Prosthetic joints often comprise a stem component which includes a body section to be received in the medullary canal of a first bone, and a head section can be received in a socket component which is implanted in a second bone to allow articulation of the first bone relative to the second bone. In the case of a hip joint prosthesis or a shoulder joint prosthesis, the head section will have a convex part-spherical surface and the socket component a corresponding concave part-spherical surface.

It is important to control accurately the location of the stem component when inserted into the medullary cavity. It is known to control the depth of the insertion of a hip joint stem component by providing a collar at or towards the proximal end of its body section, for example as disclosed in GB-1510968. The collar can rest on the resected end of the bone in which the component is implanted. When the stem is implanted with a bone cement, the collar can rest on a mantle of cement.

It can be preferable for some patients to use a stem component without a collar, for example when imperfections in the patient's bone tissue or in the bone cement might allow a void to develop around the stem component between it and the cement.

If the stem component does not have a collar which rests against bone tissue at its proximal end, it can subside into the canal into contact with the bone cement around it. GB-A- 2104391 discloses a spacer for the tip of a stem component of a hip prosthesis, which has a stopped bore in which the tip can be received. The bore defines a space around the tip, which is preserved even when the prosthesis is implanted and the bore contains bone cement. Subsidence of the stem component can then be accommodated by the space.

It can be particularly important to control the density of bone cement material that is provided in the medullary cavity around a joint prosthesis component. Weakness in the implanted prosthesis can result from unwanted voids or regions of where the cement is subjected to lower pressures as it cures by polymerising. The space defined by the stopped bore in the device disclosed in GB-A-2104391 can lead to the formations of voids in the surrounding cement mantle, in particular due to expansion of gas (normally air) in the space during the exothermic curing reaction of the cement.

The present invention provides a spacer which is a tight fit on the end of a stem component, and which can define a cement-free region around the end of the body section into which the body section can subside as necessary after implantation by displacement (including compression as discussed below) of the material of the spacer.

Accordingly, in one aspect, the invention provides an end cap spacer for use with a stem component of a joint prosthesis having an elongate body section for insertion into and alignment with the axis of the medullary cavity of a first bone in substantial alignment with the axis of the cavity, the spacer having a cavity formed therein in which the end of the stem component body section can be received and being configured to define a region around the end of the body section which remains free of bone cement when the body section is inserted into a quantity of bone cement in a medullary bone cavity, the material of the spacer being soluble in body fluids so that, after implantation, the implant is able to subside into space within the medullary cavity occupied by the spacer immediately after implantation.

The spacer provided by the present invention has the advantage that it can provide a region into which a stem component can subside if necessary after implantation, without provision of a gas-filled space within the bone cement which can lead to the formation of voids in the cement.

The use of a spacer that is made from a soluble material has the advantage that, after dissolution of the spacer, there is no interface between the bone cement and the bone cement which can otherwise give rise to stress concentrations and, potentially, failure of the cement.

The material of the spacer is selected so that dissolution is inevitable over a period after implantation of the prosthesis. It might be selected so that dissolution is stimulated by an event after implantation which causes dissolution to start or to be accelerated. For example, dissolution might be stimulated on contact between the spacer and body fluids.

Such contact might happen for example, when a failure starts in the bond between the prosthesis and the surrounding mantle of bone cement. Such failure can allow body fluids into the interface between the prosthesis and the bone cement. Subsidence of the prosthesis can be preceded by such failure. Subsidence might also result from creep of the material of the cement mantle, or from pores in the material of the cement mantle towards the distal end of the prosthesis or in some other region.

Preferably, the material from which the spacer is formed being compressible so that the stem component can subside as necessary after implantation into the said region around the end of the body section by compressing the material of the spacer. The material of the spacer might be compressed in the sense that the overall configuration of the spacer is changed without a significant reduction of the volume of the material of the spacer. For example, this might involve movement of the material of the spacer to fill a previously unfilled region in response to the applied stress. Preferably, however, the compressible material for the spacer is one which enables the volume of the material of the spacer to be reduced when the prosthesis subsides. Suitable compressible materials can have jelly-like physical properties. The compression characteristics need to be selected appropriate to provide a balance between preserving a space around the tip of the prosthesis during implantation while also allowing subsidence after implantation. Accordingly, the material should have a modulus that is less than that of the other material or materials with which the prosthesis is in contact and supported when implanted. In particular, the modulus of the material should be less than that of the bone cement material with which the prosthesis is implanted. Commonly, bone cements have a Young's modulus of about 2200 MPa. The corresponding modulus for the material of the spacer should be less than about 500 MPa, preferably less than about 250 MPa, especially less than about 100 MPa.

