A soft-surgery approach to minimize hearing damage caused by the insertion of a cochlear implant electrode: a guinea pig animal model.

Objective: A "soft surgery" technique was applied, using various types of specifically designed dummy electrodes, to mimic cochlear implantation in a guinea pig model, and the degree of hearing-preservation/cochlear damage was assessed.
Methods: Tricolor guinea pigs were divided into 3 groups: group A were implanted with electrodes without any contacts or wires (soft electrode), group B were implanted with electrodes having a metallic wire inside (stiff electrode), and group C underwent a cochleostomy procedure without implantation. Compound action potentials, in the range of 4 to 32 kHz, were used to assess electrophysiologic changes in the hearing function presurgery and postsurgery. Data were collected before surgery, at times t = 0 (immediately after surgery) and at 3, 7, 14, and 30 days.
Results: At low frequencies (4-8 kHz), an immediate elevation of hearing threshold was observed in all 3 groups. Higher threshold shifts were more consistent for group B implanted with a stiff electrode, in comparison to the other 2 groups. Animals from group C presented a recovery from hearing loss, starting 3 days after surgery. At high frequencies (16-32 kHz), the elevation of hearing threshold was higher, as compared with the data from the low frequencies. Group C animals presented oscillatory threshold shifts twice, and the recovery to normal threshold values occurred approximately at t = 14 days.
Conclusion: The data suggest that cochleostomy is minimally harmful to the inner ear and that a soft electrode might better preserve the inner ear integrity than a rigid electrode.