Abstract:
An apparatus and method for implanting and securing an implanted medical device in a recipient. The implantable medical device of the generally includes an electrode assembly that comprises an elongate carrier member having at least one stimulating electrode positioned thereon. The carrier member further has a fixation structure positioned thereon configured to interact with a portion of the rigid structure to longitudinally secure the carrier member in the recipient.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application 60/918,917, filed on Mar. 20, 2007, which is hereby incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to medical devices, and more particularly, to securing an implanted medical device in a patient. 
         [0004]    2. Related Art 
         [0005]    Hearing loss is generally of two types, namely conductive and sensorineural. The treatment of both types of hearing loss has been quite different, relying on different principles to enable sound percepts to be generated in a recipient&#39;s brain. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. In such cases, hearing is often improved with the use of conventional hearing aids. Such hearing aids amplify sound so that acoustic information reaches the hair cells of the cochlea. Typically, conventional hearing aids utilize acoustic mechanical stimulation, whereby the sound is amplified according to a number of varying techniques, and delivered to the inner ear as mechanical energy. This may be, for example, through a column of air to the eardrum, or through direct delivery to the ossicles of the middle ear. 
         [0006]    Sensorineural hearing loss is due to the absence or destruction of the cochlear hair cells which are needed to transduce acoustic signals into auditory nerve impulses. Individuals suffering from this type of hearing loss are unable to derive any benefit from conventional hearing aids regardless of the magnitude of the acoustic mechanical stimulus. In such cases, Cochlear™ implants (also referred to as Cochlear™ devices, Cochlear™ prostheses, Cochlear™ implant systems, and the like; simply “cochlear implants” herein) have been developed to provide hearing percepts to such individuals. Cochlear implants provide electrical stimulation via stimulating electrodes positioned as close as possible to the nerve endings of the auditory nerve, essentially bypassing the cochlear hair cells. The application of a stimulation pattern to the nerve endings causes impulses to be sent to the brain via the auditory nerve, resulting in the brain perceiving the impulses as sound. 
         [0007]    It is relatively common for some hearing impaired individuals to experience profound hearing loss for high frequency sounds, and yet still be able to discern middle-to-low frequency sounds. Traditionally, such individuals typically do not receive a cochlear implant, as noted above, due to the potential trauma caused by the implantation of a traditional electrode assembly into the cochlea. Rather, in the majority of such cases, such an individual receives treatment to preserve and improve hearing in the middle-to-low frequency range, typically via a conventional hearing aid. Minimal effort would be expended to restore hearing in the high frequency range. Unfortunately, such individuals generally experience poor restoration of their hearing with conventional hearing aids alone. 
         [0008]    Recently, there has been an increased interest in assisting individuals with residual hearing who do not experience adequate restoration from conventional hearing aids alone. One proposed approach for assisting these individuals is through the use of Electro-Acoustical Stimulation (EAS). So called EAS devices provide electrical stimulation of the cochlea in conjunction with acoustical stimulation. 
       SUMMARY 
       [0009]    In one aspect of the invention, an electrode assembly for implantation into a recipient through an opening in a reference structure in the recipient, comprising: an elongate carrier member, having proximal and a distal ends and at least one electrode disposed along the carrier member; and an integrated fixation structure constructed and arranged to interact with the reference structure to when the carrier member is implanted in the recipient, wherein the interaction prevents substantial translation of the carrier member. 
         [0010]    In another aspect of the invention, a method of implanting a stimulating medical device, comprising: preparing an appropriately configured opening in an internal reference structure of a recipient for implantation of an elongate carrier member therethrough; inserting the carrier member through the opening in the recipient; and allowing a fixation structure positioned on the carrier member to interact with a portion of the structure to longitudinally secure the carrier member in the recipient. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Illustrative embodiments of the present invention are described herein with reference to the accompanying figures, in which: 
           [0012]      FIG. 1  is a perspective view of an example of an implanted cochlear implant suitable for implementing embodiments of the present invention; 
           [0013]      FIG. 2A  is a side view of an electrode assembly in accordance with one embodiment of the present invention shown prior to insertion into a cochlea; 
           [0014]      FIG. 2B  is a side view of the electrode assembly illustrated in  FIG. 2A , shown inserted into a cochlea; 
           [0015]      FIG. 3  is a perspective view of a recipient illustrating the location of implantation of an electrode assembly in accordance with embodiments of the present invention; 
           [0016]      FIG. 4  is a side view of one embodiment of the electrode assembly illustrated in  FIGS. 2A and 2B ; 
           [0017]      FIGS. 5A-5H  are cross section and side views of the fixation structure illustrated in  FIGS. 2A and 2B , in accordance with certain embodiments of the present invention; 
           [0018]      FIG. 6  is a perspective view of a recipient illustrating the location and orientation of the clithridiate opening utilized with embodiments of the present invention; 
           [0019]      FIG. 7A  is a side view of one embodiment of the electrode assembly illustrated in  FIGS. 2A and 2B ; 
           [0020]      FIG. 7B  is a side view of on embodiment of the electrode assembly illustrated in  FIG. 7A ; 
           [0021]      FIGS. 8A-8D  are cross sectional and side views of embodiments of a fixation structure in accordance with certain embodiments of the present invention; 
           [0022]      FIG. 9A  is a perspective view of a recipient illustrating the location and orientation of a bracket used to cooperate with embodiments of the present invention; 
           [0023]      FIG. 9B  is a partial cross-sectional view of a recipient illustrating the location and orientation of another bracket used to cooperate with embodiments of the present invention; and 
           [0024]      FIG. 10  is a side view of one embodiment of the electrode assembly illustrated in  FIGS. 7A and 7B  cooperating with a bracket. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Embodiments of the present invention are generally directed to securing an implantable medical device in a patient (also referred to as a recipient). Certain embodiments are described herein in connection with one type of implantable medical device, a prosthetic hearing implant and, more specifically, a cochlear implant. Cochlear implants use direct electrical stimulation of auditory nerve cells to bypass absent or defective hair cells that normally transduce acoustic vibrations into neural activity. Such devices generally use multi-contact electrodes inserted into the scala tympani of the cochlea so that the electrodes may differentially activate auditory neurons that normally encode differential pitches of sound. These devices are also used to treat a smaller number of recipients with bilateral degeneration of the auditory nerve. Such devices are described in commonly owned and co-pending U.S. patent application Ser. Nos. 11/605,952 and 11/605,951, which are hereby incorporated by reference herein. For such recipients, a cochlear implant provides stimulation of the cochlear nucleus in the brainstem. Such devices, therefore, are commonly referred to as auditory brainstem implants (ABIs). 
         [0026]    Although some embodiment of the present invention are described herein with reference to a particular type of cochlear implant, it should be understood that embodiments of the present invention may be implemented in connection with all forms of cochlear implants. Furthermore, it should be understood by those of ordinary skill in the art that embodiments of the present invention may be implemented in stimulating medical devices other than cochlear implants such as neurostimulators, cardiac pacemakers/defibrillators, etc. as well as other medical devices which utilize a carrier member to temporarily or permanently implant, deliver or otherwise introduce into a recipient a therapeutic agent, sensor, electrodes or other active or passive components now or later developed. 
