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Introduction: In cochlear implant surgery, the insertion of the electrode array (EA) into the cochlea is critical as its implementation can influence the preservation of residual hearing. Insertion tests are the primary method for basic research on parameters influencing the insertion and help to further improve the design of EAs and surgical techniques. With automated insertion devices close to clinical application, a consensus on optimal insertion parameters is needed, which requires reliable testing methods. Moreover, the limited availability of EAs needs to be considered. We propose a test setup that provides high repeatability and flexibility for various research questions. Methods: Design requirements for multiple types of experiments such as variability of insertion speed or trajectory as well as cochlear geometry guided the computer aided design of the test bench. Moreover, repeated insertions with the same EA were supposed to be possible. To evaluate its functionality, insertion tests into a 3D printed cochlea model were performed and recorded. Results: The central components of the test bench are a linear actuator driving the EA and a goniometer changing the orientation of the target - a cochlea model or a specimen. A force sensor can be mounted below the target to measure forces in its frame of reference. The experimental results show high reproducibility of insertion forces for recurring trajectories with a single EA. Conclusion: The test bench enables reproducible insertion tests with a high number of repetitions and reduced EA usage. This allows a more detailed investigation of broadly discussed influences on the insertion such as the insertion speed or trajectory as well as cochlear geometry and can thereby drive future EA development.

Atraumatic insertions of electrode arrays (EA) into the cochlea aim to preserve natural structures and residual hearing. However, there is a limit as to how smooth and slow a surgeon can insert an EA. As a potential solution, we recently presented a tool (cochlea hydro drive, CHD) that makes use of an infusion pump to prompt and control the desired, continuous and very slow (< 1 mm/s) forward movement for such insertions. The present work further describes the onset, delay and cessation of the hydraulic actuation in response to different start and stop mechanisms, to better understand the safety of its application for cochlear implant surgery. Methods: Our previously designed tool was used to perform insertions of an EA into an artificial scala tympani model. The prototype is designed to hold an EA, which is then actuated by a standard infusion pump programmed to operate at 0.4 mm/ and 0.1 mm/s. A tubing system between the CHD and the pump includes a three-way valve. Ten insertions were operated using the functions of the pump and ten using the valve. Results: From the programmed start to the actual movement, we observed a larger average delay using the pump’s start function (5 s at 0.4 mm/s; 17 s at 0.1 mm/s) vs. opening the valve (< 0.7 s for both velocities). Moreover, the average cessation of movement with the valve closure was almost immediate (0.7 s for both velocities; this corresponds to < 0.1 mm with the slower tested velocity), as opposed to 60- 80 s delay when using the pump’s stop function. Conclusion: The use of a 3-way valve facilitates motion cessation to the high accuracy level required for cochlear implant surgery. These promising findings support future clinical translation of our tool.

Cochlear implants include an electrode array (EA) which needs to be inserted into the cochlea. Insertion tests using artificial cochlear models (ACM) or ex vivo specimens are widely used methods during EA development to characterize EA design properties, including insertion forces. Measured forces are directly linked to the orientation of the cochlear lumen with respect to the insertion axis of the test bench. While desired insertion directions in ACM experiments can be predefined by design, specimens are individually shaped and the cochlear lumen is embedded invisibly. Therefore, a new method for accurate, individual specimen positioning is required. A key element of the proposed method is a customizable pose setting adapter (PSA) used to adjust the specimen’s fine positioning. After rigid fixation of the specimen to a holder featuring spherical registration markers and subsequent cone beam computed tomography the desired insertion direction is planned. The planned data is used to calculate the individual shape of the PSA. Finally, the PSA is 3D printed and mounted between force sensor and specimen holder to correctly align the specimen to the test bench’s insertion axis. All necessary hard- and software have been developed including the specimen holder, a software for registration and trajectory planning, and a custom Matlab script whose output drives a parametric CAD file of the PSA. Positioning accuracy was determined in a first trial using 10 virtual trajectories and was found to be 0.23 ± 0.12 mm and 0.38 ± 0.17°. The presented stereotactic positioning procedure enables high repeatability in future ex vivo insertion experiments due to accurate, image-guided control of the insertion direction.

Introduction: Recent investigations focused on the optimization of atraumatic cochlear implant surgery have highlighted the relevance of the electrode array (EA) insertion trajectory. This is particularly studied in the context of minimally-invasive “keyhole” and robotic-assisted approaches, e.g. to avoid injuring structures inside and outside the cochlea. However, little is known about the natural, manual movements and trajectory followed during the insertion process. The present work illustrates the orientation changes within the trajectory a surgeon follows during insertions of EAs into a human cadaveric cochlea. Methods: An EA insertion tool equipped with a gyroscope was developed in our laboratory. During the insertion trials, the gyroscope captures the tool’s spatial orientation. A human head specimen and a single EA were used to perform insertions into a cochlea. A cochlear implant surgeon performed all insertion trials. The recorded orientations were compared to the initial orientation upon cochlea entry to assess the surgeon’s range of motion by calculating the angle between orientation vectors. Results: Fifteen EA insertions were performed with a median maximal deviation from the initial orientation of 7.2° (5.3 -11.1°) across trials. The largest orientation changes were seen towards the last half of each insertion trial. A negative relationship between degree of axis change and number of insertion trial was observed (r = -0.5). Conclusion: Manual EA insertions into a cadaveric cochlea revealed an insertion trajectory with maximum orientation changes of approximately < 10° degrees. The observed trend on decreasing range of motion with increasing number of insertion trials may be attributed to surgeon’s familiarization with the insertion trajectory for this specific specimen but other contributing factors (e.g. EA softening) need to be further elucidated with several EAs. Future evaluations can help determine if this orientation change is influenced by surgeon expertise.

