Anatomy of the round window and hook region of the cochlea with implications for cochlear implantation and other endocochlear surgical procedures

Abstract

Hypothesis: The goal of this study was to create a three-dimensional model of the anatomy of the hook region to identify the optimal site for cochleostomy in cochlear implant surgery.

Background: The anatomy of the hook region is complex, and spatial relationships can be difficult to evaluate using two-dimensional histological slides or cadaveric temporal bones.

Methods: The right temporal bone of a 14-year-old adolescent boy was used to create a three-dimensional model. Sections containing the round window membrane (RWM) and surrounding cochlear structures were stained, digitized, and imported into a general purpose three-dimensional rendering and analysis software program (Amira, version 4.1). Three-dimensional models of the RWM, basilar membrane, osseous spiral lamina, spiral ligament, cochlear aqueduct, inferior cochlea vein, scala media, ductus reuniens, scala vestibuli, scala tympani, and surrounding bone were generated. The relationship between these structures and the RWM and adjacent otic capsule was evaluated. Histological sections from a different temporal bone were also analyzed. This temporal bone was sectioned in a plane perpendicular to the axis corresponding to the surgical view of the RWM, seen through the facial recess.

Results: The anteroinferior margin of the RWM or adjacent otic capsule was identified as the site for a cochleostomy that will avoid damage to critical cochlear structures and allow implantation directly into the scala tympani. The model can be downloaded from: https://research.meei.harvard.edu/otopathology/3dmodels.

Conclusion: This three-dimensional model has implications for surgical procedures to the inner ear that aim to minimize insertional trauma.

FIG. 1

Illustration showing the surgical view of the RWM, seen through the facial recess. The pointer in A points in a direction perpendicular to the plane of sectioning. This plane is shown in B. Ca, carotid artery; CA, cochlear aqueduct; CT, chorda tympani; EAC, external auditory canal; FN, facial nerve; I, incus; LSC, lateral semicircular canal; PSC, posterior semicircular canal; S, stapes; SM, scala media; SSC, superior semicircular canal; ST, scala tympani; RWM, round window membrane.

FIG. 2

A, Illustration showing the RWM and surrounding structures seen through the facial recess. The red box represents the hook region of the cochlea that was modeled in this study. B, Anatomy of the round window and surrounding structures after removing the bone from the model. C, The RWM and spiral ligament have been removed from the model, and the scala tympani has been made transparent to show the intracochlear structures. BM, basilar membrane; CA, cochlear aqueduct; CT, chorda tympani; DR, ductus reuniens; EAC, external auditory canal; FN, facial nerve; I, incus; ICV, inferior cochlear vein; LSC, lateral semicircular canal; OSL, osseous spiral lamina; PSC, posterior semicircular canal; S, stapes; SL, spiral ligament; SM, scala media; SSC, superior semicircular canal; ST, scala tympani; SV, scala vestibuli; RWM, round window membrane.

FIG. 3

Histological sections showing a different temporal bone cut perpendicular to the surgical view of the round window. The plane of sectioning is shown at A. A, Section through the posterior margin of the RWM. B, Section through the middle of the RWM. C, Section through the anteroinferior margin of the RWM. D, Section through the otic capsule adjacent to the anteroinferior margin of the RWM. BM, basilar membrane; CA, cochlear aqueduct; DR, ductus reuniens; OSL, osseous spiral lamina; OW, oval window; PA, postis anterior; RWN, round window niche; RWM, round window membrane; Sa, saccule; SL, spiral ligament; SM, scala media; ST, scala tympani; SV, scala vestibuli.

FIG. 4

Graph showing the shortest distance between points placed on the RWM and otic capsule and other intracochlear structures in the hook region. CA, cochlear aqueduct; SL, spiral ligament; ST, scala tympani; SV, scala vestibuli; RWM, round window membrane.

FIG. 5

A, The RWM is shown within the round window niche. Note the relationship of the postis anterior/crista fenestra to the anteroinferior margin of the RWM. B, The optimal site of cochleostomy and direction of electrode insertion are shown. The circle represents the proximal diameter of the largest current electrode, which measures 1.3 mm. CA, cochlear aqueduct; ICV, inferior cochlear vein; MAC, mastoid air cells; PA/CF, postis anterior/crista fenestra; RWM, round window membrane; RWN, round window niche; SL, spiral ligament; ST, scala tympani; SV, scala vestibuli.