Intracochlear Position of Cochlear Implant Electrodes

Acta Otolaryngol (Stockh) 1999; 119: 229 – 233

Intracochlear Position of Cochlear Implant Electrodes WOLFGANG GSTOETTNER1, PETER FRANZ1, JAFAR HAMZAVI1, HANNS PLENK JR2, WOLF BAUMGARTNER1 and CHRISTIAN CZERNY3 From the 1Department of Otolaryngology, 2Bone and Biomaterial Research Department, Histological-Embryological Institute, and 3 Department of Radiology, Uni6ersity of Vienna, Austria

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Gstoettner W, Franz P, Hamzavi J, Plenk H, Baumgartner W, Czerny C. Intracochlear position of cochlear implant electrodes. Acta Otolaryngol (Stockh) 1999; 119: 229–233. There are many different types of cochlear implants available on the market today and they are constantly being re-evaluated and changed. Knowledge of the exact position of the electrode within the cochlea is important in order to improve the electrical stimulation of the hearing nerve by the implants. The stimulating electrodes are usually located peripherally within the scala tympani, although several attempts have been made to develop peri-modiolar located electrode arrays. In this study, our goal was to evaluate the intracochlear positions of Nucleus, Combi 40/Combi 40 + , and newly developed peri-modiolar positioned electrodes by inserting them into fresh human temporal bones. After insertion, the bones were then histologically processed with the electrodes in situ, following perilymphatic formalin perfusion and methylmethacrylate embedding. Sections of the bones 80 – 100 mm thick were prepared using a sawing, grinding and polishing technique. This technique resulted in excellent preservation of the inner ear structures and clear identification of each electrode. The different types of electrodes were then evaluated as to their insertion depth, trauma to cochlear structures and location in relation to the scala tympani walls. Key words: hearing ner6e, auditory, electrostimulation, neurostimulation, human temporal bones, Combi 40, Nucleus, perimodiolar electrode.

INTRODUCTION Many different types of cochlear implant systems are currently in use worldwide. The stimulation electrode of each device varies in the number of wires and contacts it contains, length, diameter and mechanical properties. Cochlear implants target the spiral ganglion cells and its dendrites with electrostimulation. In order to position the contacts of the stimulating electrode as close as possible to these neural elements, it is usually inserted into the scala tympani or scala vestibuli using a posterior tympanotomy-cochleostomy approach. A major goal in cochlear implantation surgery is to avoid insertional trauma as much as possible. In order to obtain information concerning the mechanisms of insertional trauma due to cochlear implant electrode insertion, a number of studies have already been performed. These studies used human cadaver temporal bones (1, 2), animal experiments (3) or evaluated histopathology of cochlear implanted patients (4–8). However, in most of these studies the electrodes had been removed prior to histological processing of the temporal bones. Therefore, little information is available today concerning the details of the position of the stimulating electrodes within the cochlea. The present study was performed in order to collect information about the insertion depth and exact intracochlear position of the Nucleus, Combi 40/ Combi 40+ and a newly developed peri-modiolar electrode when inserted in human temporal cadaver bones. A unique histological preparation technique © 1999 Scandinavian University Press. ISSN 0001-6489

was used, which allowed for sectioning and preparation of histological sections with the electrodes in situ. This technique has already been shown to be suitable for examination of human temporal bones inserted with cochlear implant electrodes (9). MATERIALS AND METHODS Three Nucleus electrodes, three Combi 40/Combi 40+ electrodes and two newly adapted Combi 40 electrodes designed for a peri-modiolar insertion have been used in this study. Eight fresh human cadaver temporal bones were harvested within 16–18 h of death and implanted immediately with one of these electrode arrays. All bones were implanted with new and previously unused electrodes, and all of the implantations were performed by the same surgeon. A standard surgical posterior tympanotomy-cochleostomy approach (10–12) was used for all specimens. Each insertion was performed to the point of first resistance, at which point the insertion depth was measured. All of the temporal bones were then Xrayed and the insertion depth in terms of degrees was calculated from the X-rays (13). With the electrode in place, the temporal bones were fixed by perilymphatic perfusion via the oval window, using buffered formalin solution. The specimens were then kept in the same fixative at 4°C for at least 24 h. All specimens were embedded in polymethylmethacrylate which permits sectioning of undecalcified bone with the implants in situ using a sawing and grinding technique. The detailed histological procedure has been described by Plenk (14). The specimen blocks were cut