         [0027]    Exemplary embodiments of a cochlear implant utilized in accordance with embodiments of the present invention include a Contour™, Freedom™, Nucleus™ or Cochlear™ implant sold by Cochlear Limited, Australia. Such devices are described in U.S. Pat. Nos. 4,532,930, 6,537,200, 6,565,503, 6,575,894, and 6,697,674, which are hereby incorporated by reference herein. Similarly, cochlear implants utilizing a short electrode array are described in commonly owned and co-pending U.S. patent application Ser. No. 11/605,952 and Ser. No. 11/605,951, which are hereby incorporated by reference herein. 
         [0028]      FIG. 1  is a cut-away view of the relevant components of outer ear  101 , middle ear  102  and inner ear  103 , with an exemplary cochlear implant  120 . In a fully functional ear, outer ear  101  comprises an auricle  105  and an ear canal  106 . An acoustic pressure or sound wave  107  is collected by auricle  105  and channeled into and through ear canal  106 . Disposed across the distal end of ear cannel  106  is a tympanic membrane  104  which vibrates in response to acoustic wave  107 . This vibration is coupled to oval window, or fenestra ovalis,  110  through three bones of middle ear  102 , collectively referred to as the ossicles  111 . 
         [0029]    Ossicles  111  comprise malleus  112 , incus  113  and stapes  114 . Bones  112 ,  113  and  114  of middle ear  102  serve to filter and amplify acoustic wave  107 , causing oval window  110  to articulate, or vibrate. Such vibration sets up waves of fluid motion within cochlea  115 . Such fluid motion, in turn, activates tiny hair cells (not shown) that line the inside of cochlea  115 . Activation of the hair cells causes appropriate nerve impulses to be transferred through the spiral ganglion cells (not shown) to auditory nerve  116  and, ultimately, to the brain where they are perceived as sound. In some persons experiencing sensorineural hearing loss, there is an absence or destruction of the hair cells. Cochlear implant  120  is utilized to directly stimulate the ganglion cells to provide a hearing sensation to such persons. 
         [0030]      FIG. 1  also shows how cochlear implant  120  is positioned in relation to outer ear  101 , middle ear  102  and inner ear  103 . Cochlear implant  120  comprises external component assembly  122  which is directly or indirectly attached to the body of the recipient, and an internal component assembly  124  which is temporarily or permanently implanted in the recipient. External assembly  122  comprises microphone  125  for detecting sound which is provided to a behind-the-ear (BTE) speech processing unit  126  that generates coded signals. The coded signals are provided to an external transmitter unit  128 , along with power from a power source (not shown) such as a battery. External transmitter unit  128  comprises an external coil  130  and, preferably, a magnet (not shown) secured directly or indirectly in external coil  130 . 
         [0031]    Internal component assembly  124  comprises an internal receiver unit  132  having an internal coil (not shown) that transcutaneously receives power and coded signals from external assembly  122 , and provides such signals to a stimulator unit  134 . In response to the coded signals, stimulator  134  applies stimulation signals to cochlea  115  via an electrode assembly  140  implanted through temporal bone  160 . Electrode assembly  140  enters cochlea  115  via an opening of the perilymphatic spaces of cochlea  115 , referred to as cochleostomy  142 , and has an array  144  of one or more electrodes  150  positioned to be substantially aligned with portions of tonotopically-mapped cochlea  115 . The delivery of stimulation signals at various locations along cochlea  115  causes a hearing percept representative of the received sound  107 . 
         [0032]    While cochlear implant  120  is described as having external components, in another embodiment, the controller, including the microphone, speech processor and power supply, may also be implantable. In such embodiments, the controller may be contained within a hermetically sealed housing or the housing used for stimulator unit  134 . 
         [0033]    Electrode assembly  140  preferably assumes an optimal electrode position in cochlea  115  upon or immediately following implantation into the cochlea. It is also desirable that electrode assembly  140  be configured such that the insertion process causes minimal trauma to the sensitive structures of cochlea  115 . Typically, electrode assembly  140  is pre-curved, held in a substantially straight configuration at least during the initial stages of the implantation procedure, then conforming to the natural shape of the cochlea during, and subsequent to, implantation. 
         [0034]      FIGS. 2A and 2B  are side views of an embodiment of electrode assembly  140 , referred to herein as electrode assembly  200 . In  FIG. 2A , the electrode assembly is illustrated prior to insertion in a recipient&#39;s cochlea;  FIG. 2B , following insertion. Electrode assembly  200  comprises a carrier member  202  having a proximal end  208  and a distal end  210 . Distal end  210  terminates in tip  211 , and is adapted to be implanted furthest into cochlea  115 . A plurality of spaced-apart electrodes  212  are mounted or disposed on or in carrier member  202 . Electrodes  212  are embodiments of electrodes  144  ( FIG. 1 ). It should be appreciated that as used herein, particular combinations of the terms mounted/disposed, in/on, etc., are not to be interpreted to refer to any particular manufacturing technique or structural relationship. 
         [0035]    Attached to or integral with carrier member  202  are a stop member  240  and a fixation structure  250 . Fixation structure  250  is positioned on carrier member  202  at or near proximal end  208  of carrier member  202 . Fixation structure  250  is described in greater detail below with reference to  FIGS. 4-10 . Stop member  240  is positioned on carrier member  202  between fixation structure  250  and all, and, in embodiments in which not all electrodes  212  are to be inserted into cochlea  115 , some of the electrodes. 
         [0036]    Extending from proximal end  208  of carrier member  202  is a lead  214 . Lead  214  physically and electrically connects electrode assembly  200  with stimulator unit  134  ( FIG. 1 ). 
         [0037]    As shown in  FIG. 2B , carrier member  202  may be implanted into cochlea  115  through an aperture in the cochlea. The aperture may be oval window  110 , round window  141  or a surgical incision  142 . In this description, reference will be made to round window  141  ( FIG. 1 ); it should be appreciated, however, that other embodiments of the present invention may be configured to be implanted in oval window  110  or other natural or man-made aperture in cochlea  115 . Carrier member  202  is inserted into cochlea  115  until stop member  240  contacts an exterior surface of cochlea  115  surrounding round window  141 . when Abutting cochlea  115  member  240  at least partially prevents perilymphatic fluid from escaping from cochlea  115 . 
         [0038]    When implanted, the surface of carrier member  202  which faces the interior of cochlea  115  is referred to herein as the medial surface  216  of carrier member  202 . The opposing side of carrier member  202 , referred to herein as lateral surface  218 , faces the external wall and bony capsule (not shown) of cochlea  115 . It should be understood that the terms medial surface, medial direction, and the like, are generally used herein to refer to the surfaces, features and directions toward the center of cochlea  115 , while the terms lateral surface, lateral direction, and the like, are generally used herein to refer to surfaces, features and directions toward the exterior of cochlea  115 . 
         [0039]    As would be appreciated by those of ordinary skill in the art, electrodes  212  may be disposed in a linear or non-linear array on or in carrier member  202 , and are typically positioned on or in carrier member  202  so as to align with predetermined regions of tonotopically mapped cochlea  115  when implanted in cochlea  115 . In alternative embodiments, electrodes  212  are implemented as described in U.S. Provisional Patent Applications 60/748,217, 60/748,273 and 60/748,314, which are hereby incorporated by reference herein. 
         [0040]    In one embodiment, electrodes  212  are half-band electrodes disposed in or on medial surface  216  of carrier member  202 . It should be appreciated, however, that any electrodes now or later developed suitable for a particular application may be used in alternative embodiments of the invention. For example, in one alternative embodiment, electrodes  212  are banded electrodes extending substantially around the circumference of carrier member  202 . In another embodiment, electrodes  212  do not laterally extend to or around the edges of carrier member  202 . Typically, each electrode  212  is arranged such that its exposed surface is substantially parallel to a longitudinal axis  224  of carrier member  202 . It should be appreciated, however, that other locations and orientations may be implemented in alternative embodiments. It should further be appreciated that the quantity of electrodes  212  may vary from as few as one or two to as many as twenty-four or more. 
