The cochlear histopathology of chronic intracochlear implantation.

A fundamental question regarding the feasibility of artificial electrical stimulation of auditory nerve in cases of sensory deafness is the fate of that nerve after electrode implantation. The results of chronic scala tympani electrode implantation in normal and neomycin-deafened cats indicate that the vast majority of primary auditory neurons will survive the initial surgical implantation and the long-term interface with the molded silastic electrode for periods of at least 30 months (the longest normal-implant survivors). While there is histologic evidence that some neurons are lost in the basal coil, especially in neomycin-deafened animals, the majority of spiral ganglion cells and their radial fibres survived neomycin administration and surgical implantation even in this region. The implantation of electrodes in neomycin-deafened cats did not result in heavy neuronal degeneration. Indeed, there was little apparent difference in nerve survival between the implanted and unimplanted ears of the neomycin animals for periods up to five months (the longest neomycin implant survivor). Traumatic electrode insertion with injury to the boney covering of the modiolus or disruption of the basilar membrane resulted in extensive nerve loss in the traumatized region. When carefully inserted by an experienced otologist, the molded-silastic scala tympani electrode permitted discrete differential stimulation of restricted segments of auditory nerve and produced little or no neural degeneration.     

Evaluation of the insertion-trauma of the Nucleus Contour Advance electrode-array in a human temporal bone model

BACKGROUND: The development of intracochlear electrode arrays is aiming at a placement close to the modiolus with an insertion as atraumatic as possible. A new perimodiolar electrode model the Nucleus Contour Advance was to be evaluated regarding the possible intracochlear trauma. METHODS: The implantation of the Contour Advance electrode was performed in 11 frozen native temporal bones. Beneath a regular insertion in 5 temporal bones in 6 cases the insertion was carried out using the "advance-off-stylett" technique with a fixed stylett. The temporal bones were embedded in metacrylate based resin for histomorphological evaluation. The evaluation was performed regarding to the intracochlear placement close to the modiolus and the damage to intracochlear fine structures (basilar membrane, osseus spiral lamina). RESULTS: In 2 out of 11 cases we found a perforation from the scala tympani to the scala vestibuli independent of the insertion-technique. A severe intracochlear trauma was observed in one case with fracture of osseus spiral lamina using the AOS-technique. A close position to the modiolus could be achieved by insertion the scala tympani without perforation of the basilar membrane. CONCLUSIONS: The Nucleus Contour Advance electrode array showed minimal trauma in human temporal bones by using a standard insertion technique. By using the freehand AOS-technique a severe cochlear trauma is possible. Therefore further development in electrode design and the use of an insertion-tool is recommended.     

Evaluating cochlear implant trauma to the scala vestibuli

Objectives: Placement of cochlear implant electrodes into the scala vestibuli may be intentional, e.g. in case of blocked scala tympani or unintentional as a result of trauma to the basilar membrane or erroneous location of the cochieostomy. The aim of this study was to evaluate the morphological consequences and cochlear trauma after implantation of different cochlear implant electrode arrays in the scala vestibuli. Design: Human temporal bone study with histological and radiological evaluation. Setting: Twelve human cadaver temporal bones were implanted with different cochlear implant electrodes. Implanted bones were processed using a special method to section undecalcified bone. Main outcome measures: Cochlear trauma and intracochlear positions. Results: All implanted electrodes were implanted into the scala vestibuli using a special approach that allows direct scala vestibuli insertions. Fractures of the osseous spiral lamina were evaluated in some bones in the basal cochlear regions. In most electrodes, delicate structures of the organ of Corti were left intact, however, Reissner's membrane was destroyed in all specimens and the electrode lay upon the tectorial membrane. In some bones the organ of Corti was destroyed. Conclusions: Scala vestibuli insertions did not cause severe trauma to osseous or neural structures, thus preserving the basis for electrostimulation of the cochlea. However, destruction of Reissner's membrane and impact on the Organ of Corti can be assumed to destroy residual hearing.     