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either in the horizontal or sagittal plane and stained in Giemsa solution. The Nucleus electrodes (Fig. 1), provided by the Cochlear Corporation, have a flexible and free-fitting array containing 32 platinum bands with the apical 22 bands active and the proximal 10 bands included for support of the Silastic carrier. The bands are each 0.3 mm wide and spaced 0.45 mm apart. The electrode is 0.6 mm in diameter and tapers to 0.4 mm at its rounded tip. The Combi 40/Combi 40+ electrodes (Fig. 1) were supplied by the Med El Corporation. The array consists of eight pairs of contacts (12 pairs for the Combi 40+ ) arranged in a twin surface configuration on a silastic silicone rubber carrier. It is a non-preformed, flexible, free-fitting electrode with a diameter of 0.5 mm at its tip, which gradually thickens to 0.6 mm. The contacts are distributed over a length of 27 mm and the 30-mm distance is marked by an annular thickening. The Combi 40+ perimodiolar electrode (Fig. 1) was provided by the Med El Company. It has the same dimensions as the Combi 40 + , with an additional microgroove located on the electrode surface and a thin nitinol wire inside this microgroove. During insertion into the scala tympani, the wire is inside the groove, and after the point of first resistance is reached, the tip of the electrode is secured in place by holding onto the wire with tweezers. The electrode is then retracted, thereby moving in position from the lateral cochlear wall closer towards the modiolus. During this movement the nitinol wire moves out of the electrode and holds its position at the lateral scala tympani wall. The electrode is then held in place by fixing it to the wire.

Fig. 1. (A) The Nucleus electrode with 32 platinum bands. (B) The Combi 40 electrode with 8 pairs of contacts. (C) The Combi 40 + electrode with 12 pairs of contacts. (D) The Combi 40 + perimodiolar electrode with its nitinol wire.

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Fig. 2. X-ray after implantation of the Nucleus electrode. Insertion depth 360°.

RESULTS The insertion depth of the Nucleus electrode into the scala tympani of the human temporal bones to the point of first resistance was between 19 and 21 mm. As seen in the X-ray of one temporal bone inserted with a Nucleus device, (Fig. 2) this is a depth of approximately 360°. With such an insertion depth all of the 22 active contacts were found to be located within the basal turn of the scala tympani. The electrodes could be clearly identified in the histological sections (Fig. 3). Different components of each electrode, such as the silastic carrier, platinum bands and wires, were recognized. In these cross-sections, the Nucleus electrode exhibited a circular shape and was found to be peripherally located within the scala tympani. However, in all three temporal bones implanted with this device, we found space between the electrode and the surrounding walls of the scala (Fig. 3), especially in the inferior and ascending portion of

Fig. 3. Temporal bone section through the basal cochlear turn. The Nucleus electrode is located peripherally in the scala tympani with space between the electrode and surrounding scala tympani walls. Bar = 0.5 mm.



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Fig. 4. X-ray after implantation of the Combi 40+ electrode. Twelve pairs of contacts are seen in the basal and middle turn of the cochlea. Insertion depth 630°.

the basal turn. In the region near the electrode tip, the Nucleus electrode was found to be in close contact with the structures of the scala tympani wall, such as the spiral ligament and basilar membrane. The insertion depth of the Combi 40 electrode to the point of first resistance was found to be 27–30 mm. As seen in an X-ray after Combi 40 electrode implantation, (Fig. 4) this corresponds to an insertion of approximately 630°. The different components of the Combi 40 electrode, such as the silicon carrier and wires, could be identified clearly in the temporal bone sections (Fig. 5). These electrodes were found to be positioned peripherally within the scala tympani of the basal and middle cochlear turn. However, in contrast to the Nucleus electrode, the Combi 40 was in close contact with the spiral ligament and the basilar membrane over the entire length of the inserted electrode. There was no space visible between this electrode and the wall of the inserted cochlea (Fig. 5). When cross-sectioned, the Combi 40 electrode appears to have an oval shape. The wires and contacts of this device were found underneath the shorter parts of this oval, which were located towards the basilar membrane. Since the contacts of the Combi 40 electrode are arranged in a twin surface configuration, the corresponding contacts and wires were found on the part of the electrode opposite the basilar membrane (Fig. 5). The new perimodiolar electrodes could be inserted to a depth of 25 to 30 mm and were then retracted until the desired perimodiolar electrode position was reached. X-rays of the temporal bones clearly demonstrate the electrode contacts in a perimodiolar position (Fig. 6). The histological temporal bone sections also show the position of the electrode close to the modiolus (Fig. 7). The nitinol wire was found to be located close to the spiral

Fig. 5. Temporal bone section through the basal cochlear turn. The Combi 40 electrode array is seen positioned peripherally within the scala tympani in close contact with the basilar membrane and the spiral ligament. Bar =1 mm.

ligament with limited trauma to this structure. However, disruption of the spiral ligament seemed to increase in the direction of the apex of the cochlea.

Fig. 6. X-ray after implantation of the Combi 40 + perimodiolar electrode array. The electrode is positioned close to the modiolus.


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Fig. 7. Temporal bone section through the basal cochlear turn. The Combi 40+ perimodiolar electrode array is seen positioned within the scala tympani close to the modiolus (large arrow). The nitinol wire (small arrow) is found peripherally in the scala tympani and appears to have torn the spiral ligament. Bar =0.7 mm.

