Extracting the Cochlea from a Human Temporal Bone: A Cadaveric Protocol

Published: August 18, 2023

doi:

¹Department of Otorhinolaryngology,Kuopio University Hospital

Summary

This article presents a reliable method for extracting the human cochlea from the cadaver temporal bone by drill-out while following distinct anatomical landmarks.

Abstract

The extraction of the cochlea from a cadaver human temporal bone may be required for different studies of the inner ear. For histological evaluations, the inner ear must be extracted from the temporal bone to facilitate histologic processing; likewise, some micro-computed tomography devices are too small to accommodate the complete temporal bones; additionally, the image quality can be enhanced when the cochlea is isolated.

The inner ear is located within the petrous part of the temporal bone. The inner ear can be divided into the osseous labyrinth or otic capsule and the membranous labyrinth inside the otic capsule. Furthermore, the inner ear can be divided into the vestibular system (the semicircular canals and the vestibule) and the cochlea. The appreciation of the location and orientation of the cochlea within the temporal bone is difficult, as it is embedded within bony structures and thus cannot be directly visualized. Nevertheless, there are distinct anatomical structures that can help guide the process to allow a reliable drill-out of the cochlea. The landmarks in the posterior parts of the cochlea are the facial nerve, semicircular canals, and the vestibule. In the middle, the inferior borders of the cochlea are identified by the round window and the basal turn of the cochlea. In the anterior border, one encounters the carotid artery; the landmark for the superior border is the genicular ganglion (GG) of the facial nerve. The medial structures are determined by the locations of the internal auditory canal, the superior semicircular canal, and the canal of the internal carotid artery.

In this article, we present a method for extracting the cochlea reliably out of the temporal bone by drill-out while following several anatomical landmarks.

Introduction

The inner ear is a delicate organ that provides us with the sense of hearing and balance. The inner ear is located at the base of the skull in the petrous part of the temporal bone (TB). The TB encases several crucial anatomical structures that twist and turn inside the bone. Thus, the TB forms a challenging anatomical entity to comprehend¹. Rask-Andersen et al., in their review²^,discuss the history of cochlea research and understanding of its microstructures.

The inner ear includes the semicircular canals, the vestibulum, and the cochlea. The three semicircular canals and the vestibulum form the vestibular system where the balance receptors are located³^,⁴. The cochlea is a shell-like structure that is connected to the vestibulum. The cochlea converts the mechanical sound waves into neural signals. The normal cochlea makes two and a half turns and ends at the apex of the cochlea. The average length of the cochlea duct is 37.6 mm; however, there is considerable variety between individuals⁵. In addition, the inner ear can be divided into the osseous labyrinth (otic capsule), formed by the bony margins of the inner ear, and the membranous labyrinth inside the otic capsule. The nerve from the inner ear passes through the internal acoustic canal (IAC) and divides into the vestibular nerve fibers and the cochlear nerve. The cochlear nerve is formed from the axons of the spiral ganglion neurons that lie in the modiolus of the cochlea³. The facial nerve (FN) makes its route also through the IAC; it passes superiorly along the cochlea, making a tight, approximately 110° turn back and downwards through the middle ear until it leaves the TB through the mastoid cavity from the foramen stylomastoid at the base of the skull¹^,⁴. There are several other significant structures in the proximity of the cochlea, e.g., the ossicles (malleus, incus, and stapes) in the middle ear, the carotid artery (ICA) (which arises inferior to the cochlea and then makes a turn medially at the level of the basal turn of the cochlea), the middle fossa plate (tegmen) and the bulbus jugulare which is inferior to the middle ear. The approach to the cochlea during surgery is generally performed through the air cells of the mastoid process. The largest air cell in the mastoid cavity is called the antrum, which communicates with the middle ear via aditus. The size and organization of these mastoid air cells vary significantly, even in terms of "normal anatomy". The prominence of the lateral semicircular canal is usually present on the floor of the antrum. The anatomy of the temporal bone with the many landmarks is presented in Figure 1 and Figure 2. The radiological anatomy of TB from the middle fossa view is presented in Figure 3.

