Symptomatic atypical femoral fractures are related to underlying hip geometry

The benefits of bisphosphonates are well documented, but prolonged use has been associated with atypical femur fractures. Radiographic markers for fracture predisposition could potentially aid in safer medication use. In this case-control designed study, we compared hip radiographic parameters and the demographic characteristics of chronic bisphosphonate users who sustained an atypical femoral fracture with a group of chronic bisphosphonate users who did not sustain an atypical femur fracture and also a group who sustained an intertrochanteric hip fracture. Radiographic parameters included were neck-shaft angle (NSA), hip-axis length (HAL) and center-edge angle (CE). Multivariate regression was used to evaluate the relationship between radiographic measures and femur fracture. Receiver-operating characteristic analysis determined cut-off points for neck-shaft angle and risk of atypical femur fracture. Ultimately, pre-fracture radiographs of 53 bisphosphonate users who developed atypical fracture were compared with 43 asymptomatic chronic bisphosphonate users and 64 intertrochanteric fracture patients. Duration of bisphosphonate use did not statistically differ between users sustaining atypical fracture and those without fracture (7.9 [±3.5] vs. 7.7 [±3.3] years, p=0.7). Bisphosphonate users who fractured had acute/varus pre-fracture neck-shaft angles (p<0.001), shorter hip-axis length (p<0.01), and narrower center-edge angles (p<0.01). Regression analysis revealed associations between neck-shaft angle (OR=0.89 [95% CI=0.81-0.97; p=0.01), center edge angle (OR=0.89 [95% CI=0.80-0.99]; p=0.03), and BMI (OR=1.15 [95% CI=1.02-1.31; p=0.03) with fracture development. ROC curve analysis (AUC=0.67 [95% CI=0.56-0.79]) determined that a cut-off point for neck-shaft angle <128.3° yielded 69% sensitivity and 63% specificity for development of atypical femoral fracture. Ultimately, an acute/varus angle of the femoral neck, high BMI, and narrow center-edge angle were associated with development of atypical femur fracture in long-term bisphosphonate users. Patients on long-term bisphosphonates should be regularly radiographically evaluated in order to assess for potential risk of atypical fracture.