GB2516662A

GB2516662A - Hip and knee joint prostheses

Info

Publication number: GB2516662A
Application number: GB1313504.1A
Authority: GB
Inventors: Andrew Clive Taylor; Michael Anthony Tuke; Anne Roques
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-07-29
Filing date: 2013-07-29
Publication date: 2015-02-04
Also published as: GB201313504D0

Abstract

A stem 21 for a prosthesis comprising a tapered connector 22 securable to a head 25 by inserting the connector 22 into a socket in the head 25, wherein the connector 22 has a body that is deformable to enhance the contact area between the connector 22 and socket. The connector may have one or more slits 26a, 26b that form fingers 27 or an axial bore. In use, the connector 22 flexes to fit a socket with a different degree of taper. The connector 22 may have proximal and distal annular contact surfaces.

Description

Hip And Knee Joint Prostheses This invention relates to hip and knee joint prostheses comprising a stem and a head.

S

Hip replacement surgery attempts to restore the function of a hip joint following, for example, rheumatoid arthritis. osteoarthritis or fracture. This may involve replacing the femoral head and/or the acetabulum. Similarly, knee replacement may involve replacing the femoral or tibial side of that joint, or both.

Currently, modular hip joint prosthesis for replacing a hip joint comprises a stem having an end for inserting into a femur and another end forming a connector onto which an artificial femoral head is secured. The head of the installed prosthesis may then be located in the original acetabulum (hemiarthroplasty) or a prosthetic acetabulum to reconstruct the joint. A modular prosthesis enables a clinician to select the most appropriate stem and head independently according to clinical conditions, thus allowing for variations to materials, head diameters and head centre offsets.

It is known for the connector to have an inwardly tapering frustroconical shape and the head to have a complementary tapering socket into which the connector is forced to secure it in place. The tapered shape is intended to produce a mechanically stable attachment. However, these connections have been linked to some issues, attributed to corrosion and/or taper mismatch. Internal surface corrosion may occur due to use of certain material combinations (e.g. involving, in particular, cobalt-chrome alloys) and material conditions, such as deformationlstiffness of the neck junction under load, taper design (e.g. length and surface finish), as well as in vivo conditions such as in particular the presence of proteins, local pH (acidity level), crevice size and taper damage. Damage observed at the taper junction has also been attributed to for example electro-mechanical erosion or electrochemical machining. Fretting may occur for instance with the presence of micromotions, sometimes attributed to the use of larger femoral head sizes and the resulting increased friction and loads transferred to the taper junction. This may be described as a toggling due to the friction forces exerted at the bearing (in flexion/extension, abduction/adduction or internal/external motions), when there may be micromotions at the taper junction.

The mechanisms underlying long-term failure of modular hip prostheses are unclear.

S Table 1 summarises proposed mechanisms of failure. In all mechanisms, following onset of initial damage, the corrosion process may accelerate.

Table 1

Document ____ Study Type Putative*Mechanism Langton et al. (2012) Bone Retrieval study of metal Corrosion Joint Res 1: 56-63 on metal joints Donell et al. (2010) JBJS(Br) -Retrieval study of metal Galvanic corrosion 923(11): 1501-1508 on metal joints Clyburn (2013) Reconstructive Retrieval study of metal Fretting corrosion Review 3(1):18-21 on plastic joints Bolland et al. (2011) JBJS(Br) Retrieval study of metal borrosion 933: 608-615 on metal joints Chana et al. (2012) JBJS(Br) Retrieval study of metal Crevice corrosion, galvanic 94B: 281-286 on metal joints corrosion and mechanically assisted Cooper et al (2012). J Bone Retrieval study of metal Crevice corrosion, fretting, Joint Surg Am 94: 1656-1 661 on plastic joints mechanically assisted crevice corrosion, production of metal ions (Go) and reaction Jacobs (1998) JBJS, 1998, Retrieval study of metal Fretting corrosion 80A: 1447-1458 on plastic joints Mathiesen etal. (1991) Retrieval study of metal Crevice corrosion JBJS(Br) 732: 569-575 on plastic joints Lindgren et al. (2011) JBJS Retrieval study of metalS *brevice corrosion more likely 932(10): 1427-1430 on plastic joints than galvanic.

