15Materials, Reliability and MRI

TSilicone and platinum-iridium

The array is a medical-grade silicone carrier holding ~25-micron Pt-Ir wires (platinum for corrosion resistance and inertness, iridium for tensile strength), Teflon/Parylene insulated and welded to recessed platinum contact plates. A larger contact surface area keeps current density low and reduces the stimulator's drive demand, linking electrode geometry to battery life.[2012]

CThe hermetic housing

The receiver-stimulator uses a hermetic titanium (and ceramic) housing with technology carried from pacemakers into Nucleus by Telectronics in the early 1980s — titanium cases, ASICs, and ceramic windows where the telemetry coil must transmit. The first inter-manufacturer reliability comparison contrasted epoxy versus titanium House/3M devices (2-yr CSR 80.8% epoxy vs 87.1% titanium).[2022]

TCumulative survival rate

Cumulative survival rate (CSR) is the standard reliability metric: S(t) = P(t) × P(t−1) × … × P(1), reported separately for children (<18 yr) and adults per model. It originated from pacemaker actuarial reporting (ISO 5841-2) and was proposed for CIs by von Wallenberg et al. (Cochlear) in 1993; all manufacturers now publish it.[2014]

CThe failure categories

The 2010 Global Consensus defines failure categories A–E (A normal; B1 in/out of spec but benefit preserved; B2 performance decrement with adverse reactions; C device failure with loss of benefit, the only category counting toward CSR and mandating regulatory report; D medical reason such as infection or upgrade; E1 lost to follow-up; E2 deceased). A European survey found failures in 488/12,856 devices (3.8%). The earlier hard/soft-failure labels were abandoned because US and European definitions of 'soft failure' conflicted.

TThe foreign-body response

Implantation triggers a foreign-body response maturing from acute edema to a fibrous capsule and possibly osteoneogenesis, driven by surgical trauma, materials biocompatibility and array stiffness; electrode impedance tracks this interface (high = open circuit, low = short circuit) and feeds the soft-failure decision tree (cross-ref Ch.23).

CThe magnet and MRI

The internal magnet holds the external coil but is the principal MRI hazard: five risks are repositioning, heating, pain, demagnetisation and image distortion. Demagnetisation depends on the field-to-magnet angle (keep <90°; ~0% below 90°, ~6.6% at 90°, ~60% above 90° at 3 T per Dubrulle 2013). Artefact spans ~6 cm at 1.5 T and ~12 cm at 3 T with the magnet in place; mitigations are magnet removal, a ≥2-layer 10 cm compression bandage, or modern self-aligning/removable magnets (cross-ref Ch.12 Imaging).