         [0041]    In certain embodiments, at least one electrode  212  has a surface that is at least adjacent medial surface  216  of carrier member  202 . Preferably, one or more electrodes  212  has a surface that is collocated with medial surface  216  of carrier member  202 . In another embodiment, the surfaces of electrodes  212  are raised above or recessed into the surface  216  of carrier member  202 . It should be appreciated, however, that any embodiment of electrodes  212  may be implemented. 
         [0042]    In certain embodiments, electrodes  212  are manufactured from a biocompatible conductive material such as platinum, although other materials or combinations of materials may be used. In certain alternative embodiments electrodes  212  are coated with a biocompatible covering that does not interfere with the transfer of stimulation signals to cochlea  115 . 
         [0043]    A variety of surgical methods may be used to implant an electrode assembly in a recipient, including a mastoidectomy and facial recess approach, a transcanal approach, or a combination thereof, depending upon the particular recipient anatomy, recipient needs and surgeon&#39;s discretion. For ease of description, embodiments of the present invention will be described with reference to implantation using a facial recess approach. 
         [0044]    Referring again to  FIG. 1 , in an implantation procedure utilizing the facial recess approach, electrode assembly  140  is inserted during an operation that usually takes between 2-3 hours, depending on the device to be implanted. An incision is made behind outer ear  101  to expose temporal bone  160 . Temporal bone  160  consists of several segments (not shown) known as the squamous, the mastoid, the tympanic, the zygomatic and the petrous segment. Typically, traditional cochlear implants require the opening of the mastoid segment of temporal bone  160  which leads to middle ear  102 . 
         [0045]    Following the opening of the incision behind outer ear  101 , a shallow recess is created in the mastoid to hold implanted receiver  132  and stimulator  134 . Next, additional amounts of the mastoid are removed. By removing this additional portion of the mastoid, the surgeon opens an area known as the facial recess. The facial recess is a concave portion of the inner side of the mastoid bone that opens to middle ear  102 , and inner ear  103 . As the facial recess is opened, the surgeon is able to access middle ear  102  and inner ear  103 . 
         [0046]    The surgeon then prepares an opening in cochlea  115  to allow implantation of electrode assembly  140  into cochlea  115 . The opening may be formed through round window  141 , oval window  110 , the promontory or through the apical turn of the cochlea. Electrode assembly  140  is then gently threaded into the shell-like structure of the cochlea. Depending in the type of implant used, the opening may either remain open to heal with scar tissue, or may be closed by the surgeon. The procedure is completed by closing the incision behind outer ear  101 . 
         [0047]      FIG. 3  illustrates a perspective view of the right side of a recipient showing the location of implantation of certain embodiments of the electrode assembly of the present invention in accordance with the facial recess approach. It should be appreciated, however, that embodiments of the present invention are equally applicable to other implantation methods. Directional arrows  330 ,  332 ,  334  and  336  illustrate general directions in relation to the recipient. Directional arrow  330  illustrates the inferior direction, and refers to a direction that is towards the feet of the recipient. Directional arrow  332  illustrates the posterior direction, and refers to a direction that is towards the back of the recipient&#39;s head. Directional arrow  334  illustrates the superior direction, and refers to a direction that is towards the top of the recipient&#39;s head. Directional arrow  336  illustrates the anterior direction, and refers to a direction that is towards the front of the recipient&#39;s head. 
         [0048]    As illustrated in  FIG. 3 , facial recess  362  is positioned between the facial nerve  310  and the cord-tympani nerve  312 . Facial nerve  310  is positioned posterior to facial recess  362 , and cord-tympani nerve  312  is positioned anterior to facial recess  362 . Visible behind facial recess  362  is round window  141  of cochlea  115 . In some embodiments of the present invention, electrode assembly  200  is configured to be implanted through facial recess  362  and into round window  141 . 
         [0049]    An implanted electrode assembly, such as electrode assembly  140 , may have a tendency to migrate out of cochlea  115  following implantation. This may be due to the materials used for the construction of the electrode assembly, as well as the bias of the electrode assembly. For example, as a straight electrode assembly is inserted into cochlea  115 , and thereby forced into a spiraled configuration, the electrode assembly attempts to return to a straight configuration. The forces applied by the electrode assembly to return to a straight configuration may cause the electrode assembly to migrate out of the cochlea. Another cause of such migration out of cochlea  115  is the inadvertent pulling of the electrode assembly during subsequent steps of the surgery. Another potential cause is when the electrode assembly has a spring force which if not properly stabilized. The spring force may tend to pull the electrode assembly out over a long period of time. 
         [0050]      FIG. 4  is a side view of one embodiment of electrode assembly  140 , referred to herein as electrode assembly  400 , configured to reduce the ability of electrode assembly  400  to exit cochlea  115  following implantation. In  FIG. 4 , electrode assembly  400  is shown in an implanted position, and is viewed from an anterior direction of the recipient. Electrode assembly  400  comprises a carrier member  202 , having proximal end  208  and distal end  210 , terminating in tip  211 . A plurality of spaced-apart electrodes  212  are disposed in carrier member  202  along medial surface  216  of carrier member  202 . The opposing side of carrier member  202  is referred to herein as lateral surface  218 . Lead  214  extends from proximal end  208 . 
         [0051]    Attached to or integral with carrier member  202  are stop member  240  as described with reference to  FIGS. 2A and 2B , and fixation structure  450 . Fixation structure  450  is positioned at or near proximal end  208  of carrier member  202  to substantially interact with at least a portion of the bone surrounding facial recess  362 . In the specific embodiment shown in  FIG. 4 , fixation structure  450  comprises a series of circumferentially-extending projections  470 . If circumferentially-extending projections  470  are viewed along a plane that extends longitudinally through fixation structure  450 , each projection may have, for example, a substantially triangular cross-sectional shape. 
         [0052]    In  FIG. 4 , circumferentially-extending projections  470  are dimensioned to extend from carrier member  202  to bone  160  surrounding facial recess  362 . The above-noted tendency of electrode assembly  400  to exit cochlea  115  places pressure on fixation structure  450  to exit the recipient. However, the pressure from carrier member  202  causes circumferentially-extending projections  470  to further interact with bone  160 . This interaction produces a longitudinal anchor force that substantially prevents longitudinal movement (that is, movement in a direction approximately parallel to the longitudinal axis of the device) of fixation structure  450  out of the recipient. This resulting longitudinal anchor force is a force along the longitudinal axis of electrode assembly  400  in the direction of cochlea  115 . The longitudinal anchor force maintains fixation structure  450  in bone  160  thereby retaining carrier member  202  in a desired position in cochlea  115 . In other words, the longitudinal anchor force prevents substantial longitudinal movement of carrier member  202  out of cochlea  115 . 
