Dimensions of the Posterior Tympanotomy and Round Window Visibility Through the Facial Recess: Cadaveric Temporal Bone Study Using a Novel Digital Microscope

To measure the width of the posterior tympanotomy in cadaveric temporal bones using the digital microscope and classify the round window visibility through it. In 17 cadaveric wet adult temporal bones, cortical mastoidectomy followed by posterior tympanotomy was performed, delineating the facial and chorda tympani nerves. Antero-posterior width of the facial recess was measured at the levels of oval window and round window with the help of a digital microscope and its software. Visibility of the round window through the facial recess was assessed and classified according to the St. Thomas Hospital classification. The mean antero-posterior width of the facial recess measured was 4.7 ± 0.6 mm at the level of oval window and 4.3 ± 0.7 mm at the level of round window. Round window visibility grading in bones studied were as follows-Type 1 (53%), Type 2a (24%), Type 2b (18%) and Type 3 (5%). Interobserver variability of the posterior tympanotomy measurements using the digital microscope was found to be 91.1% with a 95% confidence interval of 79 to 97% at the level of oval window and 94.1% with a 95% confidence interval (CI) of 87 to 98% at the level of round window. The visibility of the round window is not entirely dependent on the facial recess width at the round window level, suggesting that other factors like cochlear rotation may also contribute to its actual location. Measurements of micro distances with the help of digital microscope seems to be convenient, cost effective and accurate with good inter observer reliability.