A biomechanical study of sacroiliac rod fixation for unstable pelvic ring injuries: verification of the "within ring" concept.

Purpose: The aim of this study was to compare the fixation power of sacroiliac rod fixation (SIRF), which was developed based on our original "within ring" concept to exclude the lumbar vertebra from the fixation range, and spinopelvic fixation (SPF) in a biomechanical experiment.

Methods: SPF and SIRF were applied to the posterior element in four bones each with the pelvic ring fracture model (AO/OTA classification 61-C1.3). A 300-N axial force was loaded on the fifth lumbar vertebra of the simulated pelvis. Then the stiffness (N/m) and deformation (mm) of SPF and SIRF were determined, and the final displacement (mm) of the fracture region and angular deformity (degrees) were measured. Displacements were measured using the markers at two sites of the sacral fracture [upper margin of the sacral ala (Ala) and second sacral vertebra level (S2)] and one site of the pubic symphysis (PS), and angular deformity was measured at Ala and PS.

Results: In SPF and SIRF, the mean stiffnesses and deformations showed no statistically significant difference. Only the vertical displacement at Ala differed significantly between SPF and SIRF (p = 0. 045), and the fixing force of SPF was higher. There was no other significant difference in vertical and horizontal displacement. The mean angular deformities also showed no significant difference between the two methods.

Conclusions: In biomechanics experiments, vertical resistance was stronger in SPF-treated than SIRF-treated bone, but stiffness and deformation, horizontal resistance, and angular deformity did not differ significantly.