Biomechanical analysis of retrograde intramedullary nail fixation in distal femoral fractures.

This study employed both mechanical testing and finite element analysis to compare the stiffness variations among different intramedullary nail constructs used in the treatment of distal femoral fractures. Compressive and torsional experiments were conducted on a transversely, as well as an obliquely fractured sawbone femur restored with the retrograde intramedullary nail. Corresponding finite element models were established to evaluate the stress distributions around screw holes. The results showed that a perifracture screw could increase stiffness by 40% for the obliquely fractured femur, but that it played an insignificant role in stiffness improvement for the transverse fracture groups. Moreover, compared to proximal-screw fixation, distal-screw fixation could improve construct stiffness by 20%. The absence of one of the two distal screws would increase the screw-hole stress by 70%. Therefore, the distal screw around the metaphyseal region has a more important stabilizing effect in the femur-nail construct than does the proximal screw. A twisting stress pattern occurs on the unused screw holes of the metaphyseal region and induces a higher risk for fatigue fracture. The locking screw at the fracture site would be most effective only if it passed through the fracture gap to integrate the separated femoral pieces.