Magnet for installation in the middle ear

Abstract: A magnet assembly (100) for location around portions of bones in the middle ear comprises one portion (102) which is a magnet and another portion (104) which is a magnetic material, the portions being hinged to one another so that they are held around the bone by the magnetic field and are adapted for optimal coupling with a magnetic field produced by a coil in a magnetic induction hearing aid.

Description: The present invention relates to magnet assemblies for use in hearing aids and, more particularly, to a hearing aid using magnetic induction to reproduce sound. Hearing aids are useful in restoring lost aural perception to those persons having mild to severe loss of hearing. Conventional hearing aids have a microphone, amplifier circuitry, a battery and a speaker. The microphone receives the sound energy and transforms the sound energy into an electrical signal which is then amplified and filtered. This amplified signal is transformed back to acoustic energy by the speaker and transmitted to the person's middle ear for perception of the sound. These hearing aids can be placed behind the ear, with only the receiver being placed inside the ear canal. Alternatively, in-the-ear hearing aids are available which are placed in the outer ear and have portions extending into the ear canal. There are a number of problems with conventional hearing aids. All conventional hearing aids are visible to some extent and therefore have an undesirable cosmetic appearance. Conventional hearing aids have acoustic feedback problems because sound energy can escape from the ear canal and be detected by the microphone, generating a feedback-related whistle. Additionally, sound reproduction is often lacking in clarity because of distortions generated by standing waves existing in the closed cavity between the hearing aid and the tympanic membrane and poor mechanical reproduction by the speaker. It has been suggested that a magnetic induction hearing aid would remove many of these problems. A magnet or other item having a magnetic field is placed in the middle ear, either in contact with the tympanic membrane or in contact with other portions of the middle ear. Electrical circuitry and a coil would generate a magnetic field having the same frequency as the external sound. The magnetic field generated by the coil would interact with the field of the magnet and cause the magnet to vibrate at the same frequency as the magnetic field. The vibration of the magnet would then cause the attached portion of the middle ear to vibrate, resulting in a perception of the external sound. A magnetic induction hearing aid would overcome feedback or distortion problems of conventional hearing aids because there would be no significant air movement in the ear canal, resulting in insufficient energy escaping around the hearing aid to generate a feedback problem. There would be no standing waves generated to cause distortion because there are no appreciable sound waves at all. Attempts to use magnetic induction hearing aids have been reported. An early attempt placed a coil in conjunction with a small piece of iron on the tympanic membrane, which was excited by an external coil placed over the ear canal. This system did allow the perception of the stimulus, but had the side effect of producing discomfort and pain for the wearer. A later attempt glued a small magnet to the umbo and used an external coil placed over the ear of the wearer to cause the sympathetic vibrations of the magnet. This apparatus required approximately 7.9 ma to produce a 0 db hearing level at 1000 Hz. In an article entitled Audition via Electromagnetic Induction , Arch Otolaryngol 23 (July 1973), Goode et al describe a number of tests. One test attached a magnet to the tympanic membrane and located a coil in the ear canal 3 mm from the magnet. The coil was driven externally by an audiometer. This development required only 0.7 µa to produce a 0 db hearing level at 1000 Hz. Tests were performed for system fidelity and proved adequate. Another system tested placed the coil over the ear, drove the coil with an audiometer and had a magnet glued to portions of the middle ear, but used larger magnets than in previous tests. One version of this system placed the magnet on a Silverstein malleus clip which was connected in the normal manner. Approximately 0.7 ma was required to produce a 0 db hearing level using these arrangements. These discussions suggested that the use of electromagnetic induction to produce a hearing aid is possible, but did not teach a way to develop a practical system. The majority of tests used coils placed over the ear or adjacent to the ear. Systems using external coils are not efficient enough for use in conjunction with the low power requirements dictated by hearing aid batteries. Although one test indicated that a coil was placed inside the ear canal, an external amplifier was used to drive the coil. The tests did not result in a practical device or suggest how a totally in-the-ear device could be made. Further, the magnets described in conjunction with the above-mentioned tests were either glued to portions of the middle ear and removed after short periods of time or were connected to malleus clip and inserted for a longer duration. Neither of these attempts resulted in a magnet that could be implanted for extended periods of time with no danger of rejection by the body, have no movement in relation to the middle ear and yet have as little weight as possible. FR-A-2365267 and EP-A-0242038 disclose a magnet assembly for coupling with the magnetic coil of a hearing aid