Magnetic ossicular replacement prosthesis

Abstract: An ossicular replacement prosthesis contains a magnet (32) in the head (100) of the prosthesis P so that when used with a magnetic induction hearing aid having a coil (28) producing a magnetic field, the hearing losses resulting from insertion of the prosthesis and any other remaining hearing losses can be corrected without the difficulties associated with conventional acoustic hearing aids.

Description: Background of the Invention 1. Field of the Invention The present invention relates to hearing aids and to ossicular prostheses, and, more particularly, to a prosthesis used in conjunction with a hearing aid using magnetic induction to reproduce sound. 2. Description of the Prior Art 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 a 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. In a number of people, portions of the ossicular chain have been damaged due to trauma or other reasons and therefore a hearing loss occurs. One solution to this problem has been the use of ossicular prostheses to replace the damaged portions of the ossicular chain. The prostheses include total ossicular replacement prostheses and partial ossicular replacement prostheses depending upon the damage to the individual ossicular chain. In addition, many of the patients with damage to portions of the middle ear also have damage to the sensorineural portions of the ear contained in the cochlea. The placement of a prosthesis allows an individual to regain significant amounts of the hearing that was lost due to the damage to the ossicular chain, but in a majority of cases there is still a residual hearing loss. Correction of the residual hearing loss has typically been performed by using traditional hearing aids which have the problems discussed above. Therefore, although the addition of a prosthesis is a large step in solving a hearing loss developed due to damage of the ossicular chain, conventional hearing aids must still be used to bring hearing levels up to that existing prior to damage to the middle ear. Summary of the Invention The present invention is directed to a prosthesis preferably for use with a magnetic induction, in-the-ear hearing aid where all the elements of the hearing aid are placed within the ear canal and the middle ear. A microphone, amplifying electronics, battery and driving coil are placed within a single housing which is custom molded for each wearer and placed deep within the ear canal. The amplifier may be one of two types, either Class A or Class B, depending on volume levels required. The coils are matched to the particular amplifier type to provide optimal efficiency for a given design. The coil is formed of a number of turns of wire wound over a mumetal core, which is used to increase magnetic field strength. The coil is placed close to the magnet to allow optimal coupling of the magnet's field with the magnetic field produced by the coil. According to the invention there is provided a middle ear ossicular replacement prosthesis for use with a magnetic induction hearing aid, the prosthesis replacing at least a portion of the ossicular chain by making contact with first and second separated locations in the middle ear and comprising: a head portion for contacting the first location adjacent the tympanic membrane of the middle ear, the head portion including a magnet and; a shaft portion extending from said head portion to the second location of the middle ear. This has the advantage that hearing loss due to portions of the ossicular having been damaged are minimised without the need for conventional hearing aids, the prosthesis being inserted to replace the defective portions of the ossicular chain. Furthermore the location of the magnet in the head of the prosthesis allows the magnet to be placed adjacent the tympanic membrane and in a desirable geometric relationship to the coil of the hearing aid with which it co-operates in use. Preferably the magnet may form the entire head of the prosthesis. This has the advantage that the magnet can be the maximum size possible for a given head size and can be used to supply motion to compensate for the hearing loss due to both the use of the prosthesis and any sensorineural damage component present in the individual. Preferably, the magnet is primarily located in the head of the prosthesis with portions of the magnet optionally located in the shaft of the prosthesis if desired. The magnet installed in the prosthesis is preferably formed of samarium cobalt, which has a history of use in the body alternatively, the magnet may be formed of a neodymium-iron material allowing a very high strength magnetic field to be developed by a very small magnet. Preferably, the magnet may be coated with biocompatible material, as commonly used in implantable medical devices. The prosthesis may be largely formed of biocompatible polymers, including high-density polyethylene and other porous, biocompatible materials. There may be a pivotable