Understanding the impact of implant design on hearing performance

The inner ear, or cochlea, is a small and delicate spiral-shaped structure. Tiny hair cells within the cochlea convert sound into signals, which are then sent to the brain via the hearing nerve.

With normal hearing, the entire length of the cochlea works to create sound signals for the brain.

However, hearing with a cochlear implant is different to normal hearing.

Normal hearing works by sending sound vibrations to hair cells within the cochlea. A cochlear implant uses an electrical impulse to stimulate a different set of cells called the spiral ganglion cells. The spiral ganglion cells are located in a different area of the cochlea to the hair cells.

Spiral ganglion cells are mostly concentrated in an area that we call the ‘hearing zone’. This area is the most responsive to electrical stimulation from a cochlear implant.^1,2,3 The hearing zone does not extend deep into the cochlea.

As such, the electrode on the Nucleus^® cochlear implant has been designed to precisely stimulate the spiral ganglion cells within the hearing zone to deliver the most effective hearing outcomes.

Electrode design and hearing performance

The electrode is the part of the implant that is placed inside the cochlea. Its design is crucial for successful hearing outcomes. Cochlear uses a unique curled  design containing 22 individual contact points within the hearing zone. The electrode hugs the natural shape of the cochlea for precise stimulation.

Additionally, a Softip at the end of the electrode allows for gentle insertion to minimise the risk of damaging the delicate cochlea structures.⁴

Why does Cochlear avoid inserting electrodes too deeply?

1. Clinical studies have proven that deep insertion of an electrode array (past the hearing zone) does not necessarily increase the ability to hear low frequency sounds.⁷ In fact, some research demonstrates that hearing outcomes are frequently worse with deep insertion techniques.⁷

2. By inserting an electrode array deep into the cochlea, there is an inherent risk of damaging its delicate structures.⁸ This may also lead to an irreversible loss in remaining low frequency hearing.

3. By having a higher density of electrode contacts within the hearing zone, audiologists have greater flexibility to program the implant to suit an individual’s hearing needs as they change over their lifetime.

1. Ariyasu, L., Galey, F. R., Hilsinger, R., Jr., and Byl, F. M.:Computer-generated three-dimensional reconstruction of the cochlea. Otolaryngol Head Neck Surg. 100, 2; 87-91 1989.
2. Sridhar, Stakhovskaya, Leake, et al, “A Frequency-Position Function for the Human Cochlear Spiral Ganglion”, Audiol Neurotol. 11(supp 1):16-20 2006.
3. Stakhovskaya, O., Sridhar, D., Bonham, B. H., and Leake, P. A.: Frequency map for the human cochlear spiral ganglion: implications for cochlear implants. J Assoc Res Otolaryngol., 8, 2; 220-233 2007.
4. Roland, J.T. Jnr. A model for Cochlear Implant Electrode Insertion and Force Evaluation: Results with a New Electrode Design and Insertion Technique. Laryngoscope, 2005, 115(8), 1325-1339.
5. When used with the correct surgical technique.
6. Aschendorff A, Kromeier J, Klenzner T and Laszig R, Quality Control After Insertion of the Nucleus Contour and Contour Advance Electrode in Adults. Ear & Hearing, 28:755-795, 2007.
7. Battmer, R-D., Ernst, A. Risk and Benefit of Deeply Inserted Cochlear Implant Electrode Arrays. CIAP, Lake Tahoe 2009.
8. Adunka, O., Kiefer, J. Impact of Electrode Insertion Depth on Intracochlea Trauma. Otolarynoglogy Head and Neck Surgery, 2006, 135, 374-382.