Cannulation of the foramen ovale is often performed for treatment of trigeminal neuralgia. Despite advances in resolution and visualization, fluoroscopy and computed tomography paired with navigation technology have proven unsuccessful in cannulating the foramen. Therefore, approaches to foramen ovale cannulation warrant improvement. This study applies a geometric model to analyze the region of a foramen occupied by a cylindrical surgical tool (e.g., a cannula, stylet, catheter, or needle) inserted at an angle (ϕ) to the plane of the foramen. As the tool passes through the plane of the foramen, its cross-section is an ellipse with a major axis depending on ϕ. Accordingly, the area of the region depends on ϕ, as well as on the radius of the surgical tool. Knowing the area of the region provides a means of comparing a candidate surgical instrument to the foramen it will cannulate when the area of the foramen is known (e.g., as determined from imaging). After accounting for irregularities in the boundary of the foramen, the angle of approach ϕ, and relative orientations of the foramen and the elliptical region occupied by the tool, the geometric model proceeds to describe two new regions: one which guarantees successful cannulation and one which guarantees the tool will not cannulate. The technique predicts when cannulation of a foramen will likely be impossible. Therefore, this method may prevent adverse surgical events and improve the surgical approach and outcomes in the treatment of trigeminal neuralgia.