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.

Keywords: 3D printing; cochlear implant; insertion forces; micro-CT; scala tympani.

Figure 1

Computational workflow and the custom insertion setup. (A) A cadaveric specimen was micro-CT scanned and segmented using Stradview software. The 3D geometry was then characterised in terms of its spiral trajectory and cross-sections using a custom MATLAB script which was used to manipulate the geometry according to each experiment. This generated a 3D file of the ST which was prepared for 3D printing using another custom MATLAB script. (B) Insertion setup consists of transparent 3D-printed ST models affixed with screws to a six-axis force sensor placed on a six-axis positioning stage and a CI secured with 3D printed adapters to a one-axis force sensor placed on a stepper motor that moved the CI into the ST model at a defined speed. The setup measures the force on the ST model in the direction of implantation (x), perpendicular in the left and right directions (y), and vertically (z), shown on the right. The one-axis force sensor measured the reaction force on the implant.

Figure 2

(A) Implantation of a CI in ST models with manufacturer-recommended post-processing (left) and an additional acrylic coating (right). This demonstrates the clear imaging of the CI electrodes, vertically and horizontally, to investigate CI positioning and validate the angular insertion depth. (B) Quantification of 3D-printing accuracy using nominal–actual analysis demonstrating the 3D map (left) and histogram of surface deviation (right) compared to the original CAD file used for printing.

Figure 3

Influence of ST overall size on insertion forces. A volumetric scaling of the ST model was conducted to produce a “large” (110% the volume of the “original”) and ”small” (90% of the “original” model) models. Whereas a significant difference is seen when plotting force exerted on the CI (mean-solid line; shaded area-standard deviation) with respect to insertion distance (left), these force profiles overlap when plotting relative to angular insertion depth of the round window (middle). The exponential coefficients (right) of the Capstan model fitting to the force on the implant with respect to the angular insertion depth illustrate no significant difference (p > 0.05) between the insertion force profiles. Boxplot: red line-median, box-interquartile range (n = 10 replicates combined over N = 2 implants).

Figure 4

Manipulation of the vertical trajectory of the ST and its influence on CI insertion forces. (A) Representation of the 3D geometry (left) and the centreline z-component relative to the basal plane (right) of the different ST models, only varying in the vertical trajectories of their centrelines; these consist of the original geometry (cut off at 500° for comparison), a flat model where all centrelines are on the same horizontal plane, and two artificial non-planarities (NP1 and NP2). (B) Insertion force on the implant with respect to angular insertion depth (left; solid line-mean, shaded area-standard deviation) and respective fitting of the Capstan model exponential coefficient (middle; red line-median, box-interquartile range) for models with different vertical trajectories. Data represent n = 10 replicates per condition over N = 2 implants. Only NP2 showed a statistically significant difference (p values of 0.024, 0.010, and 0.020 compared to “original”, “flat”, and NP1 models, respectively). Vertical forces, along the z-axis, on the ST model due to CI insertion (right). Note, altering the vertical trajectory made little difference to the angular insertion depth per mm of length.

Figure 5

Influence of ST curvature on the CI insertion force. (A) Representations of the shape manipulation of the ST models. (B) Insertion force experienced by the implant for models with altered curvatures with respect to insertion distance (left) and angular insertion depth (middle; solid line-mean, shaded area-standard deviation). Statistical analysis of the exponential coefficients, which are acquired by fitting the insertion force profiles, shows no statistical significance between the models (p > 0.05; boxplot: red line-median, box-interquartile range). Data represent n = 10 replicates per condition combined over N = 2 implants.

Figure 6

Influence of ST cross-sectional area on insertion forces. (A) Manipulation of the ST geometry demonstrating the tapered cross-sectional area of the “flat” model versus the “flat-uniform cross-sectional area” model where the cross-section at 1 mm depth from the round window was used along the same horizontal trajectory. (B) Force exerted on the CI during insertion as a function of insertion distance (left) and angular insertion depth (middle; solid line-mean, shaded area-standard deviation) along with the exponential coefficient of the fitting of the force profiles (right; red line-median, box-interquartile range) demonstrates no significant effect (p > 0.05) of the uniform cross-section area on insertion force. Data represent n = 10 replicates per condition combined over N = 2 implants.