[Dorsal atlanto-axial screw fixation. A stability test in vitro and in vivo]

Clinical and biomechanical testing of the stability of atlantoaxial fusions was studied. For biomechanical testing, four different techniques for posterior atlantoaxial fusion were tested: (1) wire fixation with one median graft; (2) wire fixation with two bilateral grafts; (3) transarticular screw fixation and two bilateral posterior clamps. Ten fresh human cadaveric specimens were tested. The loads applied were 6 pure moments. The motion of C1 relative to C2 in the intact, injured and mechanically fixed spine were measured and compared. In flexion/extension the difference between Brooks, Magerl and Halifax were not significant, but each was significantly less than the Gallie-system. For the anterioposterior translation the stabilization of all fixation techniques was about equal. In axial rotation measuring the translations between C1 and C2, the screw-fixation technique proved to be the most stable. For lateral bending, there was no significant difference between the different techniques, except for Galliefixation, but the screw-fixation technique allowed the least motion. In additional in vivo tests ten patients with posterior atlantoaxial fusion by the transarticular screw-fixation technique underwent bending X-rays of the upper cervical spine as well as computertomograms. Solid fusion was achieved in all patients. Both investigations proved the reliability of the multidirectional stability of the atlantoaxial screw fixation technique.