Atypical fractures of the femur: effect of anterolateral bowing of the femur on fracture location

Bisphosphonate therapy has been associated with the development of atypical femoral fractures. The most common sites of bisphosphonate-associated fractures are at the subtrochanteric region followed by the femoral shaft. This retrospective study hypothesizes that increasing anterolateral femoral bow is associated with more distal diaphyseal fractures. Awareness of this relationship is essential in the pre-operative planning and successful surgical management of these fractures. We retrospectively reviewed twenty-one atypical subtrochanteric and femoral diaphyseal fractures and stress reactions within a 5-year period at our institution. Radiographs were assessed by two independent investigators for the degree of anterior and lateral femoral bow, and how distal the fracture was from the lesser trochanter. The relationship between the fracture position or stress reaction and degree of anterior and lateral bowing was analysed. There was a statistically significant linear relationship between anterior and lateral femoral bowing, and the fracture position along the diaphysis (correlation coefficient 0.63 (p = 0.002) and 0.684 (p = 0.001), respectively). Inter-observer reliability was highly correlated (Kappa value > 0.8). In atypical femoral fractures associated with bisphosphonate use, more distal diaphyseal fractures occurred with a higher degree of anterior and lateral femoral bow.