Risk of Distal Femur Fracture in Antegrade Intramedullary Nail Fixation of Proximal Femur Fractures: A Finite Element Study.

Surgeons often use long antegrade intramedullary nails (aIMNs) when fixing fractures of the proximal femur in geriatric patients. However, risk of peri-implant fracture in relation to aIMN termination level has not yet been studied. We therefore studied the correlation between aIMN nail tip termination level and extent of distal femur fracture in three different loading scenarios using finite element models of seven different length aIMNs. These were loaded into 3D models of synthetic osteoporotic femurs, which were loaded in single-leg stance (SLS) with body weight load at the femoral head, torsion (TOR) counterclockwise to failure with 120 Nm of torque, and distal impact/fall (DIF) simulating direct fall on the knees at 1000 mm/sec. The maximum stress in SLS and percentage of failed elements for TOR and DIF were recorded. For SLS, aIMN tip termination and maximum stress were not correlated (ρ = 0.1429, p = 0.7599) and there was no apparent trend in the data. For TOR, more proximal aIMN tip termination demonstrated a significant correlation with greater percentage of failed area (ρ = 0.9286, p = 0.0025). For DIF, aIMN tip termination was not correlated with the percentage of failed elements (ρ = -0.6071, p = 0.1482), but demonstrated a statistically significant Gaussian relationship (coeffs: $a$  = 2.22 (p = 1.13 × 10^-3), $b$  = 58.61 (p = 1.89 × 10^-5), $c$  = 17.46 (p = 1.79 × 10^-3)) where 60 mm aIMN tip termination demonstrated the greatest percentage of failed area. Therefore, we found a strong linear relationship between more proximal aIMN termination in the femur and extent of failure in TOR. There was a statistically significant Gaussian relationship between metaphyseal termination and extent of failure in DIF.