A Biomechanical Comparison of Ipsilateral and Contralateral Pedicle Screw Placement for Modified Triangular Osteosynthesis in Unstable Pelvic Fractures

Abstract

Iliosacral fixation of unstable pelvic fractures does not produce enough stability to allow for immediate postoperative weight bearing. Triangular osteosynthesis creates additional resistance to vertical displacement and rotation. A disadvantage is the loss of the L5/S1 motion segment. We propose a modification of the standard triangular osteosynthesis construct in which the contralateral S1 pedicle is used. As the ipsilateral L5 pedicle is unavailable for fixation in a saw-bones composite pelvic model, we compared ipsilateral and contralateral S1 pedicle screw constructs. We hypothesized that ipsilateral and contralateral S1 pedicle screw constructs would demonstrate no difference in displacement or rotation. Seven saw bones pelvic models were tested. A 5-mm vertical fracture gap was created through the left sacrum while the pubic symphysis was completely dissociated. Each pelvis was tested sequentially in 4 triangular osteosynthesis configurations: ipsilateral S1 screw with anterior plate, contralateral S1 screw with anterior plate, contralateral S1 screw without anterior plate, and ipsilateral S1 screw without anterior plate. Specimens were cyclically loaded from 100-200 N at 0.25 Hz for 25 cycles and then loaded up to 300 N at 10 mm/min while displacement and rotation at the sacral and pubic fracture sites were measured. There was no difference in any of the displacement measures between ipsilateral and contralateral constructs. When comparing rotation, the contralateral configuration experienced significantly less rotation than the ipsilateral configuration with and without the anterior plate applied. Within the limitations of the current model, contralateral S1 constructs for modified triangular osteosynthesis were biomechanically equal to ipsilateral constructs in preventing displacement and superior in preventing rotation.