Determination of the Round Window Niche Anatomy Using Cone Beam Computed Tomography Imaging as Preparatory Work for Individualized Drug-Releasing Implants

Abstract

Modern therapy of inner ear disorders is increasingly shifting to local drug delivery using a growing number of pharmaceuticals. Access to the inner ear is usually made via the round window membrane (RWM), located in the bony round window niche (RWN). We hypothesize that the individual shape and size of the RWN have to be taken into account for safe reliable and controlled drug delivery. Therefore, we investigated the anatomy and its variations. Cone beam computed tomography (CBCT) images of 50 patients were analyzed. Based on the reconstructed 3D volumes, individual anatomies of the RWN, RWM, and bony overhang were determined by segmentation using 3D SlicerTM with a custom build plug-in. A large individual anatomical variability of the RWN with a mean volume of 4.54 mm3 (min 2.28 mm3, max 6.64 mm3) was measured. The area of the RWM ranged from 1.30 to 4.39 mm2 (mean: 2.93 mm2). The bony overhang had a mean length of 0.56 mm (min 0.04 mm, max 1.24 mm) and the shape was individually very different. Our data suggest that there is a potential for individually designed and additively manufactured RWN implants due to large differences in the volume and shape of the RWN.

Development and In-Silico and Ex-Vivo Validation of a Software for a Semi-Automated Segmentation of the Round Window Niche to Design a Patient Specific Implant to Treat Inner Ear Disorders

“…Resulting CBCT volumes with an exposure time of 30.8 s and a computed tomography dose index of 8.04 mGy and were reconstructed in an isometric voxel size of 0.08 mm × 0.08 mm × 0.08 mm and exported as Digital Imaging and Communications in Medicine (DICOM) data using i-Dixel software version 1.68 (J. Morita Tokyo Mfg. Corp., Tokyo, Japan) [ 15 ]. The semi-automated algorithm works independently of different image resolutions and even with non-isometric voxel sizes.…”

“…Like in our earlier work [ 15 ], for each dataset, we manually fitted a model of the cochlea using three points (apex, basal turn, round window center) allowing us to define positions and directions in relation to the cochlea using the cochlear coordinate system (CCS) [ 26 ]. The CCS is defined as z-axis along the modiolus axis pointing in the direction of the helicotrema, and the x-axis pointing to the RWM.…”

“…The only two connections between the middle and inner ear are the oval and round windows that are covered by the stapes footplate and round window membrane (RWM) [ 14 ], respectively. The RWM is a semi-permeable membrane that, in human temporal bones, is located deep in a recess called a round window niche (RWN) that is formed by individually very differently shaped bones ( Figure 1 ) [ 15 , 16 , 17 , 18 , 19 ].…”

“…In the temporal bone anatomy mainly three HV have to be differentiated in the process of RWN segmentation: the bony structures surrounding the RWN have high HV, the middle ear which is air filled has low HV and the HV of the fluid filled cochlea are in between ( Figure 2 ). As shown in our previous work, the identification of the RWN and surroundings is feasible but the manual segmentation is very time-consuming [ 15 ]. In some temporal bones the identification of the RWN volume in CBCT scans may prove difficult because anatomic obstructions that can block the RWM, such as adhesions, postoperatively aroused scar tissue, the false membrane, or thickened membranes [ 24 ], can result in similar HV as the RWM or cochlea.…”

“…In our prior study an otolaryngologist used her anatomical knowledge in addition to the image intensity and manually segmented the anatomy of 50 RWN and found variations in volume and shape of the RWM and RWN [ 15 ]. However, using the same software tool for developing and subsequent clinical transfer of novel round window niche implants (RNI) is impractical due to the labor-intensive step of manual segmentation of imaging data and requires highly trained specialists for identification of the RWN anatomy.…”