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 fig1 . 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 . fig2 shows in more detail the surgical placement of the implanted receiver and stimulator package 22 of fig1 , 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 fig2 , 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 fig2 , 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 fig2 , 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 fig1 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 . fig3 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 fig3 , 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 . fig4 a and 4 b show end and side views of the embodiment depicted in fig3 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 fig4 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 . fig5 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 . fig6 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 fig4 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 fig6 , 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 .
a protective faceplate for an implantable component of a tissue - stimulating prosthesis , such as a prosthetic hearing implant . the faceplate 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 .