Table 1.
Animal Health Score.
Fig 1.
The electrodes have two active contacts (1st contact, more basal; 2nd contact, more apical) and a black marker dot at 3 mm length from apex to indicate insertion depth; A: pure silicone electrode array without DEX (0%); B: electrode array containing 1% DEX (16 ng/day delivery rate); C: electrode array containing 10% DEX (49 ng/day delivery rate).
Fig 2.
The electrodes used consist of two electrical contacts and a reference electrode. The connector was attached to the animal’s skull.
Fig 3.
Dexamethasone release rates from guinea pig electrode measured in vitro.
The first 4 mm of the electrode array were immersed into 1 ml of saline solution at 37°C. The saline solution was periodically sampled and DEX concentrations were measured by HPLC-MS (Agilent 1200 HPLC coupled with a Bruker MicrOTOF-Q II MS-detector). Representative release profile measurements for the 1% (red squares) and 10% (blue triangles) DEX loaded electrodes are shown. Over the experimental time period (91 days) the average release rate of DEX from the 1% and 10% loaded electrodes was estimated to be in the order of 16 ng/day and 49 ng/day respectively. DEX release rates were higher in the initial elution period (e.g. average over the first five days: 62 ng/day and 166 ng/day respectively for the 1% and 10% electrodes) with a slow decay over time.
Fig 4.
Mean impedance (y-axis) before and after 60 minutes of ES are plotted for all measurement days (x-axis).
Sections A-C show the impedances at the first contact (basal, near to the RWN) for each experimental group whereas sections D-F visualize the impedances at the second, more apical, contact. Statistical differences in impedances between the experimental days were denoted: * = p<0.05; ** = p<0.01; *** = p<0.001; error bars = SD.
Fig 5.
Impedances of contact one (A) and contact two (B) in all three treatment groups before (clear) and after (dashed) electrical stimulation on experimental day 91.
At contact one (A) in both DEX treated groups impedances were significantly reduced after 3 month observation time compared to the group receiving 0% DEX (** = p<0.01) and there was no difference between impedance levels in 1% DEX or 10% DEX treated individuals detected at contact one (ns = not significant). At the second contact, which was located more apical in the cochlea, no significant differences in impedance values between treatment groups were detected (B). Error bars = SEM.
Fig 6.
Hearing thresholds on experimental day 0 before and after implantation and on day 7.
The mean and SD hearing threshold (dB SPL) in the frequencies tested (1–40 kHz) did not differ between groups on experimental day 0 before or after implantation, or on day 7. The hearing threshold in all experimental animals before insertion of the electrode array was equal (red lines). Directly after implantation (straight lines) the hearing threshold increased in all groups and did not change until experimental day 7 (dotted lines).
Fig 7.
AABR threshold shifts related to day 0.
The mean and SEM AABR threshold before electrode insertion is plotted as a function of time. Significant differences in hearing thresholds are listed in the respective graphs. DEX eluting electrodes did not protect residual hearing at any frequency. At the lowest frequencies (1 and 4 kHz) and at the highest frequency tested (40 kHz), no significant differences were detected between the groups. At 8, 16, and 32 kHz DEX treated animals had an increased hearing loss compared to control (0% DEX).
Fig 8.
Shifts in hearing thresholds obtained on day 0 before (A) and after surgery (B) until day 91 are plotted (ΔdB) for all tested frequencies.
At 4, 8, 32 and 40 kHz no differences in ΔdB between treatment groups were observed for day 0 (pre-operative) and day 91. 10% DEX treatment resulted in significantly greater threshold shifts at 1 and 16 kHz than 0% DEX + ES (p<0.05) or 0% DEX without-ES at 16 kHz (p<0.05) (A). No differences between groups where observed when relating the final threshold (day 91) to the day 0 threshold measured after surgery (B). Error bars: SEM.
Fig 9.
Connective tissue formation in the scala tympani of implanted animals treated with 1%, 10% or 0% DEX (control).
Tissue growth is plotted as the percentage area filled in the scala tympani. Sector A illustrates the mean tissue growth for the whole cochlear length analyzed (basal turn) showing a significant difference between the 10% DEX treated cochleae and 0% DEX (* = p<0.05). Comparing the tissue reaction at the RWN (B), significant differences are observed for both DEX groups compared to 0% DEX. Statistical differences were denoted; ** = p<0.01; ns = not significant. Horizontal bars represent the SEM.
Fig 10.
Representative images of grinded cochleae.
The tissue reaction in 10% DEX (A), 1% DEX (B), 0% DEX + ES (C) and 0% DEX without ES (D) treated guinea pig cochleae is representatively shown. E: electrode; s.t.: scala tympani; s.v.: scala vestibuli; arrow: fibrosis; star: ossification; R: Rosenthal‘s canal; Magnification: 150fold.
Fig 11.
Correlation between fibrous tissue formation and hearing loss.
No correlation between fibrous tissue growth and hearing loss, neither for all frequencies tested (1, 4, 8, 16, 32 and 40 kHz; A)) nor for the high frequencies (32 and 40 kHz; B) were observed in ears implanted with a 0% DEX-electrode array (no-electrical stimulation).
Fig 12.
Correlation between tissue growth and impedance.
Tissue growth around the electrode array significantly correlated with the measured impedance on the basal electrode (A) as well as on the more apical electrode (B). The highest correlation was found when the tissue growth at the round window was compared to the impedance measured on the basal contact (C).