Impact of Scala Tympani Geometry on Insertion Forces during Implantation

Abstract

(1) Background: During a cochlear implant insertion, the mechanical trauma can cause residual hearing loss in up to half of implantations. The forces on the cochlea during the insertion can lead to this mechanical trauma but can be highly variable between subjects which is thought to be due to differing anatomy, namely of the scala tympani. This study presents a systematic investigation of the influence of different geometrical parameters of the scala tympani on the cochlear implant insertion force. The influence of these parameters on the insertion forces were determined by testing the forces within 3D-printed, optically transparent models of the scala tympani with geometric alterations. (2) Methods: Three-dimensional segmentations of the cochlea were characterised using a custom MATLAB script which parametrised the scala tympani model, procedurally altered the key shape parameters (e.g., the volume, vertical trajectory, curvature, and cross-sectional area), and generated 3D printable models that were printed using a digital light processing 3D printer. The printed models were then attached to a custom insertion setup that measured the insertion forces on the cochlear implant and the scala tympani model during a controlled robotic insertion. (3) Results: It was determined 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. A Capstan-based model of the CI insertion forces was developed and matched well to the data acquired. (4) Conclusion: By using accurate 3D-printed models of the scala tympani with geometrical alterations, it was possible to demonstrate the insensitivity of the insertion forces to the size and shape of the scala tympani, after controlling for the angular insertion depth. This supports the Capstan model of the cochlear implant insertion force which predicts an exponential growth of the frictional force with an angular insertion depth. This concludes that the angular insertion depth, rather than the length of the CI inserted, should be the major consideration when evaluating the insertion force and associated mechanical trauma caused by cochlear implant insertion.

Incomplete Partition Type II Cochlear Malformations: Delineating the Three-Dimensional Structure from Digitized Human Histopathological Specimens

“…Furthermore, a shorter array may limit trauma to abnormally located peripheral processes and smaller ST cross-sectional area beyond one-full turn. Insertion beyond the latter may result in larger insertion forces, with associated trauma and inflammatory sequelae (40). To determine the impact of array length, future work should investigate the IP-II frequency map, as reported by Stakhovskaya et al (9).…”

Models of Cochlea Used in Cochlear Implant Research: A Review

“…Some 3D printing techniques such as stereolithography (SLA, see Fig. 4b) [18,39,54,89,108], digital light processing (DLP) [61], and polyjet printing (PJP) [24,89] have been already exploited for the fabrication of the ST models with much better accuracy (<40 μm [61]). SLA and DLP are photopolymer-based technologies that use ultraviolet light to cure resin (liquid plastics) into 3D prints.…”

“…The cochlea has a characteristic spiral geometry which flares out from a typical Archimedean spiral at the base. This has been characterised by either a piecewise function [ 24 , 157 ], which separately describes the base and apex or a continuous double exponential function [ 16 , 61 ]. Additionally, a function to determine the height of the cochlea is highly dependent on the reference frame used.…”

“…Each cochlear shape category is unique and, in theory, could require slightly different approaches in terms of insertion site and angle to minimise trauma. However, recent evidence shows a relative independence of the overall friction force experienced by the implant to the cochlea shape [ 61 ], although shape may still play a role in the local stresses and trajectories of the implant, and where this force is concentrated.

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“…resistivity) of the cochlear bone and its contained fluids (see Table 3 ), this would greatly help in understanding current spread during cochlear electric stimulation. Having artificial models can vary significantly increase the number of repeated experiments that can be performed in a standardised model [ 61 , 88 ], as opposed to biological tissues, and thus the robustness of any conclusions; these models are mechanically more durable than cadaveric specimens and do not degrade quickly over time. Furthermore, the use of fixed cadaveric tissues may alter the electrical and mechanical properties of the sample, and the availability of fresh cadavers is often limited.…”