An Accurate and Individualized Preoperative Estimation Method for the Linear Insertion Depth of Cochlear Implant Electrode Arrays Based on Computed Tomography

OBJECTIVES

Cochlear implantation or auditory brainstem implantation is currently the only accepted method for improving severe or profound sensorineural hearing loss. The length of the electrodes implanted during cochlear implantation is closely related to the degree of hearing improvement of hearing after the surgery. We aimed to explore new methods to accurately estimate the electrode array (EA) linear insertion depth based on computed tomography (CT) images prior surgery, which could help surgeons select the appropriate EA length for each patient.

DESIGN

Previous studies estimated the linear insertion depth by measuring the length of the lateral wall of the cochlea rather than the electrode's path in the cochlea duct. Here, we determined the actual position of the EA on the CT image after cochlear surgery in order to predict the path of the EA, and the length of the predicted EA path was measured by the contouring technique (CoT) to estimate the linear insertion depth of the EA. Because CoT can only measure the length of the estimated EA path on a two-dimensional plane, we further modified the measurement by weighting the height of the cochlea and the length of the EA tail (the length of the last stimulating electrode to the end, which cannot be displayed on the CT image), which we termed the modified CoT + height + tail (MCHT) measurement.

RESULTS

Based on our established method, MCHT could reduce the error to the submillimeter range (0.67 ± 0.37 mm) when estimating the linear insertion depth of various kinds of EAs compared with the actual implant length. The correlation coefficient between the linear insertion depth as predicted by MCHT and the actual was 0.958. The linear insertion depth estimated by this method was more accurate than that estimated using the classical CoT technique ( R = 0.442) and using the modified Escudé's method ( R = 0.585).

CONCLUSIONS

MCHT is a method based on CT images that can accurately predict the linear insertion depth of cochlear implants preoperatively. This is the first report that we are aware of a method for predicting linear insertion depth before cochlear implantation with only submillimeter errors and that is tailored to different types of EAs.

Variation in cochlear size: A systematic review

BACKGROUND

Advancements in imaging and implantation technology have invited reexamination of the classic teaching that the human cochlea maintains uniform size across demographics. Yet, studies yield conflicting results and relatively few broad systematic reviews have examined cochlear size variation.

PURPOSE

The purpose of this study is to quantify cochlear variability across eight different measurement categories and suggest normative values and ranges for each with consideration of disease state and gender where possible.

METHODS

A systematic search was conducted up to October 1, 2022, using the search terms "Cochlea/anatomy and histology"[Mesh]) AND 'size'" with filters "Humans" and "English" across three databases (PubMed, CINAHL, Medline). Further inclusion criteria involved reporting of numerical measurements in any of the eight included categories.

RESULTS

Of the 625 articles manually reviewed for relevance by title and abstract, 91 were selected for full-text review and 33 met all eligibility criteria. 5,791 cochleae were included and weighted means and ranges were calculated: "A" value (defined as the distance from the round window, through the modiolus, to the oppsite lateral wall) = 9.23 mm (8.43-10.4 mm, n = 2559); cochlear duct length (CDL) = 33.04 mm (range 28.2-36.4 mm, n = 2252); cochlear height = 5.14 mm (2.8-6.9 mm, n = 2098); the basal turn lumen diameter = 2.09 mm (1.7-2.2 mm, n = 617); "B" value (defined as perpendicular to "A" value and in the same plane) = 6.52 mm (5.73-6.9 mm, n = 908); width of the basal turn = 6.4 mm (6.22-6.86 mm, n = 356); height of the basal turn = 1.96 mm (1.77-2.56 mm, n = 204); length of the basal turn 21.87 mm (21.03-22.5 mm, n = 384).

CONCLUSION

A notable size range exists across the eight different cochlear parameters considered and we provide normative values for each measurement. Females tend to have smaller CDL and "A" value than males and the sensorineural hearing loss patients had smaller CDL and "A" value but larger cochlear height than the general population.

