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Transsacral versus modified pelvic landmarks for percutaneous iliosacral screw placement--a computed tomographic analysis and cadaveric study.

The alar roots of the first sacral body are the usual confines for iliosacral screw (IS) placement when stabilizing a sacroiliac joint injury or sacral fracture. The traditional transsacral method of IS placement aligns the screw horizontally through the sacral ala on both the inlet and outlet views of the sacrum. A modified oblique method of IS placement aligns the screw in an oblique fashion, directed inferiorly to superiorly and posteriorly to anteriorly. The purpose of this investigation was to first define the S-1 segment boundaries for both methods of placement by analyzing the 3-dimensional (3-D) composites of 40 pelvic computed tomography (CT) scans, and then to evaluate the actual placement of ISs under fluoroscopy in 10 cadaveric pelves comparing the transsacral with the modified oblique techniques. Critical dimensions of 7.3 mm and 14.6 mm were considered as the diameter sizes of one and two cannulated screws, respectively. From the 3-D CT composites, the mean anterior/posterior (A/P) measurements were 10.9 mm and 18.0 mm, comparing transsacral with modified oblique methods, respectively. Moreover, 9/40 (22.5%) of the transsacral A/P measurements were <7.3 mm, while all of the modified oblique A/P measurements were >7.3 mm. The mean superior/inferior (S/I) measurements were 18.0 mm for transsacral and 26.2 mm for modified oblique placement. Out of 40 transsacral S/I measurements, 4 (10%) were <14.6 mm, while all the modified oblique S/I measurements were >14.6 mm. In the second part of this study, 10 uninjured cadaveric pelves had unilateral percutaneous IS placed under fluoroscopic guidance (inlet, outlet, and lateral projections) by one orthopedic traumatologist. The final position of all 10 screws was confirmed on fluoroscopy by two independent orthopedic trauma surgeons. The first 5 screws were placed by using transsacral pelvic landmarks. Modified landmarks guided the other 5 screws. The accuracy of final screw position was determined by "postoperative" CT scans interpreted by a blinded musculoskeletal radiologist. The screws inserted using transsacral pelvic landmarks were errant in 3 of the 5 cases. Neurovascular complications could be expected from the extraosseous position of all 3 screws. All 5 screws were located within the confines of the S-1 segment by means of the modified oblique technique. Thus, the modified oblique placement technique allowed greater accuracy and reliability over transsacral landmarks in placing percutaneous ISs. The use of the modified oblique pelvic landmarks is warranted during percutaneous iliosacral screw stabilization of the posterior pelvis.