H, the assembly comprising a magnet and means for attaching the magnet to a bone in the ossicular chain. In FR-A-2365267, the magnet is secured to a base having arms which pass through the ear-drum and whose inner ends are attached to the malleus. In EP-A-0242038, published 21.10.87, the magnet is attached to the malleus by a Silverstein malleus clip or is inserted between the ear-drum and the malleus. The present invention is characterised in that the assembly comprises first and second sections connected together by hinge means, the first section being sized and shaped to partially surround at least a portion of said bone in the ossicular chain and, the second section being sized and shaped to form in combination with the first section means to clamp the two sections onto said bone, at least one of the first and second sections comprising a magnet and the other of the first and second sections being formed of a material which is magnetically attracted to the magnet, the attraction between the first and second sections being used to clamp the magnet to said bone. It is an advantage of the invention that the magnet assembly can be simply and quickly attached to a bone in the ossicular chain without the need for crimping or bending of attachment wires or use of other materials. One piece of the assembly is comprised of a magnet, preferably samarium cobalt, while the other piece is comprised of magnetic material. This magnetic material can be paramagnetic, ferromagnetic or can be an actual magnet. The magnet is coated with biocompatible materials to prevent corrosion or other adverse body reactions. The hinge between the two components can be a separate hinge or can be a pivoting structure formed by the interaction of the two pieces. The structure is self closing in that an open gap exists on the side opposite the hinge after the structure has been placed around the incus. The existence of the gap causes the pieces that form the structure to continually try to move together because of the magnetic attraction that exists in the open gap of the structure. This allows portions of the bone to diminish in size and yet have the magnet continue to remain firmly affixed without need for further attention by a physician. The magnet is adapted for optimal shape for coupling with the field produced by the magnetic coil of the hearing aid, thereby increasing the efficiency and battery life of the hearing aid. A better understanding of the invention can be obtained when the detailed exemplary embodiment set forth below is considered in conjunction with the following drawings, in which: Fig. 1 is a cross-sectional view of a human ear with a prior art magnetic induction hearing aid placed in the ear canal; Fig. 2 is an electrical schematic diagram of one embodiment of a circuit utilizing a Class A amplifier for a magnetic induction hearing aid; Fig. 3 is an electrical schematic diagram of a second embodiment of a circuit utilizing a Class B amplifier for a magnetic induction hearing aid; Figs. 4a, 4b and 4c are schematic illustrations of induction coils for magnetic induction hearing aids; Fig. 5 is a side view of a magnet assembly according to the present invention; Fig. 6 is a perspective view of the magnet assembly of Fig. 5; and Fig. 7 is a partial cross-sectional view of a middle ear showing the magnet assembly implanted. Referring to Fig. 1, there is shown a prior art hearing aid H installed in the ear canal 34. The hearing aid H has a housing 30 enclosing a microphone 20, an amplifier 22, a volume control 24, a battery 26 and a coil 28. The hearing aid H is located deep in the ear canal 34 so that the coil 28 is located near a coated magnet assembly 32, with 2.5 mm being a desirable distance for this separation. This distance is sufficiently close to reduce the inverse relationship of distance to magnetic field strength and yet is sufficiently far that the hearing aid H can be inserted by the wearer with minimal difficulty and not be in danger of contacting the tympanic membrane 68. The installation of the hearing aid H deep within the ear canal 34 as shown in Fig. 1 eliminates any negative cosmetic effects of a hearing aid because the hearing aid H is practically undetectable. A conventional hearing aid cannot be inserted this deep in the ear canal 34 because of the standing wave and feedback problems discussed above. These problems do not occur in a magnetic induction hearing aid and therefore this deep placement is possible. Volume adjustment and battery replacement is accomplished by removing hearing aid H from the ear canal 34, appropriately adjusting the volume control 24 or replacing the battery 26 and reinserting the hearing aid H into the position shown in Fig. 1. The housing 30 is custom molded to each wearer's ear canal 34. This is necessary because each wearer has a differently sized and shaped ear canal. The hearing aid H must be sufficiently close to the magnet 32 for proper operation and the hearing aid H must be sufficiently tight within the ear canal 34 to remain in place during normal use. A class A amplifier design is shown in Fig. 2. The microphone 20 is a standard electret microphone as conventionally used in hearing aids. The amplifier 22c is class A design that is standard in hearing aid applications. This amplifier is specifically designed for low voltage operation in conjunction with a single 1.3 volt battery. The volume control 24 is connected to vary the gain of the amplifier 22c and thereby change the output signal level applied to the coil 28a. The coil 28a is designed for use with the class A amplifier 22c. Each amplifier used in hearing aids has