Assessment of the Oval Window–Facial Canal Corridor and Mastoid Pneumatization: A Preliminary HRCT-Based Radiological–Anatomical Framework

Background/Objectives: The anatomical relationship between the oval window (OW) and the facial nerve (FN) is of significant importance in the context of stapes and middle ear surgery. However, the influence of mastoid pneumatization on OW morphology and FN proximity remains to be elucidated. Methods: This retrospective study included 220 radiologically normal temporal bones from 110 adults. OW width, OW height, OW niche width, FNt–OW distance, FNm–OW distance, OW–mastoid cell distance, and OW–FNm–FNt corridor area were measured on HRCT images. Mastoid pneumatization was classified using a sigmoid sinus-based grading system. RASCI was developed by integrating OW morphology, OW niche width, and mastoid pneumatization. Results: The mean width of the OW, the height of the OW, and the width of the OW niche were 2.41 ± 0.92 mm, 1.38 ± 0.36 mm, and 1.03 ± 0.38 mm, respectively. The mean FNt–OW and FNm–OW distances were recorded as 0.93 ± 0.33 mm and 2.78 ± 0.94 mm, respectively. It was established that larger OW dimensions and FN-OW distances were associated with higher levels of mastoid pneumatization. The radiological–anatomical constraint was categorised as low in 21.4% of cases, moderate in 69.5%, and high in 9.1% of the temporal bones (RASCI). Conclusions: Mastoid pneumatization appears to be associated with OW morphology and FN–OW spatial relationships. The RASCI could provide an initial HRCT-based framework for grading the constraints of the OW-FN corridor; however, prospective surgical validation is required.