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

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 ( = 0.20, = .027), although correlation was insignificant if translocation occurred ( = 0.11, = .553). Translocation did not affect CNC score ( = .335). AID was higher for translocated electrodes (503° vs 445°, = .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 ( = .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°.