a recommended output load impedance which is normally deemed to be the speaker or receiver impedance. For optimum performance of the hearing aid H, the coil 28a should be designed to match this characteristic desired impedance across as wide a frequency band as possible. The coil 28a is a double-ended coil designed to be connected to the battery 26 and to the output of the amplifier 22c. The coil 28a is formed by winding the appropriate number of turns of wire 72 (Fig. 4a) about a high permeability core 70. Preferably, the core 70 is comprised of mumetal to increase the magnetic field strength at the ends of the coil. The maximum coil size is preferably approximately 9 mm long and 4 mm in diameter. This size limitation is used in conjunction with the optimum coil impedance in determining the number of turns of wire 72 and the gauge of the wire 72 to produce a coil of the allowed size having the desired impedance. The class A amplifier 22c is used in situations where the wearer has only a mild to moderate loss of hearing. The class A design is used in the mild loss case because the power consumption of the class A amplifier 22c is lower, but the maximum output is also lower, necessitating a higher performance or class B design for high power needs. Where the wearer has a more severe hearing loss requiring greater amplification of the sound signal, what is known as a class B amplifier design as shown in Fig. 3 is used. A class B amplifier 22b is used in the higher volume, higher amplification situations because it has a power output level higher than that of the class A amplifier 22c. The trade off for this efficiency is reduced battery life because of the higher current draw of the class B amplifier design. The microphone 20 is connected to a preamplifier stage 22a through an impedance matching and filter stage 38. The class A preamplifier 22a provides a fixed amount of gain and produces an output signal which is transmitted to filter capacitors 42 and 44 and the volume control 24. Appropriately adjusting the volume control 24 changes the output voltage of the class B output amplifier 22b which in turn drives coil 28b. As in the class A amplifier 22c, the class B output amplifier 22b has an optimal load impedance resistance which is specified by the manufacturer. The coil 28b is designed to have an impedance which matches this optimal impedance over as broad a frequency band as is necessary for the given application. The coil 28b is designed with a center tap (Figs. 4b and 4c) to allow use with the class B amplifier 22b. An appropriate number of turns of the appropriate gauge wire 74 are wound around the mumetal core 70 or other high permeability material and connected as required to the amplifier 22b. The class B amplifier 22b produces greater power because of its class B design and its push-pull operation, enabling the coil 28b to produce larger magnetic field densities. The coil 28 produces a magnetic field varying at the frequency of the sound waves received by the microphone 20. According to this invention, the magnet used with the hearing aid H can be located around the incus 98 (Fig. 7). In this preferred embodiment, the magnet assembly 100 (Figs. 5 and 6) is formed of two pieces or sections which are hinged together. A first piece 102 is generally cylindrical in shape and has a transverse groove 106 located in the interior face 105 of the piece 102. The groove 106 is adapted to contact a portion of the incus 98 when the magnet assembly 100 has been placed around the incus 98. The first piece 102 also contains a pivot section or fulcrum 108 which extends from the general body of the pivot piece 102 and has a rounded cross-section 109 at its end. A second or mating piece 104 is also generally cylindrical in shape and also contains a transverse groove 106 adapted for mating with the incus 98. The second structure 104 contains a second, generally smaller, transverse groove 110 adapted to cooperate with the pivot section 108 such that when pivot section 108 is located in the groove 110, the pieces 102, 104 are hinged at this location through their magnetic attraction and pivot in relationship to each other. Preferably, both of the pieces 102 and 104 are magnets having poles as shown in Fig. 5. Alternatively, only one of the elements 102 or 104 need be a magnet. The other piece can be a magnetic material, either paramagnetic or ferromagnetic, such that the material is a good concentrator of magnetic flux lines. If the magnetic material is a permanent magnet, the entire magnet assembly 100 has a greater magnetic field than if one piece is paramagnetic or ferromagnetic, thereby increasing the efficiency and battery life of the hearing aid. The pivot section or fulcrum 108 is further adapted such that the pieces 102, 104 have a gap 112 when attached to the incus 98. This gap 112 results in a self-closing feature of the magnet assembly 100 because the magnet assembly 100 will have a tendency to try to reduce this gap so as to concentrate the magnetic flux. This self-closing allows the magnet assembly 100 to remain in contact with the incus 98 even if the incus 98 reduces in diameter as a result of tissue resorption because of the pressure caused by the attraction between the two pieces 102, 104. Preferably the gap 112 will completely close and the pieces 102, 104 will come into contact before the incus 98 has eroded too far. When the gap 112 is closed, tissue resorption stops and the magnet assembly 100 then remains locked onto the incus 98. The magnet assembly 100 is preferably made of samarium cobalt, but can be made of neodymium-iron or other magnet materials. In the