목적: Hearing preservation (HP) in cochlear implantation (CI) is important to the patients who have residual hearing. Because by using an electrical acoustic system (EAS), patients can hear a more natural sound. To preserve residual hearing, trajectory of electrode insertion is very important. Basal turn (BT) – facial ridge (FR) angle (Choung’s angle I) predicts the risk of electrode trajectory, and this mechanism was already published (Otology & Neurotology, 2020. The other factor determining electrode trajectory is the position of posterior wall of external auditory canal (EAC) and round window (RW). The purpose of this study was to analyze of positions and related angles involving FR, EAC, RW and BT. 방법:This study included 122 patients who underwent cochlear implantation between 2011 and 2022 and had a preoperative low-frequency pure-tone average (LFPTA) of 80 dB HL or lower at frequencies of 125 Hz, 250 Hz, and 500 Hz. The preservation of residual hearing after cochlear implantation surgery was assessed using the equation developed by Skarżyński et al. It was classified as follows: complete HP (numerical scale score [S] > 75%), partial HP (25% ≤ S < 75%), minimal HP (1% ≤ S < 25%), or complete loss (S < 1%). The EAC-FR angle is defined as the angle between two lines. The first line connects the posterior EAC wall and the anterior margin of the RW, while the second line connects the FR and the posterior margin of the RW. 결과:Of the 122 LFPTA patients with ≤80dB, the preoperative average was 61.9 ± 16.4 dB. The average age was 39.2 ± 24.7 years. There were 21 slim straight electrodes, 39 slim straight electrodes from Cochlear, and 62 slim straight electrodes from MED-EL. The BT-FR angle was significantly higher in the complete-to-minimal HP group compared to the hearing loss group one month after surgery. The EAC-FR angle showed a decreasing trend from the complete to partial, minimal HP, and hearing loss groups. Notably, there were significant differences between the groups at 3 and 6 months postoperatively. At 6 months postoperatively, the EAC-FR angle was highest in the Complete HP group at 14.78°, followed by the Partial HP group at 13.99°, the Minimal HP group at 12.23°, and the Hearing Loss group, which had the lowest angle at 9.82°. The correlation between the degree of hearing loss and the EAC-FR angle showed a positive correlation with hearing preservation. 결론:The EAC-FR angle (Choung’s angle II) which can be measured easily in axial TBCT images may be useful to predict residual HP after CI. A larger angle was associated with a higher likelihood of hearing preservation. Therefore, EAC wall thinning and facial ridge lowering may help overcome these limitations. Thus, preoperative measurement of the Choung’s angle I and II may facilitate the establishment of a more individualized surgical plan.