Relations Between Scalar Shift and Insertion Depth in Human Cochlear Implantation.

OBJECTIVE: The intracochlear position of an electrode array may influence the outcome after cochlear implantation. The design of the electrode array can increase the risk of trauma causing penetration of the basilar membrane or shift of the electrode array into the scala vestibuli. The aim of the present study was to identify a scalar shift after implantation of two different electrode arrays developed by one manufacturer. STUDY DESIGN: Retrospective analysis. SETTING: Tertiary referral center. PATIENTS AND INTERVENTION: Cochlear implant recipients implanted between 2010 and 2014 and receiving either a mid-scala (n = 30) or a perimodiolar (n = 30) electrode array. MAIN OUTCOME MEASURE: Occurrence of scalar shift in association with the electrode type. RESULTS: Scalar shift occurred in 26.7% (8 of 30) of the patients implanted with a perimodiolar electrode array and in 6.7% (2 of 30) of the patients implanted with the mid-scala electrode array. The mean insertion depth in the patients experiencing scalar shift after implantation of the mid-scala electrode was much deeper (21.59 ±â€Š0.34 mm) when compared with the mean insertion depth of the patients with scalar shift after implantation with a perimodiolar electrode array (17.85 ±â€Š2.19 mm). There tends to be a correlation between the cochlear length and the occurrence of a scalar shift. However, the number of patients with scalar shift in the mid-scala group is rather small. CONCLUSION: Based on the presented data, more patients implanted with a perimodiolar electrode array have a scalar shift when compared with the midscalar electrode array.

Cochlear helix and duct length identification - Evaluation of different curve fitting techniques.

OBJECTIVE: Within the field of cochlear implantation (CIs), the role of utilizing patient-specific cochlear anatomy for choosing the optimal implant electrode is becoming increasingly important. Unfortunately, performing detailed anatomical measurements of a cochlea using clinical imaging data is rather time consuming and hence difficult to implement into the clinical routine. In order to accelerate clinical cochlear anatomy evaluations, previously developed mathematical models can be adjusted to the patient-specific anatomy by measuring just a few overall cochlear dimensions. However, the accuracy of model-based cochlear anatomy estimations is unclear, and incorrect evaluations may lead to false conclusions regarding the suitability of specific implant electrodes. METHODS: Based on 10 cochleae, an error evaluation of various commonly used curve fitting approaches for cochlear shape and duct length approximation was conducted. Spline tracings of the cochlear contours were used as reference values for the various approximations. RESULTS: Parameterized average cochlear helix models and two of five analytical approaches were found to be suitable for reconstructing the cochlear helical shape and estimating its length. DISCUSSION: Spline curve reconstructions are the most accurate and reliable method for assessing patient-specific cochlear geometry, especially in the case of anatomical irregularities. The most accurate results within the group of model-based evaluations still resulted in mean overall cochlear length deviations of approximately 5%. CONCLUSION: Spline curve reconstructions appear to be the best option for anatomical diagnostics in clinical practice. Retrospective studies can be performed to further evaluate model-based evaluations.

On the accuracy of cochlear duct length measurement in computed tomographic images.

PURPOSE: Patient specific selection of cochlear implants would benefit from pre-operative knowledge of cochlear length. Several methods for its measurement or estimation have been described in literature. This study focused on the achievable accuracy in clinically available imaging. METHODS: Five simplified cochlea models milled into porcine bone were scanned in water using clinical cone beam computed tomography. Due to their well-known dimensions these phantoms served as gold standard for the length measurements. Each phantom was measured ten times using the custom software Comet. In addition, cochleae in ten image datasets taken indiscriminately from clinical routine were measured ten times each to test the precision under realistic conditions. The results were also compared to estimations based on the diameter of the basal turn (A value) as described in literature. RESULTS: Measurement accuracy of the phantoms' lengths was high (average error: - 0.2 mm; standard deviation: 0.3 mm). The pooled standard deviation for the measurements in clinical datasets was 0.6 mm. Errors resulted mainly from problems locating the helicotrema. The estimations differed on average - 1.7 to + 0.4 mm from the manual measurements and had standard deviations between 0.5 and 0.6 mm depending on the algorithm. CONCLUSIONS: The program Comet was successfully used to accurately measure the length of the cochlea models in clinically available imaging. The lower image quality of patient scans reduced the precision of the measurement. Estimations using the A value are a quicker alternative for averagely sized cochleae in cases where the lack of accuracy is tolerable.

