Trajectory of transsacral iliac screw for lumbopelvic fixation: A 3-dimensional computed tomography study

Study Design: A 3-dimensional multi detector computed tomography study. Objective: To analyze the optimal trajectory of the transsacral iliac screw (TSIS) to overcome the disadvantages of the conventional iliac screw. Summary of Background Data: Iliac screw fixation increased lumbosacral fusion rate. However, its disadvantages have been reported including extensive muscle dissection, loss of the posterior superior iliac spine (PSIS), need of additional connecting device, screw breakage, and pain from prominence. Methods: Three-dimensionally reconstructed multi detector computed tomography images on the pelvis of 60 patients (M:F=30:30, age 20 to 50) were analyzed. The virtual trajectory started from the point where the center of S2 ala met the lateral sacral crest and targeted the superior rim of acetabulum, across the sacroiliac joint. On the basis of the virtual trajectory, the oblique sagittal, oblique axial, oblique coronal plane, and 3-dimensional surface rendering images were obtained. The optimal insertion angle in the 3 planes, the maximal length and diameter of screw were measured. Relationships between the trajectory and structures under risk were analyzed. Results: The safe insertion angle was 38±3.5 degrees in the axial plane and 86±6.5 degrees in the sagittal plane based on the posterior surface of the sacrum. The mean length and maximal diameter of optimal trajectory was 91 and 18 mm, respectively. When the screw was inserted within 25 degrees in the axial plane, and exceeding 95 degrees in the sagittal plane, the external iliac vessels and the superior gluteal arteries were at risk. Conclusions: The TSIS could be safely inserted without increasing the general risk of conventional iliac screw. On the basis of the result of this study, we consider the TSIS technique a useful option for lumbopelvic fixation to overcome disadvantages of iliac screw.