Evaluation of the HiFocus electrode array with positioner in human temporal bones

The aim of the study presented was to assess the insertion mode and possible intracochlear trauma after implantation of the HiFocus electrode with positioner in human temporal bones. The study was performed in five freshly frozen temporal bones. The position of electrodes was evaluated using conventional X-ray analysis, rotational tomography and histomorphological analysis. Insertion of the HiFocus electrode with positioner resulted in considerable trauma to fine cochlear structures including fracture of the osseous spiral lamina, dislocation of the electrode array from the scala tympani into the scala vestibuli and fracture of the modiolus close to the cochleostomy. The implication of the results regarding clinical outcome will be discussed.     

Clarion 1.2 standard electrode array with partial space-filling positioner: radiological and histological evaluation in human temporal bones

The purpose of this study was to evaluate whether use of a positioner for situating the Clarion 1.29 standard electrode array in close proximity to the modiolus, causes damage to fine intra-cochlear structures, and to provide a comparison with results obtained for insertions of the array performed without a positioner. The study was performed in seven freshly frozen human temporal bones. Electrode location and intra-cochlear trauma was analysed using cross-sectional imaging and histological analysis. Insertion of the Clarion array did not reveal major trauma. The devices inserted with the positioner showed a consistently closer location of the electron array towards the modiolus, however, insertion resulted in significant displacement of both the electrode array and the positioner resulting in severe destruction of the basilar membrane and osseous spiral lamina along the length of the basal and middle turns. The devices inserted with the positioner resulted in major trauma to the basilar membrane and osseous spiral lamina. Therefore, systematic safety studies in larger samples of human temporal bones should be performed and the results carefully evaluated before implantation can be recommended unreservedly.     

An improved cochlear implant electrode array for use in experimental studies

Experimental studies play an important role in establishing the safety and efficacy of cochlear implants and they continue to provide insight into a new generation of electrode arrays and stimulation strategies. One drawback has been the limited depth of insertion of an electrode array in experimental animals. We compared the insertion depth and trauma associated with the insertion of Cochlear Ltd's Hybrid-L (HL) array with a standard 8 ring array in cat cochleae. Both arrays were inserted into cadaver cochleae and an X-ray recorded their anatomical location. The implanted cochlea was serially sectioned and photographed at 300?μm intervals for evidence of electrode insertion trauma. Subsequently two cats were chronically implanted with HL arrays and electrically-evoked potentials recorded over a three month period. Mean insertion depth for the HL arrays was 334.8° (SD?=?21°; n?=?4) versus 175.5° (SD?=?6°; n?=?2) for the standard array. This relates to ～10.5?mm and 6?mm respectively. A similar insertion depth was measured in a chronically implanted animal with an HL array. Histology from each cadaver cochleae showed that the electrode array was always located in the scala tympani; there was no evidence of electrode insertion trauma to the basilar membrane, the osseous spiral lamina or the spiral ligament. Finally, evoked potential data from the chronically implanted animals exhibited significantly lower thresholds compared with animals implanted with a standard 8 ring array, with electrical thresholds remaining stable over a three-month observation period. Cochlear Ltd's HL electrode array can be safely inserted ～50% of the length of the cat scala tympani, placing the tip of the array close to the 4?kHz place. This insertion depth is considerably greater than is routinely achieved using a standard 8-ring electrode array (～12?kHz place). The HL array evokes low thresholds that remain stable over three months of implantation. This electrode array has potential application in a broad area of cochlear implant related research.     

Flat-panel volume computed tomography for cochlear implant electrode array examination in isolated temporal bone specimens.

HYPOTHESIS: Flat-panel based volume computed tomography could improve cochlear implant electrode evaluation in comparison with multislice computed tomography. BACKGROUND: Flat-panel based volume computed tomography offers higher spatial resolution and less metal artifacts than multislice computed tomography. Both characteristics could improve the evaluation of challenging but important questions in cochlear implantation assessment, such as an exact imaging of cochlea, osseous spiral lamina, electrode array position, and single electrode contacts. These questions are not currently fully answered by multislice computed tomography. METHODS: Four isolated temporal bone specimens were scanned in a current multislice computed tomography scanner and in two experimental flat-panel based volume computed tomography scanners before and after cochlea implantation. To compare flat-panel based volume computed tomography and multislice computed tomography, four features were rated according to the following criteria: 1) visibility of the cochlea; 2) visibility of the osseous spiral lamina; 3) discernibility of individual electrode contacts; and 4) the ability to determine the electrode array position relative to scala tympani and scala vestibuli. Layer-by-layer microgrinding pictures were used as the ground truth for verification of imaging findings. RESULTS: Flat-panel based volume computed tomography was superior to multislice computed tomography in all four features rated. The cochlea and facial nerve canal were much better delineated in flat-panel based volume computed tomography. The osseous spiral lamina and single electrode contacts were only visible in flat-panel based volume computed tomography. Assessment of implant position with regard to the cochlear spaces was considerably improved by flat-panel based volume computed tomography. CONCLUSION: Cochlear implantation assessment could be improved by flat-panel based volume computed tomography and, therefore, would be highly beneficial for cochlea implantation research and for clinical evaluation. However, these first results were shown by scanning isolated temporal bone specimens; scanning whole human skull bases might be more challenging.     

