WO2000009038A2

WO2000009038A2 - Anchoring means for a joint prosthesis or other component

Info

Publication number: WO2000009038A2
Application number: PCT/GB1999/002628
Authority: WO
Inventors: Hans Rudolf Kriek; Nicolaas Jacobus Joseph Verdonschot; Petrus Tarasius Josephus Spierings
Original assignee: Novarticulate Holdings Limited; Davies, Christopher, Robert
Priority date: 1998-08-10
Filing date: 1999-08-10
Publication date: 2000-02-24
Also published as: CA2340155A1; KR20010087178A; NL1009832C2; WO2000009038A3; AU771147B2; AU5296099A; EP1104270A2; JP2002522153A

Abstract

An anchoring means for a joint prosthesis (4) tapers in a direction away from the joint prosthesis (4) but may be mounted in a bore through an inlet opening distal from the joint to be replaced. The anchoring means may comprise a tapering pin (53) which is cemented in place. Alternatively, the anchoring means may comprise a pin (3) and one or more retaining means (2, 3) which can be placed in the bore separately and manoeuvered such that the anchoring means tapers. Thus, a tapering anchoring pin is provided for a minimally invasive joint replacement technique.

Description

ANCHORING MEANS FOR A JOINT PROSTHESIS OR OTHER

COMPONENT

This invention relates to anchoring joint a prostheses or other component in a bone and, more particularly, to anchoring means for anchoring a joint prosthesis or other component, and a method of fitting the anchoring means, in a bone. It is a well known medical technique to replace worn out, damaged or diseased joints in human and animal bodies with artificial joint prostheses . Such prostheses may, for example, comprise a replacement joint articulation, such as a metal ball and socket or other pivotal means. Alternatively, the joint prosthesis may replace only part of the joint. For example, the ball of a ball and socket joint may be replaced with a joint prosthesis comprising an artificial replacement ball designed to sit in the original socket of a natural joint.

Thus, it is intended for the purposes of this application that the term 'prosthesis' or 'joint prosthesis ' is not limited to a component which takes the form of a natural joint, but is intended to include any component for replacing part or all of the function of a natural joint.

Regardless of whether the joint prosthesis replaces all or part of a joint, the joint prosthesis, or parts of the joint prosthesis, needs to be anchored in or located on bone adjacent or near to the joint. Joint prostheses therefore generally further comprise means for anchoring the replacement articulation, joint surface or joint part in a bone. For example, a joint prosthesis for replacing a natural ball and socket joint, such as the human shoulder or hip, often has a replacement articulation comprising a replacement ball and socket . The replacement ball may be carried on or formed integrally with anchoring means comprising a pin arranged to be inserted in the medullary canal of a bone, such as the femur or humerus, after the natural ball has been removed. The replacement socket may be carried in or formed integrally with anchoring means comprising a cup which is arranged or shaped to be cemented into the bone surrounding the socket, such as the acetabulum (hip bone) or scapula (shoulder blade) . Alternatively, the replacement socket may be carried on or formed integrally with anchoring means comprising a pin which is inserted into an artificial bore in the acetabulum (hip bone) or scapula (shoulder bone) . Procedures for fitting joint prosthesis are generally very invasive. Joints are buried deep in the human or animal body, so to replace natural joints with joint prostheses and to insert anchoring means into bone around a joint requires a large amount of tissue, including muscle, ligaments and cartilage which support the joint to be cut. This leads to a long recovery time for joint replacement operations and a large failure rate. More recently, however, less or "minimally" invasive surgical techniques have been developed for joint replacements. Such a technique is disclosed in the Applicant's International Patent Application No. W098/34567, where it is disclosed to carry out a joint replacement by inserting the replacement parts through a bore in a bone. Such a bore is made through an incision distal from the joint being replaced and this reduces trauma to the tissue surrounding the joint. According to a first aspect of the present invention there is therefore provided, a hip prosthesis comprising a first fastening assembly intended for being mounted in the hip bone and an anchoring means intended for being mounted in the top end of the femur, wherein the first fastening assembly and the anchoring means are interconnected by means of a pivotable connection, wherein all parts of the hip prosthesis are so small and/or slender that they can each be arranged in the intended end position thereof via a bore in the femur, which bore extends from the lateral outer side of the femur through the femoral neck substantially in the direction of the imaginary longitudinal centre line of the femoral neck to the femoral head, wherein the anchoring means of the hip prosthesis in mounted condition comprises a tapering part, with a relatively wide, medial side of the tapering part being located adjacent the femoral neck, while a relatively narrow, lateral side of the tapering part is located adjacent the outer side of the femur.