The extraction of the cochlea from a human temporal bone may be required for different investigations of the inner ear. In histological studies, the inner ear is usually extracted from the temporal bone to facilitate its histological processing; likewise, some micro-computed tomography (micro-CT) devices have a relatively limited space to accommodate the sample and may not cope with the complete temporal bones; additionally, the image quality can be enhanced when the cochlea has been isolated⁶^,⁷^,⁸^,⁹. Especially in developing and testing new cochlear implant electrode arrays, histological processing and/or micro-CT are conducted to determine the intra-cochlear position of the electrode⁹^,¹⁰^,¹¹^,¹². In addition, histology can be performed with reduced consumption of processing solutions when the sample is small.

Nonetheless, the extraction of the cochlea requires a profound understanding of the surrounding structures, especially when the aim is to avoid excess bone in the sample. At first impressions, the anatomy of TB might seem hard to comprehend. However, ultimately, the anatomical structures inside the TB serve as boundaries around the cochlea, which can be exploited during the extraction. The accidental opening of the cochlea may lead to traumatic damage to the delicate structures inside this tissue and thus, in a flawed sample which may lead to the need to discard that cochlea.

This article presents a method for extracting the whole cochlea reliably out of the temporal bone by drill-out while watching for the following anatomical landmarks.

Protocol

This study used temporal bones (TB) gathered from the human cadaver subject post-mortem. The study was granted Institutional approval and fulfilled the Helsinki Declaration for the ethical use of human material. Approval for cadaver temporal bones was granted to the Kuopio University Hospital by the Finnish National Supervisory Authority for Welfare and Health (NRO: 9202/06.01.03.01/2013), and the study was conducted following Finnish regulations and laws. All of the bones were gathered anonymously during medical autopsy…

Representative Results

When successful, the cochlea is extracted from the temporal bone without the need to open the perilymph compartment of the cochlea. In a negative case, there is an opening inside the cochlea and damage to the tissue's membranous labyrinth. This extraction method has been used in 36 cadaveric TBs during our cochlear implant electrode investigations (Table 1). In 33 TBs, the extraction has been successful without causing any damage to the cochlea. In two out of 36 TBs, the s…

Discussion

There are several landmarks to be followed for removing the cochlea, so this procedure can be done systematically if the anatomy in the TB is normal (no malformations of the TB). The most critical parts of the removal procedure are the inferior margin and medial proportion between the IAC and the ICA. We recommend maintaining a slightly greater margin and, if necessary, precisely honing any excess bone after extracting the block. It is important to avoid opening the cochlea as it might cause trauma to its delicate inner …

Disclosures

The authors have nothing to disclose.

Acknowledgements

Matti Iso-Mustajärvi receives research grants from the Finnish government research funding (VTR), the Instrumentarium Science Foundation, the North Savo Regional Grant, and the Finnish Society of Ear Surgery. Aarno Dietz receives research grants from the Academy of Finland (Grant No. 333525) and the North Savo Regional grant.

Materials

Drillblades for Drill		N/A	See below. Drillblades should be suitable for your drill system
High speed surgical drill	Medtronic	https://www.medtronic.com/us-en/healthcare-professionals/products/neurological/powered-surgical-instruments/midas-rex-mr8.html	There are numerous providers from various different cateories for surgical drills. The one with irrigation system is recommended (e.g.,Stryker, Bbraun, Medtronic, etc.)
Operating Microscope	Zeiss	https://www.leica-microsystems.com/products/surgical-microscopes/	Microscope for microsurcigal preparation of the temporal bone. Higly recommended microscopes include Zeiss, Leica, etc.
Temporal Bone holder	Stortz	N/A	Cup to fixate the temporal bone while drilling

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