Hearing restoration using a cochlear implant requires a surgical access to the inner ear. In order to enhance patient safety, reduce trauma, and shorten the patient’s time under anaesthesia current research focusses on minimally invasive cochlear implantation surgery by drilling only a single bore hole. This demands a highly accurate surgical assistance device to guide the drill along a predetermined trajectory planned in patient’s image data. In this study a recently developed surgical targeting system was evaluated for the first time in a human cadaver trial. After screwing a reference frame on a temporal bone specimen and imaging of both, a trajectory through the facial recess was planned in order to reach the middle ear. Based on this plan a patient specific surgical template including a linear guide for the surgical drill was composed utilizing bone cement. After the hardening of the bone cement the surgical template was mounted on top of the reference frame. The drilling could be performed as previously planned without harming facial nerve and chorda tympani. The deviation of the actual drill hole to the planned trajectory was 0.17 mm at the level of the facial recess. The minimal distance of the drill hole to the facial nerve was 0.59 mm. This proof-of-concept study demonstrates the feasibility of performing the access to the middle ear in a minimally invasive manner using the mouldable surgical targeting system. The presented process allows the patient specific individualization of a drill guide under sterile conditions. This might facilitate its integration into clinical routine.

The bend and free recovery (BFR) test according to ASTM F2082 is a standard method to determine the transition temperatures of Nitinol shape memory alloys (SMAs). Unfortunately, this standard method is limited to SMA wires which are straight in its trained shape. Thus, the standard BFR test is not suitable for thermomechanical characterization of curved Nitinol SMA wires which should serve as actuators in cochlear implants in future. We developed a modified BFR measurement setup to determine the active austenite finish (AF) temperature of these very thin wires (Ø100 μm). The active AF temperature specifies the completion of the shape recovery upon heating. A parametric study of the measurement setup was carried out to investigate the influence of the heating rate on the observed active AF temperature and to verify the repeatability of the measurement setup. First, the curved wire was straightened in a cold water bath before inserting it into a water bath that is gradually heated from 5 °C to 45 °C. The shape change of the previously straightened wire was then recorded throughout the experiment using a digital microscope. Five different heating rates were employed: 0.25 K/min, 0.33 K/min, 0.5 K/min, 1 K/min as well as an unregulated maximum heating rate achievable of approximately 1.5 K/min. Furthermore, an investigation on the test-retest reliability was performed with three wires by repeating the experiment ten times with each wire. The results of this study revealed no influence of the heating rate on the thermomechanical response of the wires. Based on data from this study, a regulated heating rate of 1 K/min is suggested for future investigations, as this reduces the duration of the measurement from four hours to less than an hour. The values obtained from each wire through the test-retest reliability investigation showed a standard deviation of 1.9 K, 1.1 K and 2.1 K respectively. Our developed measurement setup demonstrates appropriate repeatability of the measurements.

For patients suffering from profound hearing loss or deafness still having respectable residual hearing in the low frequency range, the combination of a hearing aid with a cochlear implant results in the best quality of hearing perception (EAS – electric acoustic stimulation). In order to optimize EAS, ongoing research focusses on the integration of these stimuli in a single implant device. Within this study, the performance of piezoelectric actuators, particularly the dual actuator stimulation, in a scaled uncoiled test rig was investigated.

Restoration of hearing is a demanding surgical task which requires the insertion of a cochlear implant electrode array into the inner ear while preserving the delicate basilar membrane inside the cochlea for an atraumatic insertion. Already shortly after the first clinical success with early versions of cochlear implants the desire for a controlled insertion of the electrode array arose. Such a steerable electrode should be in its shape adaptable to the individual path of the helical inner ear in order to avoid any contact between the implant and the surrounding tissue. This article provides a short overview of concepts and actuator mechanisms investigated in the past and present with the objective of developing a steerable electrode array for an individualized insertion process. Although none of these concepts has reached clinical implementation, there are promising experimental results indicating that insertion forces can be reduced up to 60% compared to straight and not steerable electrodes. Finally, related research topics are listed which require considerable further improvements until steerable electrodes will reach clinical applicability.

The aim of this study was to investigate the applicability of tubular manipulators as an actuator mechanism for intracochlear positioning of the electrode array (EA) of a cochlear implant (CI). This is motivated by the vision of an atraumatic insertion of the EA into the inner ear (cochlea) without any damage to the intracochlear structures in combination with a well-defined final position. To realize this, an actuator mechanism is required which allows consideration of the patient-specific anatomy. We propose a tubular manipulator for this task. It consists of three concentric tubes: A straight outer tube serves as a guiding sleeve to enter the inner ear (cochlea) and two additional telescoping, superelastic, helically precurved tubes. By selecting helical tube parameters of both tubes prior insertion, a patient-specific curling behaviour of the tubular manipulator can be achieved. For preliminary investigation, segmentation and skeletonization of 5 human scala tympani were performed to determine their centrelines. These centrelines were considered as individual ideal insertion paths. An optimization algorithm was developed to identify suitable tube set parameters (curvature, diameter, length, torsion, stiffness) as well as configuration parameters (translation and rotation of the 2 inner tubes). Different error values describing the deviation of the shape of the tubes with respect to the insertion path were used to quantify the optimization results. In all cases tube set parameters for a final position within the cochlea were found, while keeping the maximum error below 1mm. These preliminary results are promising in terms of the potential applicability of tubular manipulators for positioning auditory prosthesis inside the scala tympani of the inner ear.