The following anatomical landmarks are identified:
Lateral border of the lateral mass
Medial border of the lateral mass
These landmarks form a square.
2. Screw entry point
The square outlined by the landmarks is divided by a horizontal and a vertical line.
The screw entry point is found slightly superomedial to the intersection of these lines.
3. Opening of the cortex
Penetrate the cortex with a thin burr or an awl.
4. Medio-lateral angulation
The drill trajectory should be aimed 25 degrees laterally to avoid the vertebral artery which is located directly anterior to the entry point.
5. Cranial-caudal angulation
To identify the cranio-caudal angulation, a Penfield elevator is inserted in the facet joint which will be included in the fusion. The drill trajectory is then then parallel to this elevator, avoiding compromise of the facet joints. Alternatively the trajectory of the facet joints may be determined by lateral fluoroscopy.
6. Monocortical or bicortical screws?
Both mono- and bicortical screws may be used.
Utilizing 14 mm screws will be safer but only monocortical purchase can be achieved.
A longer screw providing bicortical purchase will result in a more stable construct. However, the screw tip should not extend too far beyond the second cortex as it may compromise the nerve root.
If a monocortical screw is planned, the drill is set for a 14mm screw hole.
When a bicortical screw is planned, the drill is used at high speed with meticulous handling. The drill bit is advanced only for a short distance, then pulled back before advancing again. This maneuver is repeated until the second cortex can be felt and crossed. Great care is taken to avoid over penetration.
Alternatively a drill stop may be used in 2mm increments. After each increase in effective drill length, the drill hole is probed to feel for penetration of the second cortex.
A screw length is selected which corresponds to the final effective drill length required for bicortical penetration.
8. Screw insertion
A screw of appropriate diameter (3.5 mm) and length is carefully inserted into the same created trajectory.