Abstract:
A method of casting a hollow hip prosthesis head comprises forming a wax pattern of a femoral head including two cores. The first is a central socket-forming core and the second a ceramic hollow-forming core. The wax pattern is coated with a ceramic slurry to form a shell in which core extensions are captivated. The wax is melted and evacuated and the shell is fired to form a mould. Metal is cast and, when cool, the shell and cores are removed and each aperture formed in the head by the hollow-forming core is sealed.

Description:
BACKGROUND 
       [0001]    The present invention relates to replacement joint and other structural medical implants, in particular femoral head replacement implants. Currently preferred hip prostheses include a femoral head having a socket for connection to a stem, which stem is fixed in or to the patient&#39;s femur. An acetabular ball cup may or may not be provided for insertion into the patient&#39;s pelvis and adapted to receive in a close sliding fit the new artificial head. 
         [0002]    To be suitable for use in a medical device, a material must exhibit the appropriate functional properties, mainly mechanical properties, for the particular application and must be biocompatible. Biocompatibility is the ability of a material to perform with an appropriate host response in a particular application. For example, in the complex environment of the human body, metal alloys are subject to electrochemical corrosion with the bodily fluids acting as an electrolyte and, to be biocompatible, a metal alloy used in an implantable medical device must exhibit very low corrosion over the projected lifetime of the device. Metal particles released by corrosion may be concentrated locally or distributed systemically and it is important that the type and amount of material released does not pose a danger to the patient. Cobalt-chromium based alloys, developed for the aerospace industry, are used in many medical device applications, including implantable medical devices, because of their strength, corrosion resistance, and biocompatibility. For example, cobalt-chromium alloys, typified by alloys conforming to ASTM standard specifications, such as, ASTM F-75-01, STANDARD SPECIFICATION FOR COBALT-28 CHROMIUM-6 MOLYBDENUM ALLOY CASTINGS AND CASTING ALLOY FOR SURGICAL IMPLANTS, and ASTM-799, STANDARD SPECIFICATION FOR COBALT-28 CHROMIUM-6 MOLYBDENUM ALLOY FORGING FOR SURGICAL IMPLANTS, are often used as components of modular prosthetic devices such as prosthetic hip and knee joints. 
         [0003]    A prosthetic joint typically includes paired load bearing surfaces, commonly comprising a first surface of a metal alloy component paired with a second surface comprising a metal, a polymer, a ceramic, bone, or bone cement. When load bearing surfaces move relative to each other, such as during articulation of a prosthetic joint, friction can cause the surfaces to spall. The wear debris, known medically as third bodies, originating from the load bearing surfaces of an implanted medical device can initiate a histiocytic reaction in which the body&#39;s immune system is activated to release enzymes to dissolve the particles of debris. However, because the wear debris is usually a relatively hard material, such as a metal or polycarbon compound, the enzymes either fail to dissolve the debris or dissolve the debris only with the passage of considerable time. On the other hand, the enzymes do react with tissue and bone and may weaken or dissolve the bone supporting or adjacent to the medical device. In the case of a prosthetic joint, weakening of the bone or osteolysis may shorten the life of the device and may eventually render the supporting bone unusable. Further, surface erosion can eventually lead to failure of the load-bearing surfaces, requiring replacement or repair of the surfaces. In the case of implanted medical devices, replacement or repair entails expensive and risky surgery. 
         [0004]    Typically, where a polymer cup is employed, this is in the form of a lining of a metal cup that is inserted into a patient&#39;s acetabulum. As a consequence, the femoral ball is necessarily smaller in size, since the metal cup must be of limited magnitude in order to fit into the patient&#39;s acetabulum. This means that the area of the mating surfaces of the head and polymer cup is also limited, increasing the pressures between them, and hence the wear. WO-A-2005/070344 describes a method of providing a smooth hard surface of a medical implant and if, this can be achieved, then a metal-to-metal interface can be permitted since the wear between is consequently less. And in that event, the area of the mating surfaces can be maximised which further reduces the wear between them. 
