The effect of locking screw threads on the fatigue strength in intramedullary nail fixation of femur fractures

Fatigue failure of distal locking screws used in intramedullary nailing of fractures of the femur remains significant. To examine the notch effect of screw threads, the current study compares a threaded screw to an unthreaded peg according to generated stresses using the finite element method. The finite element method was chosen as analyses of this type facilitate the determination of the effect of a single controlled variable. Two analyses were run in the study. The first analysis considered the standard OrthoFix peg. The second analysis modified the peg to include a threaded portion along its length. The geometry of the nail and either the peg or screw was created in Pro-Engineer to the exact specifications of the implants. The minor (core) diameter of the screw was increased to be equal to that of the peg (i.e. there was more material in the screw than the peg). The boundary conditions simulated include a range of axial loads of 900 N to 2,400 N. According to all stress measures examined the peg will provide safer fixation when compared to the screw. The results suggest eliminating the thread from the length of the screw will increase fatigue strength by 64%. Accordingly the service life of a peg in a 270 Ib. human can be expected to be similar to that of a screw in a 164 Ib. human.