Transpedicular Screw Placement Evaluated by Axial Computed Tomography of the Cervical Pedicle

Abstract

We evaluated the trajectory of transpedicular screws in the cervical spine using axial computed tomography (CT). To provide a safe transpedicular screw trajectory by measuring the dimensions of the cervical pedicle and evaluating the entrance points and the insertion angles of transpedicular screws. The morphology of the cervical pedicle has been studied, but few in vivo CT-based studies of pedicle dimensions and transpedicular screw placement in the cervical spine have been reported. The dimensions of the pedicles (C3-C7) were determined in 30 patients with cervical spinal lesions from CT images. The space available for transpedicular screws (SAS) was defined as the distance between two parallel lines tangential to the spinal canal and the transverse foramen, respectively. SAS was evaluated at 25 degrees and 50 degrees insertion angles. SAS at a 25 degrees insertion angle (SAS-25) ranged from 4.7 to 5.4 mm. SAS at 50 degrees (SAS-50) ranged from 6.1 to 6.6 mm. SAS-25 and SAS-50 were significantly different. Four-millimeter-diameter screws would fit in all 120 C3-C6 vertebrae studied at 50 degrees, but 20 (17%) would not fit at 25 degrees. Axial CT measurements should facilitate transpedicular screw fixation in the cervical spine. We believe that the screw insertion angle should be close to 50 degrees, which is the mean pedicle transverse angle from C3-C6. The entry point of the pedicle screw should be located as laterally as possible in the posterior surface of the lateral mass.