The impact of implant design on hearing performance

Woman working in a lab

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.

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 and connected 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 implant1,2,3. The hearing zone does not extend deep into the cochlea.

As such, Cochlear’s philosophy is to deliver electric stimulation to the spiral ganglion cells located in the hearing zone to achieve optimal hearing performance, and to avoid deeper insertion in order to reduce the risk of apical stimulation or insertion trauma. This philosophy underpins the development of our range of electrodes.

Cochlear implant inserted inside the cochlea highlighted as blue spiral
demonstration of cochlear implant inserted inside the cochlea

Electrode design and hearing performance

With pre-curved, perimodiolar electrodes the stimulation contacts are placed closer to the spiral ganglion cells through a design that matches the natural shape of the cochlea. After insertion, the electrode array sits in a relaxed resting position without applying any force to either the lateral or the modiolar wall. Research has shown that perimodiolar electrodes deliver unprecedented hearing performance, a more focused stimulation and greater power efficiency4,5.

Lateral wall electrodes have also been shown to deliver excellent hearing performance in both electric-only and combined electro-acoustic stimulation modes6.

Special design features such as a basal stiffener, Softip™ smooth lateral wall surface or handle give the surgeon control over insertion depth and therefore the ability to achieve optimal coverage of the hearing zone in varying cochlear sizes.

Basal strength provides control and tactile feedback for the surgeon to increase the predictability of insertion and minimise buckling, which is often associated with significant insertion trauma. Apical flexibility is important to minimise insertion forces which may cause trauma to delicate lateral wall structures.

The design and shape of the electrode tip needs to protect the delicate internal structures of the cochlea and therefore varies in shape of perimodiolar and lateral wall electrodes.

Half-band electrode contacts featured on all of our electrodes ensure there is a smooth silicone surface facing the lateral wall to minimise friction trauma during insertion.

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 sounds7. In fact, some research demonstrates that hearing outcomes are frequently worse with deep insertion techniques7,8.

2. By inserting an electrode array deep into the cochlea where it is very narrow, there is a significant risk of damaging its delicate structures9. 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.

 

Disclaimer

Please seek advice from your health professional about treatments for hearing loss. Outcomes may vary, and your health professional will advise you about the factors which could affect your outcome. Always read the instructions for use. Not all products are available in all countries. Please contact your local Cochlear representative for product information.

Footnotes

  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. Balkany, T., Hodges, A., et al., Nucleus freedom north american clinical trial. Otolaryngology-Head and Neck Surgery, 2007. 136(5): p. 757.
  5. Cohen, L. T., Saunders, E., and Richardson, L. M., Spatial spread of neural excitation: comparison of compound action potential and forward-masking data in cochlear implant recipients. International Journal of Audiology, 2004. 43(6): p. 346-355.
  6. Skarzynski, H., Lorens, A., Matusiak, M., Porowski, M., Skarzynski, P. H., and James, C. J., Partial Deafness Treatment with the Nucleus Straight Research Array Cochlear Implant. Audiology and Neurotology, 2012. 17(2): p. 82-91.
  7. Battmer, R-D., Ernst, A. Risk and Benefit of Deeply Inserted Cochlear Implant Electrode Arrays. CIAP, Lake Tahoe 2009.
  8. Gani, M., Valentini, G., Sigrist, A., Kós, M. I., and Boëx , C., Implications of deep electrode insertion on cochlear implant fitting. JARO-Journal of the Association for Research in Otolaryngology, 2007. 8(1): p. 69-83.
  9. Adunka, O., Kiefer, J. Impact of Electrode Insertion Depth on Intracochlea Trauma. Otolarynoglogy Head and Neck Surgery, 2006, 135, 374-382.