Yerby et al. (1996) J Retrieval study of metal Crevice corrosion Arthroplasty 11(2): 157-162 on plastic joints Goldberg et al. (2002) Clin. Retrieval study of metal Fretting corrosion Orthop. Relat. Res. 401: 149-on plastic joints 161 _________________________ Document Study Type Putative Mechanism Grupp et al. (2010) BMC Retrieval study of metal Fretting corrosion Musculoskelet. Disord. 11:3 on plastic joints Kop et a]. (2009) J Arthroplasty Retrieval study of Fretting movement 24(7): 1019-1 023 modular stems Kop et a]. (2012) Clin. Orthop. Retrieval study of Crevice corrosion and fretting Relat. Res. 470(7): 1885-1894 modular stems Werner et al. (2013) J Retrieval study of Corrosion Arthroplasty 28: 543.e13-modular stems 543.e15 ___________________ ____-. __________________ Jauch et al. (2011) J Biomech. Laboratory testing of Micromotions 44: 1747-1 751 modular stems Mathew et al (2012) Cliii. Laboratory testing of Wear and corrosion synergy Orthop. Relat. Res. 470: 3109-metal on metal joints (bearing surfaces) 3117 _____ _________________ Swaminathan et al. (2012) Laboratory testing of Fretting corrosion Biomaterials 33: 5487-5503 modular stems Yan et al. (2013) J Mech Laboratory testing Wear and corrosion synergy Behavior of Biomedical (bearing surfaces) Materials 18:191-199 ________________________________________________ Any movement between the two tapered surfaces of the two-part prostheses is also more likely to permit the exchange of fluid between the inter-component cavity or crevice and the surrounding tissue. This could promote corrosion of metal components of the joint. Deformations of the head/taper, one of the possible causes listed in table 1, may exacerbate this effect by pumping fluid around the socket cavity.

Erosion and corrosion of prostheses can release particulate debris and/or metal ions, which can cause metallosis-related osteolysis and pseudotumours, and can lead to the failure of the prosthesis.

These above problems can cause patient loss of mobility and pain, necessitating costly revision surgery.

A method to improve the burst safety for ceramic heads of the stem-head prostheses is to machine a continuous spiral along the length of the stem connector. When a stem scribed in this way is forced into a head socket, the ridges of the spiral are burred over, increasing area contact and hindering separation of the joint. However, a spiral creates a continuous fluid path between the socket cavity and the surrounding tissue. It is proposed that this path may facilitate fluid and organic matter (e.g. proteins) inflow, thus promoting degradation of the joint.

The present invention seeks to overcame one or more, preferably all of the above-mentioned problems. Preferred embodiments seek to provide improvements in hip and knee prostheses.

In developing the present invention, it was found that two-part prostheses may impose a greater pressure on certain parts of the connector-socket interface. For example. greater pressure will be imposed at the proximal region of the outer face of the connector of a hip prosthesis where the connector is less tapered than the socket (Fig. 1). Conversely, greater pressure will be imposed at the distal region of the outer face of the connector where the connector is more tapered than the socket (Fig. 2).

Both can promote failure of the prosthesis, particularly in eroded or corroded joints.

According to the invention there is provided, in first aspects, a stem for a hip or knee prosthesis, comprising a tapered connector securable to a head selected from a prosthetic femoral head, a tibial tray and a femoral component by inserting the connector into a socket in the head in which the shape of the socket is imperfectly complementary to the connector, wherein the connector has a body or body portion that is deformable to enhance the contact area between the connector and socket.

In the invention, embodiments provide hip and knee joints. In medical contexts, proximal' end refers to the end nearest to the body, and distal' end refers to the end farthest from the body. The embodiments include (1) a hip prosthesis, having a femoral head and a stem, (2) a knee prosthesis, having a tibial tray and a stem and (3) a knee prosthesis, having a femoral component and a stem. Therefore, in the invention, proximal' end refers to the end at or nearest the head and distal' end refers to the end at or nearest the stem.

The invention thus provides, in separate embodiments, (1) a stem for a hip s prosthesis comprising a tapered connector securable to a femoral head, (2) a stem for a knee prosthesis comprising a tapered connector securable to a tibial tray (being a knee prosthesis for the tibia), and (3) a stem for a knee prosthesis comprising a tapered connector securable to a femoral component (being a knee prosthesis for the femur).

In use, e.g. in assembly of the prosthesis, the connector of the invention deforms when pushed firmly into the socket and can adapt to fit a socket with a mismatched degree of taper. By enhancing the contact area formable between the connector and socket, the stem-head attachment is more stable, thus reducing or preferably eliminating gaps between the components as a result of non-perfectly matching tapers and reducing possible toggling, as well as preventing or preferably reducing exchange of fluid between the socket cavity and the surrounding tissue.

Accordingly, the invention prevents or reduces the wear and long-term failure rate of a modular hip or knee prosthesis regardless of the proposed mechanism underlying the failure (Table 1).