preferred embodiment, the pieces 102, 104 have a diameter of 0.090" (2.286 mm) and a thickness of 0.030" (0.762 mm). The incus grooves 106 have a prefered radius of 0.025" (0.635 mm), a depth of 0.010" (0.254 mm) and are 0.040" (0.016 mm) wide at the piece face. The pivot section 108 is preferably 0.038" (0.962 mm) long and the pivot groove 110 has a radius of 0.007" (0.178 mm), a width of 0.021" (0.533 mm) at the piece face and a depth of 0.008" (0.2032 mm). The magnet assembly 100 is coated with a biocompatible polymer to prevent corrosion of the magnet assembly 100, to prevent rejection of the material by the body and to allow ingrowth of tissues if desired. The geometrical proportions and alignment of the magnet assembly 100 are such that the pole faces are oriented with the magnetic field produced by the coil 28 in the magnetic hearing aid H to achieve maximum coupling of the magnetic fields. The magnet assembly 100 can be adapted to be placed in other portions of the middle ear other than the incus 98, including the malleus 36 and the stapes 96, with appropriate design of the pieces 102, 104 for optimal coupling of the magnetic fields given the magnet assembly 100 location, and for hinge and groove dimensions and location given the geometry of the portion of the middle ear to which the magnet assembly 100 is being attached. Preferably the magnet assembly 100 is designed to be mounted on the incus 98 because this location is the closest, most easily accessible location relative to the tympanic membrane 68 and allows the use of basic configurations of the magnet assembly 100 for magnetic field coupling. The magnet assembly 100 is implanted by grasping the magnet assembly 100 with forceps 114 (see Fig. 5) on the sides 116 of the pieces 102, 104 away from the incus grooves 106, causing the magnet assembly 100 to pivot open. The opened magnet assembly 100 is then inserted through an incision formed in the tympanic membrane 68 until it reaches the incus 98, at which time the magnet assembly 100 is allowed to close over and clamp onto the incus 98. The magnet assembly 100 aligns itself with the incus, grooves 106 centering on the incus 98. When the magnet assembly 100 closes, it is firmly attached to the incus 98. While the pieces 102, 104 that form the magnet assembly 100 are shown in Figs. 5-7 as hinged through their magnetic attraction to each other, alternative embodiments with mechanical hinges using pins or the like could be used in accordance with the invention. Although forceps 114 can easily grasp the pieces 102, 104 as shown in Fig. 5, projections or tabs (not shown) could be added at that end of the pieces 102, 104 to provide more pronounced bearing surfaces for the forceps. The foregoing disclosure and description of the invention are illustrative and explanatory of the invention, and various changes in the size, shape and materials, as well as in the details of the illustrated construction and process may be made falling within the scope of the appended claims.

Claims: A magnet assembly (100) for coupling in use with a magnetic coil (28) of a hearing aid (H), the assembly (100) comprising at least one magnet (102 or 104) and means (104, 102) for attaching in use the or each magnet (102 or 104) to a bone in the ossicular chain, characterised in that the assembly comprises first and second sections (102, 104) connected together by hinge means (108, 110), the first section (102) being sized and shaped to partially surround at least a portion of said bone in the ossicular chain and, the second section (104) being sized and shaped to form in combination with the first section (102) means to clamp the two sections (102, 104) onto said bone, at least one of the first and second sections (102, 104) comprising a magnet (102 or 104) and the other of the first and second sections (104, 102) being formed of a material which is magnetically attracted to the magnet (102 or 104), the attraction between the first and second sections (102, 104) being used to clamp the magnet (102 or 104) to said bone. The magnet assembly of claim 1, characterized in that the said sections (102, 104) are shaped and dimensioned so that force applied to outer surfaces (116) of each section (102, 104) in the direction of the other section (102, 104) to one side of the hinge (108, 110) will cause portions of the sections (102, 104) to the other side of the hinge (108, 110) to move apart, removal of the force allowing said section portions to move together and clamp onto a portion of a bone in the ossicular chain because of the magnetic attraction between said sections (102, 104). The magnet assembly of claim 1 or claim 2, characterized in that both of sections (102, 104) are magnets with poles located so that they attract each other. The magnet assembly of claims 1, 2 or 3, characterized in that both sections (102, 104) are coated with a biocompatible polymer. The magnet assembly of any preceding claim, characterized in that the hinge means (108, 110) includes one section having a fulcrum (108) and the other section having a fulcrum groove (110) for receiving the fulcrum (108). The magnet assembly of any preceding claim, characterized in that the first and second sections (102, 104) include opposing faces and hollowed-out portions (106) designed to engage in use with a respective incus of the ossicular chain.

Inventors: BOJRAB DENNIS, GOOCH TIMOTHY D, HEIDE JORGEN, PRESCOTT ANTHONY D, BOJRAB, DENNIS, GOOCH, TIMOTHY D., HEIDE, JORGEN, PRESCOTT, ANTHONY D.
Applicants: SMITH & NEPHEW RICHARDS INC, SMITH & NEPHEW RICHARDS INC.
Application year: 1988
Main classification: H04R25
IPC codes: H04R25