The anchoring means, and indeed the whole hip prosthesis, may therefore be fitted using a minimally invasive technique, in which all access to the joint is gained through the bore in the bone. This vastly reduces trauma to the area of the joint, and therefore shortens recovery time and reduces failure rate.

A joint prosthesis similar to this can equally advantageously be used to replace other human and animal joints, in particular the human shoulder joint. However, the type of articulation can vary, as can the position of the bore. Similarly, the joint prosthesis may replace only a part of a natural joint, such as only the ball of a ball and socket joint.

Thus, according to a second aspect of the present invention there is provided a joint prosthesis having components which are sufficiently small and slender that the joint prosthesis is mountable through an extra- medullary bore in a bone from a position distal to the intended position of the prosthesis, the joint prosthesis having an anchoring means which is mountable in the bore and tapers such that its narrow end is remote from the joint when mounted. Also, according to a third aspect of the present invention there is provided a method of fitting a joint prosthesis comprising inserting the joint prosthesis through an extra-medullary bore in a bone from a position distal to the intended position of the prosthesis, and mounting a tapered anchoring means for or of the joint prosthesis in the bore such that its narrow end is remote from the joint when mounted.

The term 'extra-medullary bore' generally refers to any bore not along the same axis as the medullary canal of a bone and, in particular, includes any bore across or through a bone . These aspects of the invention are advantageous in that, in a minimally invasive procedure, the tapered anchoring means provides stable support in the direction of its main axis toward the joint. In other words, as force is exerted on the anchoring means by the joint under natural loading, such as by standing or walking on a hip, the axial force exerted on the anchoring means is supported by the taper and the anchoring means will fit more and more tightly into the bore. Thus, the joint prosthesis is held very stably in the bone. However, it will be recognised that this approach is, to an extent, counter-intuitive since the tapered anchoring means is inserted into the bore in a bone, from a position remote from the joint, wide end first. In particular, when the anchoring means is tapered before insertion into the bore (e.g. has a permanent taper) , the widest part of the anchoring means (proximal to the joint) must be inserted first, as the anchoring means is inserted into the bore from a position distal from the joint and tapers away from the joint. Thus, the bore distal from the joint must be at least as wide as the widest part of the anchoring means and, when the anchoring means is in position, the diameter of the bore in the region of the narrower end of the anchoring means (distal to the joint) is larger than the diameter of the anchoring means and the anchoring means does not fit tightly into the bore.

In a first group of embodiments of the invention, which can avoid the use of cement, it is therefore preferable for the anchoring means to be radially expandable in the bore.

This enables the anchoring means to be inserted into the bore and then wedged into position. For example, the whole anchoring means may be radially expanded. Alternatively, the anchoring means may be inserted in the bore in component parts, some of which may be tapered. The component parts may be then moved in the bore longitudinally with respect to one another such that the diameter of the anchoring at a given point along its length increases. Regardless, the effect of the radial expansion is to wedge the anchoring means in position. The bore may be uniformly cylindrical or itself tapered. A straight, uniformly cylindrical, bore is easier to make in the bone and may allow components of the joint prosthesis to be inserted through the bore into their intended position easily. A tapered bore fits a tapered anchoring means more stabily, but may have smaller dimensions at the end of the bore remote from the joint than a uniformly cylindrical bore with the result that the components of the joint prosthesis may need to be smaller and/or more slender. This type of expandable anchoring means may be used in conventional invasive surgical techniques and not just minimally invasive procedures. Furthermore, the anchoring means may be used to fix other components for fitting in bones in human or animal bodies, not just joint prostheses, in place. For example, an expandable anchoring means may be used to secure a component comprising a plate that is fitted to the outside of a broken or fractured bone for support .

Thus, according to a fourth aspect of the present invention, there is provided a component for fitting to or in a bone, the component having an anchoring means insertable into a bone cavity and radially expandable in the bone such that the anchoring means has a tapered configuration, with the narrow end of the taper furthest from the component .

The bone cavity may, in this aspect, be a naturally occurring bone canal, or may be a bore formed by a surgeon especially for the prosthesis.

