The Volume of Surgical Freedom: The Surgeon's Most Applicable Anatomical Measurement

Introduction: Surgical freedom is the most important metric at a surgeon's disposal. It should assess the maneuverability of instruments and provides the operator with insight into how realistic and appropriate using a specific access corridor is, while also allowing surgical approach comparison. Presently surgical freedom is defined as the maximal allowable working “area” at the proximal end of an X-mm probe with the distal end on the target structure. However, there are multiple scientific and application flaws associated with this definition and mathematical design. The volume of surgical freedom (VOSF) is a new methodology which produces the optimal qualitative and quantitative representation of an access corridor and provides the surgeon with an anatomical, spatially accurate and clinically applicable metric.

Methods: Overall, 12 cadaveric dissections were completed; six pterional and six supraorbital craniotomies. Data points for critical intracranial structures were measured using a neuronavigation system. The study was corrected for inter- and intrarater variability. Heron's formula and the VOSF were calculated specific to different anatomical structures of interest and their results compared. To compare the VOSF with Heron's formula, the measurements were placed on a z-scale. The one-way ANOVA was used to test the average maneuverability between surgical freedoms of instruments within the measurement methods. Measurement methods were compared for each surgical freedom by structure using paired sample t-test.

Results: The VOSF accounts for multiple defects in the established method including multiple planes, three-dimensional representation, irregular access corridors, procedural variability and mathematical inaccuracies. Key steps in calculation of the VOSF include; identification of the best fit plane, translation to a 2D coordinate system, calculation of the area of the irregular polygon, calculation of the volumetric measurement, and translation to a standardized normalized volumetric measurement. Use of the normalized measurement accounts for measurement inaccuracies produced during data acquisition due to dependency on probe length ([Fig. 1]) and provides a fixed reference metric applicable across studies.

Paired sample t-test was not significant, depicting no difference on average between standardized measurements for each structure. Pearson's correlation between the methods was strong at 0.728 ([Fig. 2]).

This methodology is the means to produce a three-dimensional anatomically and spatially accurate volumetric representation of the surgical access corridor ([Fig. 3]).

Conclusion: VOSF is a superior method of quantitative anatomical measurement. This innovative concept can develop an actual geometric model of a surgical corridor that yields better assessment and prediction of the ability to maneuver and manipulate surgical instruments. This quantitative measurement can establish surgically attainable targets specific to approaches, as well as assess the suitability of a specific surgical approach, compared with alternative operative options. VOSF produces a measure and model comparison of surgical corridor possibilities that may also account for potential anatomic structure injury or related surgical complications. Although this method correlates with results produced by Heron's formula, VOSF accounts for multiple inaccuracies in the practical and mathematical method, as well as now providing the ability to produce three-dimensional models and for this reason, is a preferable and applicable standard in assessment of surgical freedom.

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Artikel online veröffentlicht:
12. Februar 2021