joint connecting the head portion to the shaft portion. This has the advantage that it allows improved positioning of the prosthesis within the inner ear. The magnet may be of toroidal form. This has the advantage that the weight of the magnet is reduced without significant loss in the strength of the magnetic field produced by the magnet. The head of the prosthesis may have a cavity or cup. This has the advantage that cartilage may be inserted to allow easier placement of the prosthesis by a surgeon. The shaft portion may be made of permanently bendable material. This has the advantage that the shaft can be bent to better conform with the middle ear. Brief Description of the Drawings A better understanding of the invention can be obtained when the detailed exemplary embodiments set forth below are considered in conjunction with the following drawings, in which: Fig. 1 is a cross-sectional view of the human ear with a prior art magnetic induction hearing aid placed in the ear canal; Fig. 2 is an electric 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 coils forming part of a magnetic induction hearing aid; and Figs. 5a-5j are perspective views of prostheses according to the present invention. It is known, for example from European Patent Application EP-A-0242038 published 21.10.87, to provide a magnetic induction hearing aid H in an 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. 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 different sized and shaped ear canal. 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 know 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. 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. Referring to Figures 5a-5j, the letter P generally refers to a prosthesis according to the present invention containing a magnet. Figures 5a-5c show partial ossicular replacement prostheses, while figures 5d-5j show total ossicular replacement prostheses. The prosthesis P (Figure 5a) generally contains a head 100 and a shaft 102. The head 100 is located adjacent to the tympanic membrane 68 to receive the sound waves from the outer ear, which are transmitted to the inner ear by the prosthesis. The shaft 102 of the partial prosthesis contains an internal cavity 104 which is coaxial with the shaft 102 for interlocking with the stapes 103. The shaft 102 is connected at one end to the head 100 and is connected to the stapes 103 at the other end. The magnet 106 is preferably located inside the head 100 of the prosthesis P but can additionally be located in the shaft 102. The location of the magnet 106 in the head 100 provides optimal coupling of the magnetic field produced by the magnet 106 and the magnetic field produced by the coil 28 of a hearing aid H of the type hereinbefore disclosed. The magnet 106 is a samarium cobalt magnet having a diameter of approximately 0.10" (2.54 mm) and is approximately 0.034" (0.86 mm) thick. Alternate magnet materials including neodymium-iron can be used to form the magnet 106. The magnet 106 can be coated with biocompatible polymers to increase its corrosion resistance and its biocompatibility. The prosthesis P is preferably formed of a porous biocompatible material, but can be formed of other materials known in the art. In the prosthesis P of Figure 5a, the shaft 102 is coaxial with the head 100. In Figure 5b, the shaft 102 has an axis which is parallel with but offset from the axis of the head 100. Also shown in Figure 5b is a magnet 107 which has a toroidal configuration. The use of the toroidal form for the magnet allows a reduction in weight of the magnet without a significant loss in the magnetic field of the magnet 107. The weight of the magnet and the entire prosthesis has an effect on the hearing aid. The heavier the prosthesis P, whose primary weight component is the magnet, the better the coupling between the two magnetic fields and therefore the longer the hearing aid battery life, but also the greater the reduction in the high frequency response of the unit. A heavier prosthesis has a poorer high frequency response because of inertial problems due to the weight of the prosthesis. The lighter the prosthesis, primarily achieved by using a lighter magnet, the better the aural frequency response of the hearing aid but the lower the hearing aid battery life because of the reduced magnetic coupling between the two magnetic fields. The preferred embodiment uses a magnet weighing approximately 35 milligrams and prosthesis weighing approximately 43 milligrams. The head 100 can be attached to a shaft 108 (Figure 5c) via a ball 112 and socket 110 joint to allow improved positioning of the prosthesis within the middle ear. The prosthesis P can also be formed from a head 100 and a shaft 114 which is longer than the partial prosthesis shaft 102 and does not contain a central cavity. The longer shaft 114 is necessitated in a total ossicular replacement prosthesis because a portion of or the entire stapes 103 is damaged and missing when a total prosthesis is