Cochlear line: a novel landmark for hearing preservation using the anterior petrosal approach

Abstract

T he anterior petrosal approach introduced by Kawase et al. was designed to enable wider and closer surgical exposure for treatment of ventral brainstem lesions. 1 The technique provides easy access to large sphenopetroclival meningiomas, dumbbell-shaped trigeminal schwannomas, and giant acoustic neuromas located in front of the brainstem and basilar trunk aneurysms. 5,7,9 The advantage of this technique is a lower possibility of postoperative hearing loss, vestibulopathy, and facial paresis in comparison with the transcochlear approach and/ or combined middle fossa posterior approach, which both provide similar surgical fields. 1To perform the anterior petrosal approach more safely, the use of possible anatomical landmarks, including the maxillary branch of the trigeminal nerve (V3), gasserian ganglion, greater superficial petrosal nerve (GSPN), petrous internal carotid artery (ICA), cochlea, geniculate ganglion, dura mater of the internal acoustic canal (IAC), labyrinthine portion of the facial nerve, vestibule, and superior semicircular canal can also be put to good use. Of these, the cochlea is well known to be located anterior to the fundus of the IAC, inferior to the geniculate ganglion. This relationship is frequently used as an anatomical landmark. In addition, the difference in bone density between the petrous bone and the cochlea can also be another clue for determining the location of the cochlea. 4 However, surgeons may be faced with unexpected difficulty in applying these concepts in real surgical fields. In this study, based on dissection results in 5 fresh cadavers, we aimed to develop a practical landmark for the safe and easy identification of the cochlea when applying the anterior petrosal approach. MethodsThis study was approved by the institutional review board of Eulji University. Five fresh cadaver heads were dissected in the microdissection laboratory.abbreviatioNs GSPN = greater superficial petrosal nerve; IAC = internal acoustic canal; ICA = internal carotid artery; V3 = maxillary branch of the trigeminal nerve. obJect The goal of this study was to develop a practical landmark for the safe and easy identification of the cochlea when performing anterior petrosectomy based on cadaver dissection results. Methods The cochlear line was defined as the line drawn from the crossing point between the greater superficial petrosal nerve (GSPN) and the petrous internal carotid artery to the line drawn over the apex of the superior circumference of the dura of the internal auditory canal at a right angle. The validity of the cochlear line marking the anteromedial perimeter of the cochlea at the angle of the GSPN and the internal acoustic canal as a practical landmark were evaluated using 5 cadaver heads. results The mean distance (± SD) measured from the cochlear line to the margin of the cochlear cavity was 2.25 ± 0.51 mm (range 1.50-3.00 mm). coNclusioNs Anterior petrosectomy can be performed more efficiently by using the cochlear line as a key landmark to preserve the cochlea.

Middle Cranial Fossa Approach: Anatomical Study on Skull Base Triangles as a Landmark for a Safe Anterior Petrosectomy

“…According to Wang et al, 14 the IAC can be found following posteriorly the GSPN from the facial hiatus to the geniculate ganglion, pro-ceeding with bone removal along the labyrinthine segment of the facial nerve. Kim et al 23 described the cochlear line as a key landmark to preserve the cochlea; the cochlear line was defined as a line drawn from the crossing point between the GSPN and the petrous ICA to the line drawn over the apex of the superior circumference of the dura of the internal auditory canal at a right angle. However, these methods place neurovascular structures at risk because their exposure is required.…”

Real-Time Navigation-Guided Drilling Technique for Skull Base Surgery in the Middle and Posterior Fossae

“…Another reason is that the necessity of the navigation system is questionable because the presence of several landmarks, including the cochlear line and digastric point, has been reported. 5 Anterior petrosectomy can be performed more efficiently using the cochlear line as a key landmark to preserve the cochlea. 5 Understanding the exact morphology of the temporal bone increases the success of maximal removal of the posterior wall of the IAC while minimizing morbidity secondary to violation of the bony labyrinth.…”

“…5 Anterior petrosectomy can be performed more efficiently using the cochlear line as a key landmark to preserve the cochlea. 5 Understanding the exact morphology of the temporal bone increases the success of maximal removal of the posterior wall of the IAC while minimizing morbidity secondary to violation of the bony labyrinth. 8 However, the high degree of interindividual variation in the relationships between surface structures and internal anatomy is a matter of some concern.…”

“…9 Surgeons may face an unexpected difficulty in applying these concepts in the surgical field during actual operations. 5 Navigation was thought to be helpful in the middle and posterior fossa skull base surgery to achieve precise drilling of the petrous bone and to increase the safety of these approaches. 1,11 Moreover, the development of the bony surface registration technique allowed navigation with the patient in the lateral or prone position.…”

“…However, no standardized intraoperative techniques have been developed to protect the labyrinth. 5 Imaging guidance may be useful for drilling of bony structures in skull base surgery because accurate navigation is possible without intraoperative brain shift. On the other hand, use of a navigation system for drilling in skull base surgery makes it necessary for the surgeon to alternate their gaze between the surgical field and the navigation screen.…”