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Is there an optimal range of electrode array insertion angles for electric-alone stimulation?
Boyd P.J. , Gibson P.J. , Carpenter R.M.
Manchester ENT Clinic, Manchester, United Kingdom, 2Cochlear Ltd, Macquarie University, Australia
Introduction: Widespread adoption of electro-acoustic stimulation (EAS) and an increased awareness of the
importance of hearing preservation have driven development of a wide range of CI electrode arrays in recent
years, where the length of the array is one of the most important variables. There is general acceptance that
shorter arrays are more appropriate for cases where there is relatively good pre-operative residual hearing as it
is clear that hearing preservation is less successful when very long arrays are used. The optimal electrode
insertion angle for cases where there is no usable residual hearing is more uncertain, however, and the selection
of electrode for EAS cases needs to consider likely electric-alone performance if residual hearing is lost during or
following surgery.
Methods: The aim of this presentation is to examine the published literature for evidence of an optimum range of
electrode insertion angle for electric-alone stimulation. Papers providing data on influences of electrode length
on performance outcomes and hearing preservation were identified from PubMed and Medline searches and
relevant data extracted.
Results: The rationale for a relatively long electrode is to maximize the spectral information delivered, but there
is evidence from psychoacoustic, modeling and speech recognition studies that spectral information provided by
electrical stimulation towards the apex of the cochlea may be unreliable, probably because the spiral ganglion
extends only to some 1.5 to 2 cochlear turns. Several studies reporting on outcomes with relatively short arrays
(often those designed for EAS) have demonstrated reduced electric-alone performance when electrode insertion
angle is much below 360 , though interpretation of some of these studies is complicated by confounding factors
such as electrode number. On the other hand, there appears to be little evidence for any correlation between
performance outcomes and insertion angles between around 1 and 1.5 cochlear turns. There may be some
differences in this respect between perimodiolar and straight electrodes, however, the latter being currently
generally favored for hearing preservation.
Conclusions: It appears that there is a reasonably wide range of insertion angles over which optimal electricalone performance is obtained, and in cases where hearing preservation is an issue it seems logical to limit
insertion depth to the lower end of this range. Cochlear dimensions vary significantly among individuals, but an
array of appropriate length would be expected to result in a satisfactory insertion angle in the large majority of
cases, such that pre-operative estimation of cochlear size may be of little practical importance.