A biomechanical study of unilateral posterior atlantoaxial transarticular screw fixation

The purpose of this study was to investigate the fixation of C1-C2 instability with the use of a unilateral screw. Transarticular screw placement across C1-C2 may be contra-indicated in up to 20% of specimens on at least one side because of anatomic variations or other pathological processes. Hence the current study looks into unilateral screw fixation of C1- C2 instability. Eight cervical spine specimens, C1 through C5, were harvested from fresh human cadavers (4 male and 4 female) of average age 67 years (54-80). C1 and C2-C5 vertebrae were potted to allow motion only at the C1-C2 articulation. Cutting the transverse ligament on both sides of the odontoid and the tectorial membrane destabilized the specimens. Transarticular screw fixation of C1-C2 was performed in a manner similar to the technique described by Magerl. The stability was tested after fixation with one transarticular screw together with a posterior graft and wire. Placement of the screw was randomized, resulting in half the specimens receiving screws on the right side and the remaining half on the left side. The stiffness of the C1-C2 articulation was tested in rotation, lateral bending, flexion, and anterior translation in random order. The rotational stiffness was 1.44 +/- 0.44 N-m/deg, while lateral bending stiffness values were 2.33 +/- 1.14 N-m/mm (right bending) and 2.81 +/- 1.36 N-m/mm (left bending). The stiffness value in flexion was 0.813 +/- 0.189 N-m/mm and in translation 67.1 +/- 25.1 N/m. It was found that stability after unilateral transarticular screw fixation was less than that previously reported after bilateral transarticular screw fixation, but similar to that found with modified Brooks posterior wiring, which has been shown to provide better stability than other posterior wiring methods, and fusion rates of 96% have been reported. We concluded that C1-C2 unilateral posterior transarticular screw fixation with supplemental posterior graft and wiring would confer adequate stability in cases where bilateral screw placement is contraindicated.