A method for accurate and reproducible specimen alignment for insertion tests of cochlear implant electrode arrays

Abstract

Purpose: The trajectory along which the cochlear implant electrode array is inserted influences the insertion forces and the probability for intracochlear trauma. Controlling the trajectory is especially relevant for reproducible conditions in electrode insertion tests. Using ex vivo cochlear specimens, manual alignment of the invisibly embedded cochlea is imprecise and hardly reproducible. The aim of this study was to develop a method for creating a 3D printable pose setting adapter to align a specimen along a desired trajectory toward an insertion axis.

Methods: Planning points of the desired trajectory into the cochlea were set using CBCT images. A new custom-made algorithm processed these points for automated calculation of a pose setting adapter. Its shape ensures coaxial positioning of the planned trajectory to both the force sensor measuring direction and the insertion axis. The performance of the approach was evaluated by dissecting and aligning 15 porcine cochlear specimens of which four were subsequently used for automated electrode insertions.

Results: The pose setting adapter could easily be integrated into an insertion force test setup. Its calculation and 3D printing was possible in all 15 cases. Compared to planning data, a mean positioning accuracy of 0.21 ± 0.10 mm at the level of the round window and a mean angular accuracy of 0.43° ± 0.21° were measured. After alignment, four specimens were used for electrode insertions, demonstrating the practical applicability of our method.

Conclusion: In this work, we present a new method, which enables automated calculation and creation of a ready-to-print pose setting adapter for alignment of cochlear specimens in insertion test setups. The approach is characterized by a high level of accuracy and reproducibility in controlling the insertion trajectory. Therefore, it enables a higher degree of standardization in force measurement when performing ex vivo insertion tests and thereby improves reliability in electrode testing.

Keywords: 3D printing; Electrode insertion tests; Porcine cochlea; Pose setting adapter; Specimen alignment.

Fig. 1

Basic idea of the pose setting method; left: mismatching trajectory into the cochlea and insertion axis; right: alignment of the axes using a pose setting adapter

Fig. 2

Flowchart of the general workflow for creating an individual PSA

Fig. 3

Dissection process: a fresh and never-frozen porcine half skull with visible location of the cochlea without further dissection; b close-up of the otic capsule within the skull and c dissected otic capsule including the cochlea with visible round window (RW) and oval window (OW)

Fig. 4

Test setup design: a exploded views of the RSC CAD model with its components; b evaluation of 3D-printed LEGO connection using varying clearances and angles (left) and by comparing different printing orientations on the build plate of the 3D printer (right) and c design of the test setup

Fig. 5

Trajectory planning: a Cochlear specimen glued inside a removable dish and clamped on top of the RSC; b CBCT data of the specimen on top of the RSC; c sphere fitting into the registration markers and d planning points for PSA calculation

Fig. 6

PSA modeling: a visualization of the different coordinate systems and the transformation matrices; b mathematical alphaShape model of the PSA with its sub-models ‘connector sub-model’ [I], ‘cylinder sub-model’ [II] and ‘base sub-model’ [III] and c 3D-printed PSA

Fig. 7

Insertion study: a photo of the EA insertion and b CBCT scan of the porcine specimens with inserted EA

Fig. 8

Measured target points at the level of the round window