Biomechanical evaluation of an atlantoaxial lateral mass fusion cage with C1-C2 pedicle fixation.

STUDY DESIGN: A biomechanical testing protocol was used to evaluate atlantoaxial fixation techniques in a human cadaveric model.

OBJECTIVE: To compare in vitro biomechanics of atlantoaxial lateral mass fusion cage combined with C1-C2 pedicle screw technique with those of C1-C2 pedicle screw technique alone and C1-C2 transarticular screws combined with Gallie wires.

SUMMARY OF BACKGROUND DATA: An atlantoaxial lateral mass fusion cage was designed, knowing that the cage, when rigidly combined with C1-C2 pedicle screws, could offer other fusion spots for atlantoaxial stabilization in cases when the posterior arch of the atlas is absent or removed for decompression and a Gallie fixation is impossible. No comparative in vitro biomechanical test has been conducted previously to evaluate the feasibility of this method.

METHODS: Anatomic measurements of the atlantoaxial lateral masses were taken using computed tomography in normal human subjects. Six fresh-frozen human cadaveric cervical spines (C0-C4) were used in the biomechanical study. Specimens were tested in their intact condition, after destabilization via transverse-alar-apical ligament disruption, and after implantation of 3 fixation constructs: (1) transarticular screws combined with Gallie wires, (2) C1-C2 pedicle screws, and (3) atlantoaxial lateral mass fusion cage combined with C1-C2 pedicle screws. Pure moment loading up to 1.5 Nm in flexion/extension, right-left lateral bending, and right-left axial rotation was applied to the occiput, and relative intervertebral rotations were determined using stereophotogrammetry. Range of motion for the intact, destabilized, and 3 fixation scenarios were determined.

RESULTS: The anatomic data indicated that feasible cage design were in 3 sizes: 11/8, 12/9, and 13/10 mm for length/width, and 3.5, 4, and 4.5 mm for height. The biomechanical data indicated that transverse-alar-apical ligament disruption significantly increased C1-C2 motion for all directions. All the 3 fixation techniques significantly reduced motion compared with the intact and destabilized cases. There were no statistically significant differences among the 3 fixation techniques.

CONCLUSION: The biomechanical study indicated that, contrary to expectation, addition of a cage did not increase the stability compared with C1-C2 pedicle screw alone. However, the C1 + C2 + Cage technique may be a viable alternative for atlantoaxial stabilization when the posterior arch of the atlas is absent or removed for decompression and a Gallie fixation is impossible.