Promontorial cochleostomy in nonhuman primates. Is it atraumatic?

The purpose of this study is to determine if surgical approach to the inner ear is feasible without generating a hearing loss in an animal model. Five Macaca fascicularis were used as experimental animals and followed up for 27 months. Mastoidectomy, posterior tympanotomy and promontorial cochleostomy were performed on four specimens and one specimen was kept as control animal. Before and after drilling and exposing the endosteal layer and the membranous labyrinth, otoacustic emissions (dPOAE) and auditory brainstem responses (ABR) were used to test hearing. In vivo experimental studies prove it is reliable to expose the membranous labyrinth without causing hearing loss. dPOAE were present after 3, 6, 12, 24 and 26 months of follow-up. Regarding the ABR results from the four M. fascicularis in which a cochleostomy has been carried out, auditory thresholds are within the 20-30 dB interval at 27 months of follow-up. Experimental studies support clinical experiences indicating it is feasible to surgically approach the membranous labyrinth of the cochlea without damaging its hearing function.

Atraumatic surgical approach to the cochlea with a micromanipulator.

CONCLUSIONS: Our design and preliminary results show that the the micromanipulator could be a great help to the surgeon in the atraumatic surgical approach to the lateral wall of the cochlea at the promontory. OBJECTIVES: Hearing preservation in cochlear implant opens new frontiers in the treatment of sensorineural hearing loss. To preserve the membranous labyrinth intact, new surgical tools are needed, either for cochlear implantation or for other applications. The objectives of this study were to design and test a micromanipulator coupled to a drilling tool for the atraumatic exposure of the spiral ligament. The micromanipulator is designed to increase precision when drilling the otic capsule bone. MATERIALS AND METHODS: A group from the University of Navarra worked on the device design -- based on a compliant mechanism -- and in vitro test. The components and functioning of the micromanipulator are described. It was tested in 10 formalinized temporal bones after a mastoidectomy, a posterior tympanotomy, and a transcanal tympanotomy were performed. The micromanipulator was placed over the cranial surface, and used to expose the endostium, anteriorly to the round window niche. RESULTS: A combined approach through the external auditory canal was feasible, together with a posterior tympanotomy to visually control the work and make complementary manoeuvres. Drilling was easy, and visual control through the posterior tympanotomy was excellent. A high degree of drilling precision was achieved. A little disruption of the membranous labyrinth was found only in the first bone of the series.

Morphometry of the human cochlear wall and implications for cochlear surgery.

OBJECTIVE: To study the lateral wall of the cochlea in human temporal bones (TBs), in order to evaluate the feasibility of performing micro-dissection of the spiral ligament and an endosteal electrode implantation. MATERIAL AND METHODS: We reviewed the database of the TB bank of The House Ear Institute in search of bones that did not distort the otic capsule or spiral ligament, and selected 36 horizontally sectioned TBs. All bones had been removed using the en bloc technique, fixed in formalin, decalcified in EDTA, embedded in celloidin and cut into 20-pm serial sections. We evaluated sections stained with hematoxylin-eosin under a microscope, and made several measurements of the lateral wall of the cochlea using optical appliances. A total of 20 measurements were made at 3 levels: midmodiolar sections; sections at the round window niche; and sections containing the anterior border of the stapes footplate. RESULTS: The selected sections provide useful data when applied during surgery. All the numerical data were analyzed statistically and, although individual variability occurred, most of the measurements seemed to be quite homogeneous. CONCLUSIONS: A greater understanding of the microscopic anatomy and dimensions of the human TB will help to understand the surgical relationships at the lateral wall of the cochlea. Our study provides some numerical references for the dimensions of the cochlea that may help in cochlear surgery. These dimensions would be of particular interest in cochlear implantation, especially in the design of endosteal electrodes.