Table 1.

Surgical Techniques and Special Considerations for C2 Screw Fixation.

Type of Axis Fixation	Surgical Techniques30	Special Considerations—Pearls for Practice30
Pars screws	Entry point: 3-5 mm above the C2-3 junction, medial position without violating the spinal canal. Trajectory: Parallel trajectory to the pars, may be guided by lateral fluoroscopy	Average screw length is 12-18 mm. Evaluate the foramen transversarium: Generally, just anterior to the posterior vertebral line—more vertical trajectory increases the risk of vertebral artery injury
Transarticular C1-2 screws	Same entry point as a pars screw with greater craniocaudal direction. Palpating the medial and superior border of C2 pedicle to avoid breaking the cortical bone. Removal of the C1-2 articular cartilage—improves fusion rates. Trajectory: Guided by lateral fluoroscopy, through the pars into the lateral mass of C1	Average screw length is 20-30 mm. Computed tomography (CT) scan: The entire pars should be visualized on a single image slice of a parasagittal CT scan (average CT scan cut is 3 mm)—failure to identify a medial located vertebral artery may result in vascular injury C1-2 joints must be aligned on fluoroscopy Axial support in the vertex may be useful to avoid C1-2 joints distraction Excessive angulation may cause: (1) cranial—may violate the condyle-C1 joint, (2) caudal—inadequate fixation of C1, (3) medial—spinal cord injury, (4) lateral—vertebral artery injury
Pedicle screws	Entry point: The entry point in the cranio-caudal direction is an imaginary line extending the rostral border of the C2 lamina. Medial-laterally, it is 2 mm lateral to the midpoint of the pars Trajectory: Palpating the medial and rostral portion of the pedicle for guidance	True pedicle screws—cross obliquely into the pedicle toward the body of the axis CT scan: The entire pars should be visualized on a single image slice of a parasagittal CT scan (average CT scan cut is 3 mm)—failure to identify a medial located vertebral artery may result in vascular injury Up to 20% of the patients do not have pedicles large enough to allow for pedicle screw cannulation
Laminar	Entry point: Junction of the spinous process and lamina. The trajectory is directly into the lamina but avoiding a ventral breech (dorsal perforation is possible and may also improve purchase with a bicortical screw)	Free hand technique. May require of head screw extension to capture the rod Contraindicated when there is a hemilaminectomy of C2 Alternative to pedicle and transarticular screws given 20% of the patients cannot have safely placed pedicle screws With a dissector it is possible to palpate the inner portion of the lamina, which may help in selecting trajectory