Accuracy of computer-guided screw fixation of the sacroiliac joint

Computer-assisted image guidance allows precise preoperative planning and i ntraoperative localization of surgical instruments. The technique recently was validated for the insertion of pedicle screws. In the laboratory, the p recision of a surface-matching algorithm was evaluated for registration and accuracy and safety of screw placement into the vertebral bodies of SI and S2 for fixation of the sacroiliac joint. Using six plastic pelves, 24 scre w holes were made through the sacroiliac joint into the vertebral body of S I, and 12 holes were made through the sacroiliac joint into S2. The accurac y of the hole position was evaluated using a postoperative computed tomogra phy examination. The safety factor was assessed by analysis of the remainin g bone stock around the holes calculating a theoretical cylindrical volume being outside bone with increasing bore hole diameters. The registration wa s accurate with a mean error less than 1.4 turn in the posterior parts of t he pelvis. The drilling followed precisely the preoperatively planned traje ctories; perforation of the cortex of the sacrum was not observed. The safe ty factor of the SI vertebral body is higher than that of S2 allowing large r diameter screw insertion into SI. This technique provides a safe and prec ise guide for transcutaneous or open insertion of iliosacral screws in case s of iliosacral dislocation or sacral fracture.