Impact of Corridor Morphology on Internal Auditory Canal Exposure in the Retrolabyrinthine Approach

ABSTRACT Objectives The retrolabyrinthine approach is a presigmoid, potentially hearing‐preserving option for internal auditory canal (IAC) lesions, though exposure is limited by a narrow corridor. Endoscopic assistance may improve lateral visualization. This study compared projected radiographic versus postdissection IAC exposure using simulated 0° and 30° endoscopy and evaluated anatomic predictors of exposure. Methods Thirteen cadaver temporal bones underwent high‐resolution computed tomography before and after IAC dissection with posterior semicircular canal preservation. The retrolabyrinthine approach was simulated using 3D segmentation in 0° and 30° endoscopic views. Total and accessible IAC volumes and the percentage of IAC accessible were compared between projected and postdissection models. Projections were compared with observed volumes using Wilcoxon signed‐rank testing. Associations between anatomic covariates (corridor width, angle of approach, surgical freedom) and exposure were evaluated using Theil–Sen regression. Results Median IAC volume was 230.1 mm 3 (range 148.1–446.9). Surgical freedom ranged from 123.3–319.2 mm 2 (median 182.9), and angle of approach from 13.9° to 27.1° (median 19.9°). Projected exposures were 59.2% at 0° and 92.2% at 30°, versus 65.1% and 75.6% postdissection. Projection underestimated 0° exposure (−5.9%, p = 0.01) and overestimated 30° exposure (+16.7%, p = 0.048). Thirty‐degree endoscopy improved exposure ( p < 0.001). Each 1 mm increase in corridor width increased accessible volume by 8.46 mm 3 (0°) and 5.31 mm 3 (30°), and each 1° increase in angle of approach increased accessible volume by 4.98 mm 3 (0°) and 5.11 mm 3 (30°) (all p ≤ 0.04). Conclusions Endoscopic assistance improves exposure, although projections overestimate 30° exposure. Corridor width and angle of approach may inform preoperative selection. Level of Evidence N/A.