Fig. 4
![Fig. 4](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs13018-018-0810-5/MediaObjects/13018_2018_810_Fig4_HTML.gif)
Grades of screw insertion position. a Grade 1 indicates a screw positioned at the center of the pedicle without cortical bone breach. b Grade 3 indicates a screw that penetrated the cortex between one third and one half of the screw diameter
Grades of screw insertion position. a Grade 1 indicates a screw positioned at the center of the pedicle without cortical bone breach. b Grade 3 indicates a screw that penetrated the cortex between one third and one half of the screw diameter