Surfface anattomiicall llandmarks off tthe cochllea on tthe mediiall wallll off miiddlle ear:: A ttemporall bone diissecttiion sttudy

Abstract

The inner ear or the labyrinth is formed of anterior part for hearing (the cochlea) and a posterior part for balance (3semicircular canals and vestibule). The membranous labyrinth is enclosed in the osseous (bony) labyrinth. The cochlea is the organ of hearing. It spirals 2½ -2 ¾ turns around its axis, the “modiolus” and has a height 5mm. The base of the cochlea is adjacent to the fundus of the internal auditory canal and is perforated (cribrosed) allowing passage of the cochlear nerve fibers. The apex lies medial to the tensor tympani muscle. The osseous spiral lamina winds around the modiolus and along with the basilar membrane, separates the scala tympani from scala media (cochlear duct). Adjacent turns of the cochlea are separated by an interscalar septum. The medial wall of the middle ear is formed of a round eminence called the promontory that is the bulge of the basal turn of the cochlea, also it has 3 depressions: sinus tympani, OW and RW niche. The sinus tympani is defined by the ponticulus superiorly and subiculum inferiorly, the mastoid segment of the facial nerve laterally and posterior semicircular canal medially, its depth varies from shallow to deep. The oval window occupied by the stapes footplate, is located anterosuperior to the ponticulus, the RW niche is found posteroinferior to the promontory. Each turn of the cochlea is related to features on the medial wall of the middle ear, proper studying of the anatomy help finding the optimal site for cochleostomy and electrode insertion. Cochlear implant surgery is indicated in cases of hearing impairment resulting from congenital, hereditary causes, bacterial meningitis, Ménière’s disease, ototoxic drugs, perilymphatic fistula, SSNHL (sudden sensorineural hearing loss), otosclerosis, thrombosis or bleeding in labyrinthine artery, skull base fracture causing cochlear injuries. Cochlear implant surgery is performed in cases of bilateral profound hearing impairment or deafness with 3 frequencies average 500-1000-2000, pure tone audiometric threshold in the better ear equal or greater than 95d B, unable to benefit from conventional hearing aids, good physiological and physical health, motivation and consistency in attending rehabilitation program. The implant is surgically placed under the skin behind the ear. The basic parts of the device include: external parts, one or more microphones which picks up sound from the environment, a speech processor which selectively filter sound, splits the sound into channels and sends the electrical sound signals through a thin cable to the transmitter, which is a coil held in position by a magnet placed behind the external ear and transmits power and the processed sound signals across the skin to the internal device by electromagnetic induction. Internal parts, a receiver and stimulator secured in bone beneath the skin, which converts the signals into electric impulses and sends them through an internal cable to electrodes wound through the cochlea, which send the impulses to the nerves in the scala tympani and then directly to the brain through the auditory nerve system. The primary route for implanting the electrode array of cochlear implant is the scala tympani, because of its easy accessibility by traditional surgical techniques. There are 2 main routes for implantation: the first is through the RW, the second through a cochleostomy. After entry to the scala tympani, the electrode is inserted. Posterior tympanotomy is the surgical approach to the middle ear to expose the RW from behind. This is done through a triangular space between the facial nerve, chorda tympani and fossa incudis at the floor of the triangle. In cochlear implant surgery the facial nerve must be identified before making the opening, sometimes the chorda tympani nerve is abnormally placed and compromises the exposure of the RW so it should be sectioned.