see also C1 pedicle screw placement.
Check CT scan or MRI to rule-out aberrant location of vertebral artery or unusual location of foramen transversarium before placing C2 pedicle screws. Some find intraoperative image guidance systems to be helpful.
1. ENTRY palpate the medial and superior aspect of the pars with a Penfield 4 dissector. Enter at the estimated center of the surface projection of the C2 pars at the midpoint medio-laterally in the supero-medial quadrant of the surface of the C2 isthmus
2. TRAJ 20–30° medially (through the central axis of the C2 pedicle), 25° superiorly (on lateral fluoroscopy, place the screw parallel to the pars). To assist with trajectory, expose the proximal upper and medial border of the C2 pars interarticularis, and use a Penfield 4 to palpate during drilling
3. drill a shallow entry point, then drill with drill-stop set at 12 mm, monitoring progress at inter- vals under fluoro and palpating with probe, and if no breakout, then complete drilling by gradu- ally increasing drilling depth by 2 mm increments either up to 15-20 mm to stay in the pedicles, or up to ≈ 30 mm depth to perform osteosynthesis for a hangman’s fracture. If withdrawal of the drill is followed by brisk bleeding, the screw should be inserted immediately to stop the bleeding. This bleeding may be from the vertebral artery; however, it is usually due to injury to the venous plexuses, and will not have any ill e ects. In such cases it is best to not place the con- tralateral screw and to obtain an angiogram very soon post-op
4. SCREWS 3.5 mm dia. Screw length is not critical except when attempting to bridge a fracture gap (osteosynthesis) e.g. with a hangman’s fracture in which case screws of 20–30 mm length are placed to avoid penetrating anterior C2 cortex (lag screws are used for this, or the proximal bone can be overdrilled); for most purposes screw lengths of 15–20 mm length are used. Shorter screws (15-16 mm length) can still grip the pedicle with lower risk of injury.