Location of atypical femoral fracture can be determined by tensile stress distribution influenced by femoral bowing and neck-shaft angle: a CT-based nonlinear finite element analysis model for the assessment of femoral shaft loading stress.

INTRODUCTION: Loading stress due to individual variations in femoral morphology is thought to be strongly associated with the pathogenesis of atypical femoral fracture (AFF). In Japan, studies on AFF regarding pathogenesis in the mid-shaft are well-documented and a key factor in the injury is thought to be femoral shaft bowing deformity. Thus, we developed a CT-based finite element analysis (CT/FEA) model to assess distribution of loading stress in the femoral shaft.

PATIENTS AND METHODS: A multicenter prospective study was performed at 12 hospitals in Japan from August 2015 to February 2017. We assembled three study groups-the mid-shaft AFF group (n=12), the subtrochanteric AFF group (n=10), and the control group (n=11)-and analyzed femoral morphology and loading stress in the femoral shaft by nonlinear CT/FEA.

RESULTS: Femoral bowing in the mid-shaft AFF group was significantly greater (lateral bowing, p<0.0001; anterior bowing, p<0.01). Femoral neck-shaft angle in the subtrochanteric AFF group was significantly smaller (p<0.001). On CT/FEA, both the mid-shaft and subtrochanteric AFF group showed maximum tensile stress located adjacent to the fracture site. Quantitatively, there was a correlation between femoral bowing and the ratio of tensile stress, which was calculated between the mid-shaft and subtrochanteric region (lateral bowing, r=0.6373, p<0.0001; anterior bowing, r=-0.5825, p<0.001).

CONCLUSIONS: CT/FEA demonstrated that tensile stress by loading stress can cause AFF. The location of AFF injury could be determined by individual stress distribution influenced by femoral bowing and neck-shaft angle.