The compressibility of the material of the spacer should be sufficient to ensure that, when an implanted stem component is subjected to an applied stress, the spacer material compresses before the bone cement around the component is damaged. The material should be sufficiently rigid to ensure that the component can withstand the treatment to which it will be subjected during implantation, for example as it is forced into a quantity of bone cement material. A suitable material for use in the spacer of the invention can have a modulus of elasticity of at least about 3 MPa, preferably at least about 4 MPa. The material will generally have a modulus of elasticity which is not more than about 8 MPa, preferably not more than about 6 MPa.

Examples of materials that might be used for the spacer include gelatin based materials, for example containing at least about 30% by weight gelatin, preferably at least about 40% by weight. A preferred material comprises about 50% by weight gelatin. A plasticising liquid can be included to optimise the physical characteristics of the material, in an amount of at least about 15% by weight, preferably at least about 25% by weight. For example, glycerol might be included in an amount of about 30% by weight.

An example of a composition that can be used to make the end cap spacer of the present invention comprises: Component % by weight Gelatin 50 Glycerol 30 Water 20 Methyl parahydroxybenzoate 0.02 The end cap spacer will generally be made by moulding.

The spacer will generally be arranged to be a snug fit on the end of the prosthesis so as to minimise the volume of air that is trapped within the spacer between it and the prosthesis.

The precise configuration will depend on the shape of the prosthesis. It will generally be preferred for the spacer to be shaped so as to allow any gas in the space between it and the prosthesis to exude from the space as the prosthesis with the cap in place is inserted into a quantity of bone cement. This might involve for example making the edge of the cap remote from the closed end flexible, for example by reducing the wall thickness at the edge.

The spacer can be provided as a cap which fits onto the end of the prosthesis without overlapping significantly the lateral walls of the prosthesis. This has the advantage that the spacer does not reduce the thickness of the bone cement mantle around the prosthesis, between the prosthesis and the internal wall of the medullary canal in which the prosthesis is inserted.

In another aspect, the invention provides a joint prosthesis assembly which comprises: (a) a stem component having an elongate body section for insertion into and alignment with the axis of the medullary cavity of a first bone in substantial alignment with the axis of the cavity, (b) a spacer having a cavity formed therein in which the end of the stem component body section can be received and being configured to define a region around the end of the body section which remains free of bone cement when the body section is inserted into a quantity of bone cement in a medullary bone cavity, the cavity in the spacer being configured so that the spacer is a snug fit on the end of the stem component body section and the material of the spacer being compr- essible so that the stem component can subside as necessary after implantation into the said region around the end of the body section by compressing the material of the spacer, the material also being soluble in body fluids so that, after implantation, the implant is able to subside into space within the medullary cavity occupied by the spacer immediately after implantation.

The selection of the configuration of the spacer will be relatively important at the end of the prosthesis since, in the event of subsidence, stress will tend to concentrate in this region, possibly leading to failure of the bone cement due to the concentration of stress.

Accordingly, the configuration of the spacer will be selected so that the tip of the prosthesis can subside through the anticipated distance without coming into contact with bone cement. This distance could be as much as 5.0 mm, but for many applications it will be less than 3.0 mm, for example less than 2.0 mm. The distance will generally be at least about 0.1 mm, preferably at least about 0.25 mm. These distances will generally be appropriate for the thickness of the wall of the spacer on the end which faces along the axis of the medullary cavity of the bone, against which the tip of the prosthesis acts.

The external configuration of the end cap spacer will preferably be selected to minimise resistance to flow of the end cap, when located on the end of a stem component, through a body of bone cement and so that the cement is able to flow over all of the exposed surfaces of the end cap spacer and the stem component at its tip.

Preferably, the assembly includes means for locating the stem component transversely relative to the walls of the bone cavity at or towards its end. This has the advantage of facilitating a mantle of bone cement around the stem component which is not undesirably thin on one side of the component as a result of the component being located eccentrically relative to the axis of the medullary canal. Preferably, the locating means comprises a plurality of transversely extending elements arranged around the periphery of the stem component which can extend between the stem component and the internal walls of the bone cavity to hold the component away from the cavity walls. The transversely extending elements can be resiliently deformable so that they can be deformed inwardly on insertion into the medullary canal. Locating means which comprises resilient elements are disclosed in GB-1409053 and GB-A-2104391.

