Endoscopic Transorbital and Retrosigmoid Approaches to the Pre-meatal and Ambient Area: an Anatomical and Quantitative Comparative Study with Surgical Applications

The study aims to quantitatively compare the transorbital approach (TOA) and the retrosigmoid approach (RSA) in terms of surgical exposure and maneuverability, evaluating their effective complementarity or competition to the premeatal area (PMA) and ambient cistern after tentorial incision. Six sides of formalin-fixed cadaveric specimens were analyzed. Two parameters were measured using neuronavigation: (1) Area of Exposure (AoE), representing the anatomical area exposed via a given approach, and (2) Surgical Freedom (SF), an estimation of the range of motion for the surgeon’s hands. These parameters were assessed both in standard conditions and after tentorial incision extension to expose the ambient cistern, the middle tentorial incisura, and the cerebellopontine angle (CPA). Pre-extension analysis revealed that TOA exhibited superior AoE compared to RSA (p=0.22), and significantly greater SF (p=0.008). After tentorial incision, RSA achieved a larger AoE compared to TOA. Both approaches offered similar exposure to the ambient cistern (p>0.05), demonstrating that surgical extensions equalize initial exposure differences. A clinical case was also described. TOA offers greater surgical freedom, potentially improving maneuverability during neurovascular dissections in the supra-premeatal region. However, following extension, RSA achieves a larger AoE in both the supra- and inferomeatal regions, indicating its advantage for extended exposures. These findings underscore the importance of selecting an appropriate approach to optimize the surgical vector toward the middle tentorial incisura and the CPA angle. Both TOA and RSA can be employed individually or as part of a multiportal strategy, depending on the extent of the lesion.