An anatomical study defining the safe range of angles in percutaneous iliosacral and transsacral screw fixation

Percutaneous iliosacral screw fixation and transsacral fixation are challenging procedures requiring extensive knowledge of sacral anatomy to avoid damaging nearby neurovascular structures. Greater knowledge of anatomical screw trajectory and size allowances would be helpful to guide surgical placement. An anatomical study of 40 cadaveric sacra in specimens ages 18-65 was performed. Three-dimensional surface scans were obtained, and computer modeling software was used to simulate a 7.3 mm diameter screw with 1 mm buffer inserted orthogonal to the sacroiliac joint in the pelvic inlet and outlet views. Transsacral screws were also inserted into S1 and S2 vertebrae. For screws orthogonal to the sacroiliac joint, the overall mean screw insertion angle was 4.1° ± 7.5° (range, -18.3° to 22.0°) in the inlet view in the posterior to anterior direction, and 21.7° ± 5.1° (range, 8.2°-36.3°) in the outlet view in the caudal to cranial direction. Before breaching the sacrum, the range of sacral tunnel lengths was between 31.1 and 70.1 mm with a range of diameters between 9.3 and 13.3 mm. Transsacral screws inserted into either the S1 or S2 vertebrae did not breach the sacrum in 40% (16/40) at each level. 30% (12/40) of sacra could not safely accommodate both S1 and S2 transsacral screws. There is an initial screw insertion angle range of -4° to 12° in the inlet view and 16°-27° in the outlet view. There was always adequate size to accept a 7.3 mm or larger screw.