Head units for hip and knee prostheses, used with the stems of the invention, may be as supplied at present, and hence may be unchanged for use with the new stems.

Thus in improving the hips and knees only one component may be modified.

The connector of embodiments of the invention is substantially circular in cross section and is deformable at its proximal end so as to narrow its diameter, facilitating insertion further into the socket until e.g. the full length of the taper is seated.

Preferred connectors are shaped so that, in use, initial contact is at the proximal end (the small end of the conic) where the male part generally has increased tailored compliance. With increasing assembly force the entire tapered area is loaded and sealing is effected at the large end (distal end) where the stiffness is greatest.

The pre-assembled, i.e. non-deformed, shape of the connector may be designed to s be more tapered than the socket so that contact occurs primarily at the distal end of the connector when inserted into the inner section of the socket, or less tapered than the socket, so that the contact occurs primarily at the proximal end of the connector.

In both cases, the contact (proximal or distal) may be designed to consistently occur at the same end of the connector (proximal or distal) when any two parts made within the intended manufacturing tolerance ranges are assembled.

In an embodiment of the invention, the proximal end of the connector comprises one or more axial slits forming fingers which deform inwards when the connector is forced into the socket, permitting the connector to deform and adapt to the shape of the socket. These slits may pass through the centre of the connector. The slit or slits may divide the connector into 2 or more fingers, preferably 4 or more fingers, optionally 6 or more fingers, alternatively 8 or more fingers.

The formation of the slits can be achieved simply, for example by cutting from the proximal end of the connector axially, for example using wire erosion.

The slits are suitably dimensioned to permit sufficient deformation of the connector to adapt to a suitable socket, while being small enough for the fingers to retain high levels of rigidity, to maximise the stability of the assembled stem and head structure.

The slits may have a thickness that is greater than the maximum deflection required as a result of assembly. The slits design should allow full assembly throughout the design and manufacturing tolerance range of the tapers, without the need for excessive force application at assembly, and without compromising the endurance of the head/stem junction. In general this thickness may be less than 100 pm1 preferably less than 50 pm, more preferably less than 25 pm, optionally less than 13 pm. The slits are suitably at least 2 pm, preferably at least 4 pm wide.

Typically! the connector has 4 fingers produced by two slits. The slits are preferably located perpendicularly to each other and to the axis of the cone, crossing centrally.

In a further embodiment of the invention, the connector comprises an axial bore configured to set the compliance of the connector so as to deform inwards when the connector is forced into the socket. This bore is preferably located in a substantially central position. The diameter of the bore is defined to set the compliance of the connector so that an even contact is obtained between the surfaces of the connector and socket after assembly. In use of these embodiments, insertion of the connector is possible until there is initial contact between proximal surfaces of the connector and proximal internal surfaces of the socket. Further force to push the connector into the socket results in deformation of the proximal end of the connector allowing the connector to be inserted more deeply into the socket with increased area of contact between outer surfaces of the connector and internal surfaces of the socket.

The axial slit, slits or bore extend through the connector incompletely from the distal end of the connector. For example, axial slit, slits or bore may extend no more than 98%, 95%, 90%, 80% or 70% of the length from the proximal end of the connector.

This ensures a robust fit between the proximal region of the connector and the socket, thus enhancing sealing of the socket cavity from the fluid of the surrounding tissue. Preferably, contact between the distal end of the connector and the socket forms a complete seal, preferably a fluid and/or ion seal.

The connector may further comprise one or more ridges, preferably formed by scribing, which burr over on insertion of the connector into the head socket, thus producing a resilient fit. Preferably, the connector has a plurality of axially distributed ridges and each ridge continuously surrounds the connector. The continuous nature of the ridges provides an improved seal. Preferably, each ridge is located approximately on a plane perpendicular to the axis of the connector as this would provide an optimal sealing effect. The one or more ridges are preferably in the form of one or more continuous rings around the circumference of the connector, which do not form a spiral.

In a further embodiment of the invention, the connector has a rebate on the taper circumference surface configured to split the contact area formable between the connector and socket to a region at the distal end of the connector and a region at the proximal end of the connector. This distributes the pressure at the connector-s socket interface between the inner and outer parts of the joint, preferably evenly, thus preventing toggling.

The rebate may have a depth of up to 1 mm, preferable up to 500 pm, for example up to 200 pm, up to 100 pm or up to 50 pm. Typically, the rebate will be about 100 pm or about 500 pm deep.

In an embodiment of the invention, the slit(s), bore and/or rebate are in-filled with a material that is more compliant than the stem material and/or an adhesive.

