Fig 1.
Left ear cochlear implantation was performed in both the stimulated (n = 6) and non-stimulated (n = 6) groups. Serial NRT and impedance measurements began on post-operative day 7 in both groups. Electric stimulation started in the stimulated group on post-operative day 7. After post-operative day 21 and when all electrodes lost functionality (impedance > 35kOhms), cochleae were harvested for 3D x-ray microscopy and histology.
Fig 2.
The electrode array assembly (A) and schematic (B) are pictured. The electrode assembly consisted of a 2.25 mm long and 0.15 mm wide half-banded three contact electrode intracochlear array tapering to a wider extracochlear helixed lead wire with silicone insulation along with two extracochlear electrodes connected to a transcutaneous 6-pin connector. Modified polypropylene hernia mesh is affixed strategically to enable subcutaneous fixation and stabilization of the implant.
Fig 3.
Cochlear implantation surgery.
(A) Standard bullostomy (outlined in red) with exposure of the round window (outlined in blue), which is extended inferiorly in (B). After round window CI insertion, part of the implant lead wire is packed into the extended bullostomy cavity for stabilization (C).
Fig 4.
Example of the stimulation cage.
(A) Top down view shows the modified cage top with the implant emulator and processor within a metal enclosure. The emulator is connected to a sliding commutator via cabling protected by a spring-shield. The commutator is connected via a short length of cable which tethers to a transcutaneous connector in the electrode array assembly, which is stabilized by a harness (B). The translating commutator allows free movement of the mouse throughout the entire caging system, while maintaining electrical connectivity. (C) Diagram of the connectivity of the system with arrows denoting the direction of stimulus or recording flow.
Fig 5.
Mean and individual duration of electrode functionality (impedance ≤ 35 kOhms) and time of last obtainable NRT recording. The left side of the figure portrays the duration that each implant maintained the respective number of functioning electrodes. The right side of the figure portrays the last timepoint at which an NRT response could be obtained for each implant. Non-stimulated subjects are represented by blue circles and stimulated subjects by red squares. Horizontal bars represent mean group values. The red, dotted line marks the 21 day experimental endpoint threshold. 5/6 and 3/6 subjects in the no-stimulation and stimulation groups met or surpassed the 21 day threshold of maintaining at least one functional electrode, respectively. NRT responses were generally obtainable while at least 2 electrodes maintained impedance levels ≤ 35 kOhms. Error bars represent Standard Deviation.
Fig 6.
Serial impedance and NRT threshold values over time.
Individual subject impedance values per electrode and NRT threshold are plotted over time. A-F represent individual data for the non-stimulated subjects and G-L for the stimulated subjects. Day 0 represents impedance values obtained immediately prior to implantation. NRT threshold values (red) are scaled to the right sided y-axis. Impedance values are in grayscale and scaled to the left sided y-axis. The most basal electrode (E1) is black, middle electrode (E2) dark gray and most apical electrode (E3) light gray. Impedance levels of 125 kOhms represent the measurement limit of the system.
Fig 7.
Neural response telemetry (NRT) growth series.
Individual NRT responses in response to an escalating stimulus level between 90 and 120 CL is presented as an example of a typical NRT growth series.
Fig 8.
X-ray imaging shows an intracochlear position of the array in the left cochlea of a live subject with the electrode contacts labeled by a red “*” (A). The lead wire can be seen traveling across the posterior base of the skull to meet the 2 extracochlear electrode leads as they head toward the connector located on the back (not seen in this image). Fractures (red arrowhead) occurred in both the straight (B) and helixed portions (C) of the array lead wire.
Table 1.
In-vivo x-ray implant outcomes.
Fig 9.
3D x-ray microscopy and histologic co-localization.
3D x-ray microscopic images (A,C,E&G) are co-localized next to corresponding histologic sections (B,D,F and H) for non-stimulated (A-D) and stimulated (E-H) subjects. The implant tract (marked by a red “I”, where visible) with adjacent soft tissue and neo-osteogenesis is seen in the round window and basal turn of the scala tympani. Representative 500μm scale bars are provided for each image. Red arrows mark the tissue response within the scala tympani and round window area.
Fig 10.
Volumetric cochlea segmentation.
(A) 3D rendering of a volume segmented 3D x-ray microscopy cochlea image stack. Regions of interest include the implant tract (green), soft tissue response (blue), neo-ossification (red) and unoccupied scala tympani (yellow). (B) The fractional volume of the scala tympani occupied by neo-ossification or soft tissue response was quantified. Individual data for non-stimulated (blue dots) and stimulated (red squares) subjects are include with horizontal bars showing group mean values. There were no significant differences between groups (p>0.5). Error bars provide SD.
Fig 11.
Co-registration of electrode array within 3D x-ray microscopic images.
(A) 3D reconstruction of a 3D x-ray microscopic image shows the array trajectory through the scala tympani, around the modiolus (red “M”). The most apical electrode (red “E3”) and most basal electrode (red “E1”) are seen with a peri-modiolar orientation, with the middle electrode obscured by the modiolus. The red “*” denotes a foci of peri-implant neo-osteogenesis. Coronal (B) and axial (C) sections show the intrascalar orientation and trajectory of the implant with multiple areas of neo-osteogenesis (red “*”). The most basal (red “E1”), middle (red “E2”) and apical (red “E3”) are seen.
Fig 12.
ABR and DPOAE measures of hearing preservation.
Left, implanted (A) and right, non-implanted (B) ear ABR thresholds are plotted by individual subject with representative black symbols according to testing day. The black horizontal bars represent mean values. Red symbols indicate the 1 subject excluded secondary to significant hearing loss post-implantation. ABR thresholds maintained within 15 dB SPL of baseline values for all experimental subjects (black symbols) at the 8 and 16 kHz frequencies. There were no significant differences between timepoints at any frequency (p>0.05). Error bars represent SD for the 6 subjects meeting hearing preservation criteria. (C) and (D) show mean DPOAE signal to noise ratio (SNR) for the left (implanted) and right (non-implanted) ears, respectively. ‘*’ denotes statistically significant (p>0.05) difference from baseline SNR values at respective timepoints. Within our system, a SNR ≥ 9 is considered an intact DPOAE; this threshold is denoted by the red, dotted horizontal line. The solid red line represents baseline values for the 1 excluded subject and the dotted red line represents final 6-week DPOAE SNR values.