No abstract

Micro-focus fluoroscopy and high-resolution micro-focus radiographs assisted the development of the perimodiolar Contour electrode array by allowing a consistent and clear electrode position to be established, and also helped greatly in the development of appropriate surgical insertion techniques and tools for a smooth electrode insertion. ConclusionMicro-focus radiography and fluoroscopy play a vital role in the development of advanced electrode arrays such as the Nucleus Contour TM electrode array with Softip.AG HANTZAKOS, RF GRAY, The East of England Figure 1: An example of atraumatic insertion of the Contour array with the aid of an insertion tool in a temporal bone sample. (A) Cochlear view of micro-focus radiograph showing that the Contour array was located close to the modiolus; (B) side view of micro-focus radiograph showing normal course of the electrode array; and (C) photo-micrograph confirming that the Contour array is located in scala tympani and proximal to the modiolus, with no evidence of intra-cochlear trauma found. Arrowhead -outer wall of the cochlea; M -modiolus; ST -scala tympani. (A) (B) (C)

Introduction: Preservation of residual hearing in cochlear implant recipients has been demonstrated to be possible and provides the potential benefit of combined electric and acoustic auditory stimulation. A prototype 16-mm multichannel array has been designed to facilitate placement of 22 electrodes without damage to intracochlear structures. The electrode array is suitable for insertion via the round window membrane (RWM) or a small cochleostomy. Aim: To evaluate the insertion trajectory and the presence of trauma to intracochlear structures with the prototype electrode inserted by either the RWM or a scala tympani cochleostomy. Materials and Methods: Eighteen fresh frozen human temporal bones were prepared for cochlear implantation using a standard transmastoid facial recess technique. Twelve electrodes were implanted at the University of Melbourne and 6 at the Medizinische Hochschule Hannover. In Melbourne fluoroscopy was used to monitor the insertions. Twelve prototype electrodes were inserted via the RWM. A further 6 electrodes were inserted via a small scala tympani cochleostomy. The cochleostomy was sited inferior to the RWM to avoid trauma to the basilar membrane and spiral ligament. Specimens were embedded and fixed with acrylic resin and the cochleae then examined histologically at 200-µm intervals using a grinding and polishing technique. Results: Full insertion of the electrode was achieved without significant resistance in all RWM and cochleostomy specimens. In two RWM specimens fold-over of the electrode tip occurred, and in one specimen the electrode penetrated the spiral ligament to lie in an ‘endosteal ‘position. In one cochleostomy specimen the electrode was rotated within the cochlea to face laterally rather than towards the modiolus. The final electrode position differed for the two groups, with the electrodes inserted via the RWM lying in a more perimodiolar position along the first part of the basal turn. The average depth of insertion was 240° for the RWM electrodes and 255° for the cochleostomy electrodes. Histologic examination showed no damage in any specimen to the modiolus, osseous spiral lamina or basilar membrane. Conclusions: A prototype hearing preservation electrode array was inserted by either a RWM or a scala tympani cochleostomy without evidence of significant intracochlear trauma.

Increasing numbers of cochlear implant subjects have some level of residual hearing at the time of implantation. The present study examined whether (i) hair cells that have survived one pathological insult (aminoglycoside deafening), can survive and function following long-term cochlear implantation and electrical stimulation (ES); and (ii) chronic ES in these cochleae results in greater trophic support of spiral ganglion neurons (SGNs) compared with cochleae devoid of hair cells. Eight cats, with either partial (n=4) or severe (n=4) sensorineural hearing loss, were bilaterally implanted with scala tympani electrode arrays 2 months after deafening, and received unilateral ES using charge balanced biphasic current pulses for periods of up to 235 days. Frequency-specific compound action potentials and click-evoked auditory brainstem responses (ABRs) were recorded periodically to monitor the residual acoustic hearing. Electrically-evoked ABRs (EABRs) were recorded to confirm the stimulus levels were 3-6 dB above the EABR threshold. On completion of the ES program the cochleae were examined histologically. Partially deafened animals showed no significant increase in acoustic thresholds over the implantation period. Moreover, chronic ES of an electrode array located in the base of the cochlea did not adversely affect hair cells in the middle or apical turns. There was evidence of a small but statistically significant rescue of SGNs in the middle and apical turns of stimulated cochleae in animals with partial hearing. Chronic ES did not, however, prevent a reduction in SGN density for the severely deaf cohort, although SGNs adjacent to the stimulating electrodes did exhibit a significant increase in soma area (p<0.01). In sum, chronic ES in partial hearing animals does not adversely affect functioning residual hair cells apical to the electrode array. Moreover, while there is an increase in the soma area of SGNs close to the stimulating electrodes in severely deaf cochleae, this trophic effect does not result in increased SGN survival.