Anatomical Relationship of Cochlear Basal Turn and Facial Nerve in Patients With Tip Fold-Over During Cochlear Implantation Using Slim Modiolar Electrode.

Objective: To evaluate the anatomical factors contributing to tip fold-over (TFO) during electrode insertion in cochlear implantation (CI).

Study design: Retrospective study.

Setting: Tertiary care academic center.

Patients: A total of 239 ears underwent CI with Slim Modiolar Electrodes (SME) between August 2018 and December 2023.

Main outcomes and measures: Preoperative temporal bone CT scans were analyzed to identify differences between patients with and without TFO. The positional relationship between a horizontal line at the cochlear basal turn and the facial nerve was assessed, and the angle between this horizontal line and the lateral margin of the facial nerve was measured.

Results: TFO occurred in 4.2% of cases, requiring an average of 2.7 reinsertion attempts for proper placement. In the TFO group, the horizontal line at the cochlear basal turn was positioned below the facial nerve in 90% of cases, compared with only 11.3% in the non-TFO group (p < 0.001). The mean angle between the cochlear basal turn and the facial nerve was -6.7 degrees (±2.9) in the TFO group and -0.4 degrees (±2.7) in the non-TFO group (p < 0.001).

Conclusion: TFO is more likely to occur when the facial nerve is positioned laterally relative to the cochlear basal turn, causing the electrode sheath to prematurely contact the scala tympani floor. These findings highlight a potential anatomical risk factor for TFO, emphasizing the need for tailored surgical strategies in high-risk cases.