Investigation of the biomechanically optimal length of intramedullary nails in unstable intertrochanteric femur fractures 31A2.2: A finite element analysis study.

INTRODUCTION: Achieving sufficient stability in unstable intertrochanteric femur fractures (AO/OTA 31A2.2) is important for bone healing. Although implant-related factors such as nail diameter, proximal fixation configuration, and nail length contribute to mechanical stability at the fracture site, the appropriate nail length for specific fracture patterns remains unclear. This study evaluated the effect of nail length on fixation stability using finite element analysis. METHODS: Finite element models of AO/OTA 31A2.2 fractures, with and without a gap, were developed using CT data from the femur of an average-sized Japanese woman. ASULOCK® intramedullary nails ranging in length from 170 to 280 mm were virtually implanted, and stair climbing was simulated to assess interfragmentary motion and implant stress. RESULTS: Interfragmentary motion significantly decreased with increasing nail length under both gap conditions and reached a plateau at nail lengths of 260 mm or greater. Nail stress tended to be higher in the gap model, but neither nail stress nor lag screw stress showed a significant association with nail length. CONCLUSION: In an AO/OTA 31A2.2 fracture model based on an average-sized Japanese female femur, this study demonstrated that interfragmentary motion reached a plateau at nail lengths of ≥ 260 mm, suggesting that sufficient control of interfragmentary motion may be achieved in this model. Furthermore, under simulated loading conditions, the stresses applied to both the nail and lag screw remained below the referenced yield strength under the present static loading conditions.