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Achieving Hearing Preservation
through Superior Electrode Design

Over the past 10 years, a specific surgical protocol has been shown to increase the probability of hearing preservation dramatically. This surgical protocol has been particularly successful when implemented among the EAS (electric and acoustic stimulation) patient population. Research with this group has been beneficial in achieving a better understanding of the surgical techniques required to preserve residual hearing. Today, it is commonplace for these key discoveries to be applied in all cochlear implantations in order to  minimise or eliminate the trauma induced by electrode insertion.

The core philosophy guiding the design of all MED-EL electrodes is to ensure atraumatic insertion for all cochlear implant recipients. An atraumatic insertion is best evaluated in terms of the preservation of residual hearing. In other words, preservation of some or complete residual hearing after a full electrode insertion would demonstrate the high quality of the device and surgical technique. Preservation of neural tissue is regarded as critical for all CI patients, but especially for young children who may face multiple implantations in their lifetime. It is likely that any future interventions, be they device or pharmaceutical in nature, will be more successful in a cochlea that has received minimal trauma.

 

Figure 1: Waved-Shaped Wires

The key features in reducing electrode insertion trauma in the cochlea are the size and flexibility of the array. The electrode should be much smaller than the dimensions of the smallest cochlea, but not so small as to compromise the ease of insertion of the electrode. Most importantly, the electrode should be flexible enough to easily adapt to the angulations of the basal and second turns of the cochlea, regardless of lateral or medial wall positioning.

In order to make the electrode flexible, a wave-shaped (sinus) configuration of the platinum iridium wires inside the electrode silicone carrier is used (Figure 1). A collection of straight wires, even 25 microns in diameter, increases the rigidity and more than doubles the insertion force of the electrode, as compared to an array with wires in a wave-shaped configuration. Slightly reducing the diameter of the metallic wire reduces insertion forces, but not as much as forming the wire into a wave shape. Figure 2 compares the insertion force of a commercially available electrode that has a network of wave-shaped wires to the insertion force of the same electrode with straight wires.

Figure 2: Insertion Force with Wave-Shaped Wires

To further reduce electrode stiffness and increase flexibility, channel density is kept low. Having a large number of channels over a short length increases stiffness and subsequent risks of trans-scala displacement. Having fewer contacts, spaced further apart, increases flexibility. In all cases, the front end of the electrode should be especially flexible so that it can engage well into the scala tympani without deviation of the tip from the scala tympani to the scala vestibuli.

In keeping with our philosophy, MED-EL offers the FLEXEAS (21mm), Standard (31mm) and FLEXSOFT arrays (31mm ) (see Figure 3). Each of these electrode arrays features the wave-shaped wire configuration, lateral wall placement, and low channel density—all characteristics proven to reduce insertion force and cochlear trauma. This design is ideal for insertion through the round window, a technique believed by many surgeons to be the least traumatic. Pre-shaped electrodes increase trauma, are not easily inserted through the round window, and are not designed for consistent hearing preservation, due to the large number of direct scala vestibuli insertions and deviations from scala tympani to scala vestibuli associated with such electrodes. Lateral wall electrodes are less traumatic than pre-shaped electrodes and are preferred for atraumaticity and hearing preservation.

Figure 3: Standard Array, FLEXSOFT, and FLEXEAS

Hearing preservation is the result of an atraumatic electrode insertion. In order to maximise the chances of hearing preservation and reduce trauma to the cochlea during cochlear implant surgery, a flexible, lateral wall electrode array is required. Electrode arrays designed by MED-EL are the most atraumatic electrode arrays available on the market. Maintaining the integrity of the cochlea ensures that the patient will have access to future interventions, which is of particular importance in the paediatric population.
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