Adult porcine (Sus scrofa) derived inner ear cells: Characteristics in in-vitro cultures

There is a need for an animal model that closely parallels human cochlea gestational development. This study aims to document porcine inner ear anatomy, and in vitro porcine derived inner ear cell culture characteristics. Twenty-four temporal bone were harvested from 12 adult pigs (Sus scrofa). Six were formalin fixed and their maximal diameters were measured. The cochlea duct length was determined by the insertion length of a Nucleus 22 cochlear implant in two bones. Four formalin fixed bones were sectioned for histology. Cochlear and vestibular tissues were harvested from non-fixed bones, cultured and characterized at different passages (P). Gene and protein expression of multipotent stem/progenitor (Nestin and Sox2), inner ear hair (Myosin VIIa, Prestin) and supporting (Cytokeratin 18 and Vimentin) cell markers were determined. The porcine cochlea was a 3.5 turn spiral. There was a separate vestibular compartment. The cochlear mean maximal diameter and height was 7.99 and 3.77 mm, respectively. Sphere forming cells were identified on phase-contrast microscopy. The relative mRNA expression levels of KRT18, MYO7A and SLC26A5 were significantly positively correlated in cochlear cultures; and MYO7A and SLC26A5; SOX2 and KRT18; NES and SLC26A5 genes were positively correlated in vestibular cultures (p = .037, Spearman correlation [τ] = .900). Inner ear sensory and stem cell characteristics persist in passaged porcine inner ear cells. Further work is required to establish the usefulness of porcine inner ear cell cultures to the study of human inner ear disorders.

CT imaging-based approaches to cochlear duct length estimation-a human temporal bone study

Objectives

Knowledge about cochlear duct length (CDL) may assist electrode choice in cochlear implantation (CI). However, no gold standard for clinical applicable estimation of CDL exists. The aim of this study is (1) to determine the most reliable radiological imaging method and imaging processing software for measuring CDL from clinical routine imaging and (2) to accurately predict the insertion depth of the CI electrode.

Methods

Twenty human temporal bones were examined using different sectional imaging techniques (high-resolution computed tomography [HRCT] and cone beam computed tomography [CBCT]). CDL was measured using three methods: length estimation using (1) a dedicated preclinical 3D reconstruction software, (2) the established A-value method, and (3) a clinically approved otosurgical planning software. Temporal bones were implanted with a 31.5-mm CI electrode and measurements were compared to a reference based on the CI electrode insertion angle measured by radiographs in Stenvers projection (CDL_reference).

Results

A mean cochlear coverage of 74% (SD 7.4%) was found. The CDL_reference showed significant differences to each other method (p < 0.001). The strongest correlation to the CDL_reference was found for the otosurgical planning software-based method obtained from HRCT (CDL_SW-HRCT; r = 0.87, p < 0.001) and from CBCT (CDL_SW-CBCT; r = 0.76, p < 0.001). Overall, CDL was underestimated by each applied method. The inter-rater reliability was fair for the CDL estimation based on 3D reconstruction from CBCT (CDL_3D-CBCT; intra-class correlation coefficient [ICC] = 0.43), good for CDL estimation based on 3D reconstruction from HRCT (CDL_3D-HRCT; ICC = 0.71), poor for CDL estimation based on the A-value method from HRCT (CDL_A-HRCT; ICC = 0.29), and excellent for CDL estimation based on the A-value method from CBCT (CDL_A-CBCT; ICC = 0.87) as well as for the CDL_SW-HRCT (ICC = 0.94), CDL_SW-CBCT (ICC = 0.94) and CDL_reference (ICC = 0.87).

Conclusions

All approaches would have led to an electrode choice of rather too short electrodes. Concerning treatment decisions based on CDL measurements, the otosurgical planning software-based method has to be recommended. The best inter-rater reliability was found for CDL_A-CBCT, for CDL_SW-HRCT, for CDL_SW-CBCT, and for CDL_reference.

Key Points

• Clinically applicable calculations using high-resolution CT and cone beam CT underestimate the cochlear size.

• Ten percent of cochlear duct length need to be added to current calculations in order to predict the postoperative CI electrode position.

• The clinically approved otosurgical planning software-based method software is the most suitable to estimate the cochlear duct length and shows an excellent inter-rater reliability.

Synchrotron Radiation-Based Reconstruction of the Human Spiral Ganglion: Implications for Cochlear Implantation

OBJECTIVE

To three-dimensionally reconstruct Rosenthal's canal (RC) housing the human spiral ganglion (SG) using synchrotron radiation phase-contrast imaging (SR-PCI). Straight cochlear implant electrode arrays were inserted to better comprehend the electro-cochlear interface in cochlear implantation (CI).

