Abstract:
A protective faceplate ( 37 ) for an implantable component of a tissue-stimulating prosthesis, such as a prosthetic hearing implant. The faceplate ( 37 ) comprising a first or outer surface and an opposed second or inner surface. The implantable component can be removably or non-removably mountable to the second surface and adapted to extend into a cavity formed in a bone of a recipient.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional application of U.S. patent application Ser. No. 10/523,795 filed on Jun. 30, 2005, which is a national stage application of PCT/AU2003/01012, filed on Aug. 11, 2003, which claims the priority of Australian Patent Application No. 2002950754, filed on Aug. 9, 2002. In addition, this application claims the priority and makes reference to U.S. Provisional Application No. 60/590,916, filed on Jul. 26, 2004, and U.S. Provisional Application No. 60/629,578, filed on Nov. 22, 2004. The entire disclosure and contents of the above patents and applications are hereby incorporated by reference. The present application is also related to U.S. application Ser. No. 10/523,800, entitled “Fixation System for an Implantable Medical Device,” filed Feb. 9, 2005. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention resides in an improved method of mounting an implantable component of an implantable medical device, such as a prosthetic hearing implant package, securely in the head region of a recipient. 
     2. Related Art 
     In many people who are profoundly deaf, the reason for deafness is absence of, or destruction of, the hair cells in the cochlea which transduce acoustic signals into nerve impulses. These people are unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to or absence of the mechanism for nerve impulses to be generated from sound in the normal manner. 
     It is for this purpose that prosthetic hearing implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. 
     Prosthetic hearing implant systems have typically consisted of essentially two components, an external component commonly referred to as a processor unit and an internal implanted component commonly referred to as a receiver/stimulator unit. Traditionally, both of these components have cooperated together to provide the sound sensation to a user. 
     The external component has traditionally consisted of a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds, particularly speech, into a coded signal, a power source such as a battery, and an external transmitter antenna coil. 
     The coded signal output by the speech processor is transmitted transcutaneously to the implanted receiver/stimulator unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter antenna which is positioned to communicate with an implanted receiver antenna coil provided with the receiver/stimulator unit. 
     This communication serves two essential purposes, firstly to transcutaneously transmit the coded sound signal and secondly to provide power to the implanted receiver/stimulator unit. Conventionally, this link has been in the form of a radio frequency (RF) link, but other such links have been proposed and implemented with varying degrees of success. 
     The implanted receiver/stimulator unit traditionally includes a receiver antenna coil that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlear electrode assembly which applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound. 
     As mentioned above, traditional implanted receiver/stimulator units are positioned within the head of the recipient by drilling a bed into and through the posterior section of the mastoid bone lying behind the recipient&#39;s ear. Such a bed is usually made by drilling the bone down to the lining of the brain or dura mater, so that the receiver/stimulator unit is securely held in position and does not protrude excessively past the skull surface. 
     The receiver/stimulator unit manufactured by the present Applicant has a package made from titanium which houses the stimulation electronics and which is fitted into a bed created in the mastoid bone. A receiver antenna coil extends from the rear end of the package and lies superficial to the bone. Other prosthetic hearing implants have included packages made from a ceramic material which are usually placed completely within the bed drilled down to the lining of the brain. 
     Over time it has been realized that the placing of the above packages in the mastoid bone some distance behind the ear has not always been ideal and has had some problems associated therewith. In instances where young children have been implanted with a device, it has been seen that in some recipients the package has created an external protuberance in the region of the head adjacent the implant site, which has been unsightly, intrusive, and inconvenient for the recipient. In some instances, such a protuberance can prevent the placement of a behind-the-ear processor unit over the site of the implant due to the risk of skin erosion that may result. 
     Further, as the package is positioned to be facing the surface of the skull, the implant package may be subject to an impact to the head in that region either directly on top of the device or as a lateral glancing blow to the device. In this regard, such devices must be designed to withstand such an impact and remain operational. In this regard, it has been found that by designing the device to have a low profile, the risk of the device sustaining a glancing, lateral blow is less likely. It is also important that the device is designed in such a manner to ensure that it is prevented from entering the cranial cavity in the event of the device being subject to an impact of excessive force. 