Also, according to a fifth aspect of the present invention, there is provided a method of anchoring a component to or in a bone, the method comprising inserting an anchoring means into a cavity in the bone, radially expanding the anchoring means in the cavity such that the anchoring means has a tapered configuration with the narrow end of the taper furthest from the component. In particular, the anchoring means may be radially expandable such that its outermost diameter exceeds the diameter of an inlet opening of the cavity. Thus, once the anchoring means has been expanded it is unable to pass through the inlet opening and this provides particularly secure fixation.

In a preferred embodiment of the above aspects, the anchoring means comprises a pin and one or more retaining elements, at least one of the pin and/or retaining element being tapered. The pin and retaining element (s) can be inserted in the bore separately or together, but can be moved relative to one another, substantially in the direction of their major axes, such that the diameter of the anchoring means is be increased and a tapered, wedging effect is obtained. Preferably the anchoring means has two retaining elements sandwiching the pin. This provides more stable location in the bore as the pin may extend substantially along the major axis of the bore and an even load may be exerted on the retaining elements. If the anchoring means is expanded to suitable dimensions it may be fitted into the bore without the need for cement, which is used to fix most conventional joint prosthesis in place. However, the anchoring means may additionally or alternatively be cemented into position.

Cement provides secure fixing of the anchoring means into the bone as it takes the exact form of the bore, and may seep into the bone surrounding the bore, before setting. However, one difficulty in cementing an anchoring means in a bore through a bone from only one end, as required in the minimally invasive technique described above, is that it is difficult to prevent cement from leaking from the other and of the bore as it is injected, particularly as it preferable to pressurise the cement to drive air and fluid out of the bore in order to obtain proper fixation. Where the invention is adapted for use with cement, it is preferred that the anchoring means is provided with a seal for sealing the anchoring means in the bore, at an end of the bore distal to an end through which cement is injected. Thus, one end of the bore is sealed and cement can be injected under pressure from the other end of the bore without leaking from the sealed end, for example into a joint cavity.

However, this type of anchoring means could be inserted into a bore in a bone from either end. It may therefore be used in conventional invasive surgical procedures. Furthermore, whilst a tapered anchoring means is preferred, this may not be essential if the anchoring means is cemented in position, since the seal enables cement to be injected under pressure from one end of the bore and adequate fixation may be obtained with other shapes of anchoring means .

Thus, according to a sixth aspect of the present invention, there is provided an anchoring means for a component for fitting to or in a bone, the anchoring means insertable in a bore and having a seal for sealing the anchoring means in the bore at an end of the bore distal to the end through which cement is injected for cementing the anchoring means in place.

Also, according to a seventh aspect of the present invention, there is provided a method of cementing an anchoring means in a bore through a bone comprising inserting the anchoring means in the bore, sealing the anchoring means in the bore at one end, and injecting cement into the other end of the bore .

Thus, the bore is sealed in order that cement can be injected around the anchoring means in the bore without leaking past the seal, for example, into a joint cavity and interfering with a joint. The seal may also facilitate placement of the anchoring means along the central axis of the bore and hold it there while the cement sets. Thus, a tapering anchoring means can be inserted from a position distal to the joint, yet still be anchored securely in the bone.

If cement is used, it is preferable that the anchoring means comprises a single part. This provides increased strength and durability over an anchoring means comprising several sections.

Preferably, the seal is an annular ring around the end of the anchoring means distal from where cement is introduced. Alternatively, the seal may be an annular cap having an annular recess for engaging a resected end surface of the cortical bone wall of the bone. Another alternative is for the seal to comprise an inflatable "0"-ring. ^' The anchoring means may then further comprise an internal passage for inflating the "0"-ring with air or any other suitable medium. In a preferred embodiment, an additional seal may be provided at the end of the bore where cement is provided, such that there is a cavity between the seals surrounding the anchoring means into which cement may be introduced under pressure, for example via an orifice in the second seal. The second seal may also be an annular ring extending around the anchoring means in order to assist in its axial location. In the context of more standard surgical techniques in which anchoring means for prosthesis are inserted into a bone cavity or bore from the side facing the joint, the provision of the cement seal at the opening where the cement is introduced may be more significant, since the other end of the cavity or bone may terminate within the bone and therefore be effectively sealed already.

Hence, a further aspect of the invention provides a prosthesis having an anchoring means configured for mounting in an extra-medullary bore in a bone, one end of the anchoring means being provided with an annular seal through which cement may be forced into the bone in use . A further aspect of the invention provides a method of anchoring a prosthesis in an extra-medullary bore in a bone, comprising locating a tapered anchoring component in the bore, sealing the bore opening and forcing cement under pressure into the bore with the anchoring means positioned therein.

