Anatomical and computer tomographical study of the parameters of lag screw fixation of the sacroiliac joint through S_2 pedicle

Objective:To provide the anatomic basis for sacroiliac screw fixation through S-2 pedicle.Methods:Twenty-two pelvis specimens of native adults were studied.Spiral CT was used to scan 22 pelvis specimens.The axis of S-2 pedicle and the direction of sacroiliac screw were studied on axial CT scans.Then,anthropometric measurement were performed on 22 cadaveric pelvis to determine the optimal entry point for sacroiliac screw on the outer table of ilium,the direction of sacroiliac screw,the diameter of the sacroiliac screw path,the distance from entry point to the center of S-2 vertebral body,the distance from entry point to the center of S-2 pedicle,and the shape of the S-2 pedicle transverse section.Results:The average entry point for screw fixation on the outer table of ilium located 28.7 mm(15.0～34.5 mm) anterior to posterior inferior iliac spine and 15.3 mm(10.0～22.0 mm) superior to the sciatic notch.The direction of screw was perpendicular to midsagittal plane.The angle between the direction of screw and the outer table of ilium was 62.4°(55°～72°).The diameter of the sacroiliac screw path,the distance from entry point to the center of S-2 vertebral body and the distance from the entry point to the center of S-2 pedicle were 11.5 mm(8.5～14.0 mm),64.7 mm(55.3～77.6 mm) and 53.1 mm(45.3～64.8 mm) respectively.The shape of S-2 pedicle transverse section was triangle.Conclusion:The entry point locates 30 mm anterior to posterior inferior iliac spine and 15 mm superior to the sciatic notch.The direction of sacroiliac screw perpendicular to midsagittal plane.The angle between the direction of screw and the outer talble of ilium is 60°.The diameter of sacroiliac screw path through S-2 pedicle smaller than that of sacroiliac screw path through S-1 pedicle.Sacroiliac screw fixation for sacroiliac joint through S-2 pedicle should use the smaller diameter and shorter screw.