Multidirectional instabilities of experimental burst fractures of the atlas.

For the purpose of understanding the acute instability of a burst (Jefferson) fracture of the atlas, the authors produced the fractures experimentally and measured multidirectional flexibilities in seven cadaveric C0-C3 specimens. The flexibilities were measured by the authors' standardized method: they applied six types of physiologically pure moments (up to 1.5 Nm) and recorded the ensuing C0-C2 motions by stereophotogrammetry. The flexibility tests were performed before and after the production of the fracture. The greatest increase in flexibility due to the injury was in flexion-extension (+22.0 degrees, 41.7%). In lateral bending, the increase was 7.7 degrees, or 23.9%. The flexibility was mostly maintained in axial rotation (+4.8 degrees, 5.4%). The increase in motion was due to an increase in neutral zone in flexion-extension, and an increase in the elastic zone in lateral bending. These flexibility results of experimentally produced fractures reflect quite well the acute instabilities seen clinically.