In the context of a hip replacement, the bore preferably extends along approximately the imaginary longitudinal centre line of the femoral neck and the prosthesis in an artificial ball joint. These latter aspects can provide a less invasive technique and a firmer mounting than standard hip replacement procedures.

One of the fundamental advantages of the invention is the stable support provided for a joint prosthesis by a tapering anchoring means. This is particularly advantageous for hip prosthesis which may bear a large load and therefore require particularly stable support.

According to a further aspect of the present invention there is therefore provided a hip prosthesis having an anchoring means for supporting the prosthesis in an extra medullary bore in a femur, the anchoring means being tapered away from the prosthesis and extending only part way through the femur.

Thus, a relatively small and simple anchoring means may be used to support the hip prosthesis, yet this still provides stable and strong support for the prosthesis. Such an anchoring means is relatively cheap to produce and causes far less trauma to the femur than conventional femoral components for hip protheses which sit in the medullary canal of the femur or extend all the way across the bone. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a sectional view of the top of a femur in which an anchoring means according to a first exemplary embodiment of the invention has been fitted;

Fig. 2 is a cross-sectional view of the anchoring means of Fig. 1, taken on the line II- II of Fig. 1;

Fig. 3 is a sectional view similar to that of Fig. 1, showing an anchoring means according to a second exemplary embodiment of the invention;

Fig. 4 is a cross-sectional view of the anchoring means of Fig. 3, taken on the line IV-IV of Fig. 3;

Fig. 5 shows a detail of the cross-sectional view of Fig. 4; Fig. 6 is a sectional view similar to Fig. 1, showing an anchoring means according to a third exemplary embodiment of the present invention;

Fig. 7 is a sectional view similar to Fig. 1, showing an anchoring means according to a fourth exemplary embodiment of the present invention;

Figs. 8a-8e show some steps for mounting the anchoring means shown in Fig. 1 in a femur;

Fig. 9 is a sectional view similar to Fig. 1, showing an anchoring means according to a fifth exemplary embodiment of the present invention;

Fig. 10 is a close-up sectional view similar to Fig. 1, showing an anchoring means according to a sixth exemplary embodiment of the present invention; and

Fig. 11 is a close-up sectional view similar to Fig. 1, showing an anchoring means according to a seventh exemplary embodiment of the present invention. Joint prostheses may be used to replace virtually all joints in the human and animal body. Likewise, this invention may be used in the replacement of any joint in the human and animal body. However, it is particularly applicable to the replacement of hip and shoulder joints and, for convenience, preferred embodiments will be described with reference to the replacement of the human hip joint, and relate, in particular, to a hip prosthesis in accordance with than described in International Patent Application No. W098/34567. In the Figures, only an anchoring means of different embodiments of a hip prostheses is shown. An example of a possible embodiment of a complete hip prosthesis may be found in International Patent Application W098/34567, the subject matter of which is to be regarded as being inserted herein. The Figures show different exemplary embodiments for fitting an anchoring means (the second fastening assembly of W098/34567) in a femur F. As is described in detail in W098/34567, the parts of the hip prosthesis are preferably designed such that they can all be placed into their end positions through a bore in the femur, which bore extends from the lateral outer side F₀ of the femur F through the femoral neck F_n . The bore may be substantially in the direction of the imaginary longitudinal centre line of the femoral neck to the femoral head. During the positioning of the hip prosthesis, a relatively small incision is made in the leg, providing access to the lateral outer side of the femur F. Next, a bore is made in the femur F as described hereinabove. Via this bore, the femoral head is removed with a special tool, described in the above- cited International Patent Application. Next, parts of the joint prosthesis can be mounted in the hip bone via the bore in the femur F. After the positioning of the first fastening assembly, the anchoring means of the invention can be mounted. In the present exemplary embodiments, the anchoring means, in mounted condition, comprises a tapering part C. A wide, medial side CM of the tapering part C is located more adjacent the femoral neck F_n, while a narrower, lateral side CL of the tapering part C is located more adjacent the outer side F₀ of the femur F. Because the tapered part C tapers from the medial to the lateral side or away from the joint articulation, the hip prosthesis, when the leg is subjected to a normal load, is pressed into the bore in the femur F more and more tightly. Hence, the specific geometry provides the anchoring means with a self- locking action.