Development and evaluation of an improved cochlear implant electrode design for electric acoustic stimulation

OBJECTIVE: The objective of this study was to assess the intracochlear position and the extent of trauma to cochlear structures using a new prototype electrode carrier (Flex EAS). Special emphasis was placed on the practicality for combined electric and acoustic stimulation of the auditory system. STUDY DESIGN: Human temporal bones were evaluated histologically after insertion of the electrodes, and insertion forces were measured in an acrylic model of the scala tympani. METHODS: 1) Insertion forces with the regular C40+ array and the new electrode prototype were measured in an acrylic model of the scala tympani. 2) Ten human temporal bones were implanted using the same surgical procedure as in vivo. All bones underwent fixation methylmethacrylate embedding to allow cutting of the undecalcified bone with the electrode in situ. In addition, radiography of the implanted devices was performed and correlated to histologic results. Electrode positions and trauma to cochlear structures were then evaluated histologically. RESULTS: All insertions of the new electrode array were performed in the scala tympani of the cochlea. All insertions were atraumatic and covered one cochlear turn. The only effect on cochlear structures that could be observed was a slight lifting of the basilar membrane in the middle turn limited to the tip of the electrode. In three bones, basal trauma, which resulted from the cochleostomy itself, could be observed as well. All neural structures remained intact. CONCLUSIONS: The new electrode prototype provides very good mechanical properties for safe and atraumatic implantation. All criteria for the use in hearing-preservation cochlear implantation for electric and acoustic stimulation were fulfilled. Surgical measures to prevent basal trauma appear to be very important.     

Surgical implications of perimodiolar cochlear implant electrode design: avoiding intracochlear damage and scala vestibuli insertion

Abstract

Objective To review the mechanisms and nature of intracochlear damage associated with cochlear implant electrode array insertion, in particular, the various perimodiolar electrode designs. Make recommendations regarding surgical techniques for the Nucleus Contour electrode to ensure correct position and minimal insertion trauma.

Background The potential advantages of increased modiolar proximity of intracochlear multichannel electrode arrays are a reduction in stimulation thresholds, an increase in dynamic range and more localized neural excitation. This may improve speech perception and reduce power consumption. These advantages may be negated if increased intracochlear damage results from the method used to position the electrodes close to the modiolus.

Method A review of the University of Melbourne Department of Otolaryngology experience with temporal bone safety studies using the Nucleus standard straight electrode array and a variety of perimodiolar electrode array designs; comparison with temporal bone insertion studies from other centres and postmortem histopathology studies reported in the literature. Review of our initial clinical experience using the Nucleus Contour electrode array.

Results The nature of intracochlear damage resulting from electrode insertion trauma ranges from minor, localized, spiral ligament tear to diffuse organ of Corti disruption and osseous spiral lamina fracture. The type of damage depends on the mechanical characteristics of the electrode array, the stiffness, curvature and size of the electrode in relation to the scala, and the surgical technique. The narrow, flexible, straight arrays are the least traumatic. Pre-curved or stiffer arrays are associated with an incidence of basilar membrane perforation. The cochleostomy must be correctly sited in relation to the round window to ensure scala tympani insertion. A cochleostomy anterior to the round window rather than inferior may lead to scala media or scala vestibuli insertion.

Conclusion Proximity of electrodes to the modiolus can be achieved without intracochlear damage provided the electrode array is a free fit within the scala, of appropriate size and shape, and accurate scala tympani insertion is performed.     

