Topographic Anatomy of the Medial Labyrinthine Wall: Implications for the Transcanal Endoscopic Approach to the Internal Auditory Canal

Hypothesis To characterize transcanal endoscopic landmarks of the medial labyrinthine wall and correlate these with anatomical features of the fundus of the internal auditory canal (IAC). Background The transcanal transpromontorial approach (TTA) enables minimally invasive access to the IAC. The establishment of a landmark-based dissection technique for the approach is crucial to avoid injury to the facial nerve. Methods Twenty temporal bones were dissected endoscopically through the TTA. Furthermore, high-resolution computed tomography (CT) scans from ten adult normal temporal bones were analyzed and three-dimensionally reconstructed. Results A stepwise dissection technique for the TTA was demonstrated depending on a newly described landmark used in the identification of the facial nerve. The proposed landmark, which was named the intervestibulocochlear crest (IVCC), is an integrated part of the otic capsule. It can be differentiated after the excision of the lateral labyrinthine wall as a laterally based bony pyramid between the cochlea and the vestibule. Its medially directed apex blends with the central part of the falcifrom crest and points to the distal part of the meatal facial nerve. The IVCC is best detected on axial CT images at the level of the tympanic facial nerve. The union between the IVCC and the falciform crest appears radiologically as a short stem or mini-martini glass. Conclusion The proposed IVCC is a novel landmark with a consistent relationship to the IAC fundus and the facial nerve. It may be utilized in conjunction with the falciform crest to identify the facial nerve during minimally invasive transcanal surgeries.