Cochlear Implant Electrode Design and Preservation of Residual Hearing

Andreas Jäger, Jan Kiefer, Wolf-Dieter Baumgartner, Claude Jolly
MED-EL World-Wide Headquarters, Innsbruck, Austria; 2 Dept. of Otolaryngology, J.W.-Goethe-Unversity, Frankfurt/Main, Germany; 3 Dept. of Otolaryngology, University of Vienna, Austria

Author details
Dr.-Ing. Andreas Jäger
Fuerstenweg 77, 6020 Innsbruck, Austria
andreas.jaeger@medel.com



Introduction
Patients with significant residual hearing but without sufficient benefit from a hearing aid are considered candidates for combined electrical and acoustical stimulation [1, 2]. To protect residual hearing during cochlear implantation, surgical technique and electrode design are essential. The mechanical properties of electrodes influence ease of insertion and the amount of trauma induced [3]. Electrodes designed to minimize insertion forces are advantageous in this regard.

Study Design
Various electrode prototypes for the C40+ cochlear implant of MED-EL have been build. Prototype designs were intended to achieve gains in terms of following aims:

		minimized insertion trauma by high mechnical flexibility
		length of electrode array adapted to typical residual hearing

Electrode prototypes underwent different in-vitro tests and evaluations:

		measurement of mechnical flexibility
		force measurement during automated insertion into a model
		manual insertion into scala tympani model and temporal bones

Finally results of electrode prototypes have been compared to data of existing electrode variants:

fig. 1: existing MED-EL C40+ electrode variantsInsertion Force

Insertion Force Measurements
The electrode is inserted into a model of the human scala tympani. After manually aligning the tip of the electrode to the model's cochleostomy the insertion proceeds automatically by software. During this process the force required to insert the electrode into the model is measured using a precision scale.

fig. 2: insertion force measurement setup utilizing a precision scale

fig. 3: insertion force measurement results

fig. 4: insertion force ratios C40+ standard electrode vs. prototype

Surgical Evaluation
Among other parameters the ease of insertion was determined by subjective assessment of experienced surgeons using cadaver temporal bones (formalin fixed - frozen). Multiple insertions with the same electrode of various electrode designs have been performed.

tab. 1: subjective ease of insertion resuts ( * 1 = very easy to insert; 10 = impossible to insert)

Conclusions
Compared to existing commercial electrode variants the mechanical flexibility of intra-scalar electrodes can be considerably increased and insertion forces lowered. Additional criteria besides mechanical parameters have to be taken into account too when trying to choose the best design among many.

References
[1] von Ilberg C, Kiefer J, Tillein J, Pfenningdorff T, Hartmann R, Sturzebecher E, Klinke R. Electric-acoustic stimulation of the auditory system. New technology for severe hearing loss. ORL J Otorhinolaryngol Relat Spec. 1999 Nov-Dec;61(6):334-40

[2] Gantz BJ, Rubinstein JT, Tyler RS, Teagle HF, Cohen NL, Waltzman SB, Miyamoto RT, Kirk KI. Long-term results of cochlear implants in children with residual hearing. Ann Otol Rhinol Laryngol Suppl 2000 Dec;185:33-6

[3] Rebscher SJ, Leake PA, Wardrop PJC. Protective Effects of Patterned Electrical Stimulation on the Deafened Auditory System. 11th Quarterly Progress Report, Neural Prothesis Program, Project N01-DC-7-2105, 2000