The anatomy of the temporal bone, notably its posterior surface, has generated a great deal of interest in defining a roadmap that minimizes the risk of lesion of the labyrinth and the facial nerve during the pre-sigmoid approach. Although several writings propose stages of drilling the posterior part of the petrous bone, there is no consensus on the landmarks, nor on the resectable bone, volume to achieve a comfortable exposure of the petro-cerebellar and petro-clival region [1]. The pre-sigmoid retro labyrinthine approach is suitable in preserving the labyrinth, hence the hearing capacity and the facial nerve. Furthermore, this approach is less associated with cerebellum retraction as compared to the retro sigmoid approach. However, it displaces a limited corridor for the surgeon’s access, exposing him to FN and labyrinthic injuries.
The localization of The FN’s mastoid vertical segment limited cranially by its genu and inferiorly by the stylomastoid foramen is an essential is a determinant for the retro labyrinthine approach. Various landmarks include the incus, the lateral edge of the lateral semicircular canal, and the temporal spine. The depth from the outer surface of the mastoid bone to the incus corresponded to the level of the mastoid FN genu. This finding was corroborated by Boemo and Kudo [3, 4]. The average depth from the cortical mastoid bone to the mastoid (vertical) FN was 17.68mm and 13.15mm on the right and left, respectively. This measurement varies among authors; Haynes et al. found 21.4mm while reported Boemo 12.21mm [5, 6]. The mean length of the mastoid FN in the Fallopian canal was 21.02mm and 21.11mm on the left and right. This value was superior to that of 13.78mm found by Maru et al. and Yadav et al. [7, 8]. The distance from the sigmoid sinus to the medial edge of the labyrinth was measured, which represented the extent of pre-sigmoid dura available for incision during the approach. To the best of our knowledge, no similar measurements are reported in the literature. This is a narrow corridor that need to be widened to allow comfortable exposure to the CPA and petroclival region. Achieving this purpose requires additional bone drilling over the sigmoid sinus and retrosigmoid dura. This maneuver facilitates the application of brain retractors after the dura opening. The measurement of the FN depth at mid-distance of its course in the petrous bone suggests that the FN becomes more superficial as it descends towards the stylomastoid foramen. Mortazavi and al corroborates this assertion, which should be taken into account during the skeletonization of FN [9]. Similarly, Pulec and al propose that the drilling during FN skeletonization should be approached by the anterior and posteromedial rim of the Fallopian canal to avoid the nerve injury [10].
An essential step for the transposition of the FN is the exposure of the facial recess, which permits disclosure of the middle ear and the cochlea. ENTs mostly use the exposure of the facial recess during the cochlear implantation process. During this study, the mean diameter of the facial recess was 4.56mm on the left and 3.18mm on the right. This value varies significantly, depending on the development of the FN, the tympanic cavity, the position of the tympanic membrane, and mastoid cells [11]. However, drilling the facial recess may increase the risk of laceration of the tympanic chord and middle ear.
Preservation of the semicircular canal and vestibule is a primary objective during the pre-sigmoid retro labyrinthine approach, with this conserving the patient’s audition. The labyrinthine space anteroposterior dimension was an average of 8.18mm on the right and 5.74mm on the left, corroborated by Hao and al[12] .
The jugular bulb of the specimens was located at pars vascularis of the jugular foramen, with the superior rim at the tympanic cavity’s inferior edge, corresponding to the inferior limit of the retro labyrinthine exposure. Our study measured the distance from the superior border of the jugular bulb to the inferior rim of the labyrinthine space, which varied between 0 to 5.07mm, corresponding to high-riding JB as described by Sasindran and al [13]. Singla and al considered the jugular bulb to be “High-riding” if the distance between the superior rim of the jugular bulb and internal acoustic meatus was less or equal to 2mm [14]. Manjila and al proposed a CT-Scan based classification of the jugular bulb position [15]. The classification was as follows: type 1, no JB; type 2, below the inferior margin of the posterior SCC; type 3, between the inferior margin of the posterior SCC and inferior margin of the IAC; type 4, above the inferior margin of the IAC; and type 5, a combination of dehiscences.
This anatomical study has limitations such as the small size of the specimen population. Moreover, the studied measurements were irrespective to the specimen’s gender and morphotypes. However, this study provided useful metric data about the pre-sigmoid dura exposure since this is the corridor to the CPA and petro-clival regions.