Review Question - QID 591

Unstable anterior and posterior pelvic ring injuries are amenable to percutaneous treatment if reduction is able to be obtained in a closed manner and appropriate radiographic visualization is able to be achieved. In the 1996 reference by Routt et al, proper SI screw placement is described. Pelvic inlet, outlet, and lateral sacral images must be obtained to safely place a percutaneous iliosacral screw. The iliac cortical density seen adjacent to the SI joint is the anterior edge of the insertion safe zone, and is only able to be seen on the lateral image. Failure to place the screw behind this radiographic line would lead to an "in-out-in" screw (in the ilium, and then exiting anterior to the sacral ala, only to re-enter in the sacral body), which would cause direct injury to the L5 nerve root.

In the 2000 reference by Routt et al, they state "a thorough knowledge of pelvic osseous anatomy, injury patterns, deformities, and their fluoroscopic correlations are mandatory for percutaneous pelvic fixation to be effective."

Illustration A shows a representative lateral sacral radiograph, with the major anatomic landmarks labeled. Safe SI screw insertion in the S1 body should be underneath the sacral ala line to minimize risk of a "in-out-in" screw that would come out in the area of the ala and injure the L5 nerve root that sits directly on top of this structure. Dysmorphic pelvic rings will often have a more vertical sacral line, or one that starts more inferiorly.