Cochlear implants (CI) have been used successfully for more than two decades as a rehabilitative aid for severe to profound hearing loss. Over this period the expectation for increased communication capability with these devices has grown dramatically. The earliest cochlear implant recipients reported substantial benefits in lipreading performance and recognition of environmental sounds, but little or no recognition of speech using only the auditory information provided by the implant [1
]. As multichannel cochlear implants were introduced, several studies demonstrated that subjects using these devices were capable of discriminating speech without assistance from visual cues. Improvement in CI performance has continued to the present, and many current CI recipients routinely communicate via the telephone and congenitally deaf children who are implanted as infants or toddlers often develop language skills sufficient to allow them to attend mainstream schools. With the dramatic success achieved to date, one might ask what direction future research and development efforts should take to increase the performance of cochlear implants and to benefit subjects with a wider range of hearing impairments?
A cochlear implant operates as an integrated system that includes one or more microphone inputs, a software controlled digital signal processor, a transcutaneous link and an intracochlear stimulating electrode array. In this study we focus on the mechanical design of the electrode array by evaluating five different devices that have been widely implanted in human subjects and three prototype electrode designs to ascertain how specific mechanical properties of each device relate to the incidence of damage.
Three widely accepted goals for the development of future CI electrode arrays include, 1) deeper insertion into the scala tympani to access lower frequency cochlear neurons, 2) greater operating efficiency, defined as a reduction in the stimulus charge required to produce a comfortable loudness level, and 3) reduced intracochlear damage associated with surgical insertion.
CI subject testing and acoustic simulations in hearing subjects have shown that speech recognition is degraded when the frequency bands presented to a listener do not approximate the normal acoustic frequency represented at the cochlear place of stimulation [2
]. Because the tonotopic locations representing the primary speech formant frequencies are located further along the cochlear spiral, i.e. at lower frequency locations, than most fully inserted implant electrode arrays, it is clear that for most users a significant mismatch occurs between the processed frequency band assigned to each stimulus channel and the cochlear place that it excites. Thus, electrodes with mechanical characteristics that facilitate deeper insertion may be advantageous.
Until recently, determining the optimum depth of insertion and distribution of processed frequency information has been impeded by the lack of an accurate frequency-position map of the human spiral ganglion as well as a clinical method to assess where each CI stimulating site is located in relation to that map in an individual subject. Recent studies have determined the relationship between the progression of characteristic frequencies along the basilar membrane [9
] and the comparable frequency vs. position map of neurons in the spiral ganglion [10
]. Using a different experimental approach, two recent studies of CI patients with residual hearing compared the pitch percepts produced by stimulation of individual implant channels with percepts produced by acoustic stimulation of varying frequency in the non-implanted ear [12
]. Clinical methods to better estimate the frequency location of the implanted electrode array in individual subjects using modern high resolution imaging methods have also been proposed [10
]. These anatomical, psychophysical and imaging studies should help to direct the development of electrode arrays and fitting techniques that will result in a more accurate correspondence between the frequency spectra of processed sounds and the location of electrical stimulation.
The efficiency of an intracochlear electrode array is assumed to be highest when each stimulating element is positioned close to the site of neural activation. This site of activation is assumed to be the cell bodies of spiral ganglion neurons or, in some cases, surviving peripheral nerve fibers within the osseous spiral lamina. We also presume that the site of functional neural activation is partly dependent upon the duration of deafness and probably shifts gradually from peripheral dendrites in recently deaf subjects, or those with residual hearing, to the spiral ganglion cell bodies in subjects with the longest history of deafness. In any event, electrode arrays introduced within the past ten years that are designed to position stimulating contacts near the modiolus appear to operate with lower current thresholds than previous devices that were located closer to the lateral wall of the scala tympani [13
]. Examples of these perimodiolar electrode arrays include the Cochlear Contour™ and Advanced Bionics HiFocus™ electrode arrays.
Trauma to the delicate structures of the inner ear frequently occurs during insertion of cochlear implant electrodes [16
]. This damage ranges from relatively minor displacement of the basilar membrane to severe fracture of the osseous spiral lamina, tearing of the basilar membrane or spiral ligament and deviation of the electrode path from its intended location in the scala tympani to the overlying scala media and/or scala vestibuli. Even in cases of moderate severity intracochlear trauma may result in reduced numbers of functional peripheral dendrites or spiral ganglion cells, idiosyncratic distribution of these cells and large variation in the efficiency of stimulating sites along the length of the implanted array. In addition, it is possible that damage to the medial surface of the scala tympani, which separates the scala tympani from the internal auditory meatus, might act as a pathway for infection of the central nervous system.
Factors that may affect the incidence of damage include the mechanical properties of a particular electrode design, variations in the size and shape of each cochlea and the specific surgical techniques used for insertion. Because damage to intracochlear structures occurs most frequently to the partition above the scala tympani it has been proposed that an electrode array designed to minimize upward bending might reduce the incidence and severity of insertion related damage [32
]. A reduction in the incidence of damage with two electrode arrays that were designed with increased vertical stiffness was reported by Wardrop, et al [16
]. However, a more comprehensive evaluation of the mechanical characteristics of a wide range of electrode designs and their possible correlation with insertion damage has not been reported.
Skinner, et al, clearly documented significant variability in the size of the human scala tympani [33
] and this variability may be a be an important factor associated with the rate of trauma observed for cochlear implant electrodes with larger cross sectional dimensions [16
]. Two current cochlear implant manufacturers, Advanced Bionics, Inc. and Cochlear Corporation, have introduced curved electrodes which utilize an internal stylet to hold the electrode straight during all or part of the insertion process. With both devices the manufacturers recommend that the electrode and stylet be partially inserted into the scala tympani before the electrode is pushed off of the stylet to its full insertion depth. In this process the pre-coiled electrode array returns to a spiral shape intended to facilitate insertion and ultimately position the electrode array near the modiolus. This technique has been termed AOS (Advance Off Stylet). Successful insertion using this procedure requires that the tip of the electrode and stylet be positioned at the beginning of the first cochlear turn prior to pushing the electrode off of the stylet. If the electrode is advanced off of the stylet with the tip positioned too close to the cochleostomy the tightly curved tip of the array may fold back upon itself in the expanded basal cavity of the scala tympani. Conversely, if the straightened electrode and stylet are inserted too far beyond the cochleostomy the electrode tip will contact the outer wall of the first cochlear turn possibly resulting in damage to the spiral ligament or upward penetration into the scala vestibuli.
The implications of insertion trauma for subjects with residual hearing may be even more serious than those for profoundly deaf cochlear implant users, since damage to the organ of Corti could severely affect the transmission and distribution of acoustic vibration along the basilar membrane. The preliminary success of clinical trials with combined electrical and acoustic stimulation highlights the need to develop electrodes that position stimulating contacts close to the modiolus for increased efficiency and that also have the appropriate mechanical features to ensure atraumatic insertion.
Progress to Date
Manufacturers of cochlear implants have addressed these issues in several different ways with varied success. To date, no single design achieves all three objectives of deeper insertion, proximity to the modiolus and consistent atraumatic insertion. The goal of the present study is to measure the physical characteristics of several existing and prototype cochlear implant electrode arrays and to identify specific design features that directly relate to achieving these objectives.