Further embodiments of the invention comprise two or more features selected from: the slit(s); the bore; and the rebate.

In a related aspect of the invention, there is also provided a stem for a hip or knee prosthesis comprising the stem and a head, the head comprising (i) the ball of a ball and socket joint with a pelvis in a hip prosthesis, (ii) a tibial tray of a knee prosthesis or (U) a femoral component of a knee prosthesis, wherein the stem comprises an end connector that inserts into and locks into a corresponding recess in the head, and wherein upon insertion into the recess proximal portions of the end connector contact proximal inner surfaces of the recess, and upon further insertion the end connector flexes allowing further insertion until distal regions of the end connector contact distal inner surfaces of the recess.

The stem may for example be a stem for a modular hip or knee prosthesis, wherein the prosthesis comprises the stem and (i) a head which comprises or forms the ball of a ball and socket joint with a pelvis in a knee prosthesis, (U) a tibial tray of a knee prosthesis or (U) a femoral component of a knee prosthesis, wherein the stem comprises an end connector that inserts into a corresponding recess in the head, and wherein the end connector comprises proximal contact portions that upon insertion of the stem into the recess contact proximal inner surfaces of the recess, and wherein upon being urged further into the head the end connector flexes allowing further insertion until distal regions of the end connector contact distal inner surfaces of the recess, locking the stem onto the head.

In use, pushing the connector into the socket locks the stem to the head due to frictional force between the respective surfaces. The socket can be pushed on by hand or by striking with an implement e.g. a hammer or mallet, optionally using a predetermined force.

The end connector may comprise one or more flexible elements that flex with respect to the stem upon insertion into the head. In response to the assembly force, the flexing leads to increased surface area contact between the stem and head.

In particular embodiments, contact between distal regions of the end connector and distal inner surfaces of the recess seals the stem into the head, preferably forming an annular seal between the stem and the head.

In preferred embodiments, the end connector comprises two or more fingers that extend from a stem body and are spaced apart, wherein upon insertion into the recess proximal portions of the fingers contact proximal inner surfaces of the recess and wherein the fingers flex towards each other upon further insertion into the recess.

In alternative embodiments, the end connector comprises a central bore forming an open ended cylinder having wall portions that flex inwardly upon insertion into the recess.

Preferably, the end connectors comprise a distal annular contact surface that upon insertion of the stem contacts the distal inner surfaces of the recess around an annular contact surface; and a distal annular contact surface that upon further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head.

In further embodiments, the distal and proximal contact surfaces are separated by an intermediate section of the end connector the outer surface of which is cut away so that it is spaced from the inner surface of the recess upon insertion of the stem into the recess.

In specific further embodiments, the end connector comprises (i) two or more fingers that extend from a stem body and are spaced apart, wherein upon insertion into the recess proximal portions of the fingers contact proximal inner surfaces of the recess and wherein the fingers flex towards each other upon further insertion into the recess, and (ii) a distal annular contact surface that upon said further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head.

In a particular example, described in more detail below, the connector is substantially frustroconical.

Generally, all embodiments and optional and preferred features described with respect to the first aspects are similarly embodiments and optional and preferred features of the related aspects of the invention.

Stems of the invention may additionally comprise an intermediate sleeve, to be located between the stem and the head, such as that disclosed in EP 0385572. This can further improve the fit between the components.

Also provided by the invention are a hip prosthesis comprising a stem and a femoral head, a knee prosthesis comprising a stem and a tibial tray and a knee prosthesis comprising a stem and a femoral component.

The prosthesis of the invention may comprise or be made substantially entirely of cobalt-chrome, titanium, titanium alloy or stainless steel.

