title: Exposing the Fundus of the Internal Acoustic Meatus without Entering the Labyrinth Using a Retrosigmoid Approach: Is It Possible?
abstract: OBJECTIVE
  To evaluate the feasibility of performing a labyrinth-sparing neuronavigation-assisted retrosigmoid approach to the fundus of the internal acoustic meatus (IAM) and to describe the anatomy of the structures embedded in the posterior meatal wall.
  
  
  METHODS
  Ten surgical dissections were performed bilaterally on 5 fresh cadavers. Cadavers were subjected to preoperative computed tomography scans and spatial coordinates of inner ear structures were recorded. A retrosigmoid craniectomy was performed. The IAM was drilled towards the fundus until no more than 1 mm of bone covered the labyrinthine structures. Specimens underwent a new computed tomography scan to verify the length of opened IAM and the status of the labyrinth. We then opened the labyrinthine structures and recorded their coordinates using navigation. These were compared with the radiologic coordinates to verify the neuronavigation accuracy.
  
  
  RESULTS
  In 9 sides, the IAM was opened to the fundus without injuring the labyrinth; in 1 side, the vestibule was opened. The mean residual bone on the fundus was 0.97 mm. The average length of the accessible IAM was 88.95%. The best accuracy of the navigation was for the identification of the common crus, with a mean value of 0.73 mm.
  
  
  CONCLUSIONS
  This surgical technique could facilitate the opening of the IAM with preservation of inner ear structures. We opened a mean of 88.95% of the IAM without entering the labyrinthine structures in 90% of cases. These results confirm the feasibility of the retrosigmoid approach for the exposure of the IAM fundus with preservation of labyrinthine structures.
authors.affiliation: Institute of Neurosurgery, Catholic University of Rome, Policlinico A.Gemelli, Rome, Italy; Dardinger Microneurosurgical Skull Base Laboratory, Department of Neurological Surgery, Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA.
authors.forename: Alba
authors.initials: A
authors.lastname: Scerrati
authors.affiliation: Dardinger Microneurosurgical Skull Base Laboratory, Department of Neurological Surgery, Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA; Section of Neurosurgery, Department of Surgery, National Cheng Kung University Hospital, College of Medicine, National Cheng Kung University, Tainan, Taiwan.
authors.forename: Jung-Shun
authors.initials: JS
authors.lastname: Lee
authors.affiliation: Department of Radiology and Wright Center of Innovation in Biomedical Imaging, Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Dardinger Microneurosurgical Skull Base Laboratory, Department of Neurological Surgery, Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA. Electronic address: mario.ammirati@osumc.edu.
authors.forename: Mario
authors.initials: M
authors.lastname: Ammirati
mesh_terms: D002102:Cadaver; D003399:Craniotomy / Q000379:methods; D004210:Dissection; D007758:Ear, Inner / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000601:surgery*; D005240:Feasibility Studies; D006801:Humans; D038361:Neuronavigation / Q000379:methods*; D059351:Organ Sparing Treatments / Q000379:methods; D014057:Tomography, X-Ray Computed
keywords: Acoustic neuroma; Fundus; Inner ear; Internal acoustic meatus; Neuronavigation; Retrosigmoid approach; Semicircular canals