An Anatomy Study Of Basal Turn Cochleostomy Via The Middle Fossa Route For Cochlear Implant Insertion

Objective:In this study of 36 adult temporal bones we tried to highlight the anatomic site and the anatomic relationships of the basal turn cochleostomy via the middle fossa route for cochlear implant insertion. This work will provide anatomic sustention for an alternative route for cochlear implantation.Materials and methods:A total of 36 human adult temporal bones were studied. Dissection was under the operating microscope using traditional anatomic methods and standard otosurgical equipment.1.The first study was conducted on 20 dry adult temporal bones (left 10 sides, right 10 sides). Identify The important structures of the petrosal bone including the groove for the greater superficial petrosal nerve, the facial hiatus,the arcuate eminence, the facial n., the petrous ridge, and the internal acoustic meatus. The groove for the greater superficial petrosal nerve and the facial hiatus were used as landmarks in order to expose the site of the basal turn cochleostomy via the middle fossa route cochlear implant insertion. The bone was drilled to open the internal acoustic meatus until the fundus. Expose the whole basal turn of the cochlea along the cochlea nerve. The most superficial part of the basal turn was the site of cochleostomy. Mark the projection of the cochleostomy on the middle fossa floor as point A.In front of the Bill'bar was the enterence of the facial nerve canal, the uper bones was drilled to open the facial canal in labyrinthine segment and the cave of the projection of facial nerve. Drilled the bones in front of the basal turn of the cochlea and expose the canal of internal carotid a. Measurements were made of the following distances:a) between the point A and the superficial part of the basal turn, b) between the point A and the end of the facial hiatus, c) between the point A and the groove for the greater superficial petrosal nerve, d) between the point A and the facial canal in labyrinthine segment,e) between the point A and the core of modiolus, f)the angle between the groove for the greater superficial petrosal nerve and the line of point A to the facial hiatus, g) the angle between the groove for the greater superficial petrosal nerve and the facial canal in labyrinthine segment, h) the basal turn and facial canal in labyrinthine segment, i)transverse length of basal turn of the cochlea,j)thickness of the basal turn of cochlea, k) the front side of basal turn of cochlea and the canal of internal carotid a.2. The second study was conducted on 8 adult cadaver skulls,5 from male cadavers and 3 from females (temporal bones 16 sides). The skulls were fixed in 10% formaldehyde solution.1) Dissection about the middle fossa route:Skin incision was begun below the zygomatic arch,1.5 cm before the tragus. The incision was carried upward and anteriorly, then curving posteriorly to the region of squama temporalis. The temporalis muscle was sectioned and dissected from the temporal squama. A small square flake of temporal bone was cut from the skull,1/3 posterior and 2/3 anterior to the extermal auditory canal. Its length was 3×3cm. Elevated the dura from the overlying bone until the arcuate eminence and petrous ridge were identified.2) Dissection about the site of the basal turn cochleostomy:Identify the important structures of the petrosal bone including the groove for the greater superficial petrosal nerve, the facial hiatus, the arcuate eminence, the petrous ridge, and the internal acoustic meatus. The groove for the greater superficial petrosal nerve and the facial hiatus were used as landmarks to expose the site of the basal turn cochleostomy via the middle fossa route cochlear implant insertion. The bone was drilled to open the internal acoustic meatus until the fundus. Expose the whole basal turn of the cochlea along the cochlea nerve. The most superficial part of the basal turn was the site of cochleostomy. Mark the projection of the cochoeostomy on the middle fossa floor as point A.In front of the Bill'bar was the enterence of the facial nerve canal, the uper bones was drilled to open the facial canal in labyrinthine segment and the cave of the projection of facial nerve. Drilled the bones in front of the basal turn of the cochlea and expose the canal of internal carotid a. Measurements were made of the same distances as the dry adult temporal bones.Results:There is no significant difference between the skelton and cadaver, the result is get from all of the speciemen together.a) between the point A and the superficial part of the basal turn 3.28±0.53mm, b) between the point A and the back of the facial hiatus 3.25±0.52mm, c) between the point A and the groove for the greater superficial petrosal nerve 3.42±0.70mm, d) between the point A and the facial canal in labyrinthine segment 3.23±0.35mm,e) between the point A and the core of the modiolus 6.46±0.23mm,f)the angle between the groove for the greater superficial petrosal nerve and the line of point A to the facial hiatus 88.04±8.31, g) the angle between the groove for the greater superficial petrosal nerve and the facial canal in labyrinthine segment 108.34±3.25, h) the basal turn and facial canal in labyrinthine segment 0.14±0.03 mm, i)transverse length of basal turn of the cochlea 9.12±0.46mm,j)thickness of the basal turn of cochlea 3.22±0.09mm, k) the front side of basal turn of cochlea and the canal of internal carotid a 5.53±0.35mm.Conelusion:The middle fossa approach for cochlear implantation is a alternative approach which is suitable for subjects with a bilateral radical mastoidectomy cavity, chronic middle ear disease, patients suffering from middle ear malformations, and patients with partial obstruction of the cochlea in the basal turn. A new method was provided to go to the basal turn cochleostomy via the middle fossa route for cochlear implant insertion. It could reduce the injury to the blood and nerve during opration.