Biomechanical assessment of transoral plate fixation for atlantoaxial instability

STUDY DESIGN: In an experimental study using human cadaver specimens the biomechanical data of anterior atlantoaxial plating according to Harms were evaluated. OBJECTIVES: The purpose of this study was to evaluate this method biomechanically. SUMMARY AND BACKGROUND DATA: The optimum fixation method to achieve fusion at the atlantoaxial joint after odontoid resection is still a matter of discussion. Isolated posterior surgical procedures for treatment of irreducible atlantoaxial kyphosis with spinal cord compression are associated with high rates of morbidity and mortality. Transoral atlantoaxial plate fixation has been designed by Harms as a fixation technique after odontoid resection. In a modification, this procedure has been combined with the posterior wire fusion according to Brooks. METHOD: Eight human cadaver craniocervical specimens were tested in flexion, extension, rotation, and bending with a nondestructive flexibility method using a nonconstrained testing apparatus. Five different groups were examined: 1) control group (intact); 2) unstable group (after dissection of the atlantoaxial ligaments and odontoidectomy), 3) Harms group (transoral atlantoaxial plate fixation) 4) Harms-Brooks group (transoral atlantoaxial plate fixation and dorsal atlantoaxial wire fixation); and 5) Magerl group (transarticular atlantoaxial screw fixation). In a second experimental series, failure loads of the Harms-Brooks and the Magerl fixation methods were determined. RESULTS: The angular displacement of the Harms-Brooks group and the Magerl group was less than in any other group. Stiffness values at 0-3.0 Nm loads in any direction were larger for the Harms-Brooks-and Magerl-fixated specimens than for the Harms, control, or unstable specimens. No statistically significant difference was observed between Harms-Brooks and Magerl reconstruction stiffness. Ultimate failure load in the Harms-Brooks group was higher than in the Magerl group. CONCLUSIONS: Experimentally, isolated anterior atlantoaxial plating was less stable than the combined reconstruction procedures. Transoral plate fixation according to Harms in combination with posterior wire fixation according to Brooks provided a failure load and stiffness equal to transarticular screw fixation according to Magerl.