Transpedicular screw fixation relies on thorough knowledge of the pedicular anatomy and a reliable intraoperative technique. To enhance the safety and accuracy of screw insertion, computer-assisted systems have been introduced. Our in vitro study investigated the potential benefits of such a system for the preoperative planning and the intraoperative visualization. In part 1, the potentially possible range of screw paths (trajectories) through lumbar pedicles was analyzed. In part 2, the accuracy of actual pedicle-hole preparation with and without preoperative planning was assessed. It was shown that, especially in the lower lumbar regions, the possible range of trajectories is considerable, with inclinations of < or = 40 degrees in the transverse plane and a range of angulation in the sagittal plane of > 20 degrees. The computer assistance in preparation of 100 pedicle holes resulted in cortex perforation in only one case. Computer assistance therefore may be used as a valuable tool to minimize the risks of transpedicular screw insertion. It furthermore may assist in determining the desired screw orientation and length and transform this planning into the intraoperative pedicle-hole preparation.