Drilling accuracy evaluation of a mouldable surgical targeting system for minimally invasive access to anatomic targets in the temporal bone.

PURPOSE: Minimally invasive cochlear implant surgery using a micro-stereotactic surgical targeting system with on-site moulding of the template aims for a reliable, less experience-dependent access to the inner ear under maximal reduction of trauma to anatomic structures. We present an accuracy evaluation of our system in ex-vivo testing. METHODS: Eleven drilling experiments were performed on four cadaveric temporal bone specimens. The process involved preoperative imaging after affixing the reference frame to the skull, planning of a safe trajectory preserving relevant anatomical structures, customization of the surgical template, execution of the guided drilling and postoperative imaging for determination of the drilling accuracy. Deviation between the drilled and desired trajectories was measured at different depths. RESULTS: All drilling experiments were successfully performed. Other than purposely sacrificing the chorda tympani in one experiment, no other relevant anatomy, such as facial nerve, chorda tympani, ossicles or external auditory canal were harmed. Deviation between the desired and achieved path was found to be 0.25 ± 0.16 mm at skulls' surface and 0.51 ± 0.35 mm at the target level. The closest distance of the drilled trajectories' outer circumference to the facial nerve was 0.44 mm. CONCLUSIONS: We demonstrated the usability for drilling to the middle ear on human cadaveric specimen in a pre-clinical setting. Accuracy proved to be suitable for many applications such as procedures within the field of image-guided neurosurgery. Promising approaches to reach sufficient submillimetre accuracy for CI surgery have been outlined.

The Use of Clinically Measurable Cochlear Parameters in Cochlear Implant Surgery as Indicators for Size, Shape, and Orientation of the Scala Tympani.

OBJECTIVES: (1) To assess variations of the human intracochlear anatomy and quantify factors which might be relevant for cochlear implantation (CI) regarding surgical technique and electrode design. (2) Search for correlations of these factors with clinically assessable measurements. DESIGN: Human temporal bone study with micro computed tomography (µCT) data and analysis of intracochlear geometrical variations: µCT data of 15 fresh human temporal bones was generated, and the intracochlear lumina scala tympani (ST) and scala vestibuli were manually segmented using custom software specifically designed for accurate cochlear segmentation. The corresponding datasets were processed yielding 15 detailed, three-dimensional cochlear models which were investigated in terms of the scalae height, cross-sectional size, and rotation as well as the interrelation of these factors and correlations to others. RESULTS: The greatest anatomical variability was observed within the round window region of the cochlea (basal 45°), especially regarding the cross-sectional size of the ST and its orientation relative to the scala vestibuli, which were found to be correlated (p < 0.001). The cross-sectional height of the ST changes substantially for both increasing cochlear angles and lateral wall distances. Even small cochleae were found to contain enough space for all commercially available CI arrays. Significant correlations of individual intracochlear parameters to clinically assessable ones were found despite the small sample size. CONCLUSION: While there is generally enough space within the ST for CI, strong intracochlear anatomical variations could be observed highlighting the relevance of both soft surgical technique as well as a highly flexible and self-adapting cochlear implant electrode array design. Cochlear dimensions (especially at the round window) could potentially be used to indicate surgically challenging anatomies.

Common Audiological Functional Parameters (CAFPAs) for single patient cases: deriving statistical models from an expert-labelled data set.

Objective: Statistical knowledge about many patients could be exploited using machine learning to provide supporting information to otolaryngologists and other hearing health care professionals, but needs to be made accessible. The Common Audiological Functional Parameters (CAFPAs) were recently introduced for the purpose of integrating data from different databases by providing an abstract representation of audiological measurements. This paper aims at collecting expert labels for a sample database and to determine statistical models from the labelled data set.Design: By an expert survey, CAFPAs as well as labels for audiological findings and treatment recommendations were collected for patients from the database of Hörzentrum Oldenburg.Study sample: A total of 287 single patient cases were assessed by twelve highly experienced audiological experts.Results: The labelled data set was used to derive probability density functions for categories given by the expert labels. The collected data set is suitable for estimating training distributions due to realistic variability contained in data for different, distinct categories. Suitable distribution functions were determined. The derived training distributions were compared regarding different audiological questions.Conclusions: The method-expert survey, sorting data into categories, and determining training distributions - could be extended to other data sets, which could then be integrated via the CAFPAs and used in a classification task.

