Biomechanical Evaluations of Various C1-c2 Posterior Fixation Techniques

Study Design: A biomechanical in vitro study using human cadaveric spine.

Objective: To compare the biomechanical stability of pedicle screws versus various established posterior atlantoaxial fixations used to manage atlantoaxial instability.

Summary Of Background Data: Rigid screw fixation of the atlantoaxial complex provides immediate stability and excellent fusion success though has a high risk of neurovascular complications. Some spine surgeons thus insert shorter C2 pedicle or pars/isthmus screws as alternatives to minimize the latter risks. The biomechanical consequences of short pedicle screw fixation remain unclear, however.

Methods: Seven human cadaveric cervical spines with the occiput attached (C0-C3) had neutral zone (NZ) and range of motion (ROM) evaluated in three modes of loading. Specimens were tested in the following sequence: initially (1) the intact specimens were tested, after destabilization of C1-C2, then the specimens underwent (2) C1 lateral mass and C2 short pedicle screw fixation (PS-S), (3) C1 lateral mass and C2 long pedicle screw fixation (PS-L), (4) C1 lateral mass and C2 intralaminar screw fixation (ILS), (5) Sonntag's modified Gallie fixation (MG) and (6) C1-C2 transarticular screw fixation with posterior wiring (TAS 1 MG). (7) The destabilized spine was also tested.

Results: All instrumented groups were significantly stiffer in NZ and ROM than the intact spines, except in lateral bending, which was statistically significantly increased in the TAS 1 MG group. The MG group's NZ and ROM values were statistically significantly weaker than those of the PS-S, PS-L, and the ROM values of the TAS 1 MG groups. The ILS group's NZ values were higher than those of the TAS 1 MG group and for ROM, than that of the PS-S and PS-L groups. In flexion, the NZ and ROM values of the TAS 1 MG group were significantly less than those of the PS-S, PS-L, ILS, and MG groups. In axial rotation, the NZ and ROM values of the MG group were statistically significantly higher than those of the PS-S, PS-L, ISL and TAS 1 MG groups.

Conclusion: The TAS 1 MG procedures provided the highest stability. The MG method alone may not be adequate for atlantoaxial arthrodesis, because it does not provide sufficient stability in lateral bending and rotation modes. The C2 pedicle screw and C2 ILS techniques are biomechanically less stable than the TAS 1 MG. In the C1 lateral mass-C2 pedicle screw fixation, the use of a short pedicle screw may be an alternative when other screw fixation techniques are not feasible.