CT-based Anatomic and Clinical Analysis of Iliac Screw Placement During Spinopelvic Fixation

Iliac screws provide strong caudal anchorage for both long spinal fusions as well as short lumbopelvic fixations. However, anatomic based placement can be challenging, and complication rates are often underestimated.

We analysed 47 iliac screws being placed in 24 patients. Using postoperative computed tomography (CT), iliac screw placement was analysed with reference to anatomic landmarks. Iliac narrowings were described with regard to their relevance for iliac screw placement. Moreover, we analyzed clinical records for clinical complications. The latter were classified as intraoperative, postoperative, and radiological.

From starting points, described by distance to the posterior superior iliac spine (PSIS), the average iliac screw length was 71.2±13.7 mm, and the diameter was as wide as 7.9±0.7 mm. Divergence was 30.7±12.6° (transverse plane) and caudal orientation was 34.2±13.0° (sagittal orientation). General pelvic dimensions correlated significantly with each other, and certainly with the length of implanted screws. Different adverse events and complications occurred. A total of 20% of patients were found with at least partial extracortical malpositioning. The main group of complications were postoperative with painful prominence in 20% of cases, wound infection in 8.9% and wound healing disorders in 6.7%. Further complications were radiological screw loosening (11.1%). No complications were detected in 33.3% of patients.

Optimal iliac screw size relative to the individual anatomy in general is not achieved. In most cases compared to the literature, iliac screw dimensions could be both longer and thicker. Perfect anatomic placement can be challenging, which highlights the need for individual preoperative CT-based surgical planning to achieve a strong caudal anchorage in lumbopelvic fixations. In general, the diameter seems to be more important than the screw length.