Anatomic and biomechanical assessment of transarticular screw fixation for atlantoaxial instability.

The purpose of this study is to elucidate anatomically the atlantoaxial transarticular screw fixation described by Magerl in 1979 and compare it biomechanically with Gallie wiring. Five human C1-C2 specimens were tested in flexion/extension and rotation intact, then after wiring and screw fixation. Mean screw length was 39 mm, 25 mm in the C2 lamina and 14 mm in the lateral mass. Angular displacement of screwed specimens was significantly less than control or wired groups. Stiffness at 0-0.5 Nm loads was significantly greater for screwed specimens than for wired or controls (101 +/- 49 Nm, 10.3 +/- 9.2 Nm, and 1.96 +/- 0.18 Nm, respectively). All specimens withstood 5 Nm in flexion and extension without failure. Screw fixation provides stability comparable to Gallie wiring and is stiffer at low-range forces and rotational angles.