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The effect of cochlear duct length and cochlear size on hearing outcomes in hearing preservation
cochlear implantation
Kuthubutheen J. , Grewal A. , Symons S. , Nedzelski J. , Shipp D. , Lin V. , Chen J.
University of Toronto, Sunnybrook Health Sciences Centre, Toronto, Canada, 2University of Western Australia, School of Surgery, Perth,
Intro: Hearing preservation cochlear implantation relies upon the assumption of atraumatic insertion of the
electrode. Whilst it is common to utilize a single length electrode for the majority of implanted ears, it is well
known that there is variability in cochlear duct lengths within the population and even within the same individual.
This variability may result in cochleae that are more or less suited to a particular length of electrode. Insertion
depth may be related to the degree of electrode trauma and therefore hearing outcomes. The aim of this study is
to determine if cochlear duct length is a relevant factor in determining outcomes after hearing preservation
Methods: 56 adult patients undergoing hearing preservation cochlear implantation were reviewed. 35 patients
received the Flex 31 electrode (31mm) and 21 patients received the Flex 28 electrode (28mm). Full insertion was
documented through a round window approach in all patients. Preoperative high-resolution temporal bone CT
scans reformatted in axial and oblique coronal planes were used to measure the basal turn of the cochlea (A
value) and to measure the outer and mid-scalar lengths of the cochlear duct to 720 degrees. Postoperative plain
XRs were done to determine degrees of insertion and number of electrodes within the cochlea. Pure tone
average thresholds and speech discrimination at 6 months were compared between the two groups
Results: The cochlear outer wall and mid-scalar lengths are significantly correlated with the A value measured in
the oblique coronal plane (R=0.7 and 0.6 respectively, p< 0.05). Both measures of cochlear duct lengths were
highly correlated (R=0.85) and normally distributed, consistent with temporal bone studies. The Flex 28 electrode
had a greater mean insertion depth of 525 degrees and 11.3 electrodes within the cochlea compared to the Flex
31 electrode with 488.29 degrees and 10.9 electrodes. The shorter Flex 28 electrode also had a greater depth of
insertion in larger cochleae. In addition, for the Flex 28 electrode, CNC word scores at 6 months were higher in
larger cochleae (R=0.7, p< 0.05) but not for the Flex 31 electrode. Effects on hearing preservation rates will be
Discussion: This study demonstrates that standard radiological software can be used to measure the cochlear
duct length and that the basal cochlear diameter is a predictor of cochlear duct length. The shorter electrode
paradoxically had an overall greater depth of insertion and in larger cochleae, achieved a greater insertion angle
and greater speech discrimination. This may suggest postoperative migration of the longer electrode despite full
Conclusion: Cochlear duct lengths vary between individuals and longer electrode lengths may not necessarily
result in deeper insertions or better outcomes, indicating the need for individualized electrode choice.
Learning outcome: To discuss the role of cochlear size on electrode insertion and outcomes