The invention is now illustrated with references to the accompanying drawings, in which: Fig. 1 shows a cross-section of a schematic of a prior art two-part hip prosthesis and the pressure between the stem and head along the length of the tapered connector; Fig. 2 shows a cross-section of a schematic of another prior art two-part hip prosthesis and the pressure between the stem and head along the length of the tapered connector; Fig. 3 shows a part cross-section of a schematic perspective view of a prior art two-pad hip prosthesis; Fig. 4 shows a schematic of a hip prosthesis including a stem according to the invention in which the connector has 2 axial slits, forming 4 fingers; Fig. 5 shows a cross-section of the hip prosthesis of fig. 4 and the pressure between the stem and head along the length of the tapered connector; Fig. 6 shows a schematic of a hip prosthesis including a stem according to the invention in which the connector has an axial bore; Fig. 7 shows a cross-section of the hip prosthesis of fig. 6; Fig. S shows a schematic of a hip prosthesis including a stem according to the invention in which the connector has a circumferential rebate; and Fig. 9 shows a cross-section of the hip prosthesis of fig. S. A prior art two-part hip prosthesis 10, 16 has a stem 11, 17, of which only part is shown, having a frustroconical connector 12, which fits into a complementary socket in an artificial femoral head 15. Manufacturing tolerances (greatly exaggerated in the drawings) can produce stems 11 having connectors 12 that taper less than the socket in the head 15, which places an increased pressure at the proximal end 13 of the outer face of the connector 12, and forms a gap at the distal end 14 of the outer face of the connector 12. Alternatively, manufacturing tolerances (greatly exaggerated in the drawings) can produce stems 17 having connectors 12 that taper more than the socket in the head 15, which places an increased pressure at the distal end 19 of the outer face of the connector 12, and forms a gap at the proximal end 13 of the outer face of the connector 18. In use, with both of these configurations, the gap can lead to non-optimum in vivo performance through any of the mechanisms detailed by the researchers described in table 1.

is A two-pad hip prosthesis 20 comprises a stem 21 of the invention, of which only part is shown, having a frustroconical connector 22, which fits into an approximately complementary socket in a head 25. The connector 22 has two axially perpendicular slits 26a, 26b of approximately 13 pm pre-installed thickness (enlarged in the drawings for ease of viewing and hence not shown to scale). The slits were cut using wire erosion in the form of a cross, and forming four fingers 27 that extend substantially in parallel from the body of the stem. The connector 22 tapers slightly less than the socket in the head 25. Upon initial insertion, proximal portions of the fingers 27 make contact with proximal inner surfaces of the socket. However, the fingers 27 enable the connector 22 to deform by flexing inwardly to adapt to the tapered socket of the head 25 when forced into the socket, allowing further insertion deeper into the socket and distributing pressure at the joints both at and between the proximal end 23 of the connector and the corresponding portion of the socket, and also at and between the distal end 24 of the connector 22 and the corresponding inner surfaces of the socket. Thus, the contact area between the connector and the socket is larger, decreasing the pressure at any particular point thus reducing failures, increasing friction between the connector and socket thus locking the assembly more effectively, and reducing or preventing the problems associated with prior art connectors. The slits 26a, 26b extend into the connector 22 approximately 80% from the proximal end of the connector. Thus, the distal end 24 of the connector 22 mates with the head around a circumference and forms a robust fluid seal thus preventing or at least impairing fluid exchange between the socket cavity and the surrounding tissue.

An alternative two-part hip prosthesis 30 has a stem 31 of the invention, of which only part is shown, having a frustroconical connector 32, which fits into an approximately complementary socket in a head 35. The connector 32 has a bore 36 drilled in its proximal end, which has a diameter of 90% of the diameter of the end of io the connector 32. The connector 32 tapers slightly less than the socket in the head 35. Upon initial insertion, outer surfaces at the proximal end of the connector make contact with proximal inner surfaces of the socket. However, the bore 36 renders the outer cylindrical walls more flexible than in a solid connector and so enables the connector 32 to deform when forced into the socket to adapt to the tapered socket of the head 35, allowing further insertion and distributing pressure at the joints at and between the proximal end 33 and the distal end 34 of the connector 32 and inner surfaces of the socket.

A further alternative two-part hip prosthesis 40 has a stem 41 of the invention, of which only part is shown, having a frustroconical connector, which fits into a complementary socket in a head 45. The connector has a 100 pm deep rebate 46 between the proximal end 43 and the distal end 44 of the connector. The rebate 46 distributes joint pressure between the proximal end 43 and the distal end 44 of the connector. Optionally, this embodiment has slits cut per the two-part hip prosthesis of the invention 20 described above. Upon being first inserted into the socket of a head, the proximal end 43 meets inner surfaces of the socket. The contact area allows deformation at the proximal end, meaning that upon further being urged into the socket the proximal end deforms enough for the distal end to fully make contact with and lock into the socket.

The skilled person will appreciate that the dimensions and geometry of the features of the connector of the invention, in particular the slots, bores, and rebates, are tailored in embodiments to provide an even distribution of pressure against the socket along its taper for any given stem material and taper design, and within the manufacturing tolerance ranges of these devices.

Accordingly, the invention provides a stem for a hip or knee prosthesis.

