Fluoroscopically guided anterior atlantoaxial transarticular screws: a feasibility and trajectory study using CT-based simulation software.

BACKGROUND CONTEXT Anterior transarticular screw (ATAS) fixation has been suggested as a viable alternative to posterior stabilization. However, we are not aware of previous reports attempting to establish the usefulness of specific fluoroscopic landmark-guided trajectories in the use of ATAS, and we could find no reference to it in a computerized search using MEDLINE. PURPOSE To determine the anatomic feasibility of ATAS placement using defined fluoroscopic landmarks to guide screw trajectory. STUDY DESIGN Evaluation using three-dimensional screw insertion simulation software and 1.0-mm-interval computed tomographic scans. PATIENT SAMPLE Computed tomographic scans of 100 patients including 50 men and 50 women. OUTCOME MEASURES Incidence of violation of the vertebral artery groove of C1 and C2, the spinal canal, and the atlanto-occipital joint and screw lengths and lengths of C1 and C2 purchase. METHODS Four screw trajectories were determined: promontory screw (PS), single central facet (CF) screw, and medial (MF) and lateral (LF) double facet screws. Placement of a 4.0-mm screw was simulated using defined fluoroscopic landmarks for each trajectory. The previously mentioned outcome measures were evaluated and compared for the four trajectories. This study was not supported by any financial sources, and there is no topic-specific potential conflict of interest with this study. RESULTS No violation of the C1 or C2 vertebral artery groove or of the spinal canal was observed for any of the screw types. Screw lengths and the length of C2 purchase were by far the longest for PS (40.4±2.8 and 25.7±2.1 mm, respectively; p<.001 in all post hoc comparisons). The length of C1 purchase was longer for CF (16.4±2.3 mm) and LF (15.8±1.6 mm) than PS (14.7±2.0 mm) and MF (14.6±2.4 mm) (p≤.001, respectively). There was no atlanto-occipital joint violation if the length of C1 purchase was set at 12 mm for CF and LF and at 10 mm for PS and MF. CONCLUSIONS Our results suggest that it may be possible to place ATASs without violating the vertebral artery groove, spinal canal, or the atlanto-occipital joint by using the described entry points, trajectories, and fluoroscopic landmarks.