Common Audiological Functional Parameters (CAFPAs): statistical and compact representation of rehabilitative audiological classification based on expert knowledge.

OBJECTIVE: As a step towards objectifying audiological rehabilitation and providing comparability between different test batteries and clinics, the Common Audiological Functional Parameters (CAFPAs) were introduced as a common and abstract representation of audiological knowledge obtained from diagnostic tests. DESIGN: Relationships between CAFPAs as an intermediate representation between diagnostic tests and audiological findings, diagnoses and treatment recommendations (summarised as "diagnostic cases") were established by means of an expert survey. Expert knowledge was collected for 14 given categories covering different diagnostic cases. For each case, the experts were asked to indicate expected ranges of diagnostic test outcomes, as well as traffic light-encoded CAFPAs. STUDY SAMPLE: Eleven German experts in the field of audiological rehabilitation from Hanover and Oldenburg participated in the survey. RESULTS: Audiological findings or treatment recommendations could be distinguished by a statistical model derived from the experts' answers for CAFPAs as well as audiological tests. CONCLUSIONS: The CAFPAs serve as an abstract, comprehensive representation of audiological knowledge. If more detailed information on certain functional aspects of the auditory system is required, the CAFPAs indicate which information is missing. The statistical graphical representations for CAFPAs and audiological tests are suitable for audiological teaching material; they are universally applicable for real clinical databases.

Investigation of ultra-low insertion speeds in an inelastic artificial cochlear model using custom-made cochlear implant electrodes.

PURPOSE: Latest research on cochlear implantations focuses on hearing preservation during insertion of the implant's electrode array by reducing insertion trauma. One parameter which may influence trauma is insertion speed. The objective of this study was to extend the range of examined insertion speeds to include ultra-low velocities, being lower than manually feasible, and investigate whether these reduce insertion forces. METHODS: 24 custom-made cochlear implant test samples were fabricated and inserted into an artificial scala tympani model using 12 different insertion speeds while measuring the resulting insertion forces. Three commercially available slim straight electrode carriers were inserted using the same setup to analyze whether the results are comparable. RESULTS: Insertions of the test samples using high insertion speeds (2.0/2.8 mm/s) showed significantly higher insertion forces than insertions done with low insertion speeds (0.2 mm/s) or ultra-low insertion speeds (< 0.1 mm/s). The insertions with commercial slim straight electrode arrays showed significantly reduced insertion forces when using a low insertion speed as well. CONCLUSIONS: Slow insertions showed significantly reduced insertion forces. Insertion speeds which are lower than manually feasible showed even lower insertion forces.

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.

Impact of the round window membrane accessibility on hearing preservation in adult cochlear implantation.

This study was conducted to evaluate the effect of the round window membrane accessibility on the residual hearing after cochlear implantation surgery in adults. Moreover, the effects of the other demographics and intra-operative factors on the residual hearing loss have been evaluated. The hearing preservation cochlear implantation surgery was performed on 64 adults with residual hearing thresholds ≤80 dB at 250 and 500 Hz, who had referred to our tertiary academic center. All the patients underwent a standardized surgical approach with the same straight electrode inserted through the round window membrane. The hearing thresholds at 250, 500, and 1000 Hz were compared in pre-operative and 1 month postoperative pure-tone audiograms. The average hearing threshold shifts at these frequencies was used to evaluate the hearing preservation. The effects of the round window accessibility and other factors (including gender, age, side of the surgery, necessity of anterior-inferior drilling of the round window margin and average insertion speed) on hearing threshold shifts were analyzed. The mean low-frequency hearing threshold shift was found to be 17.5 dB for all the patients. The hearing preservation goal (threshold shifts ≤30 dB) was achieved in 58 patients. Among the evaluated parameters, only accessibility of the round window membrane could change the hearing threshold shifts significantly (p = 0.026), and was a predictor for the hearing loss (B coefficient = 7.5, p = 0.006). Incomplete accessibility of the round window membrane may be a predictor for increased hearing threshold shifts in short-term evaluations after cochlear implantation.

