Comparing linear and non-linear models to estimate the appropriate cochlear implant electrode array length-are current methods precise enough?

Abstract

Purpose: In cochlear implantation with flexible lateral wall electrode arrays, a cochlear coverage (CC) range between 70% and 80% is considered ideal for optimal speech perception. To achieve this CC, the cochlear implant (CI) electrode array has to be chosen according to the individual cochlear duct length (CDL). Here, we mathematically analyzed the suitability of different flexible lateral wall electrode array lengths covering between 70% and 80% of the CDL.

Methods: In a retrospective cross-sectional study preoperative high-resolution computed tomography (HRCT) from patients undergoing cochlear implantation was investigated. The CDL was estimated using an otosurgical planning software and the CI electrode array lengths covering 70-80% of the CDL was calculated using (i) linear and (ii) non-linear models.

Results: The analysis of 120 HRCT data sets showed significantly different model-dependent CDL. Significant differences between the CC of 70% assessed from linear and non-linear models (mean difference: 2.5 mm, p < 0.001) and the CC of 80% assessed from linear and non-linear models (mean difference: 1.5 mm, p < 0.001) were found. In up to 25% of the patients none of the existing flexible lateral wall electrode arrays fit into this range. In 59 cases (49,2%) the models did not agree on the suitable electrode arrays.

Conclusions: The CC varies depending on the underlying CDL approximation, which critically influences electrode array choice. Based on the literature, we hypothesize that the non-linear method systematically overestimates the CC and may lead to rather too short electrode array choices. Future studies need to assess the accuracy of the individual mathematical models.

Keywords: Angular insertion depth prediction; Insertion angle.

Fig. 1

Violin plot showing the distribution of cochlear duct lengths (CDL). CDL assessed at the lateral wall (CDL_LW) and at the organ of corti (CDL_OC). The shape of the “violin” represents a 90° rotated smoothed probability density plot of the data at different values, the horizontal lines indicate the 25% quartile, median and 75% quartile

Fig. 2

Individual cochlear duct lengths (CDL) and suitability for specific electrode array lengths. CDL assessed at the lateral wall (A) or at the organ of corti (B) for individual cochleae (represented by bars). The light grey section of the bar indicates a linear insertion depth between 70% and 80% cochlear coverage (CC) based on linear (A) and non-linear (B) models. Dotted lines indicate the linear insertion depth of a 26 mm, a 28 mm and a 31.5 mm electrode array. Below, color-coded bars indicate the suitability of specific electrode array lengths based on a 70% and 80% CC (red: not suited, green: suited). C Agreement between linear and non-linear model of suitable electrode array lengths (green: both models estimate electrode array as suitable; yellow: models do not agree on suitability of estimated electrode array; red: both models estimate electrode array as not suitable)

Fig. 3

Scatterplot showing the individually calculated electrode array lengths. Significant differences between a cochlear coverage (CC) of 70% assessed from the cochlear duct length (CDL) assessed at the lateral wall (CDL_LW) (CDL_LID-70%) and an angular insertion depth (AID) of 630° assessed from the CDL assessed at the organ of corti (CDL_OC) (CDL_AID-630°) (mean difference: 2.5; p < 0.01) (A), between a cochlear coverage (CC) of 80% assessed from the cochlear duct length (CDL) assessed at the lateral wall (CDL_LW) (CDL_LID-80%) and an angular insertion depth (AID) of 720° assessed from the CDL assessed at the organ of corti (CDL_OC) (CDL_AID-720°) (mean difference: 1.5; p < 0.01) (B) as well as between a cochlear coverage (CC) of 80% assessed from the cochlear duct length (CDL) assessed at the lateral wall (CDL_LW) (CDL_LID-80%) and an angular insertion depth (AID) of 630° assessed from the CDL assessed at the organ of corti (CDL_OC) (CDL_AID-630°) (mean difference: 1.2; p < 0.01) (C) are shown. Bar indicates mean, whiskers indicate standard deviation