[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)

1. Microphone

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

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

▲ 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

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)

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
  

Making entry to cochlea

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)