         [0053]    As noted, embodiments of electrode assembly  400  may include half-band electrodes. For optimal stimulation, an electrode assembly utilizing half-band electrodes is preferably maintained in a desired position and orientation within cochlea  115 . However, due to certain aspects of the implantation procedure, a rotational force may be created on electrode assembly  400  that causes electrode assembly  400  to twist within cochlea  115 . If electrode assembly  202  twists within cochlea  115 , the half band electrodes will no longer be in a desired orientation for optimal stimulation. In such embodiments, fixation structure  450  may be configured to produce an additional anchor force that prevents rotation of electrode assembly  400  within cochlea  115 . This additional anchor force is referred to herein as a rotational anchor force. As electrode assembly  400  attempts to twist within cochlea  115 , the torque causes circumferentially-extending projections  470  to further interact with bone  160 . This additional interaction produces a rotational anchor force that substantially prevents rotational movement of fixation structure  450 . As a result of this rotational anchor force rotational movement substantial of carrier member  202  is also prevented. 
         [0054]    In certain embodiments, fixation structure  450  comprises a flexible material having a diameter that is larger than facial recess  362 . In such embodiments, during implantation, flexible fixation structure  450  is forced in to facial recess  362  and is compressed therein. As fixation structure  450  attempts to exit cochlea  115 , the compression of fixation structure  450  by bone  160  creates the longitudinal anchor force that prevents movement of fixation structure  450  out of cochlea  115 . In such embodiments, fixation structure  450  may comprise a flexible component such as silicone, polyurethane, PTFE, etc. 
         [0055]    In other embodiments, the longitudinal anchor force created by the interaction of fixation structure  450  and bone  160  may be the result of friction. As electrode assembly  400  attempts to exit cochlea  115 , the friction between fixation structure  450  and bone  160  produces the longitudinal anchor force that prevents movement of electrode assembly  400 . In certain embodiments, fixation structure  450  may have a rough or uneven surface that increases friction with bone  160 . 
         [0056]    As would be understood to those of ordinary skill in the art, the bone surrounding facial recess  362  is typically not a smooth surface, and likely has burrs and marks resulting from its interaction with surgical tools, as well due to the structural features of bone  160 . For example, bone  160  naturally includes aerated sections that form openings in the bone. Such attributes of bone  160  tend to increase the friction between fixation structure  450  and bone  160 . In further embodiments, the surface of bone  160  may be purposefully scored to further increase the friction with fixation structure  450 . 
         [0057]    Fixation structure  450  may comprise a flexible component as described above. In an alternative embodiment, fixation structure  450  may comprise a malleable material such as a metal or a hard plastic or a shape-memory material that changes shape upon heating to body temperature or other catalyst such has IR or UV light, to anchor itself into the recess in bone  160 . In such embodiments fixation structure  450  may comprise materials such as titanium, platinum, stainless steel, chromium, nitinol, etc. In one particular embodiment, the shape-memory material comprises a shape-memory polymer. 
         [0058]      FIGS. 5A-5H  depict various embodiments of fixation structure  250  ( FIGS. 2A ,  2 B) which may take advantage of the non-smooth surface of bone  160  to increase the longitudinal and/or rotational anchor forces created by the interaction of the two. For example, in certain embodiments, fixation structure  250  comprises non-smooth surfaces that may protrude into openings or burs within bone  160  to further engage bone  160 . As the uneven surfaces of such fixation structures  250  protrude into openings in bone  160 , the uneven surface interlocks with bone  160 . This interlocking creates longitudinal anchor force that prevents substantial longitudinal movement of such embodiments of fixation structure  250  and thereby physically retaining carrier member  202  ( FIGS. 2A ,  2 B) in a desired position in cochlea  115 . 
         [0059]    In alternative embodiments, fixation structure  250  may comprise a malleable material, as described above. In such embodiments, in addition to the friction force created between fixation structure  250  and bone  160 , the pressure exerted on bone  160  by the malleable material may, over time, cause fixation structure  250  to cut into bone  160 , causing fixation structure  250  to interlock with bone  160 . This interlocking creates a longitudinal anchor force that prevents substantial longitudinal movement of such embodiments of fixation structure  250  thereby physically retaining carrier member  202  ( FIGS. 2A ,  2 B) in a desired position in cochlea  115 . 
         [0060]    In further embodiments, fixation structure  250  may be configured or treated to facilitate the in-growth of bone  160  to fixation structure  250 , further increasing the longitudinal and rotational anchor forces. 
         [0061]    As would be appreciated by one of ordinary skill in the art, circumferentially-extending projections of embodiments of fixation structure  250  such as projections  470  of fixation structure  450  do not necessarily interact with bone  160  at all surfaces of bone  160 . For example, fixation structure  450  may be positioned in facial recess  362  such that circumferentially-extending projections  470  interact with one, two, three or more surfaces of bone  160 . Accordingly, in alternative embodiments, projections may not circumferentially extend around the entire perimeter of fixation structure  250 . 
         [0062]    Alternatively, fixation structure  250  may be made of a hydrogel material to expand and lock into the slot in the bone. Preferably, the expansion of such an embodiment of fixation structure  250  is controllable so that the timing of swelling is determined by the surgeon. This may be attained, for example, by pulling a “plug” on fixation structure  250  to let moisture reach the hydrogel, or by piercing an outer silicone casing of  250  to let moisture through. 
         [0063]    The various embodiments of fixation structure  250  may also be constructed and arranged to prevent rotation of electrode assembly  200 . As noted, such embodiments, fixation structure  250  interacts with bone  160  to produce a rotational anchor force. The rotational anchor force may be created by any manner described above. For example, a rotational anchor force may be created by friction between bone  160  and fixation structure  250 , or by the interlocking of fixation structure  250  and bone  160 . 
         [0064]      FIGS. 5A-5H  each illustrate cross-sectional and (left) (right) side views of different embodiments of fixation structure  250 .  FIG. 5A  illustrates one alternative embodiment of fixation structure  250 , referred to as fixation structure  250 A. Fixation structure  250 A has a generally spherical shape that substantially surrounds carrier member  202 . As shown with reference to cross-sectional plane  588 , spherical fixation structure  250 A has an approximately circular cross section, with the center of spherical fixation structure  250 A positioned on a longitudinal axis  539  extending through the center of carrier member  202 . Spherical fixation structure  250 A is dimensioned such that at least a portion of the outer surface of spherical fixation structure  450 A is in contact with bone  160  following implantation of electrode assembly  200 . 
         [0065]    The interaction of bone  160  and spherical fixation structure  250 A produces a longitudinal anchor force that substantially maintains fixation structure  250 A in a desired location relative to bone  160 . As fixation structure  250 A is maintained in the desired location, carrier member  202  is, as noted, substantially prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between spherical fixation structure  250 A and bone  160 . For example, in one embodiment, the longitudinal anchor force resulting from the interaction between spherical fixation structure  450 A and bone  160  may be the result of friction between the two surfaces. However, as would be appreciated by one of ordinary skill in the art, fixation structure  250 A may be configured to interact with bone  160  in any manner described herein to produce the longitudinal anchor force. 
         [0066]      FIG. 5B  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 B. Fixation structure  250 B comprises a pair of radially-extending projections  572 . Radially-extending projections  572  are positioned, in this embodiment, approximately 180 degrees relative to each other and lie in a plane illustrated in  FIG. 5B  as plane  590 . Plane  590  extends laterally and longitudinally through the center of carrier member  202 . Viewing radially-extending projections  572  within plane  592 , radially-extending projections  572  each have, in this example, a substantially rectangular cross section. Radially-extending projections  572  have a length that is parallel to carrier member  202  and a width that is perpendicular to carrier member  202 , with the length being greater than the width. Radially-extending projections  572  are dimensioned to interact with bone  160  on opposing sides of carrier member  202 . 
