Optimizing iliac screw fixation: A biomechanical study on screw length, trajectory, and diameter - Laboratory investigation

Object. The authors performed a study to determine the optimal iliac screw size, length, and trajectory that produce the highest insertional torques. Methods. Ten fresh cadavers were used and 7.5 x 140-mm and 9.5 x 140-mm iliac screws were placed using 3D image guidance in a randomized fashion in 1 of 2 trajectories. The screws were inserted from the posterior superior iliac spine (PSIS) to either 1) supraacetabular bone or 2) the anterior inferior iliac spine (AIIS). Insertional torque was measured for each full revolution, and the concomitant depth for each torque measurement was recorded. Insertional torque was correlated with detailed bony anatomy. Results. There was no difference in mean peak insertional torque between the 2 trajectories (25.6 ± 16.4 in-lb [supraacetabular], 26.3 ± 18.2 lb-in [AIIS]; p = 0.8). However, there was a difference between the 2 screw diameters (21.1 ± 10.9 lb-in [7.5-mm-diameter screw], 33.7 ± 19.4 lb-in [9.5-mm-diameter screw]; p = 0.0003). The greatest mean peak insertional torques were observed at depths greater than 80 mm (12.7 ± 9.6 lb-in [≤ 80 mm], 23.7 ± 15.7 lb-in [> 80 mm]; p = 2.6 × 10^-7). Insertional torque peaks correlated with engagement of the lateral iliac cortex and the superior iliac fossa. Conclusions. Although the trajectory had no effect on insertional torque, increased torques are achievable by placing larger-diameter and longer screws in proximity to bony landmarks, most of which are at distances greater 80 mm from the entry point at the PSIS. Iliac screws longer than those commonly used in clinical practice can be safely and accurately placed using image guidance, and reproducible screw paths can be achieved.