[Screw fixation of craniocervical junction]

In recent years an increase has been observed of the use of screw techniques for the fixation of the craniocervical junction. For clinical use two techniques have been introduced: (1) transarticular screw fixation, and (2) transpedicular screw fixation. In the former the screw is inserted through the C2 lateral mass, the fissure of the C1-C2 joint, and the C1 lateral mass. (2) in the latter the screw is inserted into the C2 pedicle and anchored in C2 vertebral body. Transarticular or pedicle screws can be easily connected to longitudinal elements such as rods or plates, and combined with lateral mass screws of the remaining cervical vertebrae or occipital screws. In comparison to sublaminar wiring or interlaminar clamping the screw techniques: (a) strengthen the stiffness of the construct and speed up fusion, (b) allow fixation in the absence or deficiency of laminae as a result of trauma or laminectomy, and (c) can selectively include only the affected segments. Increased construct stiffness is due to deep anchorage of the screw in bone providing thus a solid grip on the vertebra. Both techniques require preoperative assessment of the course of the vertebral artery using imaging methods. In about 18% of cases abnormal course of the artery precludes screw use. Pedicle screw insertion requires direct control of the medial and superior walls of C2 pedicle with dissector introduced into the vertebral canal, which requires removal of the atlantoaxial ligament. Additional control can be achieved with lateral fluoroscopy. The entry point for transarticular screw is on the lateral mass of the odontoid 2-3 mm laterally to the medial margin of C2 facet and 2-3 mm above the C1/C2 articular fissure. The screw trajectory is 0-10 degrees in horizontal plane and towards the anterior C1 tuberculum in sagittal plane.