SI screw vs locking square plate fixation in sacroiliac joint disruption on composite bone models

Background: The aim of this study was to compare a locked square plate to a standard sacroiliac screw of the sacroiliac joint on a composite pelvis bone model to assess the ultimate load tolerated before failure of fixation and to describe the mode of failure of the construct. Methods: Bilateral sacroiliac (SI) joint dislocations were created in 10 composite pelvic bone models. In this descriptive comparative study, the one SI joint was fixated using a 7.3mm cannulated screw and the contralateral side fixated using a 4-hole square locking plate. The pubic symphysis was not fixed. An upward vertical load was manually applied to each respective SI joint using a hook into the sciatic notch. The ultimate load to failure and the mode of failure was recorded for both groups. Results: The mean load to failure for the SI screw group was 310 N and for the SI plate group 580 N. The ultimate load to failure was significantly lower in the SI screw group (p=0.0002). There were no hardware-related failures recorded in any of the fixations in the study. The SI screw group had failure through a fracture of the sacrum in all the specimens. In the SI plate group, fractures of the sacrum and ilium constituted, respectively, 60% and 40%. Conclusion: A locked square plate fixation is superior to a single SI screw at the ultimate load to failure when a vertical load is applied to the sacroiliac joint in a composite bone model.