CI Atlas · Preoperative Imaging · Module 12

12Cochlear measurements & electrode selection

At this point radiology stops describing and starts ordering. The cochlea is not one size, and the electrode must match it: a duct that is short or a cochlea without a modiolus calls for one kind of array, a normal cochlea with an intact modiolus for another. Measured on the scan — duct length, basal-turn diameter, modiolar integrity — these numbers choose the array family, estimate how deep it will wind, and feed straight into the device order and the surgical plan. A common cavity measured at a few millimetres needs a short array; a long cochlea suits a long one for deeper low-frequency coverage. This module is the explicit bridge from the picture to the implant.

Imaging note

Representative CT and MRI images for this chapter are being added soon. The interactive figures here are original schematic teaching diagrams; to respect copyright we do not reproduce third-party radiographs.

TCochlear duct length

Cochlear duct length (CDL) is an explicit reporting and planning parameter: normal ≥25 mm versus cochlear hypoplasia <25 mm, which defines whether a standard or short array is appropriate.[2022]

Cochlear duct length is an explicit planning parameter — normal ≥25 mm vs hypoplasia <25 mm — and together with the modiolus and basal-turn shape it chooses the array family: straight/lateral-wall full-banded arrays for malformed cochleae without a modiolus (and for common cavity, where neural elements line the periphery), short/slim arrays for hypoplasia, and perimodiolar arrays only where the modiolus is intact. CDL also estimates the angular depth of insertion (good values ~420–600°). This module is the bridge to the Devices and Surgery chapters. Schematic.

CChoosing the array family

CDL plus basal-turn morphology and modiolar integrity guide array selection: straight/lateral-wall full-banded arrays for malformed cochleae lacking a modiolus (and for the common cavity, where neural elements line the periphery); smaller/slimmer arrays for hypoplasia; and perimodiolar arrays only where the modiolus is intact (Devices chapter).

CArray lengths & worked cases

Measured cochlear size sets array length directly: a worked common-cavity case measured 7 mm; a hypoplastic cochlea 9 mm requiring a short array; full insertion is ~25–30 mm into the basal turn. Electrode-array length families run from EAS/hearing-preservation short ~10 mm to European long 19–22 mm; for a common cavity, estimate the array length from the cavity radius (2πr).

CAngular depth of insertion

CDL also informs the estimated angular depth of insertion — good values ~420–600°depending on the array — measured from a reference line (apex of the superior SCC → centre of the vestibule → round window). Imaging-driven array choice then integrates with the gusher and ossification findings (conical-stopper arrays for IP/EVA, compressed/split arrays for the ossified cochlea). The scan's numbers become the implant order.

Case 12.12 · The scan picks the array

Imaging measures a hypoplastic cochlea with a duct length of ~20 mm and an intact-looking modiolus is absent.

What array does this imply?

Self-assessment — Module 112 questions

Question 1 · Trainee

What does a cochlear duct length <25 mm indicate?

Question 2 · Clinician

When is a perimodiolar array appropriate?

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