         [0067]    The interaction of bone  160  and radially-extending projections  572  produces a longitudinal anchor force that substantially maintains fixation structure  250 B in a desired fixed location relative to bone  160 . As fixation structure  250 B is maintained in the desired location, carrier member  202  is, as noted, substantially prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between radially-extending projections  572  and bone  160 . For example, in one embodiment, the longitudinal anchor force resulting from the interaction between radially-extending projections  572  and bone  160  may be the result of friction between the outer surface of radially-extending projections  572  that lies parallel to, but away from carrier member  202 , and bone  160 . In another embodiment, radially-extending projections  572  are configured to fit into the natural openings in bone  160  to maintain fixation structure  250 B in a desired location. In still further embodiments, radially-extending projections  572  are comprised of a material that cuts into bone  160  to thereby maintain fixation structure  250 B in a desired location. However, as would be appreciated by one of ordinary skill in the art, radially-extending projections  572  may be configured to interact with bone  160  in any manner described herein so as to produce the desired longitudinal anchor force. 
         [0068]    In an alternative embodiment, radially-extending projections  572  may be flexibly biased so that they fold against carrier member  202  during insertion, and then radially extend after insertion. In one embodiment, this may be provided with shape memory material that extends upon heating to 37C. In another embodiment, a hydrogel may be incorporated into projections  572  to cause the extension. Such embodiments prevent radially-extending projections  572  from interfering with the surgeon&#39;s visibility during insertion. 
         [0069]      FIG. 5C  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 C. Fixation structure  250 C comprises four radially-extending projections  574  each having an approximately quadrilateral shape. Radially-extending projections  574  are radially positioned around carrier member  202  so as to be spaced approximately 90 degrees relative to each other. One set of radially-extending projections  574  are positioned approximately 180 degrees from each other and lie in a plane illustrated in  FIG. 5C  as plane  592 . Plane  592  extends longitudinally through the center of carrier member  202 . A second set of radially-extending projections  574  are positioned 180 degrees from each other and lie in a plane illustrated in  FIG. 5C  as plane  594 . Plane  594  extends longitudinally through the center of carrier member  202 , but is substantially perpendicular to plane  592 . Viewing radially-extending projections  574  within either plane  592  or plane  594 , radially-extending projections  574  each have a substantially rectangular cross section with a length that is parallel to carrier member  202  and a width that is perpendicular to carrier member  202 , wherein the length is greater than the width. Radially-extending projections  574  are dimensioned to provide contact with bone  160  at up to four separate locations surrounding carrier member  202  following implantation of electrode assembly  200 . 
         [0070]    The interaction of bone  160  and radially-extending projections  574  produces a longitudinal anchor force that substantially maintains fixation structure  250 C in a desired location relative to bone  160 . As fixation structure  250 C is maintained in a desired location, carrier member  202  is securely positioned within cochlea  115  and is, as noted, prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between radially-extending projections  574  and bone  160 . For example, in one embodiment, the longitudinal anchor force resulting from the interaction between radially-extending projections  574  and bone  160  may be the result of friction between the outer surface of radially-extending projections  574  that lies parallel to, but away from carrier member  202 , and bone  160 . In another embodiment, radially-extending projections  574  are configured to fit into the natural openings in bone  160  to maintain fixation structure  250 C in a desired location. In still further embodiments, radially-extending projections  574  are comprised of a material that slightly cuts into bone  160  to thereby maintain fixation structure  250 C in a desired location. However, as would be appreciated by one of ordinary skill in the art, radially-extending projections  574  may be configured to interact with bone  160  in any manner described herein so as to produce the desired longitudinal anchor force. 
         [0071]      FIG. 5D  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 D. Fixation structure  250 D comprises a plurality of radially-extending projections  576 . In this exemplary embodiment, there are four (4) radially-extending projections  576  each radially positioned on carrier member  202  so as to be spaced approximately 90 degrees from one another. Viewing radially-extending projections  576  in a cross section plane shown in  FIG. 5D  as plane  596 , each radially-extending projection  576  has an approximately cross-shaped cross section. Within plane  596 , radially-extending projections  576  each comprise a first approximately rectangular central region extending away from carrier member  202 . Extending from each of these rectangular central regions are two approximately square shaped regions. The square shaped regions extend from opposing sides of the central region substantially parallel to carrier member  202 . As such, within plane  596  each radially-extending projection  576  has the noted cross-shaped cross section. Radially-extending projections  576  are dimensioned to interact with bone  160  at up to four separate locations surrounding carrier member  202  following implantation of electrode assembly  200 . It should be appreciated, however, that more or less projections  576  may be implemented in alternative embodiments, with such projections been spaced apart by any distance and in any manner necessary to achieve a desired anchoring of fixation structure  250 D with bone  160 . For example, projections  576  may be longitudinally offset relative to each other, or have different longitudinal lengths. 
         [0072]    The interaction of bone  160  and radially-extending projections  576  produces a longitudinal anchor force that substantially maintains fixation structure  250 D in a desired location relative to bone  160 . As fixation structure  250 D is maintained in the desired location, carrier member  202  is securely positioned within cochlea  115  and is, as noted, substantially prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between radially-extending projections  576  and bone  160 . For example, in one embodiment the longitudinal anchor force resulting from the interaction between radially-extending projections  576  and bone  160  may be the result of friction between the outer surface of radially-extending projections  576  that lies parallel to, but away from carrier member  202 , and bone  160 . In another embodiment, radially-extending projections  576  are configured to fit into the natural openings in bone  160  to maintain fixation structure  250 D in a desired location. In still further embodiments, radially-extending projections  576  are comprised of a material that cuts into bone  160  to thereby maintain fixation structure  250 D in a desired location. However, as would be appreciated by one of ordinary skill in the art, radially-extending projections  576  may be configured to interact with bone  160  in any manner described herein so as to produce the desired longitudinal anchor force. 
         [0073]      FIG. 5E  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 E. Fixation structure  250 E comprises a series of longitudinally spaced disks each circumferentially-extending from carrier member  202 . In this illustrative embodiment, fixation structure  250 E comprises a series of three circumferentially-extending disks  578 , although other quantities may be implemented. Following implantation of electrode assembly  200 , a substantial portion of the outer circumference of each disk is configured to interact with bone  160 . 
         [0074]    The interaction of bone  160  and the outer surface of circumferentially-extending disks  578  produces a longitudinal anchor force that substantially maintains fixation structure  250 E in a desired location relative to cochlea  115 . As fixation structure  250 E is maintained in the desired location, carrier member  202  is securely positioned within cochlea  115  and is, as noted, substantially prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between circumferentially-extending disks  578  and bone  160 . For example, in one embodiment the longitudinal anchor force resulting from the interaction between one or more circumferentially-extending disks  578  and bone  160  may be the result of friction between the outer surface of the circumferentially-extending disks and bone  160 . In another embodiment, circumferentially-extending disks  578  are configured to fit into the natural openings in bone  160  to maintain fixation structure  250 E in a desired location. In still further embodiments, circumferentially-extending disks  578  are comprised of a material that slightly cuts into bone  160  to thereby maintain fixation structure  450 E in a desired location. However, as would be appreciated by one of ordinary skill in the art, circumferentially-extending disks  578  may be configured to interact with bone  160  in any manner described herein so as to produce the desired longitudinal anchor force. 
