Differences in femur geometry and bone markers in atypical femur fractures and the general population

This study aimed to identify differences in femur geometry between patients with subtrochanteric/shaft atypical femur fractures (AFFs) and the general population, and to evaluate the biomechanical factors related to femoral bowing in AFFs. We retrospectively reviewed 46 patients. Data on age, and history and duration of bisphosphonate use were evaluated. Femur computed tomography images were reconstructed into a 3D model, which was analyzed with a geometry analysis program to obtain the femur length, femur width and length, and femoral bowing. Patients were divided into two groups according to fracture location: the subtrochanteric and shaft AFF groups. We compared all parameters between groups, and also between each group and a general population of 300 women ≥ 60 years. Thirty-five patients had a history of bisphosphonate use (average duration, 6.1 years; range, 0.8–20 years). There was no statistical difference in bone turnover markers between the two groups. The shaft AFF group had a lower radius of curvature (ROC) (P = 0.001), lower bone mineral density (BMD, T score) (P = 0.020), and lower calcium (P = 0.016). However, other parameters and rate of bisphosphonate use were not significantly different. There were no significant differences in the parameters of the subtrochanter AFF group and the general population, but the shaft AFF group demonstrated a wider femur width (P < 0.001), longer anteroposterior length (P = 0.001), and lower ROC (P < 0.001) than the general population. Femoral bowing and width increased in shaft AFFs, but similar to subtrochanter AFFs compared to the general population. Our results highlight the biomechanical factors of femur geometry in AFFs.