Claims

Claims 1. A stem for a hip or knee prosthesis, comprising a tapered connector securable to a head selected from a prosthetic fernoral head, a tibial tray and a s femoral component by inserting the connector into a socket in the head in which the shape of the socket is imperfectly complementary to the connector, wherein the connector has a body that is deformable to enhance the contact area between the connector and socket.
2. A stem according to claim 1, wherein the connector is deformable so as to narrow its proximal end, facilitating further insertion into the socket.
3. A stem according to claim 1 or claim 2, wherein the connector is substantially circular in cross section at its proximal end and is deformable so as to reduce the is circumference of its proximal end, facilitating insertion further into the socket.
4. A stem according to any previous claim, wherein the connector comprises one or more axial slits forming fingers which deform inwards when the connector is forced into the socket.
5. A stem according to claim 4, wherein the axial slits pass through the centre of the connector.
6. A stem according to claim 4 or claim 5, comprising 2 or more fingers.
7. A stem according to claim 6, comprising 4 fingers.
8. A stem according to any of claims 4 to 7, wherein the slits have a thickness of less than 100 pm, less than 50 pm, less than 25 pm or less than 13 pm.
9, A step according to any of claims 4 to 8, wherein the connector has 4 fingers and the slits have a thickness of between 40 and 50 pm.
10. A stem according to any of claims 1 to 3, wherein the connector comprises an axial bore configured to weaken the connector to deform inwards when the connector is forced into the socket.s 11. A stem according to claim 10, wherein the bore is substantially central.12. A stem according to any preceding claim, wherein contact is made between the proximal and distal ends of the connector and the head when the connector is forced into the socket.13. A stem according to any preceding claim, wherein the axial slit, slits or bore extend through the connector incompletely from the distal end of the connector.14. A stem according to claim 13, wherein the axial slit, slits or bore extend no more than 98%, 95%, 90%, 80% or 70% of the connector length from the proximal end of the connector.15. A stem according to any preceding claim, wherein the connector body further comprises one or more ridges.16. A stem according to claim 15, wherein the connector has a plurality of axially distributed ridges and each ridge continuously surrounds the connector.17. A stem according to claim 15 or claim 16, wherein the ridge or ridges form a ring or rings around the connector.18. A stem according to any of claims 15 to 17, wherein each ridge is located on a plane perpendicular to the axis of the connector.19. A stem according to any previous claim, wherein the connector has a rebate on the circumferential taper surface configured to split the contact area formable between the connector and socket to a region at the distal end of the connector and a region at the proximal end of the connector.20. A stem according to any preceding claim, wherein contact between the distal end of the connector and the socket forms a complete seal, 21. A stem according to any previous claim comprising cobalt-chrome alloys, titanium, titanium alloy or stainless steel.22. A stem for a hip or knee prosthesis comprising the stem and a head, the head comprising (i) the ball of a ball and socket joint with a pelvis in a hip prosthesis, (U) a tibial tray of a knee prosthesis or (ii) a femoral component of a knee prosthesis, wherein the stem comprises an end connector that inserts into and locks into a corresponding recess in the head, and wherein upon insertion into the recess proximal portions of the end connector contact proximal inner surfaces of the recess, and upon further insertion the end connector flexes allowing further insertion until distal regions of the end connector contact distal inner surfaces of the recess.23. A stem for a modular hip or knee prosthesis, wherein the prosthesis comprises the stem and (i) a head which comprises or forms the ball of a ball and socket joint with a pelvis in a knee prosthesis, (H) a tibial tray of a knee prosthesis or (ii) a femoral component of a knee prosthesis, wherein the stem comprises an end connector that inserts into a corresponding recess in the head, and wherein the end connector comprises proximal contact portions that upon insertion of the stem into the recess contact proximal inner surfaces of the recess, and wherein upon being urged further into the head the end connector flexes allowing further insertion until distal regions of the end connector contact distal inner surfaces of the recess, locking the stem onto the head.24. A stem according to claim 22 or 23, wherein the connector is flexible so as to narrow its proximal end, facilitating insertion further into the socket.25. A stem according to claim 24, wherein the connector is substantially circular in cross section at its proximal end and is flexible so as to reduce the circumference of its proximal end, facilitating insertion further into the socket.26. A stem according to any of claims 22 to 25, wherein the end connector comprises one or more flexible elements that flex with respect to the stem upon insertion into the head.27. A stem according to any of claims 22 to 26, wherein contact between distal regions of the end connector and distal inner surfaces of the recess seals the stem into the head, 28. A stem according to claim 27, wherein the contact forms an annular seal is between the stem and the head.29. A stem according to any of claims 22 to 28, wherein the end connector comprises two or more fingers that extend from a stem body and are spaced apart, wherein upon insertion