Anatomic study on the placement of pedicle screw at the sacrum

To investigate the feasibility and safe zone of S1-4 for sacral screw placement, a total of 62 adult patients spiral computed tomography images of pelvis were analyzed and reconstructed using Mimics version 15.0 software. The safe zone of the pedicle screws were calculated, the entry points of sacral pedicle at S1-S4 were determined, and the screw trajectory, length, width and angle was measured in the three dimensional reconstruction of pelvis. The mean entry point of S1 pedicle screws was lateral to basilar part of S1 superior articular process. The entry points of S2-4 were approximately superior to the midpoint of a line connecting the inferior outer edge of the upper posterior foramen and superior outer edge of the lower posterior foramen. For S2-4, the entry point outward change gradually, the diameter and length of screw became smaller, the angle increase successively. There was a statistical difference in the entry point of S2 in gender group, the diameter of screw in S1 and S3 in gender group respectively also showed statistical difference (P<0.05). The results demonstrate the ideal entry points of sacral pedicle at S1-S4, and precisely describe the trajectory, length, width and angle of screws. These defined details should be taken into consideration during surgical procedures.