DESIGN

SR-PCI was used to reconstruct the human cochlea with and without cadaveric CI. Twenty-eight cochleae were volume rendered, of which 12 underwent cadaveric CI with a straight electrode via the round window (RW). Data were input into the 3D Slicer software program and anatomical structures were modeled using a threshold paint tool.

RESULTS

The human RC and SG were reproduced three-dimensionally with artefact-free imaging of electrode arrays. The anatomy of the SG and its relationship to the sensory organ (Corti) and soft and bony structures were assessed.

CONCLUSIONS

SR-PCI and computer-based three-dimensional reconstructions demonstrated the relationships among implanted electrodes, angular insertion depths, and the SG for the first time in intact, unstained, and nondecalcified specimens. This information can be used to assess stimulation strategies and future electrode designs, as well as create place-frequency maps of the SG for optimal stimulation strategies of the human auditory nerve in CI.

Mapping Cochlear Duct Length to Electrically Evoked Compound Action Potentials in Cochlear Implantation

OBJECTIVE

Objective measurements may assist in indicating cochlear implants and in predicting outcomes of cochlear implantation surgery. Using electrically evoked compound action potentials (ECAP), information about the function of the auditory nerve can be obtained by analyzing responses to electrical stimulation transmitted and derived by the recording electrode. The aim of this study was to determine whether ECAP characteristics differ depending on the stimulated intracochlear region and the size of the cochlea.

STUDY DESIGN

Retrospective cohort study.

SETTING

University Medical center, tertiary academic referral center.

PATIENTS

Patients undergoing cochlear implant surgery between 2015 and 2018.

INTERVENTION

Cochlear implantation with FLEXsoft electrode arrays (length 31.5 mm, 12 stimulating channels).

MAIN OUTCOME MEASURES

The cochlear duct length (CDL) and the cochlear coverage (CC) were measured using a new computed tomography-based software and correlated to the postoperative speech performance. Additionally, ECAP were measured and associated to the CDL.

RESULTS

A total of 59 ears of 53 cochlear implant users with a mean age of 63.6 (SD 14.9) years were included. The mean estimated CDL was 35.0 (SD 2.2) mm. The mean CC was 90.3% (SD 5.5%). A total of 4,873 ECAP were measured. A statistically significant, moderate, negative correlation between the ECAP slope and the site of stimulation was found (r = -0.29, 95% confidence interval: -0.32 to -0.27, p < 0.0001). No correlation between the CC and the speech performance was found (r = -0.08, 95% confidence interval: -0.33 to 0.18 p = 0.52).

CONCLUSION

ECAP slopes seem to be a reliable tool to identify the electrode's position inside the cochlea and also showed correlations to the anatomy of the patient. A combination of objective measurements such as anatomical parameters and ECAPs are helpful to assist the postoperative fitting and are promising tools to improve patient care.

Identification of anatomic risk factors for scalar translocation in cochlear implant patients

AIM

Microanatomical evaluation of cochlear implant (CI) patients to identify anatomical risk factors for a scalar translocation.

METHODS

CI patients with both a regular scala tympani spiralization (group A) and a scalar translocation (group B) were identified via postoperative flat-detector computed tomography (FD-CT). Then, the corresponding preoperative multislice computed tomography (MS-CT) and postoperative FD-CT datasets were assessed: First, the cochleae were separated in 6 segments of 45° each. Next, quantitative (cochlea height, length, depth, cochlear duct diameter [CD] per segment; percentual tapering of the CD per segment named cochlear geometry index [CGI]) and qualitative (identifiability of the CI model; CI-integrity; intracochlear array position) parameters were evaluated and compared for both groups. Receiver-operating-characteristics (ROC) analysis was performed for the CGI.

