Influence of the PFNA screw position on the risk of cut-out in an unstable intertrochanteric fracture: a computational analysis.

The position of the lag screw in the femoral head is a key factor to cut-out, the most reported complication in the internal fixation of intertrochanteric fractures. Considering that the best position for the lag screw remains controversial, the aim of this study was to evaluate the influence of different lag screw positions on the risk of cut-out of an unstable intertrochanteric fracture fixed with a Proximal Femoral Nail Anti-Rotation (PFNA) implant. The relationship between cut-out and the tip-apex distance (TAD) or the calcar referenced tip-apex distance (CalTAD) was also investigated. Finite element models of one male and one female femur treated with a PFNA implant were developed considering the lag screw positioned centrally and inferiorly on the anteroposterior view, and for each of these, the screw tip at 4 discrete positions along its longitudinal axis. All 8 positions simulated for each femur considered the lag screw in a centre position on the lateral view. The risk of cut-out was evaluated for two loading conditions assuming it is related with high compressive strains. The bone region at the fracture line, near the tip of the missing medial fragment, was always the most concerning regarding high compressive strains. The inferior positioning of the lag screw reduced the volume of bone susceptible to yielding compared to the centre positioning. The deep placement of the screw tip improved the outcome for both centre and inferior positions. The results suggested the inferior and deep placement of the screw to be the best position to reduce the risk of cut-out. The volume of bone susceptible to yielding was found not to be correlated to TAD or CalTAD, suggesting that further investigation is necessary to identify other, more reliable, predictors of cut-out.