Targeting a safe entry point for c2 pedicle screw fixation in patients with atlantoaxial instability

Abstract

Objective: This investigation was conducted to evaluate a new, safe entry point for the C2 pedicle screw, determined using the anatomical landmarks of the C2 lateral mass, the lamina, and the isthmus of the pars interarticularis.

Methods: Fifteen patients underwent bilateral C1 lateral mass-C2 pedicle screw fixation, combined with posterior wiring. The C2 pedicle screw was inserted at the entry point determined using the following method : 4 mm lateral to and 4 mm inferior to the transitional point (from the superior end line of the lamina to the isthmus of the pars interarticularis). After a small hole was made with a high-speed drill, the taper was inserted with a 30 degree convergence in the cephalad direction. Other surgical procedures were performed according to Harm's description. Preoperatively, careful evaluation was performed with a cervical X-ray for C1-C2 alignment, magnetic resonance imaging for spinal cord and ligamentous structures, and a contrast-enhanced 3-dimensional computed tomogram (3-D CT) for bony anatomy and the course of the vertebral artery. A 3-D CT was checked postoperatively to evaluate screw placement.

Results: Bone fusion was achieved in all 15 patients (100%) without screw violation into the spinal canal, vertebral artery injury, or hardware failure. Occipital neuralgia developed in one patient, but this subsided after a C2 ganglion block.

Conclusion: C2 transpedicular screw fixation can be easily and safely performed using the entry point of the present study. However, careful preoperative radiographic evaluation, regardless of methods, is mandatory.

Keywords: Atlantoaxial instability; C2 pedicle screw; Entry point; Technique.

Fig. 1

Drawing of the anatomical entry point for the C2 pedicle screw. A : Superior aspect of C2. The black arrow points to the transitional point from the superior line of the lamina to the isthmus of the pars interarticularis. B : The asterisk indicates the entry point for the C2 pedicle screw, 4 mm lateral to and 4 mm caudal to the transitional point. C : Schematic drawing shows inserted C2 pedicle screws without violation into the spinal canal and the vertebral foramen.

Fig. 2

Postoperative computed tomogram scan of the C2 pedicle screw. Axial scan reveals no violation into the spinal canal (A) or the vertebral foramen (B).

Fig. 3

Three dimensional computed tomogram shows suitable location of screws and wiring with cadaveric bone in the last follow-up period. A : Anteroposterior view. B : Lateral view.