Endoscopic Transcaruncular Approach for Atlantoaxial Transarticular Screw Fixation: An Anatomical Study

Introduction: Endoscopic endonasal odontoidectomy offers a direct anterior approach to the dens, similar to the anterior cervical decompression for lower levels. However, the subsequent instability created from an endoscopic endonasal odontoidectomy often requires fixation, usually through a staged posterior approach. It would be ideal to fuse the atlantoaxial joint from the same anterior corridor as attempted in an anatomical study of the endoscopic endonasal approach for placement of atlantoaxial transarticular screws. However, the endonasal corridor was limited as the trajectory was not in-line with the screw placement direction, required the use of angled instruments which in turn limited the screw length and the use of standard navigation. Alternatively, a different angle of approach through a transcaruncular corridor may offer a better trajectory for the screw placement in the atlantoaxial joints. An anatomical study to investigate the feasibility and develop the technique for transarticular screw fixation of the atlantoaxial joint through the transcaruncular approach was conducted.

Material And Methods: Five anatomical specimens with colored-latex injection and thin-sliced CT scans for navigation were dissected. An endoscopic endonasal approach to the odontoid and bilateral transcaruncular approaches with posterior ethmoidectomy, sphenoidotomy, and medial orbital wall decompression were performed to access the atlas for bilateral screw fixation (10 sides). Odontoidectomy was performed before and after screw placement in two and three specimens, respectively. Neuronavigation and fluoroscopy were used during the screw placement. Postprocedural thin sliced CT scan and three-dimensional reconstruction were performed.

Results: The trajectory from the contralateral transcaruncular approach was found to cross the atlantoaxial facet joint in all specimens. The use of straight instruments and navigation was possible during the screw placement. We developed a technique for placement of C1-C2 transarticular screw.

Technique:

Screw entry point on anterior C1 arch was determined to be at the lateral border of the spinal canal. A high-speed drill was used to open the cortical bone. Small diamond burr (2mm) was then used to pre-drill through the facet joint into the C2 lateral mass under guidance of navigation and fluoroscopy. A Kirschner wire was then placed into the created tunnel, which was taped with 3-mm cannulated reamer under fluoroscopy. A cannulated screw of 3.5 mm diameter with appropriate length was then placed over the Kirschner wire.

Postprocedural results:

Seven over 10 screws were placed successfully crossing the facet joint into the lateral mass of C2. In the other three, one screw broke the posterior wall of C1 lateral mass, and two could not pierce the facet of C2 due to an unsuitable angle of the screw trajectory. Intraoperative flexing of the joint in real clinical scenarios may overcome this situation. Screw length varied from 30 to 45 mm and quadricortical screw placement was achieved in two cases.

Conclusion: Our findings show that the transcaruncular corridor offers a straight approach that is in-line with the atlantoaxial joint from an anterior direction, and a transarticular screw fixation of the joint is feasible through this corridor when combined with endoscopic endonasal odontoidectomy.

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