Variation in human cochlear dimension must be considered when selecting a patient-suitable electrode array for cochlear implantation. A promising way of cochlear duct length (CDL) prediction utilizes statistical properties of variations in cochlear morphology. In this work, CDL values estimated by statistically derived equations were validated using three-dimensional measurements in micro-CT data sets of seven human temporal bones with implanted electrode arrays. Further, the lateral wall length (LWL) was assessed manually and compared to the prediction of Escudé's equation. Comparison showed good congruency of the measured and predicted CDL and LWL at one turn length (basal turn). Deviations of about 5 % were observed in CDL at 1.5 turn lengths, as well in LWL at 1.5 and 2 turn lengths. Results suggest that CDL prediction based on a single radiographic measurement of the cochlea could support surgeons in electrode array selection, but further investigation with increased sample size is necessary.
diameter. One cochlea was damaged during this preparation step. Electrode array insertion depth angles were found to be 606°±79° (n=7) [10].
Samples were scanned using a μCT device (μCT 40, SCANCO Medical AG, Brüttisellen, Switzerland) using a 70 kVp tube potential and 114 μA tube current. Determined by the size of the samples, a voxel size of 18 x 18 x 18 μm3 was obtained. Thereafter, the cochlea was segmented using Amira 5 visualization software (VSG, Burlington, MA, USA) starting with a region growing algorithm. Subsequently, manual correction of imaging artifacts caused by the implanted electrode arrays (exponential edge-gradient effect) was performed.
Next, a 3D surface model was generated and a zero reference angle plane [11] intersecting the modiolar axis and the center of the RW was manually aligned as seen in fig. 1. The longest distance from the RW to the lateral wall of the basal turn was then measured in this plane (distance ‘A’, fig. 1). Further, the lateral wall length (LWL) was measured manually along the surface of the cochlea, following the outermost points of the cochlear turns. Starting from the zero reference angle plane, the LWL was obtained at 1 TL (360°), 1.5 TL (540°) and 2 TL (720°), as shown in fig. 2. For CDL measurement, a 3rd order spline (500 samples per turn) was fitted in the center of the segmented electrode array (as an approximation to the position of the organ of Corti). In case of array bending in the proximal part of the basal turn, the spline was aligned with respect to the lateral wall course (fig. 3). CDL at 2 TL was determined using overlaid μCT slices in order to locate the basilar membrane position if the electrode array was not inserted deeper than 1.5 TL.
Based on the measured distance ‘A’ (mm), the CDL (mm) was computed at 1, 1.5 and 2 TL using following set of equations: Further, the LWL (mm) was estimated by applying Escudé’s equation (insertion depth angle θ in degrees) [8]: 3
Lengths of distance ‘A’ were found to range within 8.86-9.77 mm, with a mean of 9.35±0.32 mm (n=7). At 1 TL, measured values showed correspondence with the lengths estimated by eqs. (1) and (4). Measurements revealed a deviation to the estimated CDL of about 1-1.5 mm and 0.5 mm at 1.5 TL and 2 TL, respectively. LWL measurements were found to deviate about 1 mm at 1.5 TL and 1.5 mm at 2 TL (fig. 4). In this work, three-dimensional in-vitro measurements of the CDL and LWL of human cochleae were obtained and compared to values estimated by statistically derived equations. Although samples were taken from four human heads only, a high variability of the cochlear size, reflected by ‘A’, was observed. The cochlear size variation of the investigated samples (9.35±0.32 mm, n=7) are within the range reported in literature (6.8-10.3 mm, mean 8.55±0.57 mm, n=104 [6]).
The CDL was more or less congruent at 1 TL and 2 TL, whereas a deviation of approximately 5% was observed at 1.5 TL. This may be a result of either the measurement method (overestimation of the CDL at 1.5 TL), the impact of anatomical variations due to a small sample size or a deviation within the derived equations. Deviations of about 5% were observed for the LWL at 1.5 TL and 2 TL, which may again be caused by the measurement method or anatomical variation. Nevertheless, a trend of higher difference between the predicted and measured values for increasing angles can be seen.
These preliminary results suggest that a preoperative estimation of the CDL based on the measurement of a single value (distance ‘A’) is a practical approach for patient-specific electrode array selection in both cases with and without residual hearing. According to literature, the CDL of human cochleae ranges from 25 to 36 mm, with an average value of 31.5 mm (n=95) [1,2]. Currently there is no single free-fitting electrode array available which covers the whole CDL range. A short electrode array inserted into a cochlea with a long CDL is not sufficient in stimulating low frequency regions. Conversely, intracochlear damage may occur if a long electrode array is inserted into a cochlea with short CDL. Further, the risk of occurrence of extracochlear contacts is increased when electrode arrays longer than the CDL are inserted. In this context, findings of this study may help surgeons in reliably selecting a suitable electrode array length from portfolios provided by different CI manufacturers and ultimately increase the patient’s benefit.
In order to improve the method, further investigations with an increased sample size and statistical analysis are carried out. Moreover, the measurement accuracy of the presented method and the influence of lower imaging resolution of clinical data are evaluated. 5
This work was financially supported by the European Union's Seventh Framework Programme (FP7/2007-2013) under HEAR-EU grant agreement n° 304857. Electrode arrays and the software tool for CDL estimation were provided by Med-El Corporation. 6