         [0005]    However, if the head is solid, it is very heavy and that is undesirable for several reasons. A presently available design provides a hollow head cast in two parts: a hollow, hemispherical head, and a base connected to the head. The base is provided with a cup socket to permit connection to the femoral component. Between them, the base and head require considerable machining in order to permit interconnection of them by ion beam welding. However, it would be desirable to provide a hollow head that did not require machining, nor indeed, welding. Both these processes add significantly to the cost of the final product and also complicate traceability of the manufactured product. Often, the production (by casting) of the individual components may be carried out separately from any subsequent machining and/or welding. Consequently, there are more opportunities for inadequate accounting for the different stages of the production of the final product. 
         [0006]    In any event, the welding of the base to the rim of the hemispherical head results in substantial heating of the hemispherical surface near the rim, and this may adversely impact the crystal microstructure near the rim which could influence the wear and or smoothness of the hemispherical surface. 
         [0007]    U.S. Pat. No.  6 , 129 , 764  provides a relatively hollow (humeral) head with an open base to receive a variable connector to the humerus. SU-A-619179 provides a hollow cap for fitting to a reshaped femoral head. EP-A-375600 discloses forged femoral components welded together. DE-A-3907530 discloses a hollow femoral head formed from sheet material, and provided with a neck cup by welding. 
         [0008]    CH-A-274021 discloses a method of making a hollow prosthesis comprising making a destructible core from a first material and casting a second destructible material as shell around the core in a mould, removing the first destructible core leaving the shell of the second destructible material and forming a ceramic mould having the second destructible material shell as its void and casting metal therein to form a hollow metal prosthesis. 
         [0009]    It is an object of the present invention to provide a novel process of construction of a medical prosthesis head, and a head so constructed. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0010]    In accordance with the present invention there is provided a method of casting a hollow medical prosthesis head comprising the steps of:
       forming a pattern of the head, said pattern comprising a domed surface and a trunnion-seat-forming base, and a gate extension;   disposing a hollow-forming core in the head and an extension of said core through the base;   coating the pattern and said extension with a shell-forming slurry in which said core extension is captivated;   curing said shell to form a mould;   casting metal into the mould so as to form an aperture through the base of said head around said core extension;   when cool, removing said shell and at least said core extension; and   closing the or each aperture.       
 
         [0018]    Preferably, said closure is effected by weld-filling of said aperture(s), although it could be closed by a plug. Preferably, said weld-filling (or plug) uses the same metal as the cast metal. 
         [0019]    The pattern is preferably moulded from a settable fluid material such as wax or a thermoplastics material. In the case of wax, said curing step includes the step of re-melting the wax and draining the melted wax from the shell once the shell has cured to the extent that it retains its shape during said draining. Subsequently, if the shell is a ceramic mix, it may be fired to complete the curing process. 
         [0020]    Preferably, there are a plurality of said extensions and apertures formed thereby, preferably four. They may be round in section but may also be elongate in a circumferential direction for reasons explained further below. 
         [0021]    Preferably, said pattern includes a trunnion-forming part on said base and a trunnion is thereby integrally formed on the head during said casting step. A socket-forming core may be disposed in said trunnion-forming part, said socket-forming core including an extension also captivated by said shell-forming slurry. 
         [0022]    Both the socket-forming core and hollow-forming cores may be ceramic. The hollow-forming core may toroidal in shape, whereby a supporting column for the trunnion is formed. Indeed, the trunnion may be formed by the eye of said toroid, and the socket-forming core may extend into the eye. 
         [0023]    Said gate extension is conveniently off said domed surface. 
         [0024]    A method as claimed in any preceding claim, wherein all of said core is removed from the head prior to said closure. The hollow void left by removal of said hollow-forming core may be evacuated and/or filled with an inert gas before said closure is effected. 
         [0025]    The hollow-forming core is removed by shattering the core with a vibrating tool. Alternatively, it may be chemically etched to remove it. For either purpose, said extensions may usefully be elongate in section in order to facilitate access to the outside and removal of debris. A suitable dissolving agent is an alkaline solution, such as potassium hydroxide, that does not damage the cast metal. 
         [0026]    The cast metal may be a cobalt/chromium/molybdenum alloy. 
         [0027]    Said moulding of the pattern may comprise the steps of:
       positioning said core or cores in a pattern-forming mould by connection of said extension or extensions to a base of said mould;   closing said mould with a domed cover; and   injecting curable fluid material the mould.       
