Biomechanical Testing of Posterior Atlantoaxial Fixation Techniques

Study Design. An in vitro biomechanical study of C1–C2 posterior fusion techniques was conducted using a cadaveric model. Objective. To investigate and compare the acute stability afforded by a novel rod-based construct that uses direct polyaxial screw fixation to C1 and C2 with contemporary transarticular screw and wire techniques. Summary and Background Data. Acute stability of the atlantoaxial complex is required to achieve bony consolidation. Various forms of posterior wiring were the first standardized procedures advocated to achieve C1–C2 fixation, but because of insufficient construct stability, these techniques have been coupled with transarticular screw fixation. Significant technical difficulties, however, including the possibility of neurovascular compromise during implantation are associated with transarticular screw placement. A novel technique that uses direct polyaxial screw fixation to C1 and C2 and bilateral longitudinal rods was developed recently. However, there are no published reports detailing the biomechanical characteristics of this new construct. Methods. In this study, 10 fresh-frozen human cadaveric cervical spines with occiput (C0–C4) were used. Osteoligamentous specimens were tested in their intact condition after destabilization via odontoidectomy, and after two different Gallie wiring techniques. Each specimen was assigned to one of the two screw fixation groups. Five specimens were implanted with the polyaxial screw–rod construct and tested. The remaining five specimens were tested after application of bilateral C1–C2 transarticular screws with Gallie wiring (Magerl–Gallie technique). Pure-moment loading, up to 1.5 Nm in flexion and extension, right and left lateral bending and right and left axial rotation, was applied to the occiput, and relative intervertebral rotations were determined using stereophotogrammetry (motion analysis system). Range of motion data for all fixation scenarios were normalized to the destabilized case, and statistical analysis was performed using one-way analysis of variance with Fisher’s least significant difference PLSD post hoc test for multiple comparisons. Results. The data indicate that destabilization via odontoidectomy significantly increased C1–C2 motion. Both screw techniques significantly decreased motion, as compared with both Gallie wiring methods in lateral bending and axial rotation (P < 0.02 for all) and tended toward reduced motion in flexion–extension. There was no statistically significant difference between the two screw techniques. Conclusions. The results clearly indicate the screw–rod system’s equivalence in reducing relative atlantoaxial motion in a severely destabilized upper cervical spine, as compared with the transarticular screw–wiring construct. These findings mirror the previously reported clinical results attained using this new screw–rod construct. Thus, the decision to use either screw construct should be based on safety considerations rather than acute stability.