Biomechanical assessment of transoral plate fixation for atlantoaxial instability

Study Design. In an experimental study using human cadaver specimens the bi omechanical data of anterior atlantoaxial plating according to Harms were e valuated. Objectives. The purpose of this study was to evaluate this method biomechan ically. Summary and Background Data. The optimum fixation method to achieve fusion at the atlantoaxial joint after odontoid resection is still a matter of dis cussion. isolated posterior surgical procedures for treatment of irreducibl e atlantoaxial kyphosis with spinal cord compression are associated with hi gh 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 wi re 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 exami ned: 1) control group (intact); 2) unstable group (after dissection of the atlantoaxial ligaments and odontoidectomy), 3) Harms group (transoral atlan toaxial plate fixation) 4) Harms-Brooks group (transoral atlantoaxial plate fixation and dorsal atlantoaxial wire fixation); and 5) Magerl group (tran sarticular atlantoaxial screw fixation); In a second experimental series, f ailure loads of the Harms-Brooks and the Magerl fixation methods were deter mined. 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,specim ens than for the Harms, control, or unstable specimens. No statistically si gnificant difference was observed between Harms-Brooks and Magerl reconstru ction stiffness. Ultimate failure load in the Harms-Brooks group was higher than in the Magerl group. Conclusions, Experimentally, isolated anterior atlantoaxial plating was les s stable than the combined reconstruction procedures. Transoral plate fixat ion according to Harms in combination with posterior wire fixation accordin g to Brooks provided a failure load and stiffness equal to transarticular s crew fixation according to Magerl.