Effect of cochlear shape and size on cochlear implant insertion forces

This PhD thesis constitutes a comprehensive investigation into the critical factors and parameters influencing cochlear implantation outcomes, with the ultimate aim to enhance surgical practices and patient outcomes. Chapter 2 offers a detailed overview of the anatomical and physical properties of the cochlea to aid the development of accurate models for improved future cochlear implant (CI) treatments. It highlights the advancements in the development of various physical, animal, tissue engineering, and computational models of the cochlea, along with the challenges and potential future directions. Chapter 3 performs a systematic review, consolidating and scrutinising the existing literature on cochlear implantation. Firstly, it centres on the determinants of insertion forces (IFs) and intracochlear pressure (IP) during cochlear implantation, focusing on insertion depth, speed, and the role of robotic assistance. The findings underscore the necessity for standardisation across studies and offer critical insights into the factors influencing IFs and IP during cochlear implantation. The second part of the chapter assesses the influence of surgical approach and cochlear implant type on the occurrence and distribution of cochlear trauma, identifying potential areas for improvement. It indicates the significance of implant design and surgical approach in reducing cochlear trauma and enhancing patient outcomes. Chapter 4 scrutinises the precision and transparency of Stereolithography (SLA) and Digital Light Processing (DLP) 3D printing technologies in creating full cochlea and *scala tympani* models. The most accurate and transparent models were achieved using DLP technology with a 30 μm layer height combined with an acrylic coating. This provides a promising pathway for creating detailed artificial cochlea models for use in cochlear implantation surgery. Chapter 5 presents a systematic investigation of the influence of different geometrical parameters of the *scala tympani* on the cochlear implant insertion force. This was done using accurate 3D-printed models of the *scala tympani* with geometrical alterations. The results indicate that the insertion force is largely unaffected by the overall size, curvature, vertical trajectory, and cross-sectional area once the forces were normalised to an angular insertion depth. This supports the Capstan model of the cochlear implant insertion force which suggests the major factor in assessing insertion force and associated trauma are the friction, the tip stiffness, and the angular insertion depth, rather than the length of the CI inserted. This thesis provides novel insights into the dynamics of cochlear implantation, offers a comprehensive appraisal of the current state of research, provides methodologies to fabricate accurate artificial models, and identifies areas for further investigation. It is anticipated that the findings will guide future research and clinical practice to optimise cochlear implantation outcomes.Royal National Institute for Deaf People (G100138