Positioning a Novel Transcutaneous Bone Conduction Hearing Implant: a Systematic Anatomical and Radiological Study to Standardize the Retrosigmoid Approach, Correlating Navigation-guided, and Landmark-based Surgery

Hypothesis: Anatomical and radiological evaluation improves safety and accuracy of the retrosigmoid approach for positioning a transcutaneous bone conduction implant and provides anatomical reference data for standardized, landmark-based implantation at this alternative site. Background: The primary implantation site for the floating mass transducer of a novel bone conduction hearing implant is the mastoid. However, anatomical limitations or previous mastoid surgery may prevent mastoid implantation. Therefore, the retrosigmoid approach has been introduced as an alternative. Methods: Mastoid and retrosigmoid implantation sites were radiologically identified and evaluated in preoperative computed tomography scans of anatomical head specimens. Navigation-guided implantation was then performed in the retrosigmoid site (n = 20). The optimal retrosigmoid position was determined in relation to both the asterion and the mastoid notch as surgical landmarks in an anatomical coordinate system. Results: Preoperative radiological analysis revealed spatial limitations in the mastoid in 45% of the specimens. Navigation-guided retrosigmoid implantation was possible without affecting the sigmoid sinus in all the specimens. The optimal implantation site was located 1.9 ± 0.1 cm posterior/1.7 ± 0.1 cm inferior to the asterion and 3.3 ± 0.2 cm posterior/2.1 ± 0.1 cm superior to the mastoid notch. Retrosigmoid skull thickness was 6.6 ± 0.4 mm, measured anatomically, 7.0 ± 0.4 mm, measured radiologically and 6.7 ± 0.5 mm, measured with the navigation software. Conclusion: The navigation-guided retrosigmoid approach seemed to be a reliable procedure in all the specimens. Measurements of bone thickness revealed the need for spacers in 95% of the specimens. Reference coordinates of the optimal implantation site are provided and can confirm image-guided surgery or facilitate orientation if a navigation system is not available. Address correspondence and reprint requests to Heinz Arnold, M.D., Department of Otorhinolaryngology—Head and Neck Surgery, Ruhr University Bochum, St.-Elisabeth-Hospital, Bleichstrasse 15, 44787 Bochum, Germany; E-mail: heinz.arnold@rub.de This work was not sponsored by any third party outside the University. H.A., A.R., and H.L. have received travel funding by MED-EL GmbH, Innsbruck, Austria. H.A. and H.L. have lectured in training courses organized by MED-EL. H.A. has received honoraria for lectures by MED-EL. H.L. is currently receiving a research grant from MED-EL. The authors disclose no conflicts of interest. Copyright © 2018 by Otology & Neurotology, Inc. Image copyright © 2010 Wolters Kluwer Health/Anatomical Chart Company

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