목적: The slim modiolar electrode (SME) is widely used recently due to its thin and flexible design, which enhances modiolar proximity. However, there have been reports of tip fold-over occurring during electrode insertion via the round window (RW) due to the thin and flexible nature of the electrode. This phenomenon, known as tip fold-over, impedes the optimal positioning of the electrode at the desired site and can consequently impact the final outcomes. Therefore, this study aimed to investigate the characteristics of anatomical structures that contribute to tip fold-over during electrode insertion. 방법:From 2018 August to 2023 December, total 268 ears who underwent cochlear implantation with SME (Nucleus CI 532 or 632) or straight (CI 522 or 622) electrode with or without subtotal petrosectomy due to severe hearing loss. Preoperative temporal bone CT (TBCT) was conducted to evaluate the anatomy of cochlea and its adjacent structures and sought to reveal the relationship between basal turn (BT) of cochlea and facial ridge (FR). In all cases, electrode insertion was performed, and intra-operative X- rays were taken to confirm the absence of tip fold-over. The occurrence of tip fold-over during electrode insertion was investigated, and positional relationship between a horizontal line drawn at the BT and the facial nerve (FN) was assessed. Additionally, the angle between the horizontal line drawn at the BT and FR was measured. 결과:In a total of 268 ears, 7.3% of patients underwent insertion of a straight electrode based on preoperative TBCT, while the remaining 92.7% of patients received insertion of SME. Among the patients with SME insertion, tip fold-over occurred in 7.9% of cases. It was observed that the electrode was well positioned after an average of 2.9 re-insertion attempts in tip fold-over patients. In TBCT scans of the patients experiencing tip fold-over, a horizontal line drawn at the BT consistently revealed that FN positioned above the line in 100% of cases. Conversely, among patients without tip fold-over, when assessing the positional relationship between the horizontal line drawn at the BT and FN, FN was located above the line in 19.4% and below the line in 80.6% of cases. For some patients whose FN was positioned above the line drawn at the BT, straight electrode was inserted. This difference was statistically significant (p < 0.001). When measuring the angle between BT and FN using TBCT, the mean angle was -7.26° (±1.54) in the group where tip fold-over occurred and -1.04° (±2.78) in the group where it did not, with a significant difference between the two groups (p < 0.001). 결론:The anatomical relationship between cochlear BT and FR can explain the risk of tip fold-over during CI using SME. When FN was positioned above the line drawn at the BT, the electrode might contact the bone wall of the scala tympani, coming out of the sheath too quickly, resulting in a tip fold-over. It can be helpful to extend the round window to minimize the contact of the bone wall of the electrode or to change it into a straight electrode, when a tip fold-over occurs.