Lateral and Posterior Dynamic Bending of the Mid-Shaft Femur: Fracture Risk Curves for the Adult Population

Abstract

The purpose of this study was to develop injury risk functions for dynamic bending of the human femur in the lateral-to-medial and posterior-to-anterior loading directions. A total of 45 experiments were performed on human cadaver femurs using a dynamic three-point drop test setup. All 45 tests resulted in mid-shaft femur fractures with comminuted wedge and oblique fractures as the most common fracture patterns. The reaction loads were used to develop the injury criteria given that they represent the inertially compensated bending strength of the femur that is more appropriate for dummy load cell application. In the lateral-to-medial bending tests the peak reaction bending moments were 352 ± 83 Nm. In the posterior-to-anterior bending tests the peak reaction bending moments were 348 ± 96 Nm. Regression analysis was used to identify significant parameters, and parametric survival analysis was used to estimate risk functions. Femur cross-sectional area, area moment of inertia (I), maximum distance to the neutral axis (c), I/c, occupant gender, and occupant mass are shown to be significant predictors of fracture tolerance, while no significant difference is shown for loading direction, bone mineral density, leg aspect and age. Risk functions are presented for femur cross-sectional area, I/c, and a combined occupant gender and mass. The risk function that utilizes the most highly correlated (R2 = 0.77) and significant (p = 0.0001) variable, cross-sectional area, predicts a 50 percent risk of femur fracture of 240 Nm, 395 Nm, and 562 Nm for equivalent cross-sectional area of the 5th percentile female, 50th percentile male, and 95th percentile male respectively.