1Anatomy of a Cochlear Implant System

FA two-part transcutaneous system

Every modern cochlear implant is a transcutaneous two-part system: an external sound processor and an internal receiver-stimulator held together across unbroken skin by one external and one internal magnet, with no electrical conductor crossing the skin. The percutaneous skin-penetrating plugs of the early Michelson, Banfai and House-era designs were abandoned because a permanent port invites infection.[2020]

TThe external processor

The external processor has five basic parts — one or more microphones, a digital signal processor, a (usually lithium-ion rechargeable) battery, an external transmitting coil, and the external magnet. Off-the-ear single-unit processors collapse all five into one button-sized housing worn on the head with no behind-the-ear cable.[2008]

CThe internal device

The internal device has six parts — the internal coil, the internal magnet, a digital signal processor, the stimulator that generates the pulses, the lead wires, and the intracochlear electrode array. Coil, processor and stimulator together are the 'receiver/stimulator'.[2009]

FThe hermetic titanium case

The receiver-stimulator electronics are sealed in a hermetic biocompatible titanium case with a ceramic-to-metal feedthrough seal, technology borrowed from cardiac pacemakers and NASA. Clark's Nucleus design had to pass about 20 electrode leads through a single seal without fluid ingress; the modern trend is toward thinner cases that need a shallower bony well and put less tension on the skin flap.[2014]

TArray and reference electrode

The intracochlear array carries multiple platinum contacts inserted into scala tympani; in the common monopolar mode the stimulus returns to a separate extracochlear reference/ground electrode. This active-array-plus-distant-reference arrangement is conceptually identical to the very first 1957 Djourno-Eyries device, whose failure traced to a broken solder joint at the ground electrode (cross-ref Ch.1 History).

CDevice versus coding strategy

Critically, the DEVICE (hardware, electrodes, telemetry) is distinct from the CODING STRATEGY (the software rule converting sound to a pulse pattern). The same implant can run CIS, ACE or fine-structure strategies by reprogramming; the hardware merely caps electrode count, maximum pulse rate, current compliance and telemetry capability (cross-ref Ch.8 From Sound to Stimulation).

For a single, sortable side-by-side of every system available today — contacts, current sources, channels, coding strategies, processors, connectivity, water resistance and MRI conditions — see the master device comparison.