Biomechanical evaluation of 2 insertion points for ventral screw fixation of C-2 dens fractures Laboratory investigation

Object. For many Type II fractures of the dens (Anderson and D'Alonzo classification), a double anterior screw fixation is performed. If screw disruption occurs, the location is most often at the anterior caudal endplate and body of the axis and not directly at the fracture line. The authors' objective was to determine the differences in primary mechanical stability at 2 insertion points used in ventral screw fixation of Type II fractures of the C-2 dens. Methods. Screw fixation was performed on 16 formalin-fixed human C-2 dens specimens. The specimens were divided into 2 groups. For Group 1, the screws were inserted directly at the anterior lower endplates; for Group 2, the screws were inserted 2 mm dorsal to the anterior wall of the vertebral body. After a Type II odontoid fracture was created with an oscillating saw, screw fixation was performed using two 3.5-mm partially threaded lag screws with washers. Subsequently, each vertebral body was continuously loaded. The criterion for breakage was reversal of the force vector. Results. In Group 1, screw disruption occurred at the point of entry; the mean load failure was 290.5 +/- 106 N. In Group 2, no screw disruption occurred; the mean load failure was 574.2 +/- 170.5 N. These results were significant (p < 0.05). Conclusions. For double screw fixation of Type II fractures of the dens (Anderson and D'Alonzo classification), placement of the screws as far dorsal to the anterior lower endplate as possible seems to favorably affect primary stability. In actual clinical practice, care should be taken to not damage the anterior wall of the vertebral body of the axis during screw insertion.