An Alternative Method for Measuring Angles of Attack and Surgical Freedom: A Validation Study

Background: Surgical approaches in neurosurgery require precise navigation and instrument maneuverability within constrained anatomical spaces. Traditional methods to measure angles of attack (AoA) and surgical freedom (SF) necessitate placing a probe within the surgical field to test motion limits, which can be time-consuming and sometimes inaccurate due to unwarranted sliding of the instrument tip, exposing specimens’ structures to damage significantly in non-fixed cadavers. For the same reasons, no attempt has ever been made to calculate such parameters in a real surgical scenario. This study introduces an alternative method for calculating AoA and SF that eliminates the need for a testing probe.

Methods: We developed a novel approach using a straight neuronavigation probe to track linear trajectories in cardinal directions. This method identifies a target point (the same as a pivotal point in the standard AoA technique) and peripheral anatomical motion constraints (contact of the navigation probe with any anatomical boundary at the superior, inferior, medial, and lateral limits of the corridor) by collecting stereotactic points, replicating the spatial configuration required for calculating AoA and SF without explicitly moving a probe using critical structures as pivotal references. The technique was applied first on a 3D-printed scaffold, then on a cadaveric specimen, and inter-rater variability was accounted for with 10-fold measurement repetition. Points were collected using a stereotactic frameless system with a registration tolerance of less than 1 mm in all conditions. Finally, an exploratory trial was conducted in a real surgical scenario.

Results: The study compared the new method with the traditional probe-based method on a 3D-printed phantom. AoA and surgical freedom to the anterior, middle, and posterior clinoids were measured on the left side. Additionally, during an orbitozygomatic transcavernous Dolenc approach on a latex-injected formalin flushed specimen, AoA, and surgical freedom to various neurovascular structures, including the distal carotid ring, ICA bifurcation, posterior communicating artery, basilar apex, and distal posterior cerebral artery, were tested ipsilaterally and contralaterally. Surgical freedom with the novel technique was processed with an in-house script for geometric transformations in 3D space and converted to resemble the SF obtained with a 20-cm-long probe with a standard technique to allow comparisons with other studies. No statistically significant differences were found between the new and traditional methods in the 3D scaffold and cadaveric trial. Interrater consistency was high and did not constitute a potential bias in our methodology.

Conclusion: The proposed stereotaxic-based method for calculating AoA and SF provides a practical, safer alternative to traditional techniques. It simplifies the measurement process, reduces the time required, and allows a single operator to perform the calculations without affecting their accuracy. The technique's application during surgery might offer a novel opportunity to gather intraoperative measurements in the future, potentially paving the way for quantitative research in living patients.