Fatigue performance of external fixator pins

The fatigue performance of several types of commonly used external fixator pins was examined. The pins were tested under a cyclic bending mode. A strength reduction factor (SRF) was defined to quantify the effect of stress concentration caused by the pin threads as compared with the smooth portion of the pin. The half pins tested had much higher SRF values than the corresponding full pins because of the smaller root diameter of the threaded regions. The use of finer threads and a cold rolling process appeared to increase the pin fatigue strength. In some pins, the depth markers on the smooth portion and the self-tapping fluted region were sites of significant stress concentration. When certain half pins are severely loaded under clinical conditions, they may fracture because of fatigue.