Temporal bone investigations on landmarks for conventional or endosteal insertion of cochlear electrodes

CONCLUSION: Our anatomical findings place special emphasis on the requirement to follow an infero-anterior approach to the round window, to expose the scala tympani safely for 'normal' cochlear implantation. It is also known how easily the basilar membrane may be accidentally damaged, despite exercising considerable caution in the approach used. With regard to an 'endosteal electrode' it can be stated that there are no really specific indicators to locate the spiral ligament, or each of the scalae, on the lateral aspect of the tissue layer encasing the cochlea. For the concept of an endosteal electrode, however, the soft tissue layer of the lateral aspect of the cochlea is considered to be sufficiently thick to serve as a physical barrier between the electrode and the inner ear fluid. OBJECTIVES: To re-evaluate surgical techniques of gaining access to the scala tympani for cochlear implantation (cochleostomy, 'fenestration'). There are two reasons for this study. First, recent publications show that in a significant number of patients the electrode array was unintentionally inserted into the 'wrong' scala (sc. vestibuli). Second, dealing with an alternative concept proposed by Lehnhardt for patients with residual hearing ('endosteal electrode'), the anatomical site of the spiral ligament should be known. In a study on human temporal bones the topography of the middle and inner ear is revised with regard to the presence of anatomical or surgical landmarks that may guide the surgeon. MATERIALS AND METHODS: Anatomical examinations were performed on 10 temporal bones (5 fresh specimens and 5 fixed in formalin), in which the bone of the promontory was carefully milled. The consistency of identification and the relative location of specific surgical indicators or landmarks such as 'blue lines' and 'gray lines' were evaluated for 10 temporal bones. Furthermore, the projection of the lateral attachment of the basilar membrane on the promontory was determined with regard to round window anatomy. RESULTS: In all cases, a major blue line indicated the lateral aspect of the basal cochlear turn while milling the promontorial bone. In a limited number of cases (20%), an additional gray line potentially indicated the spiral ligament before the last shell of bone was removed. In 80% of the cases it was possible to remove the bony layer and leave the endosteum intact as a precondition for a potential endosteal electrode insertion. In addition, through the examination of these models, the relative anatomical location of structures, such as the scala vestibuli, scala tympani, spiral ligament, and basilar membrane, is reviewed.     

The Nucleus Contour electrode array: a radiological and histological study

OBJECTIVES: To evaluate the handling and insertion trauma of the recently developed Nucleus perimodiolar Contour electrode array (Cochlear Ltd., Pty, Lane Cove, New South Wales, Australia) in human temporal bones compared with the Nucleus standard straight electrode array. STUDY DESIGN: E-perimental control group. METHODS: Twenty-nine fresh-frozen bones were implanted with different electrode arrays by an experienced cochlear implant surgeon, and evaluated both radiologically and histologically. RESULTS: Intracochlear insertion of the standard Nucleus straight electrode array was found to be atraumatic, confirming previous findings in the literature. Insertion of the Nucleus Contour electrode array resulted in instances of localized basilar membrane penetration causing the electrode array to move from the scala tympani into the scala vestibuli. However, this trauma did not result in any observable damage to the osseous spiral lamina or the modiolus. Basilar membrane penetration was observed in six of eight cochlear bones when a standard cochleostomy size (approximately 0.8 mm) and site (anterior and superior to the round window) were used. However, when the surgical technique was modified to use a slightly larger cochleostomy ( approximately 1.8 mm) situated closer to the round window and employ a partial stylet withdrawal technique during electrode insertion, the frequency of penetrations was restricted to two of seven bones. This trauma rate is comparable to that observed with other cochlear implants designs. CONCLUSIONS: Following our results, the design of the Nucleus Contour electrode appears to fulfill the safety requirements for an intracochlear electrode array, provided that the surgical insertion technique is modified in the manner outlined.     

Surgical anatomy of the round window with special reference to cochlear implantation

When the multi-channel cochlear implant electrode is inserted into the scala tympani through the round window the operation is best performed via a posterior tympanotomy. The view of the round window membrane, however, is incomplete because of its orientation and the fact that it has a conical shape. Nevertheless, a good view along the basal turn is obtained after the antero-inferior overhang of the round window niche and the crista fenestrae have been removed. It might be damaging to drill away the postero-superior overhang as the osseous spiral lamina lies extremely close to the round window membrane.     

