Morphometric measurements of safety zones and orientation angles for freehand placement of revision screws in atlas lateral mass.

To demonstrate the possibility of revision screw placement to the atlas, as well as define the safety zones and orientation angles. This retrospective study analysed the records of four patients who were operated for AAI earlier. Because they needed revision of Atlas screws, they were re-operated after obtaining the measurements mentioned in this study. In addition, measurements of 50 healthy subjects were included in the study as the control group. Maximum screw lengths were also measured. Safe zone in the ideal sagittal direction were wider. As the screw projection becomes more cephalic direction in the sagittal plane, the safe zone for the screw becomes narrower. With the sagittal angle moving forwards cranially, the screw length becomes longer. Atlas lateral mass screws could be safely revised whenever needed. The fact that needs to be considered is that the angular range becomes narrower, and the screw length becomes longer when the screw is directed more cranially.