7Where Is the Array? Verifying Electrode Position

FWhat "correct" actually means

A good placement satisfies five criteria at once: the array lies in the scala tympani, it has not translocated into the scala vestibuli, it is inserted to the intended depth, the tip has not folded back on itself, and the lead is not kinked or buckled at the cochleostomy/round window. Scala tympani is the target because it sits below the basilar membrane and osseous spiral lamina, keeping the electrode away from the delicate hearing structures and close to the spiral ganglion. Translocation (the array crossing the basilar membrane from scala tympani into scala vestibuli, usually in the upper basal turn) is the single most damaging mechanical event short of frank fracture, and it is invisible to the naked eye on the table. Tip fold-over and kinking shorten the effective active length, distort the place-frequency map, and concentrate trauma at one point. Cross-reference Ch.16 (the surgical insertion itself) and the Complications chapter (electrode misplacement and migration) for management once a problem is found.[2008][2021][2009]

TReading the array with electrophysiology

Impedance telemetry is the first sanity check: open circuits suggest a lead break or an electrode still in air/fluid outside the cochlea, while uniformly very high impedances can flag an extracochlear or air-trapped array. ECAP (neural response) thresholds confirm that the electrodes are stimulating auditory neurons and give a coarse profile along the array; absent responses across the board raise suspicion of malposition. Spread-of-excitation and transimpedance-matrix (TIM) measurements probe the geometry: an abrupt discontinuity or reversal in the expected smooth gradient along the array is a recognised electrical signature of a tip fold-over, often detectable before imaging. Electrocochleography (ECochG) recorded from an apical contact during and after insertion tracks the cochlear response in real time; a drop in the ECochG amplitude can signal trauma or translocation as it happens (see Ch.16/Ch.13). Electrophysiology is fast and needs no extra equipment in the field, but it is indirect: it infers position from function and must be read together with imaging.[2020][2021]

TImaging and the surgeon's hands

Intraoperative imaging—plain transorbital/Stenvers radiographs, fluoroscopy, or cone-beam/flat-detector CT—directly shows the coiled array, and is the reference standard on the table for confirming full insertion, excluding tip fold-over, and (with CT) judging scalar position. Tactile feedback during insertion remains a genuine signal: smooth, low resistance suggests an unobstructed scala-tympani path, whereas sudden increased resistance, buckling, or the array stopping short warns of obstruction, fibrosis, or a wrong scala. The combined toolkit is complementary: electrophysiology samples function continuously, imaging confirms geometry at a moment in time, and the surgeon's hands integrate both during the act of insertion. If verification reveals a problem, the array can often be withdrawn and reinserted on the table—far better than discovering malposition on the postoperative scan weeks later. Postoperative imaging (Ch.12) remains the definitive scalar-location check, but on-table verification is what allows immediate correction.[2015][2008]

CWhy position predicts outcome

Scalar translocation rates differ markedly by array type: meta-analysis pools roughly 7% for lateral-wall (straight) arrays versus about 43% for perimodiolar (pre-curved) arrays, a difference driven by the perimodiolar tip's tendency to lift across the basilar membrane in the upper basal turn. Translocation is not a cosmetic finding: it is associated with poorer speech-perception scores (weighted means around 41% vs 55% in pooled data) and with loss of residual hearing. Scala-tympani placement and shallower angular insertion depth correlate with better preserved low-frequency acoustic hearing, linking position directly to EAS candidacy. Position is one of the few outcome variables the surgeon controls on the day, which is why verification—not just insertion—is part of a complete operation. The choice between array families is therefore partly a trade-off between perimodiolar proximity to the neurons and the lower translocation risk of lateral-wall designs (see Ch.13 devices/EAS).[2021][2015][2008]