into the recess proximal portions of the fingers contact proximal inner surfaces of the recess and wherein the fingers flex towards each other upon further insertion into the recess.30. A stem according to any of claims 22 to 29, wherein the end connector comprises a central bore forming an open ended cylinder having wall portions that flex inwardly upon insertion into the recess.31. A stem according to any of claims 22 to 30, wherein the end connector comprises a proximal annular contact surface that upon insertion of the stem contacts the proximal inner surfaces of the recess around an annular contact surface: and a distal annular contact surface that upon further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head.32. A stem according to claim 31, wherein the distal and proximal contact surfaces are separated by an intermediate section of the end connector the outer surface of which is cut away forming a rebate that it is spaced from the inner surface of the recess upon insertion of the stem into the recess.33. A stem according to any previous claim, wherein the end connector comprises two or more fingers that extend from a stem body and are spaced apart, wherein upon insertion into the recess proximal portions of the fingers contact proximal inner surfaces of the recess and wherein the fingers flex towards each other upon further insertion into the recess a proximal annular contact surface that upon said further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head.34. A stem according to any preceding claim, wherein the connector is substantially frustroconical.35. A stem according to any preceding claim for a hip prosthesis, comprising a tapered connector securable to a prosthetic femoral hip head.38. A stem according to any preceding claim for a knee prosthesis, comprising a tapered connector securable to a tibial tray.37. A stem according to any preceding claim for a knee prosthesis, comprising a tapered connector securable to a femoral component.38. A hip prosthesis comprising a stem according any of claims 1-34 and a prosthetic femoral head.39. A knee prosthesis comprising a stem according any of claims 1-34 and a tibial tray.40. A hip prosthesis comprising a stem according any of claims 1-34 and a prosthetic femoral head.41 A hip or knee prosthesis according to any of claims 38-40 further comprising a deformable sleeve located between the connector and the head.42. A stem for a hip prosthesis substantially as hereinbefore described with reference to any of Figs 4 to 9.43. A hip prosthesis substantially as hereinbefore described with reference to any of Figs 4 to 9.Amendments to the Claims have been filed as follows:-Claims 1. A stem for a hip or knee prosthesis, comprising a tapered connector securable to a head selected from a prosthetic femoral head, a tibial tray and a femoral component by inserting the connector into a socket in the head in which the shape of the socket is imperfectly complementary to the connector, wherein the connector has a body that is deformable to enhance the contact area between the connector and socket, wherein the connector comprises one or more axial slits forming fingers which deform inwards when the connector is forced into the socket.2. A stem according to claim 1, wherein the connector is deformable so as to narrow its proximal end, facilitating further insertion into the socket.3. A stem according to claim 1 or claim 2, wherein the connector is substantially circular in cross section at its proximal end and is deformable so as to reduce the circumference of its proximal end, facilitating insertion further into the socket.4. A stem according to any preceding claim, wherein the axial slits pass through the centre of the connector.5. A stem according to any preceding claim, comprising 2 or more fingers.6. A stem according to claim 5, comprising 4 fingers.7. A stem according to any preceding claim, wherein the slits have a thickness of less than 100 pm, less than 50 pm, less than 25 pm or less than 13 pm.8. A step according to any preceding claim, wherein the connector has 4 fingers and the slits have a thickness of between 40 and 50 pm.9. A stem according to any preceding claim, wherein the connector comprises an axial bore configured to weaken the connector to deform inwards when the connector is forced into the socket.10. A stem according to claim 9, wherein the bore is substantially central.
11. A stem according to any preceding claim, wherein contact is made between the proximal and distal ends of the connector and the head when the connector is forced into the socket.
12. A stem according to any preceding claim, wherein the axial slit, slits or bore extend through the connector incompletely from the distal end of the connector.
13. A stem according to claim 12, wherein the axial slit, slits or bore extend no more than 98%, 95%, 90%, 80% or 70% of the connector length from the proximal end of the connector.O
14. A stem according to any preceding claim, wherein the connector body further Q) comprises one or more ridges. (415. A stem according to claim 14, wherein the connector has a plurality of axially distributed ridges and each ridge continuously surrounds the connector.16. A stem according to claim 14 or claim 15, wherein the ridge or ridges form a ring or rings around the connector.17. A stem according to any of claims 14 to 16, wherein each ridge is located on a plane perpendicular to the axis of the connector.18. A stem according to any previous claim, wherein the connector has a rebate on the circumferential taper surface configured to split the contact area formable between the connector and socket to a region at the distal end of the connector and a region at the proximal end of the connector, 19. A stem according to any preceding claim, wherein contact between the distal end of the connector