12The Three Array Families: Lateral-Wall, Perimodiolar, Mid-Scala

TThree resting positions

Three families are defined by intended resting position: lateral-wall (straight) arrays rest against the outer wall under the basilar membrane; perimodiolar (precurved) arrays hug the modiolus near the spiral-ganglion cells; mid-scala arrays aim for the median position. The defining trade-off is modiolar proximity (lower thresholds, less channel interaction) versus atraumatic outer-wall sparing (hearing preservation, lower translocation risk).[2014]

CThe perimodiolar rationale

Perimodiolar proximity is the efficiency rationale: contacts near the spiral-ganglion somata in Rosenthal's canal are theoretically more selective and need lower current, with lower ECAP thresholds and wider dynamic range. But Frijns modelling cautions that beyond the first turn perimodiolar contacts may preferentially excite modiolar 'fibres of passage' (axons), so the benefit concentrates in the basal turn.[2012]

TGeometry sets length

Geometry sets length: lateral-wall arrays must be longer (insertable up to ~720–760°) while perimodiolar arrays achieve meaningful modiolar apposition only to ~360–420°, beyond which the modiolus is too thin. The spiral ganglion spans only ~1.75 turns, and ideal first-formant placement is ~540° (~1.5 turns).[2008]

CEach family's failure mode

Each family has a characteristic failure mode: straight arrays buckle and ride up the outer wall, injuring the spiral ligament, basilar membrane or osseous spiral lamina; perimodiolar arrays risk tip foldover, scalar translocation and over-insertion injury during the rigid straightened delivery phase, and a curled array can compress lateral-wall draining veins.[2005]

TThe mid-scala compromise

Mid-scala arrays (AB HiFocus Mid-Scala) were designed to occupy the median position, avoiding both modiolar tip-foldover and outer-wall trauma — a deliberate compromise between the two extremes.

CMapping makers to families

Manufacturer mapping guides choice: lateral-wall = MED-EL FLEX/Standard, AB 1J/SlimJ, older Cochlear straight arrays; perimodiolar = Cochlear Contour Advance and Slim Modiolar, AB Helix; mid-scala = AB Mid-Scala. Perimodiolar arrays show a slightly lower extrusion rate, and tighter perimodiolar wrap correlates with better CNC word scores independent of manufacturer.