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

Fatigue failure of threaded distal locking screws used in intramedullary nailing of fractures of the femur is a significant problem. The current study compares fatigue strength of a threaded screw to an unthreaded peg according to generated stresses using the finite element method. The first analysis considered the unthreaded OrthoFix peg. The second analysis modified the peg to include a threaded portion along its length. The boundary conditions simulated include a range of axial loads of 900 N to 2,400 N. The results demonstrate that using a smooth peg versus a threaded screw will increase the fatigue strength by 72%. The peg will last well past the required ten weeks at all of the loads considered. For the screw, a 270 lb. human can expect two weeks of safe fixation before screw breakage while a 202 lb. body can expect only seven weeks of safe fixation.