The effect of thread length and location on extraction strengths of iliosacral lag screws

Although iliosacral lag screws are an established technique for fixation of sacroiliac joint dislocation and sacral fractures, there is a paucity of data on the relative strength of fixation of screws in the sacral ala and body. The purpose of this study was to quantify and compare the extraction strength of cancellous screws in the sacral ala and body. Twelve fresh frozen cadaveric human pelves (mean age 76) were used to test the extraction strength of three groups of 7.0 mm cannulated cancellous screws: long-threaded in the sacral body, short-threaded in the body and short-threaded in the ala. The mean extraction strengths were 925, 374 and 71 newtons (or 92, 37, and 7 kg) respectively. The differences between the three groups were highly significant (all P < 0.0025). These data strongly recommend that the goal in iliosacral lag screw fixation should be to insert a long-threaded screw into the sacral body, if safely feasible. Fixation in the ala is inferior and should be avoided in the elderly.