Objective: Modiolus and modiolar base abnormalities in patients with incomplete partition-II anomaly (IP-II) increase the risk of intraoperative cerebrospinal fluid (CSF) leakage. This study aimed to classify modiolar abnormalities and define objective radiological measures for preoperatively evaluating intraoperative CSF leakage risk. Study Design: Retrospective case series. Setting: Tertiary referral center. Patients: The study included 73 patients with IP-II that underwent cochlear implant surgery between 2002 and 2017. Intervention: Analysis of preoperative temporal bone computed tomography (CT) scans and surgical records. Main Outcome Measures: Preoperative CT modiolar anomalies and intraoperative CSF leakage status. Results: Among the 73 patients (41 men and 32 women), mean age at cochlear implant surgery was 11.4 (range, 0–42) years. Preoperative CT-based modiolar base anomaly classification was as follows: grade 1 (complete modiolus, n = 1), grade 2 (thin plate of bone in the modiolar base + partial modiolus, n = 14); grade 3 (thin plate of bone in the modiolar base, n = 53); grade 4 (total modiolar base defect, n = 5). The gusher rate was 8%. All patients with grade 4 anomaly had an intraoperative gusher. Patients with grade 3 anomaly accounted for 86% of oozing cases. Oozing, pulsation, and no CSF leakage rates were similar in those with grade 2 anomaly. Conclusions: The modiolus and modiolar base must be carefully evaluated in patients with IP-II. Gushers primarily occur in IP-II patients with grade 4 anomaly. A thin plate of bone in the modiolar base most commonly prevents gushers.