         [0075]      FIG. 5F  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 F. Fixation structure  250 F comprises a cylindrical extension radially extending from carrier member  202 . As shown with reference to cross sectional plane  591 , cylindrical fixation structure  250 F has an approximately circular cross section, with its center positioned on longitudinal axis  549  extending through the center of the length of carrier member  202 . The diameter of cylindrical fixation structure  250 F is substantially constant, although it may vary in alternative embodiments of the invention. Cylindrical fixation structure  250 F is shaped and sized such that at least a portion of the outer surface of cylindrical fixation structure  250 F is in contact with bone  160  following implantation of electrode assembly  400 . 
         [0076]    The interaction of bone  160  and cylindrical fixation structure  250 F produces a longitudinal anchor force that substantially maintains fixation structure  250 F in a desired fixed location relative to bone  160 . As fixation structure  250 F is maintained in the desired location, carrier member  202  is secured within cochlea  115  and is, as noted, substantially prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between cylindrical fixation structure  450 F and bone  160 . For example, in one embodiment the longitudinal anchor force resulting from the interaction between cylindrical fixation structure  450 F and bone  160  may be the result of friction between the two surfaces. However, as would be appreciated by one of ordinary skill in the art, fixation structure  250 F may be configured to interact with bone  160  in any manner described above with reference to  FIG. 4  to produce the longitudinal anchor force. 
         [0077]      FIG. 5G  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 G. Fixation structure  250 G comprises a series of circumferentially-extending projections  582 . A plane  595  extends longitudinally through the center of carrier member  202 . Viewing a circumferentially-extending projection within plane  595 , each projection  582  has a cross section that includes three sides. The first side of the cross section is adjacent to, and parallel to carrier member  202 . The other two sides of the cross section comprise a first arcuate slope joined to a second slope that is substantially perpendicular to carrier member  202  by a substantially rounded apex positioned apart from the first side of the cross section. Viewing a circumferentially-extending projection  582  within plane  593 , the projection has an approximately circular cross section. 
         [0078]    The interaction of bone  160  and circumferentially-extending projections  582  produces longitudinal anchor force that substantially maintains fixation structure  250 G in a desired fixed location relative to cochlea  115 . As fixation structure  250 G is maintained in the desired location, carrier member  202  is, as noted, substantially prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between circumferentially-extending projections  582  and bone  160 . For example, in one embodiment the longitudinal anchor force resulting from the interaction between circumferentially-extending projections  582  and bone  160  may be the result of friction between the outer surface of circumferentially-extending projections  582  and bone  160 . In another embodiment, portions of circumferentially-extending projections  582 , particularly the rounded apex of each cross section, are configured to fit into the natural openings in bone  160  to maintain fixation structure  250 G in a desired location relative to bone  160 . In still further embodiments, circumferentially-extending projections  582  are comprised of a material that slightly cuts into bone  160  to thereby maintain fixation structure  250 G in a desired location. However, as would be appreciated by one of ordinary skill in the art, circumferentially-extending projections  582  may be configured to interact with bone  160  in manner described above with reference to  FIG. 4  so as to produce the desired longitudinal anchor force. 
         [0079]      FIG. 5H  illustrates another embodiment of fixation structure  250 , referred to as fixation structure  250 H. Fixation structure  250 H comprises a generally cylindrical shape extending from carrier member  202 . The diameter of cylindrical shaped fixation structure  450 H varies along the length of carrier member  202  to produce a wavy-shaped surface of fixation structure  250 H. As shown with reference to cross sectional plane  591 , cylindrical fixation structure  250 H has a generally circular cross section having its center positioned on the longitudinal axis  559  extending through the center of the length of carrier member  202 , but with a diameter that varies. Cylindrical fixation structure  250 H is shaped and sized such that at least a portion of the outer surface of cylindrical fixation structure  250 H is in contact with bone  160  following implantation of electrode assembly  400 . 
         [0080]    The interaction of bone  160  and cylindrical fixation structure  250 H produces a longitudinal anchor force that substantially maintains fixation structure  250 H in a desired fixed location relative to cochlea  115 . As fixation structure  450 H is maintained in the desired location, carrier member  202  is, as noted, prevented from longitudinal movement toward or away from cochlea  115 . Also as described above, this longitudinal anchor force may be the result of several different interactions between cylindrical fixation structure  250 H and bone  160 . For example, in one embodiment the longitudinal anchor force resulting from the interaction between cylindrical fixation structure  250 H and bone  160  may be the result of friction between the two surfaces. In another embodiment, portions of circumferentially-extending projections  582  are configured to fit into the natural openings in bone  160  to maintain fixation structure  250 H in a desired location. However, as would be appreciated by one of ordinary skill in the art, fixation structure  250 H may be configured to interact with bone  160  in any manner described above with reference to  FIG. 4  to produce the longitudinal anchor force. 
         [0081]    As would be appreciated by one of ordinary skill in the art, the embodiments described with reference to  FIGS. 5A-5H  are equally capable of producing the rotational anchor force to thereby prevent rotational movement of carrier member  202  within cochlea  115 . 
         [0082]      FIG. 6  is a perspective view of the right side of a recipient demonstrating the location of implantation of an electrode assembly in accordance with certain of the embodiments shown in  FIGS. 7A-8D . As described above with reference to  FIG. 3 , facial recess  662  is located between the facial nerve  310  and the cord-tympani nerve  312 . Facial nerve  310  is positioned posterior to facial recess  662 , and cord-tympani nerve  312  is positioned anterior to facial recess  662 . Visible behind facial recess  662  is round window  141  of cochlea  115 . 
         [0083]    In the embodiment shown in  FIG. 6 , facial recess  662  has a narrow slot  670  on the inferior side of facial recess  662 . Slot  670  forms a channel running from facial recess  662  in an inferior direction. As shown in  FIG. 6 , facial recess  662  is a contiguous opening that has an approximately clithridiate, or key-hole shape. 
         [0084]      FIG. 7A  is a side view of one embodiment of electrode assembly  140 , referred to herein as electrode assembly  700 , shown in an implanted configuration via facial recess  662  illustrated in  FIG. 6 . In  FIG. 7A , cochlea  115  and electrode assembly  700  are viewed from the anterior direction of the recipient. 
         [0085]    Electrode assembly  700  comprises a carrier member  202 , having proximal end  208  and distal end  210 , terminating in tip  211 . A plurality of spaced-apart electrodes  212  are mounted on or in carrier member  202  along medial surface  216  of carrier member  202 . The opposing side of carrier member  202  is referred to herein as lateral surface  218 . Lead  214  extends from proximal end  208  to stimulator unit  134  ( FIG. 1 ). 
         [0086]    Attached to or integral with carrier member  202  are stop member  240  as described herein with reference to  FIGS. 2A and 2B , and fixation structure  750 . Fixation structure  750  is positioned at or near proximal end  208  of carrier member  202  to substantially interact with at least a portion of the bone surrounding facial recess  362 . Illustrative embodiments of fixation structure  750  are described below with reference to  FIGS. 8A-8D . 
         [0087]    As described above, in embodiments utilizing facial recess  362 , a surgeon inserts electrode assembly  700  through bone  160 , across middle ear  102  and into cochlea  115  through round window  141  until stop element  240  contacts cochlea  115 . Stop element  240  is substantially similar as described above with reference to  FIGS. 2A and 2B . 
         [0088]    After stop element member  240  contacts cochlea  115 , the surgeon positions proximal end  208  of carrier member  202  into slot  670  of facial recess  662 . As shown in  FIG. 7A , fixation structure  750  is positioned on carrier member  202  such that after placement of proximal end  208  into slot  670 , fixation structure  750  contacts bone  160  surrounding slot  670 . 
