Fig. 5.

Assessment of the starting point in the lateral view. Three views are mandatory for fluoroscopically controlled SI screw fixation: lateral view, inlet view and outlet view. The techniques for obtaining correct lateral, inlet and outlet views have been previously described in this journal [6]. In addition, the ideal angles for intraoperative fluoroscopic inlet and outlet views can be estimated from preoperative midsagittal computed tomography (CT) reconstructions [4]. It has to be kept in mind that ideal fluoroscopic inlet and outlet view angles are not orthogonal to each other and that the arc of angulation between both views rather averages about 67° [8]. a 55-year-old man with a minimally displaced transforaminal sacral fracture after a fall from height. The preoperative assessment shows no signs of sacral dysmorphism with a safe transverse MC corridor (i. e. both the starting point and the endpoint are located in the MC zone in the lateral view). There is also a safe inlet-oblique corridor (MC—MA), a safe outlet-oblique corridor (MC—UC) and an inlet-outlet-oblique corridor (MC—UA). It is planned to perform an SI screw fixation via a transverse MC corridor. b In the lateral view a central MC starting point is chosen for a planned strict transverse screw trajectory (MC). c The guide wire is advanced to the SI joint for a secure wire purchase in the bone. This additionally allows for assessing the wire trajectory in a lateral view. In the present case fluoroscopic control shows a slight cranial deviation from the planned strict transverse trajectory. Since the outlet-oblique corridor (MC—UC) was preoperatively assessed as safe as well, this trajectory is accepted for further advancement of the wire. U upper, M middle, L lower, A anterior, C central, P posterior