters, with a dichotomy at 8 cm. The proximal (subtrochanteric) cluster comprised 25 patients who were generally younger (median 71 years) than the 104 patients in the cluster with shaft fractures (median 80 years). The 95% CI for the difference between medians was 4–11 years. Of the patients with subtrochanteric fractures, 18 of 25 used bisphosphonates as compared to 89 of 104 with shaft fractures. INTERPRETATION: The younger age and possibly smaller proportion of bisphosphonate users in the subtrochanteric cluster may be compatible with a greater influence of mechanical stress in the underlying pathophysiology of proximal fractures.