RESULTS

In total, 40 preoperative MS-CT and postoperative FD-CT datasets (n_A=20; n_B=20) were analysed. Model "CI 512" was successfully identified and CI-integrity has been confirmed in all cases. Quantitative analysis showed a significant difference of both the CD at 0° (CD_A0°= 2.06± 0.23mm; CD_B0°= 2.19±0.18mm; p_0°= 0.04) and the CGI of the first segment (CGI_A0°-45°= 18.87±6.04%; CGI_B0°-45°= 28.89±8.58%; p_0°-45°= 0.0001). For all other 5 cochlear segments there was no significant difference of CD and CGI; there was no significant difference of external cochlea diameters. The area under the curve (AUC) of the CGI_0-45° was 0.864 with 24.50° as the optimal cut-off value to discriminate patients with a scala tympani spiralization and a scalar translocation. CGI_0-45° of> 24.50° allowed the correct identification of 85% of patients with a scalar translocation.

CONCLUSION

CI insertion trauma is associated with a significantly higher narrowing of the proximal basal cochlea turn (BCT). The CGI as percentual tapering of the BCT turned out as reliable, clinically applicable parameter for identification of patients with an increased risk for a scalar translocation.

Computed Tomography-Based Measurements of the Cochlear Duct: Implications for Cochlear Implant Pitch Tuning

OBJECTIVES

To determine the sources of variability for cochlear duct length (CDL) measurements for the purposes of fine-tuning cochlear implants (CI) and to propose a set of standardized landmarks for computed tomography (CT) pitch mapping.

DESIGN

This was a retrospective cohort study involving 21 CI users at a tertiary referral center. The intervention involved flat-panel CT image acquisition and secondary reconstructions of CIs in vivo. The main outcome measures were CDL measurements, CI electrode localization measurements, and frequency calculations.

RESULTS

Direct CT-based measurements of CI and intracochlear landmarks are methodologically valid, with a percentage of error of 1.0% ± 0.9%. Round window (RW) position markers (anterior edge, center, or posterior edge) and bony canal wall localization markers (medial edge, duct center, or lateral edge) significantly impact CDL calculations [F(2, 78) = 9.9, p < 0.001 and F(2, 78) = 1806, p < 0.001, respectively]. These pitch distortions could be as large as 11 semitones. When using predefined anatomical landmarks, there was still a difference between researchers [F(2, 78) = 12.5; p < 0.001], but the average variability of electrode location was reduced to differences of 1.6 semitones (from 11 semitones.

CONCLUSIONS

A lack of standardization regarding RW and bony canal wall landmarks results in great CDL measurement variability and distorted pitch map calculations. We propose using the posterior edge of the RW and lateral bony wall as standardized anatomical parameters for CDL calculations in CI users to improve pitch map calculations. More accurate and precise pitch maps may improve CI-associated pitch outcomes.

Cochlear duct length and cochlear distance on preoperative CT: imaging markers for estimating insertion depth angle of cochlear implant electrode

OBJECTIVES

Preoperative estimation of the insertion depth angle of cochlear implant (CI) electrodes is essential for surgical planning. The purpose of this study was to determine the cochlear size using preoperative CT and to investigate the correlation between cochlear size and insertion depth angle in morphologically normal cochlea.

METHODS

Thirty-five children who underwent CI were included in this study. Cochlear duct length (CDL) and the diameter of the cochlear basal turn (distance A/B) on preoperative CT and the insertion depth angle of the CI electrode on postoperative radiographs were independently measured by two readers. Correlation between cochlear size and insertion depth angle was evaluated. Interobserver agreement was calculated using the intraclass correlation coefficient (ICC).

RESULTS

The mean CDL, distance A, and distance B of 70 ears were 36.20 ± 1.57 mm, 8.67 ± 0.42 mm, and 5.73 ± 0.32 mm, respectively. The mean insertion depth angle was 431.45 ± 38.42°. Interobserver agreements of CDL, distance A/B, and insertion depth angle were fair to excellent (ICC 0.864, 0.862, 0.529, and 0.958, respectively). Distance A (r = - 0.7643) and distance B (r = - 0.7118) showed a negative correlation with insertion depth angle, respectively (p < 0.0001). However, the correlation between CDL and insertion depth angle was not statistically significant (r = - 0.2333, p > 0.05).

CONCLUSIONS

The CDL and cochlear distance can be reliably obtained from preoperative CT. Distance A can be used as a predictive marker for estimating insertion depth angle during CI surgery.

