Influence of the inclination angle of the S1 pedicle on screw malposition and operative revision in percutaneous iliosacral screw fixation of posterior pelvic ring fractures.

Introduction: Due to aging society, osteoporotic posterior ring fractures of the pelvis are gaining importance. Percutaneous iliosacral screw fixation can reduce pain if conservative therapy is not sufficient. One of the surgical complications is the malposition of the screws in neuroforamen. The aim of this study is to correlate the measured pedicle angle with the screw position in postoperative CT scans.

Materials And Methods: In this retrospective study (level of evidence: III), 97 patients with posterior ring fractures were treated with 137 percutaneous iliosacral screws. The inclination angles of the S1 pedicle were measured in the preoperative pelvic CT scan of each patient. Patients were divided up into pedicle angles from 10 - 19.9° (Group 1), 20 - 29.9° (Group 2), 30 - 39.9° (Group 3) and 40 - 49.9° (Group 4). We correlated the pedicle inclination angle to radiologically described screw position in the L5/S1 neuroforamen and the need for revision because of screw malposition.

Results: A total of 10 pedicle inclination angles were from 10 - 19.9° (7.3%), 96 from 20 - 29.9° (70.1%), 28 from 30 - 39.9° (20.4%) and three from 40 - 49.9° (2.2%). Of the 137 screws used, 19 were intraforaminal (13.9%). There were no intraforaminal screws in the Group 1, eight in Group 2, 10 in Group 3 and one in Group 4. Five of the screws needed revision (3,6%). There were no revisions in Groups 1 and 4, two in Group 2 and three in Group 3. Groups 3 and 4 had a higher percentage of intraforaminal screw positions. Patients with steeper angles S1 pedicles showed a significantly higher probability of intraforaminal screw location and revision (p<0.01).

Discussion: Complications such as intraforaminal screw position and revision surgery are more frequent in patients with steeper S1 pedicles, making this a detrimental prognostic marker.