[Forms of fracture of the dens axis in the application of ventral flexion force]

In fracture experiments in 220 cast models of the axis and atlas, an attempt was made to separate two different factors influencing the position of the dens fracture (superior and inferior type): the direction of the force application and the inclination of the dens axis. For this purpose, two extreme forms of dens inclination in the sagittal plane were used, namely a form with a ventrally inclined axis (kyphotic form) and a dorsally oriented axis (lordotic form). The force was applied in ventral flexion in 11 single steps between 10 and 75 degrees. The fracture types occurring under natural conditions could be reproduced with this experimental paradigm. Depending on the direction of the force application, both types of dens fracture occurred in ventral flexing traction alone. Independent of the inclination of the dens axis, this ventral flexion led to an inferior fracture type with a shallow angle, and a superior fraction type with a steep angle. With 75 degrees flexion, no longer the dens fracture, but the Hangman's fracture was the usual fracture type in both dens forms. With fundamental agreement of both dens forms with regard to the direction-dependent genesis of the fracture pattern, the lordotic dens and kyphotic dens displayed differences in the details which are discussed exhaustively. In this connection, the differences in the size of the horizontal thrust component due to the primary form of the dens with the same direction of force application is pointed out. According to the present investigations, a classification of dens fractures should be made more precise by specification of the dens form.