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

Laboratory investigation

Restricted access

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.

Article Information

Address correspondence to: Ludwig Oberkircher, M.D., Department of Trauma, Hand and Reconstructive Surgery, Philipps University Marburg, Baldingerstrasse, D-35043 Marburg, Germany. email: oberkirc@med.uni-marburg.de.

Please include this information when citing this paper: published online April 5, 2013; DOI: 10.3171/2013.3.SPINE12745.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Intraoperative images after screw fixation, lateral views. A: Group 1 (point of insertion directly at the anterior lower endplate). B: Group 2 (point of insertion 2 mm dorsal to the anterior lower endplate). Insertion points are marked by arrows.

  • View in gallery

    Intraoperative images after screw fixation of a previously generated Type II fracture according to the Anderson and D'Alonzo classification. A: Anteroposterior view. B: Lateral view.

  • View in gallery

    Vertebral body being subjected to load by an Instron 5566. The maximum pressure point was at the midpoint of the dens.

  • View in gallery

    Interruption of loading after reversal of the force vector. A: Group 1 (point of insertion directly at the anterior lower endplate). The screws are clearly breaking loose ventrally at the base of the body of the axis. B: Group 2 (point of insertion 2 mm dorsal to the anterior lower endplate). Note there is no fracture at the ventral wall of the axis.

  • View in gallery

    Mean load failure, in newtons, of the different insertion points. *p < 0.05.

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