Three-dimensional modeling of the cochlea by use of an arc fitting approach.

PURPOSE: A cochlea modeling approach is presented allowing for a user defined degree of geometry simplification which automatically adjusts to the patient specific anatomy. Model generation can be performed in a straightforward manner due to error estimation prior to the actual generation, thus minimizing modeling time. Therefore, the presented technique is well suited for a wide range of applications including finite element analyses where geometrical simplifications are often inevitable. METHODS: The method is presented for n=5 cochleae which were segmented using a custom software for increased accuracy. The linear basilar membrane cross sections are expanded to areas while the scalae contours are reconstructed by a predefined number of arc segments. Prior to model generation, geometrical errors are evaluated locally for each cross section as well as globally for the resulting models and their basal turn profiles. The final combination of all reconditioned features to a 3D volume is performed in Autodesk Inventor using the loft feature. RESULTS: Due to the volume generation based on cubic splines, low errors could be achieved even for low numbers of arc segments and provided cross sections, both of which correspond to a strong degree of model simplification. Model generation could be performed in a time efficient manner. CONCLUSION: The proposed simplification method was proven to be well suited for the helical cochlea geometry. The generated output data can be imported into commercial software tools for various analyses representing a time efficient way to create cochlea models optimally suited for the desired task.

Visualization, measurement and modelling of the cochlea using rotating midmodiolar slice planes.

PURPOSE: Cross-sectional visualization of anatomical structures in DICOM viewers is usually presented in parallel slices. For visualizing the inner ear, this concept is unfavourable due to the spiral shape of the cochlea. Radial slicing through its central axis (known as midmodiolar view) is advantageous. Therefore, a custom DICOM viewer was developed, which allows the visualization of the cochlea in a midmodiolar slice plane that rotates around the central axis of the cochlea, always cutting the latter radially. METHODS: The program was written in C++ using the open-source libraries ITK, VTK, GDCM and Qt. The rotation axis is defined by placing two points in the modiolus within a conventional slice visualization of the dataset. A midmodiolar visualization is calculated based on this axis. Scrolling the mouse wheel rotates slice plane around the axis, displaying midmodiolar slices at variable angles. Measurement options are provided as well as interactive placement of marker points whose coordinates can be exported for post-processing in other programs. RESULTS: The program can be used in multiple applications including the determination of cochlear dimensions, especially its length, and post-operative positions of cochlear implant (CI) electrode carriers. Computer-aided design models of the cochlea can be generated from exported marker points. CONCLUSION: The proposed DICOM viewer directly focuses on the needs of cochlear visualization, thus making it a valuable tool in CI related research. The ease of use facilitates future clinical use, e.g. for pre-operative selection of optimal CI electrode carrier length based on the patient's cochlear length.

Configuration optimization and experimental accuracy evaluation of a bone-attached, parallel robot for skull surgery.

PURPOSE: Minimally invasive cochlear implantation is a novel surgical technique which requires highly accurate guidance of a drilling tool along a trajectory from the mastoid surface toward the basal turn of the cochlea. The authors propose a passive, reconfigurable, parallel robot which can be directly attached to bone anchors implanted in a patient's skull, avoiding the need for surgical tracking systems. Prior to clinical trials, methods are necessary to patient specifically optimize the configuration of the mechanism with respect to accuracy and stability. Furthermore, the achievable accuracy has to be determined experimentally. METHODS: A comprehensive error model of the proposed mechanism is established, taking into account all relevant error sources identified in previous studies. Two optimization criteria to exploit the given task redundancy and reconfigurability of the passive robot are derived from the model. The achievable accuracy of the optimized robot configurations is first estimated with the help of a Monte Carlo simulation approach and finally evaluated in drilling experiments using synthetic temporal bone specimen. RESULTS: Experimental results demonstrate that the bone-attached mechanism exhibits a mean targeting accuracy of [Formula: see text] mm under realistic conditions. A systematic targeting error is observed, which indicates that accurate identification of the passive robot's kinematic parameters could further reduce deviations from planned drill trajectories. CONCLUSION: The accuracy of the proposed mechanism demonstrates its suitability for minimally invasive cochlear implantation. Future work will focus on further evaluation experiments on temporal bone specimen.

