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Transcript
RT14 Siebold memorial session – Japanese-German friendship
RT14-2
Estimation of the cochlear duct length for MED-EL standard electrode arrays
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Yamamoto N. , Hiraumi H. , Sakamoto T. , Okano T. , Yamazaki H. , Ito J.
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Graduate School of Medicine, Kyoto University, Dept. of Otolaryngology, Head and Neck Surgery, Kyoto, Japan
The standard electrode array of MED-EL is 31.5 mm long and that covers almost full length of the cochlear duct
when fully inserted. According to the MED-EL company, this full coverage of the cochlear duct contributes to
better hearing by stimulating an entire cochlea. In contrast, the length of human cochlear duct is variable. The
anatomical analysis of human temporal bone specimen revealed that the cochlear duct length (CDL) in human
varied from 25.2 to 35.5 mm and 29 out of 69 cases have CDLs less than 31.5 mm (Hardy et al., 1938). This
result suggests that full insertion of the standard electrode array from MED-EL is impossible at least in some
cases even if the cochlea does not have any malformation.
Actually, one case could not obtain full insertion of the electrodes among 24 cochlear implantation cases with
normal cochleae using MED-EL standard electrode array in Kyoto University hospital from 2007 to 2012.
In this study, we retrospectively estimated the CDLs of standard electrode array cases using preoperative CT
scan images and tested if it is possible to predict the full insertion, in other word CDLs, preoperatively. 21 out of
24 cases, where CT data with DICOM format were available, were included in this study. CDLs were estimated
using double-oblique paracoronal reformatted images of cochleae (Escudé et al., 2006). A measurement in the
images from the round window to the opposite wall of the basal turn through the midmodiolar axis (distance A)
(Escudé et al., 2006) was used to calculate CDLs. We used two different equations that are calculating the CDLs
along lateral walls (LW) (Escudé et al., 2006) and organs of Corti (OC) (personal communication with Dr. Claude
Jolly). Distance A ranged from 6.75 to 10.07 mm (average: 8.62 mm, median: 8.62 mm) and estimated CDLs
ranged from 27.9 to 41.5 mm (average: 35.6 mm, median: 35.6 mm) for LW and from 24.1 to 37.9 mm (average:
31.9mm, median: 31.9 mm) for OC. The estimated CDLs for OC were almost comparable to CDLs reported by
Hardy et al. (25.3 - 35.5 mm, average: 31.5 mm). The CDLs for the case that full insertion was not obtained were
27.9 (LW) and 24.1 (OC) mm. In the case that has the second shortest estimated CDLs (LW: 33.0 mm, OC: 29.3
mm), full insertion was difficult in re-implantation after hard failure. By the CDL estimation based on LW, all but
one case had CDLs longer than 31.5 mm, which is consistent with the results of full-insertion achievement. In
contrast, eight cases had estimated CDLs shorter than 31.5 mm by the estimation based on OC length.
These results suggested that estimated CDLs along LW are more preferable to predict the CDL for MED-EL
standard electrode array. This may be due to the position of MED-EL electrode arrays that sit near the lateral
wall of the cochlea. Although 28 mm electrode array is more popular choice currently, the estimation of CDLs
based on preoperative images is still important because some cases have CDLs less than 28 mm.
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