Scalar localization of the electrode array after cochlear implantation: a cadaveric validation study comparing 64-slice multidetector computed tomography with microcomputed tomography.

HYPOTHESIS: Improved resolution available with 64-slice multidetector computed tomography (MDCT) could potentially be used clinically to localize the cochlear implant (CI) electrode array within the basal turn. BACKGROUND: In CI surgery, the electrode array should be inserted into and remain within the scala tympani to avoid injury to Reissner's membrane and the scala media. Correlating the position of the electrode in the basal turn with surgical technique and implant design could be helpful in improving outcomes. METHODS: After a standard left mastoid exposure of the round window niche through the facial recess performed on a cadaver head, an electrode array from a Nucleus Softip Contour CI was fully inserted through a cochleostomy. The head was then scanned axially on a 64-slice MDCT with 0.4-mm slice thickness and reconstructed into the oblique axial, oblique coronal, and oblique sagittal planes of the cochlea. The temporal bone was then harvested and imaged on a microcomputed tomographic scanner using 20-microm slice thickness. Identical reconstructions were made and compared with the 64-slice images to confirm exact location of the electrode array. RESULTS: The 64-slice MDCT accurately localized the electrode array to the scala tympani. This was best demonstrated in the oblique sagittal plane, identifying the electrode array in the posterior inferior portion of the basal turn, posterior to the spiral lamina. CONCLUSION: This ex vivo validation study suggests that 64-slice MDCT has the potential to allow accurate localization of the CI electrode array within the basal turn of the cochlea.     

International consensus on middle ear implants

Conclusion. Our anatomical findings place special emphasis on the requirement to follow an infero-anterior approach to the round window, to expose the scala tympani safely for ‘normal’ cochlear implantation. It is also known how easily the basilar membrane may be accidentally damaged, despite exercising considerable caution in the approach used. With regard to an ‘endosteal electrode’ it can be stated that there are no really specific indicators to locate the spiral ligament, or each of the scalae, on the lateral aspect of the tissue layer encasing the cochlea. For the concept of an endosteal electrode, however, the soft tissue layer of the lateral aspect of the cochlea is considered to be sufficiently thick to serve as a physical barrier between the electrode and the inner ear fluid. Objectives. To re-evaluate surgical techniques of gaining access to the scala tympani for cochlear implantation (cochleostomy, ‘fenestration’). There are two reasons for this study. First, recent publications show that in a significant number of patients the electrode array was unintentionally inserted into the ‘wrong’ scala (sc. vestibuli). Second, dealing with an alternative concept proposed by Lehnhardt for patients with residual hearing (‘endosteal electrode’), the anatomical site of the spiral ligament should be known. In a study on human temporal bones the topography of the middle and inner ear is revised with regard to the presence of anatomical or surgical landmarks that may guide the surgeon. Materials and methods. Anatomical examinations were performed on 10 temporal bones (5 fresh specimens and 5 fixed in formalin), in which the bone of the promontory was carefully milled. The consistency of identification and the relative location of specific surgical indicators or landmarks such as ‘blue lines’ and ‘gray lines’ were evaluated for 10 temporal bones. Furthermore, the projection of the lateral attachment of the basilar membrane on the promontory was determined with regard to round window anatomy. Results. In all cases, a major blue line indicated the lateral aspect of the basal cochlear turn while milling the promontorial bone. In a limited number of cases (20%), an additional gray line potentially indicated the spiral ligament before the last shell of bone was removed. In 80% of the cases it was possible to remove the bony layer and leave the endosteum intact as a precondition for a potential endosteal electrode insertion. In addition, through the examination of these models, the relative anatomical location of structures, such as the scala vestibuli, scala tympani, spiral ligament, and basilar membrane, is reviewed.     