Does severity of cerebral MRI lesions in congenital CMV infection correlates with the outcome of cochlear implantation?

The objective is to investigate whether there is a correlation between the severity of typical brain lesions in congenital cytomegalovirus (cCMV) infection and cochlear implant (CI) outcome. The design of the study is a retrospective single-institutional chart review (2005-2015), performed in a tertiary academic referral center. 23 children with typical signs of cCMV infection on cerebral magnetic resonance imaging (MRI) and bilateral severe-to-profound sensorineural hearing loss were retrospectively evaluated. They were graded in three groups according to the severity of brain involvement. The average implantation age of the first CI is 1.8 years (range 0.6-5.8). Five patients were implanted unilaterally, 18 bilaterally. The average follow-up time after implantation was 3.3 years (range 0.6-6.9). Hearing performance was assessed using the Categories of Auditory Performance (CAP), and speech development was assessed using Speech Intelligibility Rating (SIR). The outcome in each group showed great variation. The majority of children achieved moderate-to-good auditory and speech rehabilitation. The children with severe MRI changes had comparatively better auditory than speech scores. There were children with good auditory performance (CAP ≥6) both in grades II and III, while poor performers (CAP ≤3) were encountered in each group. The severity of brain lesions on its own does not directly correlate with the outcome of cochlear implantation. Despite good retrospective diagnostic evidence of cCMV infection through MRI patterns, this has no predictive role for future hearing and speech rehabilitation.

Insertion forces and intracochlear trauma in temporal bone specimens implanted with a straight atraumatic electrode array.

The aim of the study was to evaluate insertion forces during manual insertion of a straight atraumatic electrode in human temporal bones, and post-implantation histologic evaluation of the samples to determine whether violation of intracochlear structures is related to insertion forces. In order to minimize intracochlear trauma and preserve residual hearing during cochlear implantation, knowledge of the insertion forces is necessary. Ten fresh frozen human temporal bones were prepared with canal wall down mastoidectomy. All samples were mounted on a one-axis force sensor. Insertion of a 16-mm straight atraumatic electrode was performed from different angles to induce "traumatic" insertion. Histologic evaluation was performed in order to evaluate intracochlear trauma. In 4 of 10 samples, dislocation of the electrode into scala vestibuli was observed. The mean insertion force for all 10 procedures was 0.003 ± 0.005 N. Insertion forces measured around the site of dislocation to scala vestibuli in 3 of 4 samples were significantly higher than insertion forces at the same location of the cochleae measured in samples without trauma (p < 0.04). Mean force during the whole insertion process of the straight atraumatic electrode is lower than reported by other studies using longer electrodes. Based on our study, insertion forces leading to basilar membrane trauma may be lower than the previously reported direct rupture forces.

Determination of optimal excitation patterns for local mechanical inner ear stimulation using a physiologically-based model.

Within the field of hearing prosthetics it is known that patients with sufficient residual hearing benefit from the simultaneous employment of hearing aid and cochlear implant. Several attempts have been proposed to combine the sources of the corresponding acoustic and electric stimuli in a single, implantable device. However, since only little is known about the effect of also applying the acoustic stimulus locally from within the inner ear, the current state of research lacks detailed knowledge on the optimal stimulation at the corresponding bionic interface. Within this manuscript, a simple but yet physiologically-based inner ear model is presented which was designed specifically for the analysis of local acoustic or mechanical inner ear stimulation. A detailed model analysis is performed showing that it is capable of mirroring the known mechanical phenomena of this particular stimulation approach. Using the model, it is demonstrated how amplitude and phase shift values of stimuli applied from within the inner ear should be chosen for optimal inner ear stimulation.