[NPE] Cochlear implant – Electrode

Elements and functions they replace

1. Sound collection & mechanical amplification
  • microphone, automatic gain control
2. Tonotopy of basilar membrane
  • Bandpass filter array(filter bank)
  • Fast Fourier Transform
3. Damaged inner hair cells
  • Multichannel elctrode sites
4. Auditory nerve activation by neurotransmitter release(chemical link)
  • Electrical stimulation of the auditory nerve (electraical link)

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1. Microphone

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directional, 방향성이 있는 microphone을 사용한다.

  • Requirements of good miucrophone
    • Broad frequency response
    • minimize response to low-frequency vibrations (ex. head movement, walking)
    • good performance under adverse condition (ex. cafeteria noise)
  • How to address adverse condition?
    • Directional microphone
    • Multiple microphone
      • selectivity of the directional pattern is increased compared to single microphone
      • Sounds originating between and in front of microphones are emphasized, otherwise suppressed
    • SNR improved

2. Cochlear Electrode Array

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Scala Tympani(ST)에 공간이 있기 때문에 이곳에 cochlear electrode를 삽입할 수 있다.

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▲ 오른쪽 그림에서 11개의 전극이 삽입되어 있는 것을 확인할 수 있다.

Requirements for cochlear electrode

  • biocompatible: remain over lifespan
  • mechanically stable
  • facilitate atraumatic insertion
    • flexible arrays, narrow cross-sectional area
    • use lubricant (ex. hyaluronic acid)
  • good spatial specificity of stimulation

Insertion trauma
Insertion force & Extraction force

  • More force is needed to advance in to the ST

Insertion trauma

  • Sharp edge or stiffness of the electrode may cause damages to surrounding tissues.

Insertion trauma grading

  • 0: No observable trauma
  • 1: elevation of BM
  • 2: rupture of BM
  • 3: electrode in scala vestibuli
  • 4: fracture of osseous spiral lamina or modiolar wall

Multi channel, it should be
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Spatial specificity of stimulation depends on..

  • Whether neural processes peripheral to the ganglion cells are present or not
  • The number and distribution of surviving ganglion cells
  • The proximity of the electrodes to the target neurons
  • The electorde coupling configuration (monopolar, bipolar)

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8개의 채널만으로도 충분한 성능을 낼 수 있다.

Pre-curved vs. Straight
Straight types

  • deep insertion
  • far from target cells
  • lateral wall insertion

Pre-curved types

  • close to target cells
  • using stylet(safety problem)
  • perimodiolar insertion

잘 십입되었을 경우 Pre-curved type이 더 성능이 좋다. Target cell에도 가깝고, modiolus에도 가깝게 위치시킬 수 있기 때문이다.

Peri-modiolar placement

Positioning of electrodes in ST
Place close to inner wall of ST to minimize the distance between electrodes and SG

  • maximize the number of largely non-overlapping popuiations of neurons
  • imporve spatial specificity of stimulation
  • reduce threshold voltage
  • increase battery life
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Making entry to cochlea

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  1. Cochleostomy approach

    • straight entry
    • relatively deep insertion depth
    • hard to drill
    • damaging to HC
  2. Round window approach

    • using natural window
    • curved entry
    • relatively shallow insertion depth
    • less drilling necessary
    • saving residual HCs
    • used in EAS(Combined electric and acoustic stimulation) : 전기자극과 함께 residual hair cell을 통해 음성신호도 전달할 수 있다.

Electrode Length

  • Electrical only (Long)
    • deep insertion
    • apex stimulation (low frequency)
    • high insertion trauma
  • Combined electrical acoustic stimulation (Short)
    • acoustic + eletrical
    • low insertion trauma

We wish we could do better in positioning

Limitation in positioning of electrodes

  • SG is the target
  • Inner wall of ST is not always close to SG
  • SG has 1.75 turns while ST has 2.5 turns
  • Distance between the innser wall of the ST and SG increases with increasing distance from the round window(toward the apex)

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