Patent Publication Number: US-2015073205-A1

Title: Magnet Arrangement for Bone Conduction Hearing Implant

Description:
This application is a continuation of co-pending U.S. patent application Ser. No. 13/721,408, filed Dec. 20, 2012, which in turn claims priority from U.S. Provisional Patent Application 61/578,953, filed Dec. 22, 2001, both of which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical implants, and more specifically to a novel transcutaneous auditory prosthetic implant system. 
     BACKGROUND ART 
     A normal ear transmits sounds as shown in  FIG. 1  through the outer ear  101  to the tympanic membrane (eardrum)  102 , which moves the ossicles of the middle ear  103  (malleus, incus, and stapes) that vibrate the oval window  106  and round window  107  membranes of the cochlea  104 . The cochlea  104  is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea  104  forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the cochlear nerve  105  reside. In response to received sounds transmitted by the middle ear  103 , the fluid-filled cochlea  104  functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve  105 , and ultimately to the brain. 
     Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea  104 . To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear  103 , a conventional hearing aid or middle ear implant may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea  104 , a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode. 
     Middle ear implants employ electromagnetic transducers to convert sounds into mechanical vibration of the middle ear  103 . A coil winding is held stationary by attachment to a non-vibrating structure within the middle ear  103  and microphone signal current is delivered to the coil winding to generate an electromagnetic field. A magnet is attached to an ossicle within the middle ear  103  so that the magnetic field of the magnet interacts with the magnetic field of the coil. The magnet vibrates in response to the interaction of the magnetic fields, causing vibration of the bones of the middle ear  103 . See U.S. Pat. No. 6,190,305, which is incorporated herein by reference. 
     U.S. Patent Publication 20070191673 (incorporated herein by reference) described another type of implantable hearing prosthesis system which uses bone conduction to deliver an audio signal to the cochlea for sound perception in persons with conductive or mixed conductive/sensorineural hearing loss. An implanted floating mass transducer (FMT) is affixed to the temporal bone. In response to an externally generated electrical audio signal, the FMT couples a mechanical stimulation signal to the temporal bone for delivery by bone conduction to the cochlea for perception as a sound signal. A certain amount of electronic circuitry must also be implanted with the FMT to provide power to the implanted device and at least some signal processing which is needed for converting the external electrical signal into the mechanical stimulation signal and mechanically driving the FMT. 
     One problem with implantable hearing prosthesis systems arises when the patient undergoes Magnetic Resonance Imaging (MRI) examination. Interactions occur between the implant magnet and the applied external magnetic field for the MRI. The external magnetic field from the MRI may create a torque on the implant magnet, which may displace the magnet or the whole implant housing out of proper position and/or may damage the adjacent tissue in the patient. The implant magnet may also cause imaging artifacts in the MRI image, there may be induced voltages in the receiving coil, and hearing artifacts due to the interaction of the external magnetic field of the MRI with the implanted device. 
     Thus, for existing implant systems with magnet arrangements, it is common to either not permit MRI or at most limit use of MRI to lower field strengths. Other existing solutions include use of a surgically removable magnets, spherical implant magnets (e.g. U.S. Pat. No. 7,566,296), and various ring magnet designs (e.g., U.S. Provisional Patent 61/227,632, filed Jul. 22, 2009). Among those solutions that do not require surgery to remove the magnet, the spherical magnet design may be the most convenient and safest option for MRI removal even at very high field strengths. But the spherical magnet arrangement requires a relatively large magnet much larger than the thickness of the other components of the implant, thereby increasing the volume occupied by the implant. This in turn can create its own problems. For example, some systems, such as cochlear implants, are implanted between the skin and underlying bone. The “spherical bump” of the magnet housing therefore requires preparing a recess into the underlying bone. This is an additional step during implantation in such applications which can be very challenging or even impossible in case of very young children. 
     U.S. patent application Ser. No. 13/163,965, filed Jun. 20, 2011, and incorporated herein by reference, described an implantable hearing prosthesis two planar implant magnets connected by a flexible connector member which are fixable to underlying skull bone. Each of the implant magnets was in the specific form of a center disk having magnetic polarity in one axial direction. Around the disk magnet was another ring magnet having an opposite magnetic polarity in a different direction. This ring/disk magnet arrangement had less magnetic interaction with an external magnetic field such as an MRI field. 
     SUMMARY 
     Embodiments of the present invention are directed to an implantable magnet arrangement for a hearing implant in a recipient patient. A pair of implant magnets are fixable in a common plane beneath the skin of the patient to underlying skull bone. One or both of the magnets is adapted to transform a magnetic drive signal from an external signal drive coil into a corresponding mechanical stimulation signal for delivery by bone conduction of the skull bone as an audio signal to the cochlea. Each implant magnet includes a pair of internal magnets lying in parallel planes which meet along a common junction with repelling like magnetic polarities facing towards each other. 
     The arrangement may further include a connector member flexibly connecting and positioning the implant magnets a fixed distance from each other. At least one of the implant magnets may be adapted for fixed attachment to the skull bone by a pair of radially opposed bone screws. Both of the implant magnets are adapted to transform the magnetic drive signal from the external signal drive coil into a corresponding mechanical stimulation signal for delivery by bone conduction of the skull bone as an audio signal to the cochlea. Each internal magnet may have a planar disk shape. 
