This was an anatomical cadaveric study conducted in the temporal bone dissection lab of the otolaryngology department of a medical college hospital. The objective of this study was to determine the dimensions of the posterior tympanotomy as well as grade visualization of the round window through it, using a novel digital microscope and assess its reliability and accuracy. Newer equipment like stereo-microscopes available for the same purpose are quite expensive. Direct measurements of this small recess are usually difficult, with most of the previous studies in literature employing either measurement on histological cut section slides or on computed tomography (CT) reconstruction images. Adequate visualization of the round window membrane most often requires drilling of the postero-superior bony niche overhang of the round window and the antero-inferior margin called crista fenestrae. Narrow recess has risk of injury to the facial or chorda tympani nerves, whereas a wider recess facilitates easy passage of the implant electrode with the correct angle. With regard to cochlear implantation, the width of the facial recess and the exposure of the round window (RW) niche through it are major determinants for insertion of the implant electrodes. Though superior boundary is the incus buttress and medial boundary is mastoid segment of facial nerve, definitions of classical and extended posterior tympanotomy have varied depending upon whether the lateral boundary of this space is chorda tympani nerve or the tympanic annulus respectively. Posterior tympanotomy, first described by Jansen is the otological surgical approach that provides trans-mastoid access to the tympanic cavity via a triangular space called as the facial recess (FR). Measurements of micro distances with the help of digital microscope seems to be convenient, cost effective and accurate with good inter observer reliability. 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. 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. Round window visibility grading in bones studied were as follows-Type 1 (53%), Type 2a (24%), Type 2b (18%) and Type 3 (5%). 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. Visibility of the round window through the facial recess was assessed and classified according to the St. 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. In 17 cadaveric wet adult temporal bones, cortical mastoidectomy followed by posterior tympanotomy was performed, delineating the facial and chorda tympani nerves. To measure the width of the posterior tympanotomy in cadaveric temporal bones using the digital microscope and classify the round window visibility through it.
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