Working-Volume: A Novel Parameter to Characterize Surgical Exposures to the Anterior Skull Base

Objective: Surgical approaches are classically characterized using angles, working areas, or distances. These parameters are useful endpoints of comparison but do not express three-dimensional surgical freedom and may overlook benefits of bone removal to expand skull base approaches. To capture the three-dimensional character of surgical approaches, we have developed a novel system using frameless neuronavigation, aimed at measuring intraoperative “working-volume.”

Design: Simulations in surgical phantom and intraoperative measurements.

Materials and Methods: Phantom was calibrated against a frameless neuronavigation system. Coordinates were acquired along the anatomical borders of an eyebrow craniotomy without or with orbital bar removal. The target area was the anterior clinoid process. Working-volume was computed using a 3D alpha-shapes technique. We also acquired intraoperative pilot measurements in a single eyebrow craniotomy case.

Results: The working-volume for the classic eyebrow craniotomy was 12.08 cm (111 points) and increased to 18.48 cm (129 points) with orbital bar removal. Although we acquired more than 100 points to compute the working-volume, an iterative analysis (1000 iterations) showed approximately 20 data points enabled calculating a simplified working-volume with >95% accuracy and < 2.5 coefficient of variance in both approaches. Intraoperative working-volume was 15.56 cm, computed from 26 points in a patient with residual craniopharyngeoma.

Conclusion: Our simulations demonstrate that removal of the orbital bar yields approximately a 53% increase in working-volume. We are in the process of confirming this with intraoperative measurements.