Reprint of Corrosion casting of the temporal bone: Review of the technique

The temporal bone has the most sophisticated anatomy of the whole skeleton. Its study is a challenge for students and surgeons. An inverse model of the visually obscured cavities and canals can facilitate better three-dimensional orientation and investigation. This can be made by means of corrosion casting, which is an established technique first documented on the temporal bone at the beginning of the nineteenth century. The prepared specimens are suitable not only for teaching purposes but also for research on the fascinating topography of the osseous labyrinth and the whole temporal bone. Many important studies on temporal bone anatomy are based on this technique. An extensive review of the pertinent literature is provided in relation to each method available.

Direct measurement of cochlear parameters for automatic calculation of the cochlear duct length

BACKGROUND

Cochlear morphology and cochlear duct length (CDL) play important roles in the selection of appropriate electrodes. Cochlear parameters such as diameter (A value) and width (B value) are used as inputs for calculating the CDL. Current measurements of these parameters are inefficient and time consuming. Recently developed otological planning software (OTOPLAN) allows surgeons to directly measure these parameters and then automatically calculate the CDL.

OBJECTIVES

The primary objective was to validate this new software for measuring the cochlear parameters and CDL. The secondary aim was to investigate the correlation between each cochlear parameter with the calculated CDL.

DESIGN

Retrospective.

SETTINGS

Ear specialist hospital.

PATIENTS AND METHODS

The measurement of cochlear diameter (A value) was chosen as the validation parameter. To do this, the A value was measured by a neurotologist on the new OTOPLAN planning software and was validated to the one measured on the currently used DICOM viewer. Upon the validation of the OTOPLAN software, the other two cochlear parameters, namely width (B value) and height (H value) were measured, and CDL was automatically calculated. Finally, the correlation of all parameters with the CDL was statistically analyzed.

MAIN OUTCOME MEASURES

Validation of OTOPLAN and CDL estimation.

SAMPLE SIZE

88 ears.

RESULTS

There was no significant difference between the A-value measured on the DICOM viewing software and that on the new planning software by the two independent neurotologists (P=.27). Both A-and B-values showed a high positive correlation to the CDL. However, the B-value showed a stronger correlation to the CDL than the A-value (r=0.63 for A, and r=0.96 for B).

CONCLUSION

The direct measurement of cochlea parameters and automatic calculation of the CDL could improve the efficiency of clinical workflow and make otology surgeons more independent. Moreover, the cochlear width (B) has a strong correlation to the CDL. Thus, we suggest using the combination of A and B to accurately estimate the CDL rather than using only one.

LIMITATIONS

Single center and small sample size.

CONFLICT OF INTEREST

None. No relationship with manufacturers.

Corrosion casting of the temporal bone: Review of the technique

The temporal bone has the most sophisticated anatomy of the whole skeleton. Its study is a challenge for students and surgeons. An inverse model of the visually obscured cavities and canals can facilitate better three-dimensional orientation and investigation. This can be made by means of corrosion casting, which is an established technique first documented on the temporal bone at the beginning of the nineteenth century. The prepared specimens are suitable not only for teaching purposes but also for research on the fascinating topography of the osseous labyrinth and the whole temporal bone. Many important studies on temporal bone anatomy are based on this technique. An extensive review of the pertinent literature is provided in relation to each method available.

CT-scan contouring technique allows for direct and reliable measurements of the cochlear duct length: implication in cochlear implantation with straight electrode-arrays

OBJECTIVES

The advent of hybrid electro-acoustic implants requires precise positioning of the electrode-array (EA) within the cochlea. The cochlea size, that is, the length of the cochlear scala tympani, is often indirectly estimated from distance A by Escudé's method. This technique has been confirmed by anatomical studies, in a bunch of cadaveric specimens, but it is not yet widely established in the field of computed tomography (CT). We compared cochlear duct length obtained by Escudé's method to those directly acquired on CT images.

MATERIALS AND METHODS

The lengths of cochlear scala tympani were directly measured on CT scans by contouring the external cochlear wall (contouring technique-CoT). In fifteen patients implanted with a straight EA, the length of the EA and the measured length of the cochlea by the CoT were compared, to check the reliability of the CoT. Then, in 200 CT-scans, the length of the cochlear duct was measured by the CoT then compared to Escudé's method.