 
         [0031]    Said domed cover may conveniently have a gate aperture forming said gate extension and said injection of said fluid material may be through said gate aperture. 
         [0032]    The method is particularly suitable when said hollow medical prosthesis head is a hip prosthesis head. Nevertheless, although described herein in relation to, and being particularly suitable for, hip replacement prostheses, the present invention is not limited thereto and the principles could be employed in other prostheses such as shoulder joints. 
         [0033]    In another aspect of the present invention, a medical prosthesis head comprises a casting of metal provided with a domed surface and a base, and having an integrally cast hollow void, an integrally cast trunnion seat, and closed core pin aperture or apertures in the base of the head, said aperture(s) being of smaller dimensions than the void. 
         [0034]    By “smaller dimensions” is meant that the core cannot be withdrawn through the aperture or apertures after casting but have either to be left behind, which is a perfectly feasible option, or be fluidised so as to flow out of one or more of the apertures, whether under the effects of gravity, vibration-spillage or pressurised ejection. In any event, while the apertures are desirably large for this purpose, they are also desirably small to reduce the effects of subsequently having to close them afterwards. 
         [0035]    The prosthesis head may have an integrally cast trunnion on said trunnion seat. This also is preferred in order to reduce post-casting operations. Indeed, said trunnion preferably includes an integrally cast socket. The trunnion is advantageously supported by an integrally cast column spanning said void and connected with said domed surface. 
         [0036]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
         [0037]    Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0038]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0039]    An embodiment of the present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: 
           [0040]      FIG. 1  is a side section through a human hip region showing a hip prosthesis of the type to which the present invention relates: 
           [0041]      FIG. 2  is a section through a wax pattern casting mould, with cores in place, partly in section; 
           [0042]      FIG. 3  is a section through a ceramic shell incorporating the wax pattern prior to metal casting; 
           [0043]      FIG. 4  is a perspective view of a cast prosthesis prior to complete removal of all core material and gate; and, 
           [0044]      FIG. 5  is a section through an alternative form of head also in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0045]    Referring in detail to the drawings where similar parts of the invention are identified by like reference numerals, and, more particularly to  FIG. 1 , an artificial prosthetic hip joint  20  includes a pair of interfacing load bearing surfaces arranged to move relative to each other. The prosthetic hip joint  20  typically includes a ball  22  that is connected to a body  24  comprising a neck  26  and a stem  28 . The stem  28  may be held in place in the femur  30  by a variety of methods, including the use of cementing agents  29 , an interference fit, a threaded attachment mechanism, or biological fixation. 
         [0046]    A cup-shaped socket  32  is anchored in the pelvis  34  by any of a variety of known techniques, such as cementing; press fitting; the use of screws; the use of a textured, knurled, or threaded exterior; the use of a biological fixation mechanism or by a combination of biological and mechanical fixation. The ball  22  is positioned adjacent to the concave surface of the socket  32 . A socket insert  36 , commonly comprising a polymer, such as an ultra-high molecular weight polyethylene (UHMWPE) or an ultra-high molecular weight, cross linked polyethylene (UHMWXLPE), is disposed within the socket  32  to reduce friction between the ball  22  and the socket and to increase the life of the joint. On the other hand, the socket insert  36  may comprise ceramic or metal or a ceramic or metal socket may be used without a socket insert. In some cases, the second load bearing surface in contact with a surface of a medical device component of cobalt-chromium comprises bone or bone cement. The convex outer surface of the ball  22  interfaces with the concave load bearing surface of the socket insert  36  or socket  32 , as appropriate, to allow the joint to rotate and articulate simulating the movement of the natural hip joint. 
         [0047]    A metal to metal interface, provided the surfaces are smooth and defect-free potentially offers the longest lifetime and, if the interfacing surfaces can be formed directly on the head and socket respectively, the largest area can be provided for the interface, thereby reducing the pressures between them. 
