Orientation of the Round Window Membrane: A normative study of inner ear anatomical orientation using 2D projections of 3D volumes

INTRODUCTION: Orientation of the Round Window Membrane (RWM) is an important metric to establish if utilized as a potential access for targeted delivery of magnetically guided nanomedicines to the inner ear. Orientation with respect to an internal reference frame (such as the planes defined by the semicircular-canals [SCC]) may provide an internally consistent basis if the basis is orthogonal and consistent (from patient to patient). MATERIALS AND METHODS: Utilizing a micro computed tomography (CT), 20 temporal bones are scanned for anatomical information. The scanned data sets are loaded into an imaging program to provide volumetric reconstruction and segmentation. Volumetric models of the anatomical relationships between the inner ear SCC and the RWM are utilized to get normative projection angle information and are statistically analyzed. RESULTS: Micro-CT shows low to moderate reliability for reproducibility, intraobserver, and interobserver measurements; in addition, it provides mean values (+/-SD) for the various measured angles. The combined mean angular values for surface orientation of the RWM, with respect to the SCC basis (quasi-orthogonal spherical coordinate system), was 57.0 degrees +/- 20.9 degrees as measured from the line defining the posterior SCC plane in the direction of the line defining the superior SCC plane. An angle of 65.2 degrees +/- 19.1 degrees was measured for an angle away from the line defining the horizontal SCC plane.