     International PCT Application No. PCT/AU00/00936 discloses an implant package capable of being located within the mastoid cavity of a recipient. This application introduces the utilization of the naturally occurring gutter lying between the sigmoid sinus, posterior osseous ear canal, the mastoid tip and the floor of the middle fossa to protect and maintain the implant package in place. This application discloses a suitably shaped implant casing capable of fitting wholly within the mastoid cavity, having a receiver coil connected thereto via flexible arms. Such a package design may have problems associated with stability of the implant within this cavity region, which could be greatly dependant upon the anatomy of the patient and the particular surgical approach used by the surgeon. Should the package be not firmly secured within the cavity, the implant may move following implantation causing tissue erosion and/or movement of the attached electrode arrays, possibly resulting in the need for re-implantation of the device. 
     Therefore, there is a need to provide a prosthetic hearing implant package that is capable of addressing at least some of the concerns with prior art devices. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     SUMMARY 
     According to one aspect of the present invention, a faceplate for protecting an implantable component is provided. The faceplate having dimensions slightly larger than the dimensions of the implantable component securely positioned within a cavity formed in a bone, the faceplate being capable of resting securely on the bone so as to substantially cover the cavity. The faceplate comprises a planar member having an upper and lower surface; and at least one flange extending outwardly beyond the perimeter of the implantable component. 
     According to another aspect of the present invention, a method of using a faceplate to protect an implantable component secured within a cavity formed in a bone, the implantable component having an upper and lower surface is provided. The method comprises the steps of forming a cavity in a bone of a recipient with dimensions such that the implantable component is capable of being positioned securely within the cavity; positioning the implant in the cavity such that the upper surface is at least in substantial alignment with at least a portion of the surface of the bone surrounding the cavity; and positioning the faceplate to rest securely on the bone, the faceplate having dimensions slightly larger than the dimensions of the implantable component. 
     According to another aspect of the present invention, a method of using a faceplate to protect an implantable component secured within in a cavity formed in a bone is provided. The method comprises the steps of: forming a cavity in a bone of the recipient with dimensions such that the implantable component is capable of being positioned securely within the cavity; selecting a faceplate having a planar member having an upper and lower surface, the faceplate having a dimensions slightly larger than the dimensions of the implantable component; and positioning the faceplate with the implantable component mounted thereto over the cavity, the faceplate being capable of resting securely on the bone so as to substantially cover the cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, a preferred embodiment of the invention is now described with reference to the accompanying drawings, in which: 
         FIG. 1  is a pictorial representation of a conventional prosthetic hearing implant system; 
         FIG. 2  is a representation of a conventional receiver/stimulator unit positioned in a bed fashioned in the mastoid bone according to conventional surgical techniques; 
         FIG. 3  is a simplified view of the receiver/stimulator unit and faceplate of a prosthetic hearing implant device according to a preferred embodiment of the present invention; 
         FIG. 4   a  is an end view of the unit and faceplate of  FIG. 3  depicted implanted in the mastoid of a recipient; 
         FIG. 4   b  is a side view of the unit and faceplate arrangement of  FIG. 3 ; 
         FIG. 5  is a simplified view of another embodiment of a faceplate and receiver/stimulator unit according to the present invention; and 
         FIG. 6  is an end view of yet another embodiment of a receiver/stimulator unit and faceplate of a prosthetic hearing implant device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing the features of the present invention, it is appropriate to briefly describe the construction of one type of known prosthetic hearing implant system with reference to  FIG. 1 . 
     Known prosthetic hearing implants typically consist of two main components, an external component including a speech processor  29 , and an internal component including an implanted receiver and stimulator package  22 . The external component includes a microphone  27 . The speech processor  29  is, in this illustration, constructed and arranged so that it can fit behind the outer ear  11  and is held in place behind the outer ear  11  via an ear-hook arrangement (not shown). Alternative versions may be worn on the body. Attached to the speech processor  29  via a cable  13  is a transmitter antenna coil  24  that transmits electrical signals to the implanted package  22  via a radio frequency (RF) link. 
