Impact of electrode design and surgical approach on scalar location and cochlear implant outcomes

Abstract

Objectives/hypothesis: Three surgical approaches: cochleostomy (C), round window (RW), and extended round window (ERW); and two electrodes types: lateral wall (LW) and perimodiolar (PM), account for the vast majority of cochlear implantations. The goal of this study was to analyze the relationship between surgical approach and electrode type with final intracochlear position of the electrode array and subsequent hearing outcomes.

Study design: Comparative longitudinal study.

Methods: One hundred postlingually implanted adult patients were enrolled in the study. From the postoperative scan, intracochlear electrode location was determined and using rigid registration, transformed back to the preoperative computed tomography which had intracochlear anatomy (scala tympani and scala vestibuli) specified using a statistical shape model based on 10 microCT scans of human cadaveric cochleae. Likelihood ratio chi-square statistics were used to evaluate for differences in electrode placement with respect to surgical approach (C, RW, ERW) and type of electrode (LW, PM).

Results: Electrode placement completely within the scala tympani (ST) was more common for LW than were PM designs (89% vs. 58%; P < 0.001). RW and ERW approaches were associated with lower rates of electrode placement outside the ST than was the cochleostomy approach (9%, 16%, and 63%, respectively; P < 0.001). This pattern held true regardless of whether the implant was LW or PM. When examining electrode placement and hearing outcome, those with electrode residing completely within the ST had better consonant-nucleus-consonant word scores than did patients with any number of electrodes located outside the ST (P = 0.045).

Conclusion: These data suggest that RW and ERW approaches and LW electrodes are associated with an increased likelihood of successful ST placement. Furthermore, electrode position entirely within the ST confers superior audiological outcomes.

Level of evidence: 2b.

Keywords: Cochlear implant; cochleostomy; electrode; round window; sensorineural hearing loss.

Fig. 1

(A) Software-generated, three-dimensional reconstructed image and (B) microdissection demonstrating an electrode array entering the scala tympani (ST); after approximately 180° (black arrow indicates the array moves from ST through the basilar membrane and enters scala vestibuli [SV]), the electrode traverses the interscalar partition to enter the SV (white arrow indicates the clear silicone tip of the array, which is above the basilar membrane on the middle turn). (C) Three-dimensional reconstructed image. (D) Microdissection revealing an electrode completely residing within the ST without violation of the osseous spiral lamina or basilar membrane.

Fig. 2

Computed tomography of a right temporal bone following cochlear implant electrode insertion. (A) Axial, (B) magnified axial, and (C) magnified coronal views showing the cochlear implant electrode situated completely within the scala tympani (ST). The ST is outlined in red and the scala vestibuli is outlined in blue.

Fig. 3

Three-dimensional reconstructed images showing a cochlear implant completely within the scala tympani (ST). (A) Medial-to-lateral view with the ST is shown in semitransparent red. (B) The scala vestibuli is shown in opaque blue.

Fig. 4

Computed tomography of a right temporal bone following cochlear implant electrode insertion. (A) Axial, (B) magnified oblique axial, and (C) magnified oblique coronal views demonstrating the implant beginning in the scala tympani (outlined in blue) and crossing over into the scala vestibuli (outlined in red) at approximately 180°.

Fig. 5

Three-dimensional reconstructed images demonstrating scala crossover from the scala tympani (ST) to the scala vestibuli (SV). (A) Inferior-to-superior, medial-to-lateral view of the ST (red) demonstrating the electrode array crossing the basilar membrane at approximately 180°. (B) View parallel to the basal turn shows electrode crossover from the ST (semitransparent red) to the SV (semitransparent blue).