Fixation Strength of Unicortical Versus Bicortical C1-2 Transarticular Screws
C1-C2 transarticular screws increase the stability of posterior wiring constructs and have become popular adjuncts to atlantoaxial fusion techniques. However, transarticular screws are technically demanding and place neurovascular structures at risk. The vertebral artery was injured in 4.1% of patients in one multicenter study. Bicortical fixation places the hypoglossal nerves and internal carotid arteries at risk on the anterior aspect of the C1 lateral mass. The purpose of this study was to compare the fixation strength of unicortical and bicortical C1-C2 transarticular screws. The mean pullout strength for the bicortical screws was 1049 N (range 434 to 1490 N) versus 939 N (range 452 N to 1697 N) for the unicortical screws. The difference was not statistically significant (p=0.22). A post-hoc power analysis indicated that with the observed variability, there was 80% power to detect differences of 256 N or greater. In 6 of 15 cases, the strength of the unicortical screw was superior to the strength of the bicortical screw.
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