     Each implant magnet may further include a magnet housing, for example of titanium material, enclosing the pair of internal magnets and holding them together against each other. In addition or alternatively, there may be a magnet connector nut and bolt combination holding the internal magnets together along the common junction. Embodiments may also include a magnet spacer insert lying along the common junction and separating the internal magnets. 
     Embodiments of the present invention also include a hearing implant system having an implantable magnet arrangement according to any of the foregoing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows anatomical structures of a typical human ear. 
         FIG. 2  shows a cross-sectional view of an implantable hearing prosthesis arrangement according to an embodiment of the present invention. 
         FIG. 3  shows a cross-sectional view of a different embodiment of an implantable hearing prosthesis. 
         FIG. 4  A-B shows examples of arrangements for holding the magnetically opposing internal magnets together. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are directed to a magnetic arrangement for an implantable hearing prosthesis system which is compatible with MRI systems.  FIG. 2  shows a cross-sectional view of an implantable hearing prosthesis arrangement having an implant holding magnet  201  and an implant transducer magnet  202  which are fixable in a common plane beneath the patient skin  207  to underlying skull bone  208 . A flexible connector member  206  connects and positions the implant holding magnet  201  and the implant transducer magnet  202  a fixed distance from each other. The implant transducer magnet  202  is fixedly secured to the skull bone  208  by a pair of radially opposed bone screws  205 . 
     The implant holding magnet  201  and the implant transducer magnet  202  are each enclosed within a titanium housing which contains a pair of internal magnets  203  and  204  in the shape of planar disks that lie in parallel planes which meet along a common junction with repelling like magnetic polarities facing towards each other. Thus, the internal magnets  203  and  204  within the housing of the implant transducer magnet  202  face each other with south magnetic fields facing towards each other and north magnetic fields facing outward. The magnetic polarities of the internal magnets  203  and  204  within the implant holding magnet  201  are reversed from those of the implant transducer magnet  202  so that north magnetic fields face towards each other and south magnetic fields face outward, and the magnet housing holds them together against each other. 
     The external elements of the system include a processor lobe  209  and a drive coil lobe  210  connected by a flexible connector  211 . The processor lobe  209  contains a signal processor  212  that produces a communications signal to the implanted components and an external holding magnet  213  in the shape of a planar disk having a magnetic polarity opposite to the outermost internal magnet  204  of the implant holding magnet  201  so as to maximize the magnetic attraction between the two. The drive coil lobe  210  contains an external drive magnet  214  in the shape of a planar disk having a magnetic polarity opposite to the outermost internal magnet  204  of the implant transducer magnet  202  so as to maximize the magnetic attraction between the two. And because the outermost internal magnet  204  has different directions in the implant holding magnet  201  and the implant transducer magnet  202 , that helps ensure that the processor lobe  209  aligns into proper position directly over the implant holding magnet  201  and the drive coil lobe  210  aligns into proper position over the implant transducer magnet  202 . 
     An external drive coil  215  surrounds the outer perimeter of the external drive magnet  214 . The external drive coil  215  receives the communications signal produced by the signal processor  212  and produces a corresponding electromagnetic drive signal that travels transcutaneously through the patient skin  207  where it interacts with the magnetic field of the outermost internal drive magnet  204  of the implant transducer magnet  202 . This in turn causes the implant transducer magnet  202  to produce a corresponding mechanical stimulation signal for delivery by bone conduction of the skull bone  208  as an audio signal to the cochlea, which the patient perceives as sound. 
     To summarize, the magnetic polarity of the outermost internal magnet  204  in each of the implant magnets is closer to the skin surface and dominates in the near field so that there is magnetic attraction with the magnets in the external device. But with regards to an external far field magnetic field such as from an MRI, the magnetic polarities of the internal magnets  203  and  204  oppose and cancel each other, as does the opposing overall magnetic polarities of the implant holding magnet  201  and the implant transducer magnet  202 . This net minimizing of the magnetic fields of the implant magnets reduces their magnetic interactions with the external MRI field to minimize adverse effects such as torque forces and imaging artifacts. 
       FIG. 3  shows a cross-sectional view of a different embodiment of an implantable hearing prosthesis having a second processor drive coil  302  surrounding a processor drive magnet  301  in the processor lobe  209  of the external device. Thus the external device has two external drive coils  214  and  301  respectively, which magnetically interact with their respective implant magnets as shown, each of which generates a portion of the mechanical stimulation signal coupled into the skull bone  208 . 
       FIG. 4  A-B shows examples of different arrangements for holding the magnetically opposing internal magnets together.  FIG. 4A  shows an embodiment of an implant magnet  400  where the internal magnets  403  and  404  are enclosed within and held against each other by a titanium housing  402 . The embodiment shown also includes a magnet spacer insert  405  that lies along the common junction and separates the internal magnets  403  and  404 , thereby assisting in their easy assembly.  FIG. 4  B shows another arrangement where a combination of a magnet connector nut  407  and a magnet connector bolt  406  hold the internal magnets  403  and  404  together along their common junction for ease of assembly. 
     Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of the invention without departing from the true scope of the invention.