A fatigue loading model for investigation of iatrogenic subtrochanteric fractures of the femur

BackgroundBiomechanics of iatrogenic subtrochanteric femur fractures have been examined. Previously-described loading models employed monotonic loading on the femoral head, which is limited in emulating physiological features. We hypothesize that cyclic loading combined with the engagement of abductor forces will reliably cause iatrogenic subtrochanteric fractures.MethodsFinite element analysis determined the effects of adding the abductor muscle forces to the hip contact force around holes located in the lateral femoral cortex. Finite element analysis predictions were validated by strain gage measurements using SawbonesTM femurs (Pacific Research Laboratories, Inc., Vashon, Washington, USA) with or without abductor muscle forces. The newly developed physiologically-relevant loading model was tested on cadaveric femurs (N = 8) under cyclic loading until failure.FindingsFinite element analysis showed the addition of the abductor muscle forces increased the maximum surface cortical strain by 107% and the strain energy density by 332% at the lateral femoral cortex. Strain gages detected a 72.9% increase in lateral cortical strain using the combined loading model. The cyclic, combined loading led to subtrochanteric fractures through the drill hole in all cadaveric femurs.InterpretationFinite element analysis simulations, strain gage measurements, and cyclic loading of fresh-frozen femurs indicate the inclusion of abductor forces increases the stress and strain at the proximal-lateral femoral cortex. Furthermore, a cyclic loading model that incorporates a hip contact force and abductor muscles force creates the clinically encountered subtrochanteric fractures in vitro. This physiologically-relevant loading model may be used to further study iatrogenic subtrochanteric femur fractures.