Cochlear implantation in rats: a new surgical approach

The laboratory rat has been used extensively in auditory research but has had limited use in cochlear implant related research due mainly to the surgically restricted access to the scala tympani. We have developed a new surgical method for cochlear implantation in rats. The key to this protocol was cauterizing the stapedial artery (SA) and making a small cochleostomy near the round window in order to enlarge the surgical access to the scala tympani. Five normal hearing Hooded Wistar rats were used to investigate the effect of cauterizing the SA on hearing and auditory nerve survival. Results showed that cauterizing the SA was surgically feasible, afforded excellent exposure of the round window niche for cochleostomy, and did not adversely affect acoustic thresholds measured electrophysiologically. Moreover, there was no difference in spiral ganglion cell densities for any cochlear turn when compared with the contralateral control ears. Three deafened rats were subsequently implanted with a scala tympani electrode array using this new surgical approach. Electrically evoked auditory brainstem responses using bipolar stimulation, and subsequent cochlear histopathology demonstrated that cochlear implantation using a custom-made rat electrode array was safe and effective. The surgical approach presented in this paper presents a safe and effective procedure for acute or chronic cochlear implantation in the rat model.     

Tracing neurotrophin-3 diffusion and uptake in the guinea pig cochlea

Neurotrophin therapy in the cochlea can potentially slow or reverse the degeneration of the auditory nerve that occurs during progressive deafness. Studies were performed to trace the diffusion and uptake of neurotrophin-3 (NT-3) following infusion into the cochlea. NT-3 labeled with (125)I or coated onto fluorescent microspheres was introduced into the basal turn of normal hearing and deafened guinea pig cochleae via a single slow-rate injection. Cochleae were examined between 2 h and 28 days post-infusion by autoradiography or fluorescent microscopy to determine the number of turns labeled by NT-3, identify individual cells and tissues receiving NT-3 and quantify the proportion of signal in each tissue. In general, long-term infusions were required for all cochlear turns to receive NT-3. (125)I NT-3 signal was strongest in cells lining the perilymphatic space of the scala tympani, basilar membrane, osseous spiral lamina and spiral ligament. Signal in the peripheral nerve tract and Rosenthal's canal was only 1.3-2.1 times background levels of radiation. NT-3 microspheres were detected within neural areas of the cochlea (nerve tract and Rosenthal's canal) in all cases, but not within neuronal cell bodies. NT-3 microspheres remained in the cochlea for at least 28 days, suggesting a low clearance rate within cochlear tissues.     

Local pO2- and pH2-measurements with needle electrodes for the examination of the hydrogen maintenance and microcirculation of the cochleae (author's transl)]

The local partial pressure of oxygen (pO2) and the rate of hydrogen elimination were measured in the three scalae of the basal turn of 28 Guinea-pig cochleae under conditions of normoxia, hyperoxia and hypercapnia and with acoustical stimulation with the needle electrodes developed by Baumg?rtl and Lübbers. In the scala tympani a pO2 decrease from the round window toward Corti's organ was registered and pO2 values of over 100 mm Hg were measured near the membrane of the round window and of 10-40 mm Hg near the basilar membrane depending on how deeply the electrode penetrated into the scala tympani. The pO2 profiles were changed or reversed when the animal breathed a mixture of 95% oxygen and 5% carbon dioxide and when the round window membrane was covered with agar-agar or paraffine and exteriorly flooded with nitrogen. Acoustical stimulation with a white noise of 85 dB caused a considerable pO2 drop in the perilymph of the scala tympani while in the endolymph of the scala media we observe only a slight decrease. Intravenous application of dextran of low viscosity leads to a pO2 increase when the original oxygen value in the scala tympani was low. The half-life of hydrogen in the scala tympani amounts to about 4 min. The results permit the conclusion that, in the area of the cochlear basis, Corti's organ receives its oxygen supply via the capillary system as well as via the membrane of the round window.     

A silastic positioner for a modiolus-hugging position of intracochlear electrodes: electrophysiologic effects