         [0089]    As described above, an implanted electrode assembly has a tendency to exit cochlea  115  following implantation. In the illustrated embodiment, as electrode assembly  700  attempts to exit cochlea  115 , fixation structure  750  is pressed against bone  160 . As such, fixation structure  750  interacts with bone  160  and a longitudinal anchor force is exerted on electrode assembly  700  in the direction of cochlea  115 . This longitudinal anchor force thereby securely locks carrier member  202  into a desired position within cochlea  115  by preventing if from exiting the cochlea. To remove carrier member  202  from cochlea  115 , a surgeon may lift proximal end  208  of carrier member  202  from slot  670  and exit fixation structure  750  through facial recess  662 . 
         [0090]      FIG. 7B  is an alternative view of the electrode assembly illustrated in  FIG. 7A  spiraling into cochlea  115 . In  FIG. 7B , electrode assembly  700  and cochlea  115  are viewed from a superior direction of the recipient. Cochlea  115  spirals to the apex  762  of cochlea  115 . 
         [0091]    As noted above, electrode assemblies in accordance with certain embodiments of the present invention include half-band electrodes. For optimal stimulation, an electrode assembly utilizing half-band electrodes is preferably maintained in both a desired position and orientation within cochlea  115 . As such, in additional embodiments, carrier member  202  and fixation structure  750  may be configured to collectively lock electrode assembly  700  into both a desired position and orientation in cochlea  115 . In such embodiments, proximal end  208  may comprise planar surfaces on at least two parallel edges of carrier member  202 , shown in  FIG. 7B  as parallel planar surfaces  790 . Proximal end  208  having surfaces  790  is placed into slot  670  with surfaces  790  each abutting and parallel to bone  160  that partially surrounds slot  670 . Once placed within slot  670 , if carrier member  202  would attempt to twist within cochlea  115 , surfaces  790  would be forced against bone  160 . The interaction of bone  160  and surfaces  790  produces a rotational anchor force that prevents axial rotation of carrier member  202 . 
         [0092]      FIGS. 8A-8D  are cross section and side views of embodiments of a fixation structure  750  in accordance with embodiments of the present invention, as described with reference to  FIGS. 7A and 7B . Specifically, the fixation structures shown in  FIGS. 8A-8D  may be utilized to lock carrier member  202  into cochlea  115 . 
         [0093]      FIG. 8A  illustrates one embodiment of fixation structure  750 , referred to as fixation structure  850 A. Fixation structure  850 A has a generally spherical shape that substantially surrounds carrier member  202 . As shown with reference to cross sectional plane  890 , spherical fixation structure  850 A has an approximately circular cross section, with the center of spherical fixation structure  850 A positioned on the longitudinal axis  819  extending through the center of the length of carrier member  202 . Spherical fixation structure  850 A is dimensioned such that at least a portion of the outer surface of spherical shaped fixation structure  850 A is in contact with bone  160  following implantation of electrode assembly  700 . Any movement of carrier member  202  out of cochlea  115  is prevented by a longitudinal anchor force back towards cochlea  115  that is produced by the interaction of bone  160  and spherical shaped fixation structure  850 A. Spherical shaped fixation structure  850 A may be of any diameter that is sufficient to achieve desired interaction with bone  160 . For example, spherical shaped fixation structure  850 A may have a diameter that is larger than the width of slot  670 . 
         [0094]      FIG. 8B  illustrates another embodiment of fixation structure  750 , referred to as fixation structure  850 B. Fixation structure  850 B has a diamond shape that substantially surrounds carrier member  202 . As shown with reference to cross sectional plane  890 , diamond shaped structure  850 B has an approximately diamond cross section, and is positioned such that the longitudinal axis  829  extending through the center of the length of carrier member  202  also extends through the center of fixation structure  850 B. Diamond shaped fixation structure  850 B is dimensioned such that at least a portion of the outer surface of fixation structure  850 B is in contact with bone  160  following implantation of electrode assembly  700 . Any movement of carrier member  202  out of cochlea  115  is prevented by a longitudinal anchor force back towards cochlea  115  produced through the interaction of bone  160  and diamond shaped fixation structure  750 B. Diamond shaped fixation structure  750 B may have any dimensions that are sufficient to achieve desired interaction with bone  160 . For example, the distance between the center of diamond shaped fixation structure  750 B and any vertex of the diamond may be larger than the width of slot  670 . 
         [0095]      FIG. 8C  illustrates a further embodiment of fixation structure  250 , referred to as fixation structure  850 C. Fixation structure  850 C has a generally cube or cuboidal shape that substantially surrounds carrier member  202 . As shown with reference to cross sectional plane  890 , spherical fixation structure  850 C has an approximately square cross section, with the center of spherical fixation structure  850 C positioned the longitudinal axis  839  extending through the center of the length of carrier member  202 . Fixation structure  850 C is shaped and sized such that at least a portion of the outer surface of cuboidal fixation structure  850 C is configured to be in contact with bone  160  following implantation of electrode assembly  400 . Any movement of carrier member  202  out of cochlea  115  is prevented by a longitudinal anchor force back towards cochlea  115  produced through the interaction of bone  160  and cuboidal shaped fixation structure  850 C. Cuboidal fixation structure  850 C may be of any height  898  and width  896  that is sufficient to achieve desired interaction with bone  160 . For example, cuboidal shaped fixation structure  850 C may have a height  898  and width  896  that is larger than the width of slot  670 . 
         [0096]      FIG. 8D  illustrates a still other embodiment of fixation structure  250 , referred to as fixation structure  850 D. Fixation structure  850 D comprises four radially-extending projections  870  each having an approximately quadrilateral shape. For example, in the illustrated embodiment, radially-extending projections  870  each have an approximately rectangular cross section as described below. Radially-extending projections  870  are positioned approximately 90 degrees from each other. One set of radially-extending projections  870  are positioned approximately 180 degrees from each other and lie in a plane illustrated in  FIG. 8D  as plane  893 . A second set of radially-extending projections  870  are positioned 180 degrees from each other and lie in a plane illustrated in  FIG. 8D  as plane  891 . Plane  891  extends longitudinally through the center of carrier member  202 , but is substantially perpendicular to plane  893 . Viewing radially-extending projections  870  within either plane  893  or plane  891 , radially-extending projections  870  each have an approximately rectangular cross section. Radially-extending projections  870  have a length that is parallel to carrier member  202  and a width that is perpendicular to carrier member  202 , with the length being greater than the width. Radially-extending projections  870  are shaped and sized to provide contact with bone  160  at four separate locations surrounding carrier member  202  following implantation of electrode assembly  400 . Any movement of carrier member  202  out of cochlea  115  is prevented by a longitudinal anchor force back towards cochlea  115  produced through the interaction of bone  160  and radially-extending projections  870 . Radially-extending projections  870  have sufficient length and width to prevent carrier member  202  from exiting through slot  670 . 
         [0097]    As described above with reference to  FIGS. 7A and 7B , the embodiments illustrated in  FIGS. 8A-8D  may be further modified to prevent rotation of carrier member  202 . 
