Study design: This study analyzed the sacroiliac articulation at the level of the second sacral vertebra (S2). Anthropometric measurements were performed on 20 cadaveric pelves to determine the optimal starting point for lag screw fixation of the sacroiliac joint at S2.
Objectives: The measurements were utilized to identify a region on the outer table of the posterior ilium which will provide a starting point for consistent safe placement of a lag screw across the sacroiliac joint into the ala of S2.
Summary of background data: Previous studies have defined the optimal starting point on the outer table of the ilium for the projection of lag screws into the ala of S1. No data are available for lag screw fixation of the sacroiliac joint at S2.
Methods: Twenty human cadaveric pelves, disarticulated at the sacroiliac joint and fixed in a holding frame designed to maintain the sacrum and ilium in anatomic reduction, were utilized to identify a point on the outer table of the posterior ilium at which an interfragmentary screw could be inserted into the center of the pedicle of the second sacral vertebra.
Results: The starting point on the posterolateral ilium for screw insertion into the center of the S2 pedicle was found to exist 1.5 +/- 0.31 cm superior and 2.5 +/- 0.3 cm posterior to the apex of the greater sciatic notch only when the screw or guide pin was advanced at an angle perpendicular to the long axis of the sacrum.
Conclusion: During lag screw fixation of posterior pelvic ring disruptions, aberrant screw placement may impose considerable risk to adjacent vascular, visceral, or neural structures. After anatomic reduction of the sacroiliac joint, safe and accurate screw fixation can be achieved by utilizing the starting point and insertion trajectory described in this paper.