     The implanted component includes a receiver antenna coil  23  for receiving power and data from the transmitter coil  24 . A cable  21  extends from the implanted receiver and stimulator package  22  to the cochlea  12  and terminates in an electrode array  20 . The signals thus received are applied by the array  20  to the basilar membrane  8  and the nerve cells within the cochlea  12  thereby stimulating the auditory nerve  9 . The operation of such a device is described, for example, in U.S. Pat. No. 4,532,930, the contents of which is incorporated herein by reference. 
       FIG. 2  shows in more detail the surgical placement of the implanted receiver and stimulator package  22  of  FIG. 1 , according to conventional practices. The package  22  is in the form of a capsule, for example a titanium capsule, which houses the necessary circuitry required for the implant to operate as desired. The receiver coil  23  is shown encapsulated in a material, such as silicone rubber, to provide a protective body and ensure fatigue resilience. A magnet  30  is shown positioned within the coil to assist in the alignment of the transmitter antenna coil  24  with the receiver antenna coil  23  as discussed previously. As is shown, a bed is drilled into the bone  31  to maintain the package  22  in position. This bed is typically round or ovoid to match the shape of the package. The bed is typically made in the mastoid bone and mastoid angle of the parietal bone in the region of the asterion. Typically, the bed is fashioned initially with a cutting burr, and then completed with a diamond paste burr and a template is typically used to ensure that the bed is fashioned to the correct size. As is shown, the bed may be drilled down to the lining of the brain, or dura mater  32 , particularly for young children with thin skulls. It is for this reason that a diamond paste burr may be used when approaching the dura and when the dura is exposed, to minimise the risk of tearing of the dura  32 . 
     As can be seen from  FIG. 2 , once the receiver and stimulator package  22  is secured in place in the mastoid bone, it remains rather unprotected, with only a layer of skin (not depicted) covering the skull protecting the package from any direct impact. Further to this, it can be appreciated that any impact in the direction shown by the arrow A of  FIG. 2 , has the potential for the package to tear the dura  32  and enter the cranial cavity, potentially causing damage to the sensitive structures of the brain. As can also be appreciated from  FIG. 2 , an impact to the head region of the recipient, particularly in the direction shown by arrow B, has the potential to dislodge the implant from its bed within the skull bone. Such dislodgement can cause damage to the area of the head adjacent the device as well as discomfort to the recipient. Any dislodgment of the device also has the potential to require further surgical procedures to relocate the device in the desired position within the head of the recipient. 
     The present invention aims to address the above potential problems by positioning the receiver/stimulator package in the head in a manner whereby the package preferably has a low profile and its contents are afforded some protection from impact and from being subsequently damaged and/or dislodged. 
     During a typical surgical procedure for implanting a conventional cochlear prosthetic hearing device, such as this shown in  FIGS. 1 and 2 , a mastoidectomy and posterior tympanotomy are typically employed to obtain access to the middle ear. The mastoidectomy procedure typically requires removal of material from the mastoid bone behind the ear of the patient via a cutting burr or drill. Typically, the cortex of the mastoid superior and posterior to the external meatus is removed and the excavation is deepened and air cells are removed superior and posterior to the meatus, exposing the mastoid antrum and the middle ear via the tympanotomy. Following the tympanotomy, the round window should be accessible, thereby allowing a cochleostomy to be performed and the electrode array inserted. 
     It can be understood that by performing a mastoidectomy, a cavity is created which could thereby house the receiver/stimulator package at a location remote from the exterior wall of the skull. It is considered that by allowing the mastoid cavity to house the implant package, considerable advantages can be obtained in relation to the protection and safety of positioning the implant package and the other advantages as discussed previously. 
