The Retrotympanum Revisited By a Volumetric Approach Using Synchrotron-Based X-Ray Phase-Contrast Imaging

Hypothesis: We hypothesize a large anatomic variability with a positive correlation between retrotympanic recess volume and depth classification and an inverse correlation between recess volume and facial nerve distance. Background: The retrotympanum consists of various bony recesses and is of particular interest given its frequent involvement in cholesteatoma. As the structures are only partly accessible with established imaging techniques due to their size, we propose here to systematically analyze the different retrotympanic subsites and describe topographic relationships using synchrotron-based X-ray phase contrast imaging (SR X-PCI). Methods: Ten fresh-frozen human temporal bones underwent SR X-PCI at the TOMCAT beamline at the Swiss Light Source. Microtomographic data sets were acquired and segmented for visual inspection. In each specimen, sinus tympani, facial recess, posterior sinus, and lateral tympanic sinus were present and were identified for further analysis. Results: Among the volumes of retrotympanic recesses, the posterior sinus was on average the smallest and sinus tympani the largest. A positive correlation was observed between the volumes of the sinus tympani and facial recess and their respective A-B-C classifications. Moreover, an inverse correlation between volume and distance to the facial nerve was found for the different retrotympanic recesses. In addition, a previously undescribed recess located above the facial recess, termed the suprafacial recess, was identified in 7 specimens, and its volume was quantified. Conclusion: SR X-PCI enabled 3D visualization and volumetric analysis of retrotympanic subspaces. Our findings confirmed the complex and variable anatomy of this region and revealed a consistent presence of a previously underreported suprafacial recess.