Anatomic studies of the Human Cochlea: Implications for Cochlear Implantation

Anatomical studies of the human cochlea are of value for cochlear implantation (CI) and may also increase our understanding of the function of the human hearing organ. Hearing loss is the most common sensory deficit.

Cochlear implantation is a major achievement in modern medicine. CI electrodes on the market are mostly designed either for a position along the inner modiolar wall (so-called peri-modiolar electrodes) or along the outer wall of scala tympani. Anatomic variations of the cochlea may influence its final position relative to the cochlea place/frequency map. With less invasive surgical techniques and shorter electrodes the fragile inner ear structures may also be conserved Preservation of residual hearing is now a goal in all cochlear implant surgery and better knowledge about anatomical variation may limit intra-cochlear damage.

Our collection of plastic human inner ear molds contains 325 specimens (Fig. 1). The applied method of casting temporal bone specimens was described by Wilbrand et al.(Wilbrand et al. 1974) Wadin (Wadin 1988) and Rask-Andersen et al. (Rask-Andersen et al. 1977). 

Uppsala collection of 325 plastic molds of human inner ear.
Fig.1 Uppsala collection of 325 plastic molds of human inner ear.  

Corrosion cast of a left human cochlea (axial-pyramidal view). The reference points used for estimating the length of the various turns of the cochlea are shown.
Fig. 2. Corrosion cast of a left human cochlea (axial-pyramidal view). The reference points used for estimating the length of the various turns of the cochlea are shown. 

A hemi sectioned human cochlea in a scanning electron microscope.
Fig. 3 At our laboratory we also investigate the fine structure of the human cochlea. Here we see the hemi sectioned human cochlea in a scanning electron microscope. Fine details can be visualized and the conditions for nerve regeneration is considered. The yellow structure is the human auditory nerve.

Unique details of the human organ of hearing can be visualized three-dimensionally.
Fig. 4 Unique details of the human organ of hearing can be visualized three-dimensionally for the first time in well preserved human specimens.
Illustration: the molecular structure and organization of various proteins in the human cochlea can be revealed by immunohistochemistry
Fig. 5 The molecular structure and organization of various proteins in the human cochlea can be revealed by immunohistochemistry including confocal microscopy and super resolution structured illumination microscopy (Liu et al. 2017).

Since the last four years we also investigate the human inner ear anatomy using high resolution synchrotron imaging (SR-PCI). This is done together with Canadian researcher and imaging is performed in Saskatoon in Saskatchewan in Canada. This technique gives unique information about both the bony and soft tissue structures. It may help to design better CI electrodes and cast new light on the structure of the human inner ear.

Illustration: the human cochlea with the auditory nerve and basilar membrane.
Fig. 6 Image showing the human cochlea with the auditory nerve (yellow) and basilar membrane (green). At lower right a CI electrode is also shown (Li et al. 2020).
mage showing the vascular supply of the human spiral ganglion
Fig. 7 Image showing the vascular supply of the human spiral ganglion (Mei et al. 2020). Our findings show that arteries (red) to the auditory neurons (yellow) are sheltered inside the bone and that there is a double venous (blue) drainage that may explain the unique preservation of the nerve in man. According to the authors this could partly explain frequent spectacular results with CI despite severe cochlear changes. Brown channels contain both arteries and veins.

References

Li H, Schart-Morén N, Rohani SA, Ladak HM, Rask-Andersen H, Agrawal S. 2020. Synchrotron Radiation-Based Reconstruction of the Human Spiral Ganglion: Implications for Cochlear Implantation. Ear Hear. 41(1):173–81

Liu W, Schrott-Fischer A, Glueckert R, Benav H, Rask-Andersen H. 2017. The Human “Cochlear Battery” – Claudin-11 Barrier and Ion Transport Proteins in the Lateral Wall of the Cochlea. Front. Mol. Neurosci. 10:

Mei X, Glueckert R, Schrott-Fischer A, Li H, Ladak HM, et al. 2020. Vascular Supply of the Human Spiral Ganglion: Novel Three-Dimensional Analysis Using Synchrotron Phase-Contrast Imaging and Histology. Sci. Rep. 10(1):5877

Rask-Andersen H, Stahle J, Wilbrand H. 1977. Human cochlear aqueduct and its accessory canals. Ann. Otol. Rhinol. Laryngol. 86(suppl.42, 5II):1–16

Wadin K. 1988. Imaging contributions to the temporal bone anatomy (high jugular fossae). Scand Audiol Suppl. 30:145–48

Wilbrand HF, Rask-Andersen H, Gilstring D. 1974. The vestibular aqueduct and the para-vestibular canal:An anatomic and roentgenologic investigation. Acta radiol. 15(4):337–55

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