     As discussed previously and disclosed in International PCT Patent Application PCT/AU00/00936, anatomical dissections have shown that there exists a “gutter” lying between the sigmoid sinus, posterior osseous ear canal, the mastoid tip and the floor of the middle fossa. This gutter can also form an ideal location to place the implantable receiver/stimulator package, in a position not exposed above the surface of the bone and protected by the pinna. 
       FIG. 3  is a view of one preferred embodiment of the present invention. In this embodiment, the mastoid cavity is shown by the fine dotted line  35 , which is shown as being located behind the pinna  11 . An upper surface in the form of a top faceplate  37  of the implanted receiver/stimulator unit  38  (heavy dotted line) is shown positioned above and over the mastoid cavity  35 . As is evident in  FIG. 3 , the receiver/stimulator unit  38  has a lower surface that is shaped to be sunk into the mastoid cavity. The depicted faceplate  37  has flanges  39  which extend outside the perimeter of the implanted receiver/stimulator unit  38  to enable securing of the unit to the skull via surgical screws  36 . A receiver antenna coil  40  is shown external of the unit  38  and faceplate  37 , in much the same manner as a conventional design as discussed previously. 
       FIGS. 4   a  and  4   b  show end and side views of the embodiment depicted in  FIG. 3  and where appropriate, the same reference numerals are used. As can be clearly seen in these figures, the receiver/stimulator unit  38  extends into the mastoid cavity  35  and is protected by the faceplate  37  which acts as a protecting shield for the unit  38  as well as a stabiliser and means for securing the unit  38  in place. A lead  41  connects the receiver/stimulator unit  38  to the intracochlear electrodes (not shown) which deliver the electrical stimulation to the nerves within the cochlea. 
     The flanges of the faceplate  37  can be a simple extension of the upper surface of the receiver/stimulator unit and made from the same material as the rest of the receiver/stimulator unit. This material can, for example, be titanium, preferably a malleable titanium. Alternatively, a titanium flange may be attached to the titanium case of the receiver/stimulator unit  38  by an appropriate welding or other method. 
     The flanges  39  are formed so as to be relatively robust whilst also sufficiently malleable so that the entire faceplate  37  can be formed to the shape of the skull surrounding the mastoid cavity by the surgeon using finger pressure only. As the anatomy of this region of the head varies somewhat from individual to individual, it is desirable to form the flanges  39  so that they adopt a flush fit in abutment against the skull. 
     As the faceplate  37  provides protection for the receiver/stimulator unit  38 , it is advantageous to form the faceplate from one of a number of different thicknesses of titanium sheet. In order to withstand impacts of considerable force it is desirable to form the faceplate  37  out of a suitable material such as titanium having a thickness of between 0.3 to 1 mm. As the flanges  39  must be malleable to enable a surgeon to alter their shape with a minimum of force, the flanges  39  are, in the depicted embodiment, made from a thinner material than that of the faceplate  37 . Alternatively, the desired conformability of the flanges  39  could be achieved by altering their geometry rather than their thickness. In this regard, the flanges  39  could be of the same thickness as the faceplate  37 , provided that the flanges are in a narrower strip form rather than a wide flange form. However, in a preferred embodiment, the flanges  39  may be formed from a material, such as titanium, having a thickness of, for example, 0.1 to 0.2 mm. 
     The lead  41  is preferably pre-coiled so that it can settle into the mastoid cavity  35 , below the receiver/stimulator unit  38 . As is shown in  FIGS. 4   a  and  4   b , the lead  41  exits the receiver/stimulator unit  38  from a bottom surface thereof. This facilitates routing of the lead to the cochlea via the posterior tympanotomy, which is at the bottom of the mastoid cavity. However, it is envisaged that the lead exit point and the form can have many other geometries and still remain within the spirit of the invention. For example, the lead  41  may exit from the side of the receiver/stimulator unit and may be straight. 
     In order to prevent tissue erosion, the faceplate  37 , flanges  39  and screws  36  are preferably coated in a silicone rubber or other elastomeric material. In such a case, the screws  36  would be accessed by means of a slit or hole in the silicone above the screw  36 . 
