In vitro investigations of internal fixation systems of the upper cervical spine. I. Stability of the direct anterior screw fixation of the odontoid.

Fresh odontoid fractures of types II and III and in some cases nonunions of the odontoid can be repaired by direct anterior screw fixation preserving C1-C2 motion. The goal of this experimental study was to investigate the biomechanical stability of the fragment achieved by this direct odontoid osteosynthesis according to Böhler. Sixteen human C1-C2 segments with fractures of type II or type III were biomechanically tested in vitro under standardized conditions. Flexion and extension moments, anterior and posterior shear forces were applied, and the motion of the refixed odontoid fragment relative to C2 was determined. The results show that direct screw fixation of the odontoid under the experimental conditions provides sufficient stability for the dens fragment.