Displacement after simulated pelvic ring injuries: a cadaveric model of recoil

Determining pelvic ring stability after a fracture is vital to treatment decisions. Commonly used information includes the displacement seen on initial radiographs. Static imaging studies may misrepresent the maximal amount of traumatic displacement at injury. We hypothesized that postinjury radiographs do not reveal maximal displacement of pelvic ring fractures. We also sought to determine whether different injury patterns and varying severity of displacement lead to different amounts of passive recoil.

In 15 cadaveric pelvic specimens, unilateral anteroposterior compression (n = 7) or lateral compression (n = 8) injury patterns were experimentally created. A motion-tracking system was used to record rotational deformity of each hemipelvis before, during, and after fracture creation. The absolute and relative magnitudes of pelvic displacement and recoil after force relaxation were determined.

In the simulated AO/OTA Type 61-B1.1 patterns (open book, rotationally unstable), maximal symphyseal diastasis recoiled by 48% +/- 18% (p < 0.05). In the AO/OTA Type 61-C1.2 patterns (open book, completely unstable), diastasis passively recoiled by 44% +/- 7% (p < 0.05). Lateral compression injuries (AO/OTA Type 61-B2.2) had maximal hemipelvis rotation of 41 degrees +/- 7 degrees and subsequently recoiled by 80% to 8 degrees +/- 6 degrees (p < 0.001).

In this cadaveric model of simulated pelvic injury, a significant magnitude of passive recoil occurred after removal of the deforming force. The amount of recoil varied based on different injury patterns. However, the degree of recoil among specimens with similar injury patterns was generally consistent. In a clinical scenario, this suggests that only a portion of the maximal displacement that occurs at the time of injury is seen on initial plain radiographs. Injury severity should not be minimized based on pelvic displacement seen on initial static radiographs and computed tomographic scans.