     It should be appreciated that the screws  36  used in the present invention may have a number of design variations to satisfy the design requirements of the present application. For example, the screws  36  may be countersunk for low profile, may have a round head, and may even be resorbable screws. Resorbable screws would assist in holding the implant in place for a short period until the fibrous tissue surrounds and secures the device in place. 
       FIG. 5  depicts an alternative embodiment of the present invention. In this embodiment, the faceplate has relatively narrow flanges  39  that are adapted to assist in enabling the faceplate  37  to conform to the contours of the skull. Further to this, extra screw holes are provided to allow some redundancy in the variations in patient anatomy and the mastoidectomy performed. Also, if there is a problem with securing the device at one screw site, such as a cavity from a past surgery or a skull growth line, then that screw may be omitted and an alternative screw site used. It should be stressed that this aspect of the present invention is important particularly as it is recommended against fixing the device with screws on both sides of the natural growth lines of the skull. In this embodiment, the basic size of the faceplate  37  is designed to be just larger than the size of the mastoid cavity  35 , allowing the faceplate  37  to be stabilised on the rim of the mastoidectomy. To assist in this stabilisation, the rim of the mastoid cavity may be easily flattened by the surgeon, for example by drilling, to create a stable seat for the faceplate  37 . 
       FIG. 6  depicts yet another embodiment of the present invention. In this embodiment, the faceplate and flanges are not fixedly attached to the receiver/stimulator unit  38 . The primary difference between this embodiment and that described in  FIGS. 4   a  and  4   b  is that the faceplate  37  is provided with mechanical catches or clips  45  to hold and maintain the receiver/stimulator unit  38  in place. In this manner, the receiver/stimulator unit is ‘snap-fit’ into the faceplate  37  for securing in place. 
     The benefit of this embodiment is that the use of the faceplate and flanges to secure the implant in place is optional and can be decided upon at the time of surgery. Further, the securing mechanism can be used with non-metallic receiver/stimulator units as there is no need for the faceplate and flanges to be welded onto the unit casing. This enables the present device and method to be employed with ceramic cased implants. It is also envisaged that with a detachable system as shown in  FIG. 6 , the faceplate/flange combination could be made from a non-metallic material such as a biocompatible plastic, as welding to the implant case would not be required. Such a feature would avoid the need to coat the surface of the faceplate and flanges with a coating of silicone rubber and the like to prevent tissue erosion. For example, the plate could be made of polypropylene or polytetrafluoroethylene (PTFE) which have the properties suitable for such an application. 
     In each of the above-described embodiments of the present invention, the receiver/stimulator unit  38  is shown as an arbitrarily shaped unit capable of fitting within the bone cavity. It is considered that the receiver/stimulator unit  38  could also be conformable such that the shape of the unit  38  may be altered during the procedure to conform to the specific shape of the bone cavity. In this regard, the unit  38  can be made of a conformable material that allows the shape and form of the unit to be changed without effecting the hermiticity of the unit  38 . 
     In each of the above-described embodiments, the procedure associated with implanting a device according to the present invention could generally be as follows:
     1. A mastoidectomy would be performed in the same manner as a conventional procedure;   2. Device placement would be determined using a template shaped like the actual implant device;   3. Drill holes would be marked for securing the device in place following the insertion of the electrode array;   4. A posterior tympanotomy and cochleostomy would be performed in the same manner as a conventional procedure;   5. The electrode array would be inserted into the cochlea;   6. The implant package would be placed in position. In this step, the coil connecting the package to the electrode array inserted into the cochlea would preferably coil itself up into the mastoid cavity due to the preformed coil in the lead; and   7. The implant package would be secured in place via screws or the like.   

     In this manner, the process for implanting a device of the present invention would in no way complicate a conventional procedure and would eliminate the need to drill an additional bed in the mastoid bone for receiving the implant. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail may be made therein without departing from the scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.