Fig 1.
Preoperative planning of screw trajectories.
The trajectories and the coordinates for bone entry points and the tips of screws were obtained for the atlas (A, B, C), cranial axis (D, E, F), and caudal axis (G, H, I). The trajectories of each screw were assessed in a 3D manner using multiplanar reconstructed images.
Fig 2.
Design and fabrication of patient-specific drill guide templates.
Drill guide templates for the atlas (A and B) and axis (D and E) were designed using 3D modeling software. Contact between the fabricated drill guide templates and bone models was assessed preoperatively (C and F). An arrowhead in B and C shows the location of an opening at the base of the cylindrical sleeve.
Fig 3.
Simulation of AA joint realignment and titanium plate designing.
Using 3D modeling software, the luxated AA joint (A) was realigned to a normal anatomical relationship (B). This was typically achieved by moving the atlas dorsally in order for the ventral arch to make contact with the dens (C). On the realigned AA joint, the surface area of the ventral arch and axial vertebral body that makes contact with the plate was examined (D) and a 1.5-mm-thick plate was designed (E and F).
Table 1.
Clinical information and findings of axial.
Fig 4.
Illustration of a representative case (Case No.2).
Preoperative CT (A) and MR (B) images demonstrating AA subluxation and spinal cord compression by the dorsally deviated dens. The dens appeared to be slightly hypoplastic, and, thus, may have been predisposed to AAI. Spinal fracture was not detected on CT images. During surgery, the drill guide template was firmly attached to the ventral surface of either the atlas or axis for accurate drilling (C). A custom-made titanium plate was fixed to the realigned atlas and axis with cortical screws (D). Postoperative CT images (E, reconstructed sagittal plane; F, transverse plane). The luxated atlantoaxial joint was reduced and fixed to the titanium plate. The left lateral screw, shown on the right side in F, was inserted more laterally than planned.
Table 2.
Specific data of deviations in screw entry points and exit points for the atlas and axis.
Fig 5.
A case with C2 screw loosening (Case No.18).
Preoperative CT (A) and MR (B) images demonstrating AA subluxation and spinal cord compression by the dorsally deviated axis. The dens appeared to be non-union. An immediate postoperative reconstructed sagittal image (C) and axial image (D). Screws in the atlas and axis are placed as planned and the luxated atlantoaxial joint was reduced. A postoperative radiograph obtained 33 days after surgery (E). The screws placed in C2 were loosened. A postoperative radiograph obtained 60 days after surgery (F). Further displacement of the C2 screws and dislocation of the AA joint were observed. This case underwent second surgery 91 days after the first surgery.