and the socket forms a complete seal.20. A stem according to any previous claim comprising cobalt-chrome alloys, titanium, titanium alloy or stainless steel.21. A stem for a hip or knee prosthesis comprising the stem and a head, the head comprising (i) the ball of a ball and socket joint with a pelvis in a hip prosthesis, (ii) a tibial tray of a knee prosthesis or (ii) a femoral component of a knee prosthesis, wherein the stem comprises an end connector that inserts into and locks into a corresponding recess in the head, and wherein upon insertion into the recess proximal portions of the end connector contact proximal inner surfaces of the recess, and upon further insertion the end connector flexes allowing further insertion until distal regions of the end connector contact distal inner surfaces of the recess, wherein the end connector comprises a proximal annular contact surface that upon insertion of the stem contacts the proximal inner surfaces of the recess around an annular contact surface; and a distal annular contact surface that upon further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head, and wherein the distal and proximal contact surfaces are separated by an intermediate section of the end connector the outer surface of which is cut away forming a rebate that it is spaced from the inner surface of the recess upon insertion of the stem into the recess.22. A stem for a modular hip or knee prosthesis, wherein the prosthesis comprises the stem and (i) a head which comprises or forms the ball of a ball and socket joint with a pelvis in a knee prosthesis, (ii) a tibial tray of a knee prosthesis or (ii) a femoral component of a knee prosthesis, wherein the stem comprises an end connector that inserts into a corresponding recess in the head, and wherein the end connector comprises proximal contact portions that upon insertion of the stem into the recess contact proximal inner surfaces of the recess, and wherein upon being urged further into the head the end connector flexes allowing further insertion until distal regions of the end connector contact distal inner surfaces of the recess, locking the stem onto the head, wherein the end connector comprises a proximal annular contact surface that upon insertion of the stem contacts the proximal inner surfaces of the recess around an annular contact surface; and a distal annular contact surface that upon further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head, and wherein the distal and proximal contact surfaces are separated by an intermediate section of the end connector the outer surface of which is cut away forming a rebate that it is spaced from the inner surface of the recess upon insertion of the stem into the recess.23. A stem according to claim 21 or claim 22, wherein the connector is flexible so as to narrow its proximal end, facilitating insertion further into the socket.24. A stem according to claim 23, wherein the connector is substantially circular in cross section at its proximal end and is flexible so as to reduce the circumference of its proximal end, facilitating insertion further into the socket.25. A stem according to any of claims 21 to 24, wherein the end connector comprises one or more flexible elements that flex with respect to the stem upon insertion into the head.26. A stem according to any of claims 21 to 25, wherein contact between distal regions of the end connector and distal inner surfaces of the recess seals the stem into the head.27. A stem according to claim 26, wherein the contact forms an annular seal between the stem and the heath 28. A stem according to any of claims 21 to 27, wherein the end connector comprises two or more fingers that extend from a stem body and are spaced apart, wherein upon insertion into the recess proximal portions of the fingers contact proximal inner surfaces of the recess and wherein the fingers flex towards each other upon further insertion into the recess.29. A stem according to any of claims 21 to 28, wherein the end connector comprises a central bore forming an open ended cylinder having wall portions that flex inwardly upon insertion into the recess.30. A stem according to any previous claim, wherein the end connector comprises two or more fingers that extend from a stem body and are spaced apart, wherein upon insertion into the recess proximal portions of the fingers contact proximal inner surfaces of the recess and wherein the fingers flex towards each is other upon further insertion into the recess a proximal annular contact surface that upon said further insertion of the stem contacts the distal surfaces of the recess around an annular contact surface, sealing the stem into the head. a) (431. A stem according to any preceding claim, wherein the connector is substantially frustroconical.32. A stem according to any preceding claim for a hip prosthesis, comprising a tapered connector securable to a prosthetic femoral hip head.33. A stem according to any preceding claim for a knee prosthesis, comprising a tapered connector securable to a tibial tray.34. A stem according to any preceding claim for a knee prosthesis, comprising a tapered connector securable to a femoral component.35. A hip prosthesis comprising a stem according any of claims 1 to 31 and a prosthetic femoral head.36. A knee prosthesis comprising a stem according any of claims I to 31 and a tibial tray.s 37. A hip prosthesis comprising a stem according any of claims 1 to 31 and a prosthetic femoral head.38. A hip or knee prosthesis according to any of claims 35 to 37 further comprising a deformable sleeve located between the connector and the head.39. A stem for a hip prosthesis substantially as hereinbefore described with reference to any of Figs 4 to 9.40. A hip prosthesis substantially as hereinbefore described with reference to any ofFigs4to9. a) (4