Postoperatively, all patients undergo computed tomography of the operated area. On the basis of this computed tomography the number of pedicle perforations as well as their extent should be determined according to the simplified Laine classification. These results are to be statistically evaluated with the question of whether there are significant differences between the two techniques with respect to the absolute and individual number of pedicle perforations, as well as their extent. It is also to be examined whether these results show a dependence on the level of experience of the surgeon.