DISCUSSION The actual physical components of the various cochlear implants used in this study differ in many aspects (length, diameter, shape, number of wires and contacts, as well as mechanical properties). Therefore, in our examination of these devices, we found differences in insertion depth into the scala tympani, intracochlear electrode position, and mechanism of trauma to the cochlear structures. The histological processing method with cross-sections of the electrodes evaluated in situ proved to be a very effective technique, as the shape and position of these devices could then be identified easily. Both the Nucleus and the Combi 40/Combi 40+ electrodes are easy to insert since they slide along the lateral wall of the scala tympani. However, we found differences in intracochlear electrode position between the two devices. The Combi 40 electrode was seen to lie close to the spiral ligament and basilar membrane throughout its whole intracochlear extent (Fig. 5). The Nucleus electrode was found to have close contact to the lateral cochlear wall especially at the region of the electrode tip. In more basal regions of the cochlea the Nucleus electrode was less close to the spiral ligament and we found space between the electrode and surrounding cochlear walls (Fig. 3). The reason for the differences in intracochlear position between these two devices may exist in the fact that the Nucleus and Combi 40/Combi 40+ electrodes have different mechanical properties. The Combi 40 electrode is soft and oval in shape and adjusts easily to the turn of the lateral cochlear wall. Due to a greater number of wires and contacts the Nucleus electrode is stiffer and shows a certain dis-


tance between the device and the lateral cochlear wall, except at its tip. The differences in insertion depth between the two electrodes may also be due to their different mechanical properties. However, when insertion is limited to the point of first resistance, both electrodes showed only minimal trauma to cochlear structures. Insertion attempts beyond the point of first resistance also reflect the differences in mechanical properties between the two electrode arrays. The tendency of the tip of the Nucleus electrode to imbed into the outer cochlear wall increases during forced insertion (1). Forced insertion of the Combi 40/Combi 40+ device, however, results in the electrode kinking within the basal turn, which increases trauma in this region of the cochlea (9). Insertion of the new perimodiolar electrode into human temporal bones resulted in an electrode position close to the modiolus as seen in the histological sections (Fig. 7) and the X-ray (Fig. 6). Since the nitinol wire which holds the electrode tip in place buckles against the spiral ligament trauma in this region increases (Fig. 7). However, this type of electrode is still under development. Further studies must be undertaken before this new type of electrode can be used in 6i6o.

REFERENCES 1. Kennedy DW. Multichannel intrachochlear electrodes: mechanism of insertion trauma. Laryngoscope 1987; 97: 42 – 9. 2. Welling DB, Hinojosa R, Gantz BJ, Lee J. Insertional trauma of multichannel cochlear implants. Laryngoscope 1993; 103: 995 – 1001. 3. Shepherd RK, Clark GM, Xu S, Pyman BC. Cochlear pathology following reimplantation of multichannel scala tympani electrode array in the macaque. Am J Otol 1995; 16: 186 – 99. 4. Nadol JB, Ketten DR, Burgess BJ. Otopathology in a case of multichannel cochlear implantation. Laryngoscope 1994; 104: 299 – 303. 5. Zappia JJ, Niparko JK, Oviatt DL, Kemink JL, Altschuler RA. Evaluation of the temporal bones of a multichannel cochlear implant patient. Ann Otol Rhinol Laryngol 1991; 100: 914 – 21. 6. O’Leary MJ, Fayad J, House WF, Linthicum FH. Electrode insertion trauma in cochlear implantation. Ann Otol Rhinol Laryngol 1991; 100: 695 – 9. 7. Fayad J, Linthicum FH, 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 – 11. 8. Marsh MA, Coker NJ, Jenkins HA. Temporal bone histopathology of a patient with a Nucleus 22-channel cochlear implant. Am J Otol 1992; 13 (3): 241 – 8. 9. Gstoettner W, Plenk H, Franz P, et al. Cochlear implant deep electrode insertion: extent of insertional trauma. Acta Otolaryngol (Stockh) 1997; 117: 274–7.



10. Gstoettner W, Baumgartner W, Hamzavi J, Franz P. The implantation of the Combi 40 cochlear implant. Cent East Eur J Otorhinolaryngol Head Neck Surg 1996; 1: 214–8. 11. Gstoettner W, Baumgartner W, Hamzavi J, Franz P. Surgical experience with the Combi-40 cochlear implant. Adv Otorhinolaryngol 1997; 52: 143–7. 12. Gstoettner W, Baumgartner W, Franz P, Hamzavi J. Cochlear implant deep insertion surgery. Laryngoscope 1997; 107: 544–6. 13. Cerny C, Steiner E, Gstoettner W, Baumgartner W, Imhof H. Postoperative radiographic assessment of the Combi 40 cochlear implant. Am J Roentgenol 1997; 169: 1689–94.

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14. Plenk H. The microscopic evaluation of hard tissue implants. In: Williams DF, ed. Techniques of biocompatibility testing. Vol. 1. Boca Raton, FL: CRC Press, 1986: 35 – 81. Address for correspondence: Wolfgang Gstoettner, MD University of Vienna — Medical School ENT Department AKH-HNO Waehringer Guertel 18-20 AT-1097 Vienna Austria Tel: + 43 1 40400/3308 Fax: + 43 1 40400/3332 E-mail: [email protected]