Preferably, locating means for locating the stem component transversely relative to the walls of the bone cavity at or towards its end comprise transversely extending elements which are formed from a substantially rigid material. This has the advantage of providing accurate control over the location of the stem component. In particular, rigid elements can reduce the likelihood of a stem component being displaced from a desired location within the medullary cavity by a transverse force which might deform elements formed from a deformable material.

Rigid locating means can be made from a polymeric material. It will be selected according to the physical requirements placed on it when in use. It should also be compatible with other materials with which it will come into contact when in use, in particular the materials of the prosthesis and of the bone cement, and the patient's natural tissue materials. An example of a suitable material for the locating means is a polymethylmethacrylate. The locating means is preferably made by moulding.

The locating means, and especially the transversely extending elements thereof, will preferably be configured so that they minimise disruption to flow of bone cement past the locating means as the stem component with the locating means located on it is inserted into a medullary canal containing the cement. The transversely extending elements will preferably have a small area when the locating means is viewed along the axis of the prosthesis. This has been found not to affect adversely the performance of the locating means since the transversely extending elements are not subject to significant bending forces during implantation.

The locating means can comprise a collar on which the transversely extending elements are mounted. The collar can receive the end of the stem component, generally before the end cap spacer is fitted on it. The locating means might be arranged so that it can slide along the stem component, to a location not more than about 5 cm from its tip, preferably not more than about 3 cm from the tip.

When the locating means is formed from a rigid material, it should be arranged to fit appropriately into the prepared medullary cavity. The locating means might be capable of being trimmed to an appropriate size. For example, it might have a plurality of lines of weakness on which it can be trimmed (by means of a tool such as a knife or by breaking it).

Alternatively, the surgeon might be provided with a plurality of different locating means with a range of dimensions, from which the surgeon can select a component to suit the requirements for a particular patient.

Preferably. the locating means is provided by a component that is separate from the end cap spacer. This has the advantage of allowing the two components to be made from different materials, which are selected to optimise the characteristics of the components for their respective functions. It has the further advantage of allowing the surgeon to select the components which are suitable for a particular application, for example according to the size and configuration of the stem component and according to the size and other characteristics of the medullary canal. It further allows the surgeon to decide to use just one, or both, of the end cap spacer and the transverse locating means.

Preferably, the stem component is one which is collarless and comprises an elongate body section for insertion into and alignment with the axis of the medullary cavity of a first bone in substantial alignment with the axis of the cavity, and a head section which extends generally laterally away from the body section, for reception in a socket component which is implanted in a second bone to allow articulation of the first bone relative to the second bone. At least the body section is tapered inwardly towards its tip when viewed from one side along a line that is perpendicular to the axis of the body section and lies in the plane which contains the body section and the head section, and at least the head section is tapered inwardly when viewed in transverse cross-section, from a point towards its edge which faces away from the body section towards its edge which faces towards the body section.

The use of a collarless stem component has the advantage that its tendency to subside into the medullary cavity of a bone after implantation in that cavity, for example due to imperfections in the patient's bone tissue or in the bone cement, is reduced compared with stem components which are not tapered as specified above. This reduced tendency to subside can be obtained with only small degrees of taper of the stem when viewed along the laterally extending viewing line specified above. Preferably, the ratio of the reduction of the lateral extent of the body section when viewed along the first axis to the length of the section of the body section over which it is tapered is at least about 0.015, more preferably at least about 0.02. Preferably, the said ratio is not more than about 0.075, more preferably not more than about 0.45. In a preferred construction, the value of the said ratio is about 0.03.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a sectional elevation along the anterior-posterior axis of a stem component, having an end cap spacer according to the present invention on its tip, and means for locating the stem component transversely within a medullary canal, Figure 2 is an enlarged side elevation view of the end cap spacer shown in Figure 1, Figure 3 is an end view of the locating means shown in Figure 1, Figure 4 is a side elevation view of another embodiment of locating means, Figure 5 is an end view of the locating means shown in Figure 4.

