Oval Window Niche Height: Quantitative Evaluation with CT before Stapes Surgery for Otosclerosis

Abstract

Stapes surgery for otosclerosis can be challenging when the oval window niche is narrow. We analyzed the reliability of CT to evaluate the height of the OWN and propose a quantitative criterion to distinguish normal and narrow OWNs. Fifty-six patients were scheduled for primary stapes surgery and, with available preoperative CT scans, were prospectively enrolled in the study at a tertiary care hospital. OWN height was measured on coronal CT and qualitatively evaluated during surgery. CT findings and surgical observations were matched to determine the preoperative imaging criterion of a narrow OWN. OWN was found to be narrow during surgery in 8 of 56 patients (14%). On CT, mean OWN height measurement was 1.1 mm for the narrow group and 1.8 mm for the normal OWN surgical cases. The cutoff between normal and narrow OWN was computed at 1.3 mm by using discriminant analysis and at 1.4 mm with boxplot analysis. These CT cutoff values allowed a correct classification of "normal" and "narrow" OWN, compared with visual evaluation during surgery. Measurements of the OWN height provide an accurate and relevant evaluation of this region before otosclerosis surgery. A width below 1.4 mm should be considered at risk for technical difficulties during the stapes footplate approach.