The positioning of the screws inside the pedicles will be evaluated by means of tomographic cuts of 2mm. Correct positioning will be considered when the pedicle screw is fully encircled by the vertebral pedicle and the cortical vertebra is not perforated. The perforation will be classified as small (≤2mm), medium (2.1-4mm) and severe (> 4mm), and the perforation site: medial, lateral, superior, posterior will also be evaluated.