Study Design. Radiologic evaluation of computed tomography (CT) scans using screw insertion simulation software.
Objective. To investigate the optimal entry point and trajectory of atlantooccipital transarticular screws.
Summary of Background Data. To our knowledge, no large series focusing on the placement of atlantooccipital transarticular screws have been published.
Methods. We used 1.0-mm sliced CT scans and 3-dimensional screw trajectory software to simulate 4.0-mm screw placement. Four entry points were evaluated. Screw placement success rate, safe range of medial angulation, and screw length using each entry point were determined.
Results. CT scans of 126 patients were evaluated, for a total of 252 screws for each entry point. On simulation, the 2 lateral entry points showed significantly higher success rates and safe range of medial angulation than the 2 middle points. The 2 lateral entry points had similar success rates (98.0% for anteriolateral (AL) point and 97.6% for posteriolateral (PL) point). Although the safe range of medial angulation was significantly wider for the AL point (26.1°) than for the PL point (23.7°), the screw lengths were significantly longer for the PL point (32.6 mm) than for the AL point (29.4 mm). For both points, 30° of medial angulation led to highest rate of successful screw placement, but the rate was only 79.4% and 80.2%, respectively.
Conclusion. Although there was no significant difference in success rates between AL and PL points, PL is likely the best entry point. Although 30° medial and ∼5° upward angulation led to the highest rate of successful screw placement, the rate was only around 80%. Given the wide individual variation, we recommend that a preoperative 3-dimensional CT scan be obtained when attempting atlantooccipital transarticular screw fixation.