         [0048]    For strength, corrosion resistance, and biocompatibility, the ball  22  comprises a cobalt-chromium alloy. Additional components of the prosthetic hip joint  20 , including the body  24  and the socket  32  may also comprise a cobalt chromium alloy. Cobalt-chromium alloys are alloys comprising significant portions of cobalt and chromium and, commonly, also include a significant portion of molybdenum. Cobalt-chromium alloys used in medical devices are typified by alloys complying with ASTM standard specifications, ASTM F-75-01, STANDARD SPECIFICATION FOR COBALT-28 CHROMIUM-6 MOLYBDENUM ALLOY CASTINGS AND CASTING ALLOY FOR SURGICAL IMPLANTS, and ASTM-799, STANDARD SPECIFICATION FOR COBALT-28 CHROMIUM-6 MOLYBDENUM ALLOY FORGING FOR SURGICAL IMPLANTS. 
         [0049]    Cobalt-chromium alloys also include alloys that have higher minor portions of carbon or nitrogen and comply with an ASTM F-75 Modified specification. In addition, as used herein, cobalt-chromium alloys include other proprietary alloys that contain cobalt and chromium and resemble alloys conforming to the ASTM-F75, modified ASTM-F75, and ASTM-799 standard specifications. 
         [0050]    The ball head  22  and neck  26  are frequently separate components provided with a stem and socket interconnection (not shown), whereby the same ball head  22  can be applied to stems such as body  24 , or to a stem fixed directly on the femur  30 , or even to the bone itself. Moreover, different shapes and sizes of body  24  can be employed. 
         [0051]    Cobalt-chromium, and more particularly cobalt-chromium-molybdenum, are desirably cast so that the microcrystalline structure is most favourable for the purposes of resisting corrosion, providing the smoothest possible surface and reducing any tendency to suffer fatigue cracks after an extended period of use within the patient&#39;s body. Moreover, the subsequent machining and welding operations should be minimised to reduce cost and also to reduce adverse consequences by inadvertent altering of the crystal structure. 
         [0052]    With reference to  FIG. 4 , a part completed head  22 ′ is shown which has a spherical (“domed”) surface  10  from which extends a gate  12 , explained further below. A base surface  14  of the head  22 ′ has a central socket  16  to receive a plug (not shown) on the end of the neck  26  of the body  24 . The head  22 ′ is formed in a casting process which begins (see  FIG. 2 ) with the moulding of a wax pattern in an aluminium mould  100 . The mould  100  is in two parts  100   a,b , lower part  100   a  comprising a hemispherical former  102  having an integral gate-forming bore  104  through which a mould injection port  106  connects. The complimentary part  100   b  of the mould  100  includes blind bores  108 , 110   a, b  adapted to receive extensions  112 , 114  of cores  116 , 118  respectively. 
         [0053]    Extension  112  of core  116  has a circumferential groove  120 , which is engaged by a pin  122  that retains the core  116  in the bore  108  once the pin  122  is inserted from the side of the mould part  100   b.  Likewise, extensions  110   a,b  (typically there will be four of such extensions, only two of which are visible in  FIG. 2 ) are also provided with grooves for engagement by pins  124  that retain the core  118  in engagement with the mould part  100   b.    
         [0054]    Once assembled as shown in  FIG. 2 , molten wax (or other suitable alternative) is injected through the port  106  to fill the void between the cores  116 , 118  and the mould parts  100   a,b.    
         [0055]    In  FIG. 3 , pins  122 , 124  have been removed and the mould parts  100   a,    100   b  separated in the direction of the arrow A in  FIG. 2  to expose a moulded wax pattern  150 . The separation of the mould parts  10   a,b  occurs, of course, once the wax has solidified. The cores  116 , 118  are then captivated within the wax pattern  150 . 
         [0056]    As is well known in the art, a layer  130  of ceramic paste is applied to the outside of the patent  150  by sequentially dipping in powder and slurry, to build up a mould shell for metal casting. At this point, the wax that remained in injection port  106  has been removed and the gate pattern  104 ′ has been connected to a wax runner system  132  (not shown in detail) whereby a number of patterns  150  are connected together on a tree. 
         [0057]    Once the shell  130  is built up, by consecutive dipping in slurry and powder, as is known in the art, the shell is allowed to dry and harden and subsequently is placed in a kiln. When the kiln is heated, the wax melts and is allowed to drain from the casting mould, leaving the cores  116 , 118  in place, retained in the shell  130  by their respective extensions  112 , 114  captured by the shell  130 . 
