Accuracy of Iliac Screws Insertion in Adult Spinal Deformity Surgery: Relationship between Misplacement and the Iliac Morphologies.

STUDY DESIGN: Retrospective study.

OBJECTIVE: To investigate the accuracy of freehand iliac screw insertion and to determine how this can be done safely.

BACKGROUND AND METHODS: Seventy-seven adult scoliosis patients with an average age of 70.1 years who underwent spinal deformity surgery with spinopelvic fixation using bilateral iliac screws were enrolled. Penetration of the iliac table was assessed using postoperative computed tomography (CT). Screw penetration of the iliac table or screw insertion from the sacroiliac joint was considered misplacement. Screw positioning was classified as the screw being in the proper position (group C), the screw penetrating the outer table (Group O), and the screw penetrating the inner table (group I). The iliac opening angle and the distance between the posterior superior iliac spines (PSISs) were measured using preoperative CT. The angle between the sacral slope and the iliac screw, termed as the sagittal screw angle, was measured using postoperative lateral lumbar radiography.

RESULTS: Of the 154 iliac screws in 77 patients, 14 screws in 12 patients penetrated the outer table and 12 screws in 11 patients penetrated the inner table. The total proportion of misplacement was 18.8%, although there were no major complications. With regard to iliac morphology, the iliac opening angle was 24.2°±4.3° and distance between the PSISs was 90.6±7.7 mm. The distance between the PSISs correlated negatively with the iliac opening angle. The iliac opening angle was smaller in group O than in group C (p<0.05). The sagittal screw angle in group I was smaller than that in group C (p<0.01).

CONCLUSIONS: Screw penetration of the outer iliac table possibly occurred in patients with a narrow iliac opening angle. Screw penetration of the inner table occurred when the screw was inserted more cranially than the sacral slope. Therefore, the iliac screw should be inserted approximately parallel to the sacral slope.