In vitro investigations of internal fixation systems of the upper cervical spine. II. Stability of posterior atlanto-axial fixation techniques.

Unstable C1-C2 segments are generally treated surgically. Depending on the indication a direct screw fixation of the odontoid or a C1-C2 arthrodesis is a possible technique. In this experimental in vitro study the three different atlantoaxial fusion techniques by Gallie, Brooks, and Magerl were compared biomechanically. Sixteen human C1-C2 segments with odontoid fractures of type II and III were investigated under standardized conditions. Flexion and extension moments, anterior, and posterior shear forces, left and right torsional moments were applied, and the motion of C1 relative to C2 was determined. The results of this investigation show clearly that the segments treated with the technique by Magerl with transarticular screws achieved the highest stiffness, compared to the wiring methods of Brooks and Gallie. These differences were most evident for posterior shear forces and for torsional moments. For these load conditions the ratio of stiffness Magerl:Brooks:Gallie was about 10:2:1. Significant differences for the plastic deformation of the differently fixed C1-C2 segments were found within the first few load/unload cycles, which give information about the relationship between primary and long-term stability.