necessitated. The total prosthesis can have the shaft centered (Fig. 5d), offset (Fig. 5e), or pivoted (Fig. 5g). Alternate versions of a total ossicular replacement prosthesis include versions having non-cylindrical heads 116 (Fig. 5f), prostheses having cupped heads 120 (Fig. 5h) and prostheses having internal cores (Fig. 5i). The cupped head 120 contains a cavity or cup 122 in which cartilage is inserted to allow easier placement of the prosthesis by the surgeon. The shaft 114 and head 100 of the prosthesis P can also contain a core 124. The core 124 preferably includes an enlarged head 126 located in the prosthesis head 100 and a core shaft 128 located substantially coaxial with the prosthesis shaft 114. The core head 126 is preferably substantially concentric with the prosthesis head 100 and is substantially flat. The prosthesis shaft 114 and the core shaft 128 are made of permanently bendable biocompatible materials such as stainless steel so that the shaft 114 can be bent to better conform with or be implanted in the middle ear. As an alternative to having the magnet 106 located inside the head 100, the magnet 130 (Fig. 5j) can form the entire head of the prosthesis P. This embodiment allows maximum magnet size for a given head size. The magnet 130 can be coated with biocompatible polymers to increase its corrosion resistance and its biocompatibility. The shaft 114 is attached to the magnet 130 and can be formed of porous polymers, hydroxyapatite or other materials known in the art. The use of the magnet in the prosthesis P allows additional vibration to be transmitted to the cochlea to compensate for hearing loss due to the placement of the prosthesis and due to any damage which has occurred to the cochlea. In this manner an individual having a prosthesis inserted can develop normal hearing levels in an injured ear without the need for using a conventional hearing aid and the problems associated therewith. The inclusion of the magnet in the prosthesis allows the magnet shape and location to be optimized for best coupling with the magnetic field from the hearing aid, for improved efficiency and longer battery life. 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 which are contemplated as falling within the scope of the appended claims.

Claims: A middle ear ossicular replacement prosthesis for use with a magnetic induction hearing aid, the prosthesis replacing at least a portion of the ossicular chain by making contact with first and second separated locations in the middle ear and comprising: a head portion (100,116,120) contacting the first location adjacent the tympanic membrane of the middle ear, the head portion including a magnet (106,107,130) and; a shaft portion (102,108,114) extending from said head portion (100,116,120) to the second location. A prosthesis as claimed in claim 1, comprising a pivotal joint (110,112) connecting said head portion (100) to said shaft portion (108). A prosthesis as claimed in either claim 1 or claim 2, wherein said shaft portion includes a magnet. A prosthesis as claimed in any of any claims 1 to 3, wherein said shaft portion (102) is coaxial with said head portion (100). A prosthesis as claimed in any of claims 1 to 3, wherein the axis of said shaft portion (114) is parallel with the axis of said head portion (100). A prosthesis as claimed in any of claims 1 to 5, wherein said shaft portion (102,108) is hollow. A prosthesis as claimed in any of claims 1 to 6, wherein said head portion (100) is generally cylindrical. A prosthesis as claimed in any of claims 1 to 7, wherein said magnet (106,130) is generally cylindrical. A prosthesis as claimed in any of claims 1 to 8, wherein said magnet (107) is generally toroidal. A prosthesis as claimed in any of claims 1 to 9, wherein said shaft portion (114) and said head portion (100) include a core (124) encased within said shaft portion (114) and said head portion (100), said core (124) including an elongated core shaft aligned substantially coaxially within said shaft portion (114) and further including a first end forming an enlarged and substantially flat head (126) positioned within and substantially concentrically within said head portion (100), said shaft portion (114) and said core (124) being permanently bendable to allow said prosthesis to be permanently bent to a specific shape during implantation thereof to conform to the person's middle ear structure, said core (124) being stationary with respect to said head portion (100) and said shaft portion (114). A prosthesis as claimed in any of claims 1 to 9, wherein said head portion is a magnet (130). A prosthesis as claimed in claim 11, wherein said magnet is coated with a biocompatible material. A prosthesis as claimed in claim 1, wherein the head (120) of the prosthesis (P) has a cavity or cup (122).

Inventors: TATGE GARY, TATGE, GARY
Applicants: SMITH & NEPHEW RICHARDS INC, SMITH & NEPHEW RICHARDS INC.
Application year: 1988
Main classification: H04R25
IPC codes: H04R25, A61F2