A Comparison of MEDEL Standard and Perimodiolar Cochlear Implant Electrodes

1
A Comparison of MEDEL Standard and
Perimodiolar Cochlear Implant Electrodes
Adrien A. Eshraghi MD, MSc, Thomas J. Balkany MD,
FAACUniversity of Miami School of Medicine,
Miami, Florida -WPB VA Medical Center, West Palm
Beach, Florida
Introduction Recent designs of cochlear implant
electrodes are intended to position contacts
closer to the modiolus in order to reduce power
consumption and increase stimulation selectivity.
However, this may result in increased damage to
the cochlea in some cases. Because of
reimplantation, implantation with residual
hearing, possible future bilateral implantation,
the prevention of damage to cochlear structures
during implantation is of greater importance than
it has been in the past. The purpose of this
study is to compare the insertional trauma as
well as the electrode position in the cochlea, of
the MED-EL standard electrode (C40) with the
MED-EL developmental perimodiolar electrode
(C40PM).
Horizontal section of the cochlea by the
cryomycrotome
Temporal bone, Horizontal section at the level
of cochlea, midmodiolar
Material and methods Electrode designs Two
electrodes were examined the standard Combi 40
and the experimental Combi40PM. For the
experimental design, a restraining arm is placed
within a microgroove on the antimodiolar
electrode surface. After insertion into scala
tympani, the electrode is slightly withdrawn
while holding the restraining arm in place. This
maneuver positions the electrode contacts closer
to modiolus. Electrode insertion Eight cadaver
temporal bones underwent surface preparation to
remove all excess bone and were implanted with
either the standard or perimodiolar cochlear
electrode arrays by MED-EL Corporation. All
insertions (3 for standard and 5 for perimodialar
electrode design) were performed under
microscopic visualization, image-enhanced
videofluoroscopy was used to assess insertion
dynamics. Cryosectioning technique After fluid
immersion and air extraction, the cochlea was
placed in the freezer to allow the fluid within
the cochlea to solidify. Then the cochlea was
immersed in a methyl cellulose to form a large
block of reinforced ice that could be positioned
on a cryomicrotome tissue holder. A Leica CM3 600
cryomicrotome was used to section the
undecalcified cochlea with the electrode in
place. The temporal bones were examined and
photographed at 200 microns intervals. Analysis
of trauma and electrode position The extent of
cochlear trauma was rated on a scale of 0 to
4. Insertion depth and position of each electrode
contact are assessed using computer-assisted
morphometric analysis of the photographic image.
Cryohistology photograph of temporal bone
implanted with Combi40 Grade of trauma 0 No
observable damage
Cryohistology photograph of temporal bone
implanted with Combi40PM Grade of trauma 4
Fracture of spiral lamina
Grade of trauma for each implanted temporal
bone. The trauma severity was evaluated on a
scale of 0 to 4 where 0 no major trauma, 1
elevation of basilar membrane, 2 rupture of
basilar membrane, 3 electrode in scala
vestibuli, and 4 severe trauma such as fracture
of spiral lamina.

Mean distances from the modiolus ( standard
deviation) for electrode contacts presented for
Combi 40PM. The distances for Combi40 is not
presented, all electrodes were positioned in the
outer wall of the cochlea.
Discussion The developmental perimodiolar design
was effective in positioning the electrode
contacts closer to the modiolus than the standard
electrode. However, the insertion trauma caused
by this electrode was greater than the standard
MED-EL electrode. While the examination of the
cryohistology sections of the two Combi40
standard arrays do not show any observable damage
to the cochlea, three out of five experimental
arrays show various degrees of trauma (from
elevation of basilar membrane to fracture of the
spiral lamina). A technical failure resulted in
loss of data from one of the standard arrays.
Videofluoroscopic observations of the electrode
insertions do demonstrate the dynamics of some of
the trauma. In retropositioning the developmental
MED-El Combi40PM, trauma was generally caused by
the restraining arm.
Cadaver temporal bone after surface preparation
to remove all excess bone before implantation
Fluoroscopic image of cochlear implant array
inserted into human cochlea
References 1.Roland JT Jr, Fishman AJ, Alexiades
G, Cohen NL. Electrode to modiolus proximity a
fluoroscopic and histologic analysis. Am J Otol
2000 21 2 18-225. 2.Tykocinski M, Cohen LT,
Pyman BC, Roland JT Jr, Treaba C, Palamara J, et
al. Comparison of electrode position in the human
cochlea using various perimodiolar electrode
arrays. Am J Otol 2000 21 205-211. 3.Shepherd
RK, Htsushika S, Clark GM. Electrical stimulation
of the auditory nerve the effect of electrode
position on neural excitation. Hearing Research
199366108-1206. 4.Balkany TJ, Eshraghi AA. Yang
N. Modiolar proximity of three new perimodiolar
cochlear implant electrodes. Acta
Otolaryngologica (Stockholm)2002 122356-362
. 5.Linthicum FR Jr, Galey FR. Histologic
evaluation of temporal bones with cochlear
implants. Ann Otol Rhinol Laryngol 1983 92
610-13. 6.Kennedy DW. Multichannel intracochlear
electrodes mechanism of insertion trauma.
Laryngoscope 1987 97 42-49. 7.Fayad J,
Linthicum FR Jr, Otto SR, Galey FR, House WF.
Cochlear implants histopathologic findings
related to performance in 16 human temporal
bones. Ann Otol Rhinol Laryngol 1991 100
807-811. 8.OLeary MJ, Fayad J, House WF,
Linthicum FR Jr. Electrode insertion trauma in
cochlear implantation. Ann Otol Rhinol Laryngol
1991 100 695-699. 9.Gstoettner W, Franz P,
Hamzavi J, Plenk H Jr, Baumgartner W, Czerny C.
Intracochear position of cochlear implant
electrodes. Acta Otolaryngol (Stockh) 1999 119
229-233. 10.Nadol JB, Shiao JYS, Burge BJ, et al.
Histopathology of cochlear implants in humans.
Ann Otol Rhinol Laryngolo 2001110883-891.
Cochlea immersed in a methyl cellulose to form a
large block of reinforced ice
A Leica CM3 600 cryomicrotome was used to
section the undecalcified cochlea with the
electrode in place.