Characterization of the human helicotrema: implications for cochlear duct length and frequency mapping

Abstract

Background: Despite significant anatomical variation amongst patients, cochlear implant frequency-mapping has traditionally followed a patient-independent approach. Basilar membrane (BM) length is required for patient-specific frequency-mapping, however cochlear duct length (CDL) measurements generally extend to the apical tip of the entire cochlea or have no clearly defined end-point. By characterizing the length between the end of the BM and the apical tip of the entire cochlea (helicotrema length), current CDL models can be corrected to obtain the appropriate BM length. Synchrotron radiation phase-contrast imaging has made this analysis possible due to the soft-tissue contrast through the entire cochlear apex.

Methods: Helicotrema linear length and helicotrema angular length measurements were performed on synchrotron radiation phase-contrast imaging data of 14 cadaveric human cochleae. On a sub-set of six samples, the CDL to the apical tip of the entire cochlea (CDL_TIP) and the BM length (CDL_BM) were determined. Regression analysis was performed to assess the relationship between CDL_TIP and CDL_BM.

Results: The mean helicotrema linear length and helicotrema angular length values were 1.6 ± 0.9 mm and 67.8 ± 37.9 degrees, respectively. Regression analysis revealed the following relationship between CDL_TIP and CDL_BM: CDL_BM = 0.88(CDL_TIP) + 3.71 (R² = 0.995).

Conclusion: This is the first known study to characterize the length of the helicotrema in the context of CDL measurements. It was determined that the distance between the end of the BM and the tip of the entire cochlea is clinically consequential. A relationship was determined that can predict the BM length of an individual patient based on their respective CDL measured to the apical tip of the cochlea.

Keywords: Basilar membrane; Cochlear apex; Cochlear duct length; Cochlear implant; Frequency mapping; Helicotrema; Helicotrema size; Synchrotron radiation.

Fig. 1

a Illustration of the fiducials placed along the LW from the end of the BM to the apical tip of the cochlea (orange) on a 3D model of the middle and apical turns. The fiducial placed at the modiolar axis is additionally visible (blue). ϴ is a visual representation of the HAL. b A cross-sectional image slice from SR-PCI data. The BM is visible and annotated. When progressing apically from this slice, the BM collapses towards the middle turn and quickly reaches its end-point. c A cross-sectional slice after the BM end-point. Fiducials are seen along the LW at the approximate level of the BM end-point (orange), and at the modiolar axis location (blue). LW denotes lateral wall; HAL denotes helicotrema angular length; SR-PCI denotes synchrotron radiation phase-contrast imaging; BM denotes basilar membrane

Fig. 2

3D volume rendering of the cochlear apex. Fiducials (orange) are placed along the LW from the end of the BM to the tip of the cochlea. The BM is coloured in green, Reissner’s membrane is coloured in red, and the spiral ligament is coloured in blue. LW denotes lateral wall; BM denotes basilar membrane

Fig. 3

Plot displaying the regression result between HLL and HAL. Individual pairs of HLL and HAL values are plotted as points, and the linear curve of best fit is illustrated in blue. HLL denotes helicotrema linear length; HAL denotes helicotrema angular length

Fig. 4

Plot displaying the regression result between CDL_TIP and CDL_BM. Individual pairs of CDL_TIP and CDL_BM values are plotted as points, and the linear curve of best fit is illustrated in blue. CDL_TIP denotes length measured to the apical tip of the cochlea; CDL_BM denotes BM length