Effect of Scala Tympani Height on Insertion Depth of Straight Cochlear Implant Electrodes

Abstract

Objective: Studies suggest lateral wall (LW) scala tympani (ST) height decreases apically, which may limit insertion depth. No studies have investigated the relationship of LW ST height with translocation rate or location.

Study design: Retrospective review.

Setting: Cochlear implant program at tertiary referral center.

Subjects and methods: LW ST height was measured in preoperative images for patients with straight electrodes. Scalar location, angle of insertion depth (AID), and translocation depth were measured in postoperative images. Audiologic outcomes were tracked.

Results: In total, 177 ears were identified with 39 translocations (22%). Median AID was 443° (interquartile range [IQR], 367°-550°). Audiologic outcomes (126 ears) showed a small, significant correlation between consonant-nucleus-consonant (CNC) word score and AID (r = 0.20, P = .027), although correlation was insignificant if translocation occurred (r = 0.11, P = .553). Translocation did not affect CNC score (P = .335). AID was higher for translocated electrodes (503° vs 445°, P = .004). Median translocation depth was 381° (IQR, 222°-399°). Median depth at which a 0.5-mm electrode would not fit within 0.1 mm of LW was 585° (IQR, 405°-585°). Median depth at which a 0.5-mm electrode would displace the basilar membrane by ≥0.1 mm was 585° (IQR, 518°-765°); this was defined as predicted translocation depth (PTD). Translocation rate was 39% for insertions deeper than PTD and 14% for insertions shallower than PTD (P = .008).

Conclusion: AID and CNC are directly correlated for straight electrodes when not translocated. Translocations generally occur around 380° and are more common with deeper insertions due to decreasing LW ST height. Risk of translocation increases significantly after 580°.

Keywords: cochlear implantation; scala tympani height; straight electrodes; translocation.

Figure 1.

Scala tympani measurement tool. On left, distance from lateral wall is measured: red circle with diameter 0.5mm sits 0.11mm from lateral wall of scala tympani. On right, basilar membrane displacement is measured.

Figure 2.

Histogram of angular insertion depth for all electrode arrays. X-axis labels correspond to the center of bins with a width of 45°.

Figure 3.

Histogram of angular depth of translocation for translocated electrodes. X-axis labels correspond to the center of bins with a width of 45°.

Figure 4.

Risk of translocation at given angular insertion depth (AID) is represented using an inverted Kaplan-Meier curve. At their final AID, arrays were considered events if translocation occurred or censored if it did not.

Figure 5.

Distance (in mm) from the lateral wall of the scala tympani where an electrode with diameter 0.5mm would sit.

Figure 6.

Basilar membrane displacement (in mm) caused by an electrode with diameter 0.5mm sitting against the lateral wall of the scala tympani.

Figure 7.

For patients without postoperative residual hearing, consonant-nucleus-consonant (CNC) score and angular insertion depth were positively correlated for non-translocated electrodes. There was no correlation for translocated electrodes, but these slopes were not significantly different.