The Anatomy Of Facial Nerve Decompression Through Transmastoid, Extralabyrinthine, Subtemporal Approach

Objective: To validate the feasibility and security of the facialnerve decompression through transmastoid, extralabyrinthine, subtemporal approach;To get the data and provide the safe scope about this approach and cultivate the otoneurosurgery skill on the cadaveric skull.Materials and methods:1. The first study was conducted on 20 adult temporal bones. After sawed , the bones was drilled to open the lateral semicircular canal with its ampulla, facial canal in horizontal segment, anterior semicircular canal with its ampulla , geniculate ganglion fossa, facial canal in labyrinthine segment and last open the basal turn of the cochlea in front of labyrinthine segment. Observe the anatomy of the specimens and snap with digital camera. Various distances were measured with dividers and read against a vernier caliper.2. The second study was conducted on 20 sides of cadaveric skulls. Decalcified with 10% hydrochloric acid, the cadaveric skulls were dissected through two ways: ①through middle cranial fossa to expose geniculate ganglion, labyrinthine segment , the fundus of internal auditory meatus and the proximal part of horizontal segment , ②through mastoid to expose the vertical segment and the horizontal segment of facial nerve. Observe the anatomy of the specimens and snap with digital camera. Various distances and angles were measured with dividers , protractor and vernier caliper.Results:1.20 adult temporal bones (X + SD): ① the average distance betweencholeariform process and the angle point of proximal end to distal end in geniculate ganglion was 3. 80±O.72mm. (2) the average length of horizontal segment was 11. 14 ± 0.73mm. (3) the average length of labyrinthine segment was 3. 35 ± 0.68mm. ?the average shortest distance between basal turn of the cochlea and the labyrinthine segment was 1. 04±0.14mm. (5) the average tegmen thickness in most distal point of labyrinthine segment was 2.82 ± 0.64mm. ?the average shortest distance between facial canal in horizontal segment and the lateral semicircular canal was 1. 27±0.21mm. ?the average distance between facial canal in horizontal segment and middle point of anterior semicircular canal ampulla was 2. 47±O.21mm.2.20 sides of cadaveric skulls ( X ± SD): ?the average distancebetween long process of incus and horizontal segment was 2. 33 + 0.26mm. (2) the average distance between short process of incus and facial nerve was 3. 06+1.01mm. (3) the average distance between capitulum of malleus and the horizontal segment was 2. 17±0.87mm(4) the average distance between capitulum of malleus and tegmen tympani was 2. 31±0.87mm(5) the average length of vertical segment was 16. 27±0.58mm.?the average angle of arcuate eminence to sagittal surface was 33. 6±2.19° . (7) the average angle of proximal end to distal end in geniculate ganglion was 74. 3 ± 2.47 ° . (8) the average angle of pyramidal segment was 110. 2±8.3O° .Conclusion: This study provided the safe scope, manipulation room,and the angle of transmastoid, extralabyrinthine, subtemporal approach, and these data could be reference for the surgeon. Through transmastoid, extralabyrinthine, subtemporal approach the distal part of labyrinthine segments of facial nerve can be exposed withoutperformance of a craniotomy while cochlear function is spared. The near-total facial nerve decompression can be done without cranial complication , this operation can be an alternative method for surgeon who should be proficient in temporal bone anantomy and otoneurosurgery skill .