BackgroundThe aim of this study was to compare a locking square plate to a standard sacroiliac (SI) screw of the sacroiliac joint on a composite pelvic bone model to assess the ultimate load tolerated before failure of fixation and describe the mode of failure of the construct. MethodsBilateral sacroiliac joint dislocations were created in 10 composite pelvic bone models. In this descriptive comparative study, the one SI joint was fixated using a 7.3 mm cannulated screw and the contralateral side 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. ResultsThe median load to failure for the SI screw group was 310 Newton (N) (range 280–390 N) and for the SI plate group 580 N (range 380–760 N). The ultimate load to failure was significantly lower in the SI screw group (p = 0.0002). No hardware-related failures were recorded in any of the fixations. 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 60% and 40%, respectively. ConclusionA locking 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.