Locked plates reduce displacement of vertically unstable pelvic fractures in a mechanical testing model

Background: The purpose of this study is to compare a locked screw construct to a single iliosacral screw for fixation of a vertically unstable pelvic ring injury in a transforaminal sacral fracture model. Methods: Orthopaedic Trauma Association type 61-C1.3a2c5 fractures were created in 10 fresh frozen cadaveric pelvis specimens. Specimens were divided into two groups of five. In both groups, the anterior ring was stabilized with a six-hole 3.5-mm reconstruction plate. In the locked plate (LP) group, the posterior injury was stabilized using a two-hole locking plate with one solid 5.0-mm locking iliosacral screw directed onto the S1 body and a second locking screw directed into the lateral sacral ala. In the iliosacral screw group, the posterior injury was stabilized using a single cannulated 7.3-mm screw. Testing was conducted on a Materials Testing System. Values for displacement and rotation were recorded. Each pelvis was axially loaded with a compressive sine wave from 175 N to 350 N for 10,000 cycles to simulate limited weight bearing, with data recorded at 1,000 cycle increments. Results: Two specimens in the iliosacral screw group displaced more than 1 cm during the first 1,000 cycles. These two specimens displayed gross fracture motion in all planes. All five specimens in the LP group completed 10,000 cycles of testing with less than 1 cm of displacement. A vector displacement calculation from the plane displacement data revealed that the LP group had significantly less displacement (median 1.9 mm) than the specimens in the iliosacral group (median 6.7 mm; p = 0.008) after 10,000 cycles. Conclusions: A two-hole plate locked head screw construct resulted in less displacement than a single iliosacral screw in a transforaminal sacral fracture model.