         [0098]    As would be appreciated by one of ordinary skill in the art, fixation structure  750  may comprise any shape or element that serves to interact with bone  160  to prevent longitudinal movement of carrier member  202  out of cochlea  115 . For example, fixation structure  750  may comprise any of the embodiments of fixation structure  450  illustrated in  FIGS. 5A-5H . All of the embodiments illustrated in  FIGS. 5A-5H , utilized as described with reference to  FIGS. 7A and 7B , would serve to interact with the cochlea-side of bone  160  to lock carrier member  202  into cochlea  115 . Similarly, other embodiments for fixation structure  750  may be envisioned that have not been illustrated herein. 
         [0099]    As would be further appreciated, in certain embodiments fixation structure  750  may comprise flexible materials such as silicone, polyurethane, PTFE, rubber etc. In alternative embodiments, fixation structure may comprise a malleable material such as metals or hard plastics. Specifically, the fixation structure may comprise materials such as titanium, platinum, stainless steel, chromium, nitinol etc. 
         [0100]    In other aspects of the present invention utilizing embodiments of fixation structure  750 , a surgeon does not cut slot  670  into a recipient prior to implantation of electrode assembly  700 . Rather, a surgeon may desire to implant a bracket in the recipient to interact with fixation structure  750 .  FIGS. 9A and 9B  illustrate partially cross sectional views of a recipient that demonstrate the location of brackets implanted by a surgeon. The brackets can be formed of any material(s) and have any dimensions appropriate for the application. For example, in one embodiment, the bracket is a shape memory alloy that is heated to adopt the final desired shape. 
         [0101]    As shown in  FIG. 9A , a bracket  930 A is implanted on the posterior side of facial recess  362 . Bracket  930 A shown in  FIG. 9A  comprises a metal plate that is attached to bone  160  of the recipient with a bone screw  932 . Similarly, as shown in  FIG. 9B , bracket  930 B is also implanted on the posterior side of facial recess  362 . In this illustrated embodiment, bracket  930 B comprises a malleable wire bracket secured to bone  160  with screw  932 . 
         [0102]    As would be appreciated by one of ordinary skill in the art, bracket  930  may be implanted at any location around facial recess  362  such that it at least partially extends over facial recess  362  and so long as it does not interfere with any nerves near facial recess  362 . 
         [0103]      FIG. 10  is a side view of one embodiment of the electrode assembly illustrated in  FIGS. 7A and 7B , referred to as electrode assembly  1000 , viewed from the anterior direction of the recipient. In the illustrated embodiment, electrode assembly  1000  interacts with a bracket  1030 . 
         [0104]    Electrode assembly  1000  comprises a carrier member  202 , having proximal end  208  and distal end  210 , terminating in tip  211 . A plurality of spaced-apart electrodes  212  are mounted on or in carrier member  202  along the medial surface  216  of carrier member  202 . Lead  214  extends from proximal end  208 . 
         [0105]    Attached to or integral with carrier member  202  are stop member  240  as described with reference to  FIGS. 2A and 2B , and an embodiment of fixation structure  750 , referred to herein as fixation structure  1050 . Fixation structure  1050  is positioned at or near proximal end  208  of carrier member  202  to substantially interact with bracket  1030 . As would be appreciated, fixation structure  1050  may comprise any fixation structure described above with reference to  FIGS. 8A-8D  and  5 A- 5 F. 
         [0106]    Prior to commencing implantation of electrode assembly  1000 , a surgeon loosely attaches bracket  1030  to bone  160  with bone screw  1032 . Bracket  1030  is swung away from facial recess  362  to allow the surgeon access to facial recess  362 . As described above, in embodiments utilizing facial recess  362 , a surgeon inserts electrode assembly  1000  through bone  160 , across middle ear  102  and into cochlea  115  through round window  141 , until stop element  240  contacts cochlea  115 . Stop element  240  is substantially the same as described above. The surgeon then swings bracket  1032  back over facial recess  362  and secures bracket  1032  to bone  160  with screw  1032  to prevent movement of bracket  1032 . The surgeon then positions proximal end  208  of carrier member  202  into receiving slot of bracket  1030  (not shown). Fixation structure  1050  is positioned on carrier member  202  such that after placement of proximal end  208  of carrier member  202  into the receiving slot of bracket  1030 , fixation structure  1050  is in contact with bracket  1030 . 
         [0107]    As described above, an implanted electrode assembly may have a tendency to exit cochlea  115  following implantation. In the illustrated embodiment, as electrode assembly  1000  attempts to exit cochlea  115 , fixation structure  1050  is pressed against bracket  1030 . As such, fixation structure  1050  interacts with bone  160  and a longitudinal anchor force is exerted on electrode assembly  700  in the direction of cochlea  115 . This longitudinal anchor force thereby locks carrier member  202  into a desired position within cochlea  115  by preventing an exit there from. To remove carrier member  202  from cochlea  115 , the surgeon must lift proximal end  208  of carrier member  202  from bracket  1020  and exit fixation structure  750  through facial recess  362 . 
         [0108]    Furthermore, as would be appreciated, carrier member  202  may be modified as described above with reference to  FIGS. 7A and 7B  to interact with bracket  1030  to prevent rotation of electrode assembly  1000  within cochlea  115 . 
         [0109]    As would be understood by one of ordinary skill in the art, stop element  240  is not necessary for the present invention. In all embodiments stop element  240  may be omitted if desired. 
         [0110]    As would be appreciated by one of ordinary skill in the art, in the above embodiments the fixation structures may be positioned in or on the carrier member during manufacture of the electrode assembly. However, in alternative embodiments the fixation structure could be chosen by the surgeon at some time following manufacture. For example, a surgeon could choose the fixation structure that is believed to be the most fitting for the particular recipient during the implantation procedure. In such embodiments, the surgeon could prepare the insertion opening, and then fasten, attach, adhere or connect a properly shaped, sized and suited fixation structure to fit the prepared opening. It is also envisioned that a surgeon could attach the fixation structure prior to commencing the implantation procedure. 
         [0111]    All documents, patents, journal articles and other materials cited in the present application are hereby incorporated by reference. 
         [0112]    Although the present invention has been fully described in conjunction with several embodiments thereof with reference to the accompanying drawings, it is to be understood that various changes and modifications may be apparent to those skilled in the art. For example, in the exemplary application above embodiments of the present invention have been described in the context of a cochlear implant. It should be appreciated, however, that embodiments of the present invention may be implemented in other medical devices now or later developed, including those that do not provide stimulation. Furthermore, embodiments of the present invention are not limited to any particular cochlear implant or cochlear implant function or modality. For example, embodiments of the present invention may be implemented in cochlear implants that perform operations to suppress tinnitus, provide vestibular treatments, and so on, It should further be appreciated that although in the above embodiments the present invention is implemented in connection with an electrode carrier member, embodiments of the present invention may be implemented in connection with other types of carrier members, other types of implantable devices and components, and so on. As another example, in the embodiments of the fixation structure such as those illustrated in  FIGS. 5E and 5G , the structure is shown to be completely within the facial recess. It should be appreciated, however, that in other embodiments the fixation structure has a length that enables it to extends beyond the medial wall of the recess. This will allow the structure to expand and effectively create a lock against the medial wall of the facial recess, similar to the arrangement illustrated in  FIG. 7B . It should further be appreciated that the reference structure in the recipient may not be bone as in the above exemplary applications. And, as noted above, it need not be biological. It may be man-made, or a combination of a man-made material injected into, applied to the surface of or otherwise used to treat a biological structure. Also, the rigidity of the reference structure may vary depending on, for example, the permissible movement of the controlled medical device; for example, the electrode carrier member in the above examples. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart there from.