CI Atlas · Intraoperative Monitoring and Hearing Preservation · Module 06

6Catching the Fold-Over: Intraoperative Imaging and Field Telemetry

A tip fold-over or scalar translocation that goes unseen costs the patient a second operation. Caught before the wound is closed, it costs only a few minutes — the array is simply withdrawn and reinserted. This module is about the tools that catch it on the table: the X-ray, the cone-beam CT, and the implant's own field telemetry.

CWhat we are trying to catch, and why before closure

Tip fold-over (the apical electrodes doubling back on themselves) and scalar translocation (the array crossing from scala tympani into scala vestibuli) are placement errors that degrade outcome and, untreated, are corrected only by revision surgery. The whole rationale of an intraoperative check is the same-sitting fix: a fold-over identified before closure is reversed by withdrawing and reinserting the array immediately, avoiding a second anaesthetic. The mechanisms and downstream consequences of malposition are detailed in the Complications chapter; here the focus is intraoperative DETECTION. Pre-curved perimodiolar arrays (e.g. slim modiolar) carry the highest fold-over risk and are the arrays for which on-table screening matters most.[2022]

TPlain radiograph and fluoroscopy: the classic on-table view

An intraoperative plain radiograph in a modified Stenvers / cochlear (trans-orbital) projection unrolls the cochlear spiral so the array is seen end-on as a smooth curl; a fold-over appears as a hairpin or crossed loop at the tip. Fluoroscopy gives the same view in real time, allowing the surgeon to watch insertion and immediately confirm a smooth spiral or spot a buckling tip. These methods are widely available and quick but carry ionising radiation, need theatre radiography setup, and depend on correct projection to be read confidently. Reader agreement for fluoroscopic fold-over detection is good but not perfect, which is one reason electrophysiological screening has gained ground as an adjunct.[2021]

Transimpedance (or spread-of-excitation) matrices exploit one fact: electrodes that are physically close couple strongly, so the recorded voltage is brightest along the diagonal where stimulating and recording contacts are near-neighbours. A correctly spiralled array gives a clean diagonal-dominant matrix. When the tip folds over, the most apical contacts become physical neighbours of basal ones, so a bright off-diagonal patch appears away from the main diagonal — a fingerprint of fold-over readable on the table before closing. Deterministic and schematic.

TFlat-panel and cone-beam CT: resolving the scala

Intraoperative cone-beam / flat-panel CT (e.g. an O-arm) reconstructs the array in three dimensions, reliably identifying misplacement or fold-over and quantifying insertion depth angle on the table. Unlike a single plane film, volumetric imaging can resolve the SCALAR compartment, distinguishing scala tympani placement from a translocation into scala vestibuli that a plain view may miss. Flat-panel CT resolves intra-scalar position with less metallic artifact and lower dose than multi-detector CT, making it suitable for the operating environment. The trade-off is equipment availability and footprint; many centres reserve volumetric CT for difficult anatomy or ambiguous radiographs rather than every case.[2018]

On a cochlear (Stenvers / trans-orbital) view the array should sweep round as a single smooth spiral with the electrode numbers ascending in order from the round window at the base to the apical tip. Trace the contacts: if the numbers keep climbing along one clean curve the insertion is normal, but if the most apical contacts reverse direction and overlap near the tip the array has folded over and should be withdrawn and reinserted. The plain radiograph is the classic intra-operative confirmation, corroborated by the transimpedance matrix. Schematic radiograph, not a real film.

CField telemetry: the implant images itself

The transimpedance matrix (TIM) stimulates each electrode in turn and records the resulting voltage on every other electrode, producing a heat-map of the intracochlear field that needs no X-ray. A normally-spiralled array gives a smooth diagonal-dominant heat-map (each electrode is closest to its immediate neighbours); a tip fold-over produces a characteristic REVERSED / off-diagonal signature where folded tip electrodes read as electrically adjacent to more basal contacts. Spread-of-excitation, the related ECAP-based field measure, shows the same logic — an off-pattern, mirrored excitation profile betrays electrodes that are physically close despite being numerically distant. In series using pre-curved arrays, TIM detected every fold-over confirmed by fluoroscopy, is fast, requires no extra equipment, and can be run before closure — making it an increasingly first-line, radiation-free intraoperative check.[2022][2021]

Case 18.6 · Catching the Fold-Over

During implantation with a pre-curved slim modiolar array, insertion felt slightly resistant near the end. The surgeon wants to confirm the array is fully and correctly spiralled before closing, but the centre wants to minimise radiation and theatre time.

Which intraoperative tool can detect a tip fold-over fastest and without ionising radiation, before wound closure?

Self-assessment — Module 63 questions

Question 1

On a transimpedance matrix heat-map, what is the signature of a tip fold-over?

Question 2

Why is detecting a fold-over BEFORE wound closure so valuable?

Question 3

Which intraoperative imaging tool best resolves whether the array sits in scala tympani versus scala vestibuli?

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