Posterior sacroiliac fixation using a sacral pedicle targeting device: an anatomical study.

Abstract

This study compares the accuracy of posterior sacroiliac (SI) screw placement using a "free-hand" method, without the use of fluoroscopy, versus a specially developed targeting device. Posterior SI screws were inserted after exposing the iliac wing in five cadavers on one side, and inserted percutaneously with a unique targeting device on the opposite side. Fluoroscopy was not used for screw or pedicle placement on either side. Computed tomography and dissection results were then used to grade screw placement for both sides. A statistically significant difference between the sides was found. More importantly, three screws on the free-hand side violated major neurovascular structures. The regional anatomy was defined: structures most at risk are the iliac vein ventrally and the sacral canal dorsally. A highly variable "safe zone" (mean arc 43 degrees at the S1 level and 30 degrees at the S2 level) was established. Inclination of the SI joint was also defined (mean 29 degrees at the S1 level and 17 degrees at the S2 level). SI screw placement using the specially developed targeting device is technically less demanding, requires less soft tissue dissection, allows variable placement, and poses minimal risk to major neurovascular structures. Our limited clinical experience with the device is encouraging. The potential application of this technique to unstable vertical shear fractures is appealing.