Biomechanical simulation of the anteroposterior compression injury of the pelvis. An understanding of instability and fixation.

Seven fresh cadaveric pelvic specimens were biomechanically analyzed. Testing was first performed on intact pelves and then after progressive disruption of the (1) symphysis pubis, (2) unilateral anterior and interosseous sacroiliac ligaments and capsule, (3) ipsilateral sacrospinous and sacrotuberous ligaments; and fixation with a 4.5-mm narrow dynamic compression plate at the symphysis pubis, or a 4.5-mm narrow dynamic compression plate at the anterior sacroiliac joint with and without the symphysis pubis plate, or a 7.0-mm sacroiliac lag screw anchored into the S1 vertebral body with and without the symphysis pubis plate. Symphyseal gapping occurred after isolated symphysis pubis disruption. With additional disruption of the unilateral sacroiliac joint ligaments, symphysis pubis displacement was unaffected; however, the injured sacroiliac joint gap displacement, and sacroiliac joint flexion angulation on both intact and injured sides increased as compared to the specimen in the intact state. Further disruption of the ipsilateral sacrotuberous and sacrospinous ligament complex produced little additional motion at either symphysis pubis or sacroiliac joints. Plate fixation of the symphysis pubis alone reduced symphysis pubis motion, but not sacroiliac motion. Use of sacroiliac fixation alone without a symphysis pubis plate did not affect symphysis pubis motion. The symphysis pubis plate is the key to stabilizing symphysis pubis motion, and similarly, sacroiliac joint fixation is required to control sacroiliac joint motion. Both single iliosacral screws and plates produced equivalent decreases in sacroiliac joint motion.