3How Implant Surgery Injures the Labyrinth

TThe histopathologic footprint

In 11 implanted temporal-bone pairs, vestibular end-organ damage was present in 72%, mostly the saccule; 75% of bones with basal-turn damage also had saccular damage In 19 temporal bones, cochlear hydrops was found in 83%, saccular collapse in 56% and saccular hydrops in 22%, suggesting post-op endolymph accumulation and Meniere-like risk Insertion trauma can cause otolith-membrane distortion or collapse, reactive neuromas and vestibulofibrosis.[2002][2006]

CThe pressure-transient mechanism

Vestibular dysfunction is judged unlikely to result from direct histological trauma to the vestibular apparatus; transmitted intracochlear pressure variation during insertion is the more probable cause A bad insertion may cause a transient intracochlear pressure rise of ~100 Pa and a poor insertion possibly ~2.0 kPa, roughly 133-160 dB SPL — comparable to a gunshot or blast adjacent to the ear Cadaveric recordings during insertion reached up to ~169 dB SPL peak, with pressures significantly HIGHER in the lateral semicircular canal than in the cochlea, at acoustic-trauma levels.[2022][2018]

TWhere the array presses hardest

Electrode contact and insertion forces concentrate in the basal turn, the region of first resistance, where studies model potential pressure/trauma points along the spiral The saccule, lying closest to this insertion path, is more vulnerable than the utricle or the canals — consistent with cVEMP being the most frequently abolished test after implantation Cochleostomy can provoke a fibro-osseous reaction and scala-vestibuli fibrosis leading to vestibular endolymphatic hydrops.[2009][2017]

CFrom mechanism to delayed injury

Proposed pathways include direct insertion trauma, intraoperative perilymph loss, endolymphatic-flow disturbance generating hydrops, and a delayed foreign-body reaction with labyrinthitis and vestibular fibrosis These overlapping mechanisms explain both immediate (surgical) and delayed (hydrops-like, weeks-to-months) dizziness Round-window or extended-round-window approaches are reported as less traumatic to inner-ear structures than antero-inferior cochleostomy.[2018][2022]