Morphometric Study of the Facial Nerve in the Presigmoid Retrolabyrinthine Approach

Background: The pre-sigmoid retro labyrinthine space is a narrow corridor that provide a shorter approach to the cerebellopontine angle lesion while preserving the audition and the facial nerve function. However, during this approach, the labyrinth and the mastoid segment of the facial nerve are particularly at risk. Although anatomical landmarks have been reported to minimize such risks, in this study, we aimed at producing morphometric data to facilitate the delineation of the vertical segment of the mastoid facial nerve, labyrinth, and jugular bulb during a pre-sigmoid retro labyrinthine approach. Methods Pre-sigmoid retro labyrinthine approach was performed on five cadaveric specimens to evaluate the metric relationships of the labyrinth, the mastoid (vertical) facial nerve and the sigmoid sinus within the mastoid bone. Results The distance from the sinuso-dural angle to the FN's vertical segment ranged between 10-16mm.The genu of the FN has a mean distance of 12-18.5mm from the cortical mastoid bone. The anteroposterior diameter of the labyrinth from the lateral edge of the lateral semicircular canal to the posterior edge of the posterior semicircular canal ranges from 5-10mm.The the pre-sigmoid dura between the sigmoid sinus and the medial edge of the labyrinth varied between 4 to 19mm. Conclusions When working within the narrow pre sigmoid retro labyrinthine corridor, accurate measures of the relationship between the facial nerve, labyrinth and sigmoid sinus are critical for surgeons.