Optimal trajectory for the atlantooccipital transarticular screw

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.