RESULTS

In the 200 CT-scans which served for cochlear length measurements, a significant variability between the cochleae were observed, as expected. At 360°, the correlation between the measurements of the length of the cochlear scala tympani between the two techniques differed, with a difference of 0.2 ± 0.7 mm at 360° (extreme: 2 mm; p < 0.001) and 2.2 ± 1.2 mm at 540° (extreme: 5.6 mm; p < 0.001).

CONCLUSION

The CoT can predict with accuracy the length of EA-insertion depth, more precisely than estimation methods such as Escudé's.

Variations in cochlear duct shape revealed on clinical CT images with an automatic tracing method

Cochlear size and morphology vary greatly and may influence the course of a cochlear implant electrode array during insertion and its final intra-cochlear position. Detailed insight into these variations is valuable for characterizing each cochlea and offers the opportunity to study possible correlations with surgical or speech perception outcomes. This study presents an automatic tracing method to assess individual cochlear duct shapes from clinical CT images. On pre-operative CT scans of 479 inner ears the cochlear walls were discriminated by interpolating voxel intensities along radial and perpendicular lines within multiplanar reconstructions at 1 degree intervals from the round window. In all 479 cochleas, the outer wall could be traced automatically up to 720 degrees. The inner wall and floor of the scala tympani in 192 cochleas. The shape of the cochlear walls were modelled using a logarithmic spiral function including an offset value. The vertical trajectories of the scala tympani exhibited a non-monotonous spiral slope with specific regions at risk for CI-related insertion trauma, and three slope categories could be distinguished. This presented automatic tracing method allows the detailed description of cochlear morphology and can be used for both individual and large cohort evaluation of cochlear implant patients.

The cochlea in skull base surgery: an anatomy study

OBJECTIVE The object of this study was to examine the relationships of the cochlea as a guide for avoiding both cochlear damage with loss of hearing in middle fossa approaches and injury to adjacent structures in approaches directed through the cochlea. METHODS Twenty adult cadaveric middle fossae were examined using magnifications of ×3 to ×40. RESULTS The cochlea sits below the floor of the middle fossa in the area between and below the labyrinthine segment of the facial nerve and greater petrosal nerve (GPN) and adjacent to the lateral genu of the petrous carotid. Approximately one-third of the cochlea extends below the medial edge of the labyrinthine segment of the facial nerve, geniculate ganglion, and proximal part of the GPN. The medial part of the basal and middle turns are the parts at greatest risk in drilling the floor of the middle fossa to expose the nerves in middle fossa approaches to the internal acoustic meatus and in anterior petrosectomy approaches. Resection of the cochlea is used selectively in extending approaches through the mastoid toward the lateral edge of the clivus and front of the brainstem. CONCLUSIONS An understanding of the location and relationships of the cochlea will reduce the likelihood of cochlear damage with hearing loss in approaches directed through the middle fossa and reduce the incidence of injury to adjacent structures in approaches directed through the cochlea.

CT and MR imaging cochlear distance measurements may predict cochlear implant length required for a 360 degrees insertion

BACKGROUND AND PURPOSE

A preoperative prediction of the 360 degrees point insertion depth would aid the planning of electric acoustic stimulation (EAS) implantation. The purpose of this study was to establish whether the distance between the round window and the opposite cochlear wall on CT or MR imaging may be used to predict the length of a cochlear implant electrode array required to be inserted to the 360 degrees point of the basal turn.

MATERIALS AND METHODS

CT and MR imaging data were studied in 19 patients undergoing cochlear implantation. Distances were measured between the round window and the opposite outer cochlear wall on an oblique paracoronal reformatted image. Adjusted distance measurements were applied to a spiral function to estimate the length of an electrode array extending between the round window entry point and the 360 degrees point. This was compared with measurements of implant length to this insertion depth on postoperative CT.

RESULTS

Intraobserver reproducibility for each of the 2 observers was r = 0.85/0.55 for CT and r = 0.87/0.67 for MR imaging. Interobserver reproducibility was r = 0.68 for CT and r = 0.84 for MR imaging. There was no bias between CT and MR imaging measurements, with a mean difference of less than 0.1 mm. CT and MR imaging estimates markedly correlated with the actual length of the electrode array extending to the 360 degrees insertion depth (SD between the estimated and actual length was 0.84 mm for CT and 0.87 mm for MR imaging).