Mechanical characterization of bone anchors used with a bone-attached, parallel robot for skull surgery.

Bone-attached robots and microstereotactic frames, intended for deep brain stimulation and minimally invasive cochlear implantation, typically attach to a patient's skull via bone anchors. A rigid and reliable link between such devices and the skull is mandatory in order to fulfill the high accuracy demands of minimally invasive procedures while maintaining patient safety. In this paper, a method is presented to experimentally characterize the mechanical properties of the anchor-bone linkage. A custom-built universal testing machine is used to measure the pullout strength as well as the spring constants of bone anchors seated in four different bone substitutes as well as in human cranial bone. Furthermore, the angles at which forces act on the bone anchors are varied to simulate realistic conditions. Based on the experimental results, a substitute material that has mechanical properties similar to those of cranial bone is identified. The results further reveal that the pullout strength of the investigated anchor design is sufficient with respect to the proposed application. However, both the measured load capacity as well as the spring constants vary depending on the load angles. Based on these findings, an alternative bone anchor design is presented and experimentally validated. Furthermore, the results serve as a basis for stiffness simulation and optimization of bone-attached microstereotactic frames.

An experimental evaluation of loads occurring during guided drilling for cochlear implantation.

PURPOSE: During guided drilling for minimally invasive cochlear implantation and related applications, typically forces and torques act on the employed tool guides, which result from both the surgeon's interaction and the bone drilling process. Such loads propagate through the rigid mechanisms and result in deformations of compliant parts, which in turn affect the achievable accuracy. In this paper, the order of magnitude as well as the factors influencing such loads are studied experimentally to facilitate design and optimization of future drill guide prototypes. METHODS: The experimental setup to evaluate the occurring loads comprises two six degree of freedom force/torque sensors: one mounted between a manually operated, linearly guided drill handpiece and one below the specimens into which the drilling is carried out. This setup is used to analyze the influences of drilling tool geometry, spindle speed as well as experience of the operator on the resulting loads. RESULTS: The results reveal that using a spiral drill results in lower process loads compared with a surgical Lindemann mill. Moreover, in this study, an experienced surgeon applied lower interaction forces compared with untrained volunteers. The measured values further indicate that both the intraoperative handling of the bone-attached drill guide as well as the tool removal after completing the hole can be expected to cause temporary load peaks which exceed the values acquired during the drilling procedure itself. CONCLUSIONS: The results obtained using the proposed experimental setup serve as realistic design criteria with respect to the development of future drill guide prototypes. Furthermore, the given values can be used to parameterize simulations for profound stiffness analyses of existing mechanisms.

Temporal bone borehole accuracy for cochlear implantation influenced by drilling strategy: an in vitro study.

PURPOSE: Minimally invasive cochlear implantation is a surgical technique which requires drilling a canal from the mastoid surface toward the basal turn of the cochlea. The choice of an appropriate drilling strategy is hypothesized to have significant influence on the achievable targeting accuracy. Therefore, a method is presented to analyze the contribution of the drilling process and drilling tool to the targeting error isolated from other error sources. METHODS: The experimental setup to evaluate the borehole accuracy comprises a drill handpiece attached to a linear slide as well as a highly accurate coordinate measuring machine (CMM). Based on the specific requirements of the minimally invasive cochlear access, three drilling strategies, mainly characterized by different drill tools, are derived. The strategies are evaluated by drilling into synthetic temporal bone substitutes containing air-filled cavities to simulate mastoid cells. Deviations from the desired drill trajectories are determined based on measurements using the CMM. RESULTS: Using the experimental setup, a total of 144 holes were drilled for accuracy evaluation. Errors resulting from the drilling process depend on the specific geometry of the tool as well as the angle at which the drill contacts the bone surface. Furthermore, there is a risk of the drill bit deflecting due to synthetic mastoid cells. CONCLUSIONS: A single-flute gun drill combined with a pilot drill of the same diameter provided the best results for simulated minimally invasive cochlear implantation, based on an experimental method that may be used for testing further drilling process improvements.