Referring to the drawings, Figure 1 shows a stem component 2 of a hip prosthesis. The stem component comprises a body section 4 which is for implantation in the medullary cavity of the femur, and a head section 6. The head section has a plug 8 on which a spherical bearing component can be fixed. The bearing component can be received in a spherical socket which is implanted in the patient's acetabulum. The head section 6 has an axis 10 which is arranged at an angle a (about 130 to about 150°) to the axis 12 of the body section 4.

Each of the body section 4 and the head section 6 is tapered inwardly towards its tip. The body section of the stem component is taped inwardly towards the distal end of the component. The head section is tapered inwardly towards the proximal end of the component. The edge 13 of the body section which is directed away from the head section is substantially straight. The edge 14 which is directed towards the head section is generally rounded, in particular towards the proximal end of the body section.

The taper of the stem component when viewed along both the lateral-medial and anterior- posterior axes extends from the region 18 which is located when the stem component is implanted at the head of the bone after resection, to the distal tip of the stem component The stem component is formed from a high nitrogen stainless steel by a process which involves warm forging followed by appropriate finishing techniques which include grinding and polishing. The surface of the body section is finished so that it has a surface roughness (Ra) of less than 100 nm. This surface finish gives the body section a highly polished appearance.

The stem component has an end cap spacer 20 mounted on its end. The spacer is formed from a gelatin material having a composition as described above. Its modulus of elasticity is about 5 MPa. It has a hollow space 21 formed in it in which the tip of the stem component can be received with a snug fit on the end of the stem component so that there is no significant space within the spacer between it and the surface of the stem component.

The edge 22 of spacer remote from the closed end 24 is maintained flexible by reducing the wall thickness at the edge, so that any small amount of gas within the spacer can exude out under pressure from a body of liquid bone cement as the component with the spacer is inserted into the cement.

The gelatin material of the spacer 20 is capable of dissolving in body fluids such as blood on prolonged exposure. It is configured to provide a space into which the stem component 2 can subside in the event of failure of the bond between the component and the mantle of bone cement. The thickness of the end wall 26 of the spacer is about 5 mm. The use of a gelatin material allows the prosthesis to subside by compression of the material because of the relatively low modulus of the material (compared with that of other materials by which the prosthesis is supported, notably the bone cement). In this way, subsidence through as much as 1,2 or even 3 mm can be accommodated before the tip of the component comes into contact with bone cement. After degradation involving substantially complete dissolution of the gelatin material, the stem component can subside by as much as 5 mm.

In use, the spacer component 20 is positioned on the end of the stem component 2 prior to insertion into the medullary cavity within a patient's bone 31. The cavity will previously have been loaded with a quantity of a bone cement 28 which will create a mantle around the component to bond it to the internal surface of the bone: flow of cement within the medullary cavity is controlled by means of a cement block 29. On insertion of the stem component with its end cap spacer into the bone cement, the cement flows around the spacer into sealing contact with all of the exposed surfaces of the spacer and the stem component.

The spacer presents a small volume around the end of the stem component which is preserved free of bone cement, in particular while the cement hardens. Subsequently after implantation of the stem component with its end cap, exposure of the end cap to body fluids can cause the material of the end cap to degrade so that a void is formed around the end of the stem component into which the component can subside.

The locating means 40 shown in Figures 1 and 3 comprises a collar 42 which can fit on to the end of the stem component 2. The collar bears a plurality of laterally extending fins 44, each of which is tapered towards its laterally directed edge 46. The locating means is formed from a rigid material such as a polymethylmethacrylate by moulding. The material is substantially rigid so that it can locate the stem component positively within the medullary cavity. The locating means is selected according to the internal dimensions of the cavity to provide a sliding fit within the cavity.

Figures 4 and 5 shows another embodiment of locating means 30 which comprises a collar 32 and three wings 34 arranged symmetrically around the collar. Each of the wings is capable of being resiliently deformed inwardly. The locating means comprising the collar and the wings is formed by moulding from a polymeric material with appropriate resilient properties, such as a high molecular weight polyethylene. In use, the collar is positioned around the stem component towards its end and the assembly comprising the stem component 2, the end cap spacer 20 and the collar 30 is inserted into the medullary cavity of a patient's bone. The collar with its wings is selected so that it is too big transversely to fit into the cavity. Consequently. the wings are deformed inwardly on contact with the internal surfaces of the medullary cavity, to hold the stem component away from the walls of the cavity and therefore to maintain a mantle of bone cement around the stem component.