CONCLUSIONS

CT and MR imaging measures of cochlear distance (CD) were used to predict insertion depths to 360 degrees , and these were markedly concordant with the actual length of the electrode array required to reach this point. MR imaging measurements were more precise and similar in accuracy to those obtained with CT.

Role of electrode placement as a contributor to variability in cochlear implant outcomes

HYPOTHESIS

Suboptimal cochlear implant (CI) electrode array placement may reduce presentation of coded information to the central nervous system and, consequently, limit speech recognition.

BACKGROUND

Generally, mean speech reception scores for CI recipients are similar across different CI systems, yet large outcome variation is observed among recipients implanted with the same device. These observations suggest significant recipient-dependent factors influence speech reception performance. This study examines electrode array insertion depth and scalar placement as recipient-dependent factors affecting outcome.

METHODS

Scalar location and depth of insertion of intracochlear electrodes were measured in 14 patients implanted with Advanced Bionics electrode arrays and whose word recognition scores varied broadly. Electrode position was measured using computed tomographic images of the cochlea and correlated with stable monosyllabic word recognition scores.

RESULTS

Electrode placement, primarily in terms of depth of insertion and scala tympani versus scala vestibuli location, varies widely across subjects. Lower outcome scores are associated with greater insertion depth and greater number of contacts being located in scala vestibuli. Three patterns of scalar placement are observed suggesting variability in insertion dynamics arising from surgical technique.

CONCLUSION

A significant portion of variability in word recognition scores across a broad range of performance levels of CI subjects is explained by variability in scalar location and insertion depth of the electrode array. We suggest that this variability in electrode placement can be reduced and average speech reception improved by better selection of cochleostomy sites, revised insertion approaches, and control of insertion depth during surgical placement of the array.

Frequency map for the human cochlear spiral ganglion: implications for cochlear implants

The goals of this study were to derive a frequency-position function for the human cochlear spiral ganglion (SG) to correlate represented frequency along the organ of Corti (OC) to location along the SG, to determine the range of individual variability, and to calculate an "average" frequency map (based on the trajectories of the dendrites of the SG cells). For both OC and SG frequency maps, a potentially important limitation is that accurate estimates of cochlear place frequency based upon the Greenwood function require knowledge of the total OC or SG length, which cannot be determined in most temporal bone and imaging studies. Therefore, an additional goal of this study was to evaluate a simple metric, basal coil diameter that might be utilized to estimate OC and SG length. Cadaver cochleae (n = 9) were fixed <24 h postmortem, stained with osmium tetroxide, microdissected, decalcified briefly, embedded in epoxy resin, and examined in surface preparations. In digital images, the OC and SG were measured, and the radial nerve fiber trajectories were traced to define a series of frequency-matched coordinates along the two structures. Images of the cochlear turns were reconstructed and measurements of basal turn diameter were made and correlated with OC and SG measurements. The data obtained provide a mathematical function for relating represented frequency along the OC to that of the SG. Results showed that whereas the distance along the OC that corresponds to a critical bandwidth is assumed to be constant throughout the cochlea, estimated critical band distance in the SG varies significantly along the spiral. Additional findings suggest that measurements of basal coil diameter in preoperative images may allow prediction of OC/SG length and estimation of the insertion depth required to reach specific angles of rotation and frequencies. Results also indicate that OC and SG percentage length expressed as a function of rotation angle from the round window is fairly constant across subjects. The implications of these findings for the design and surgical insertion of cochlear implants are discussed.

The Size of the Cochlea and Predictions of Insertion Depth Angles for Cochlear Implant Electrodes

AIMS

To establish normative data on the size of the basal turn of the cochlea using high-resolution computed tomography of the temporal bone in adults and children. To determine whether final insertion depth angle for a perimodiolar cochlear implant electrode varies according to cochlear size.

METHODS

Forty-two patients screened for cochlear anomaly using computed tomography were randomly selected from patients with otologic disease. Reconstruction of the full basal turn was performed for both ears using a 1.0-mm layer, minimum intensity projection. The largest distance from the round window to the lateral wall (distance A) and the perpendicular distance (B) were measured. Distances were averaged between ears for each individual. In addition, 15 patients were implanted with the Nucleus 24 Contour Advance electrode array using a linear insertion depth of either 17 mm (n = 9) or 19 mm (n = 6). Postoperative X-rays were analyzed using the method of Xu et al. [Am J Otol 2000;21:49-56] to obtain the insertion depth angles for individual electrodes.