         [0058]    Once the wax has fully drained, the kiln is further heated to fire the ceramic shell and burn off any remaining hydrocarbon components of the wax. 
         [0059]    Finally, the desired alloy is cast into the mould formed by the conjoined shells  130 . Once the metal has solidified and cooled in an appropriate quenching cycle, the shell  130  is broken and removed and the gate  12  cut to separate the cast heads  22 ′ from the runner tree. The cores  116 , 118  are then removed, again by means known in the art. The core  116  can be withdrawn and, indeed, it is not necessarily ceramic and could be employed again in appropriate circumstances. However, the core  118  most likely is ceramic and of the type that shatters to a fine dust when vibrated with an appropriate tool. Once extensions  114  have been snapped off, a tool can be inserted through the apertures  140  so formed in the base  14  of the head  22 ′. The core  118  is then shattered and can be withdrawn through the apertures  140 . Alternatively, the ceramic or alternative core material can be dissolved using an appropriate etching medium, again known in the art, for removal from the void within the head  22 ′. Such a medium may comprise a caustic solution such as potassium hydroxide. The apertures  140  need not be round, but could be elongate in a circumferential direction, as shown at  140 ′. This might facilitate removal of the core. 
         [0060]    Once the void is empty of core, the apertures  140  are sealed by welding using the same material as the casting of the head  22 ′. Indeed, it is arranged that the apertures  140  are spaced from the edge  142  between the base  14  and domed surface  10 . This ensures that the inevitable heating caused by the welding does not raise the temperature of the domed surface  10  to such an extent that the desirable microstructure is adversely affected. Alternatively, the core is not removed at all. This reduces the weight advantage of the present invention, but still reduces the cost compared with solid metal, and is still much less dense. Also, although weld filling is preferred, if a larger aperture  140 , 140 ′ is employed, it may be preferred to fit and weld in place a plug of the appropriate metal. Although the same metal as used in the casting is preferred, this is not essential. 
         [0061]    Finally, once the apertures  140  are sealed, the gate  12  is removed so that the domed surface  10  can be formed with a perfect spherical surface, and any hardening process prior, to final polishing, can be effected. By this process, the required machining of the head  22 ′ can be reduced to perhaps nothing more than adjustment of the socket  16 . Indeed, the socket  16  can be cast as a rough, somewhat tapered aperture, employing the ceramic shell to form it, in which event a simple machining operation can refine its dimensions. 
         [0062]    Although the hollow interior of the head  22 ′ is isolated from the exterior it is quite possible to effect the sealing of the apertures  140  in an inert atmosphere, such as argon, so that it is that gas which fills the interior of the head in the unlikely event of any leakage therefrom in time. 
         [0063]    The hollow-forming core  118  is shown as a single component. There is no reason why this should be essential. It could comprise several components spaced from one another. The resulting spaces between them would provide webs in the final product linking the trunnion  16   a  (as the socket-forming part of base is referred to) to the domed surface, thereby increasing the rigidity of the arrangement. Indeed, in  FIG. 2 , a column-forming bore  119  is shown in dotted lines in the core  118 . This not only provides support for the trunnion  16   a  in the finished product, but also provides a better route for the molten metal as it enters the shell mould  130  and follows into the trunnion-forming part of the mould. 
         [0064]    It is a feature of the present invention that at least a trunnion seat  16   b  (see  FIG. 5 ), if not the trunnion  16   a  itself, is moulded integrally with the rest of the head. In  FIG. 5 , an alternative embodiment of the present invention provides only a trunnion seat  16   b  on the cast head  22 ″. The trunnion  16   a ′ is separately formed and subsequently welded to the trunnion seat. The trunnion  16   a ′ is shown as a cup with a weld flange  16   c,  but it could, of course, be a simple hollow cylinder. In this embodiment, the casting gate  12 ′ is shown inclined and connected to the base  14 ′ (formed by repositioned gate/port  106 ′ shown in dotted lines in  FIG. 2  at the interface between mould parts  100   a,b ). 
         [0065]    While separate formation of the trunnion is not especially preferred (because it reduces traceability of the components and increases the separate operations performed in the manufacture of the head, it still has the advantage that the welding edge  16   c  can be positioned well away from the edge  142 ′. 
         [0066]    All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0067]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0068]    The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.