HYPOTHESIS: It was postulated that an electrode array that achieved a close modiolar proximity would result in reduced threshold levels and amplitude slopes, as measured with electrically evoked auditory brainstem responses (EABRs). BACKGROUND: Quality and quantity of auditory information transmitted by a cochlear implant to patients with sensorineural hearing loss depend on spatial and temporal resolution achieved by the electrical intracochlear stimulation. METHODS: To improve spatial resolution, a new electrode system was developed by Advanced Bionics Corp., with the intention of obtaining greater modiolar proximity. The implant version specified for animal experiments consists of a straight electrode array of seven embedded platinum discs and a so-called Silastic-positioner. The Silastic positioner is shaped to follow the dimensions of the scala tympani with a concave (triangular) inner side, which fits the form of the electrode array. The aim of the study was to evaluate the influence of a modiolus-hugging electrode position in contrast to a conventional electrode position on EABR in short-term animal experiments. Short-term electrophysiologic studies were performed on six adult cats. After local intracochlear application of neomycin solution (50 mg/mL), electrodes were inserted into the scala tympani. Electrically evoked auditory brainstem response threshold levels and EABR amplitude slopes were systematically investigated with and without the positioner. RESULTS: Electrically evoked auditory brainstem response measurements revealed a distinct apicobasal threshold shift, with increasing thresholds toward the basal end of the electrode. After insertion of the positioner, this shift diminished or was inverted and EABR thresholds and amplitude slopes were reduced significantly. CONCLUSIONS: Threshold and amplitude slope data emphasize the functional benefit of the positioner system, especially for the stimulation of electrodes in the more basal channels.     

Anatomy of the round window and hook region of the cochlea with implications for cochlear implantation and other endocochlear surgical procedures.

HYPOTHESIS: The goal of this study was to create a three-dimensional model of the anatomy of the hook region to identify the optimal site for cochleostomy in cochlear implant surgery. BACKGROUND: The anatomy of the hook region is complex, and spatial relationships can be difficult to evaluate using two-dimensional histological slides or cadaveric temporal bones. METHODS: The right temporal bone of a 14-year-old adolescent boy was used to create a three-dimensional model. Sections containing the round window membrane (RWM) and surrounding cochlear structures were stained, digitized, and imported into a general purpose three-dimensional rendering and analysis software program (Amira, version 4.1). Three-dimensional models of the RWM, basilar membrane, osseous spiral lamina, spiral ligament, cochlear aqueduct, inferior cochlea vein, scala media, ductus reuniens, scala vestibuli, scala tympani, and surrounding bone were generated. The relationship between these structures and the RWM and adjacent otic capsule was evaluated. Histological sections from a different temporal bone were also analyzed. This temporal bone was sectioned in a plane perpendicular to the axis corresponding to the surgical view of the RWM, seen through the facial recess. RESULTS: The anteroinferior margin of the RWM or adjacent otic capsule was identified as the site for a cochleostomy that will avoid damage to critical cochlear structures and allow implantation directly into the scala tympani. The model can be downloaded from: https://research.meei.harvard.edu/otopathology/3dmodels. CONCLUSION: This three-dimensional model has implications for surgical procedures to the inner ear that aim to minimize insertional trauma.     

Electrical impedance measurements of cochlear structures using the four-electrode reflection-coefficient technique

In individuals with severe-to-profound hearing loss, cochlear implants (CIs) bypass normal inner ear function by applying electrical current directly into the cochlea, thereby stimulating surviving auditory nerve fibers. Although cochlear implants are able to restore some auditory sensation, they are far from providing normal hearing. It has been estimated that up to 75% of the current injected via a CI is shunted along scala tympani and is not available to stimulate auditory neurons. The path of the injected current and the consequent population of stimulated spiral ganglion cells are dependent upon the positions of the electrode contacts within the cochlea and the impedances of cochlear structures. However, characterization of the current path remains one of the most critical, yet least understood, aspects of cochlear implantation. In particular, the impedances of cochlear structures, including the modiolus, are either unknown or based upon estimates derived from circuit models. Impedance values for many cochlear structures have never been measured. By combining the hemicochlea preparation, a cochlea cut in half along its mid-modiolar plane, and the four-electrode reflection-coefficient technique, impedances can be measured for cochlear tissues in a cochlear cross section including the modiolus. Advantages and disadvantages of the method are discussed in detail and electrical impedance measurements obtained in the gerbil hemicochlea are presented. The resistivity values for the cochlear wall in Ωcm are, 528 (range: 432–708) for scala media 3rd turn, 502 (range: 421–616) for scala tympani 3rd turn and scala vestibuli 2nd turn, 627 (range: 531–759) for scala media 2nd turn, 434 (range: 353–555) for scala tympani 2nd turn and scala vestibuli basal turn, 434 (range: 373–514) for scala media basal turn, and 590 (range: 546–643) for scala tympani basal turn. The resistivity was 455 Ωcm (range: 426–487) for the modiolus.