RESULTS

Mean distance A was 9.23 mm (SD = 0.53, range 7.9-10.8 mm). Perpendicular distance B was significantly correlated with distance A (r2 = 0.57, p < 0.001). The mean difference in insertion depth angle between the 17 and 19 mm groups was 80 degrees . A statistically significant correlation (r2 = 0.51) was found between distance A and the insertion depth angle for the 17 mm group.

CONCLUSIONS

The cochlear size measure distance A was repeatable to within the resolution of the high-resolution computed tomography image data. The basal turn of the normally formed cochlea is variable in size. These variations in size would produce >5.0 mm variation in the length of the lateral wall to the point consistent with an insertion depth angle of 360 degrees . Cochlear size influenced final insertion depth angles obtained for the perimodiolar Nucleus 24 Contour Advance electrode.

Inner ear fluid volumes and the resolving power of magnetic resonance imaging: can it differentiate endolymphatic structures?

Magnetic resonance imaging (MRI) can accurately recognize minute volumes as small as 1 mm3. The volumes of the utricle and saccule of the inner ear are within the resolving power of MRI, but these structures cannot be recognized because the endolymph and perilymph signals are identical. To clarify the interpretation and description of inner ear structures on MRI, we measured and calculated the volumes of the perilymphatic and endolymphatic spaces of the human ear. We found the total volume of the bony labyrinth to be approximately 192.5 mm3 (endolymph, 34.0 mm3; perilymph, 158.5 mm3).

Dimensions of the human vestibular and tympanic scalae

The cochlear scalae provide a practical access route for the insertion of cochlear implant electrodes. A microanatomical study was carried out on 25 human temporal bones obtained from cadavers. These bones were dissected with the aid of an operation microscope, in which their perilymphatic spaces were filled with coloured latex and further prepared in a formalin stain. Each of the rubber moulds was removed from the osseous matrix using standard otosurgical equipment, and subsequently cut into 1 mm segments. The height and width of the vestibular and tympanic scalae were measured. The results, presented in diagrams, indicate that the vestibular scala is less prominent than the tympanic scala in the basic and middle coil of the cochlea and in the upper coil, they display greater dimensions which could serve as a place for electrode insertion in cochlear implant procedures. In addition, the vestibular and tympanic scalae present alternate dominance in their width and height as corroborated by the calculated coefficients. The results obtained in this study supplement our knowledge of the anatomy of the cochlea thus far lacking a full investigation of the scalae, and could serve as a basis for other studies dealing with the physiology of the organs of hearing.

Comparative review of the human bony labyrinth

The bony labyrinth inside the petrous part of the temporal bone houses the organs of hearing and balance. Being functionally linked with sensory control of body movements and located in a part of the basicranium that is closely associated with the brain, this structure is of great interest in the study of human evolutionary history. However, few aspects of its morphology have been studied in nonhuman primates. This review compares the bony labyrinth of humans with that of the great apes and 37 other primate species based on data newly acquired with computed tomography combined with previous descriptions. With body mass taken into account, consistent differences are found between the size of the semicircular canals in humans, the great apes, and other primates. In particular, the arcs of the anterior and posterior canals are larger in humans than in the African apes. The functional implications of semicircular canal dimensions for registering angular head motion are evaluated in relation to locomotor behavior. Biophysical models, comparative studies, and some neurophysiological experiments all support a link between semicircular canal size and agility, or more specifically the frequency contents of natural head movements, but the evidence on the exact nature of this link is ambiguous. It is concluded that any link between the characteristic dimensions of the human canals and locomotion will be more complex than a simple association with the broad categories of quadrupedal vs. bipedal behavior. The functionally important planar orientations of the semicircular canals are similar in humans and the African apes as well as in many other species. In contrast, other aspects of the human labyrinth differ markedly in shape, following a pattern that seems to reflect the characteristic architecture of the human basicranium. Indeed, it is found that labyrinthine and basicranial shape are interspecifically correlated in the sample, and in most respects the human morphology is consistent with the general trend among primate species. Differences in brain growth and development are proposed as the predominant factor underlying both the unique shape of the human labyrinth as well as the interspecific labyrintho-basicranial correlations.