In all exemplary embodiments shown in Figs. 1-7, the anchoring means comprises a pin 3, respectively 13, 33, 43. The pin 3, 13, 33, 43 has a free, medial end thereof provided with means for forming at least a portion of the pivotable connection. In the present exemplary embodiments, this portion is designed as a ball 4, 14, 34, 44 of a ball joint. It is readily understood that other pivotable connections are also possible. For this, reference is made to W098/34567.

The anchoring means of the present exemplary embodiments also comprises two retaining elements, respectively 1, 2; 11, 12; 31, 32; 41, 42. The retaining elements 1, 2; 11, 12; 31, 32; 41, 42 in mounted condition bound a hole into which the pin 3, 13, 33, 43 is at least partially inserted. In the present exemplary embodiments, the two retaining elements 1, 2; 11, 12; 31, 32; 41, 42 taper from a medial end CM proximal to the pivotable connection, to a lateral end CL. The retaining elements 1, 2; 11, 12; 31, 32; 41, 42 are dimensioned such that they can be moved through the bore in the femur F as long as the pin 3, 13, 33, 43 is not yet fitted or inserted far enough into the hip joint cavity or bore. The exemplary embodiments of Figs. 1-6 comprise a pin 3, 13, 33 having a constant sectional profile at least over a part of its length. The hole bounded by the two retaining elements 1, 2; 11, 12; 31, 32 likewise has a substantially constant sectional profile, into which the part of the pin 3, 13, 33 with the constant sectional profile is at least partially inserted. Such construction has the advantage that the retaining elements 1, 2; 11, 12; 31, 32 can be moved into their end positions, whereupon the pin 3, 13, 33 can be positioned.

Because in such embodiments, the pin 3, 13, 33 can still move relative to the retaining elements 1, 2; 11, 12; 31, 32, means for connecting the retaining elements 1, 2; 11, 12; 31, 32 and the pin 3, 13, 33 is provided.

In Fig. 1, these means are formed by screw thread 5 provided on the pin 3. The retaining elements 1, 2 have their lateral ends provided with internal screw thread mating with the screw thread 5 on the pin 3. To prevent the pin 3 from moving after it is rotated into a specific position, a locking means is provided which in the present exemplary embodiment comprise a sleeve 6 having and end wall. The sleeve 6 embraces or surrounds the lateral ends of the retaining elements 1, 2. Means for connecting the sleeve 6 to the pin 3 is also provided and, in the present exemplary embodiment, the means for connecting the sleeve 6 to the pin 3 comprise a bolt 7. Provided in the end wall of the sleeve 6 is an end wall bore 9. In line with this end wall bore 9, a pin bore 8 is provided in the pin 3, substantially along the major axis of the pin 3. The bolt 7 passes through the end wall bore 9 and engages the pin bore 8. As the bolt 7 is tightened, the sleeve 6 is drawn further over the lateral ends of the retaining elements 1, 2. Because these lateral ends 10 are of a slightly tapered design and, moreover, the inner circumferential wall of the sleeve 6 is also of tapered design, the retaining elements 1, 2 are pressed together when the bolt 7 is tightened, causing the pin to be clamped between the retaining elements 1, 2. Moreover, the pulling force exerted by the bolt 7 prevents rotation of the pin 3 relative to the retaining elements 1, 2, by the friction occurring in the screw thread connection 5.

In the exemplary embodiment shown in Fig. 3, the pin 13 has a constant sectional profile, viewed in the longitudinal direction of the pin 13. As Fig. 4 clearly shows, in the transverse direction, the pin 13 is provided with tapered faces 15 which are receivable in the hole bounded by the retaining elements 11, 12. In the present exemplary embodiment, the tapered faces 15 and the hole formed by the retaining elements 11, 12 are provided with serrated or snap edges preventing an axial displacement of the pin 13 relative to the retaining elements 11, 12. To ensure that the tapered faces 15 of the pin 13 are properly pressed into the associated opening in the retaining elements 11, 12, this exemplary embodiment, too, has locking means comprising a sleeve

16 having an end wall with an opening 19 provided therein. The sleeve 16 is tightened by means of a bolt

17 engaging a pin bore 18. Because this sleeve 16, too, comprises tapering circumferential walls 20, the retaining elements 11, 12 will be pressed together during tightening of the bolt 17. When the retaining elements 11, 12 are being moved together, the tapered faces 15 of the pin 13 engage the associated tapered faces in the retaining elements 11, 12 more and more tightly. The serrated snap edges of the tapered faces 15 which are clearly shown in the detail of Fig. 4 shown in Fig. 5 provide a firm connection between the pin 13 and the retaining elements 11, 12. The tapered outer surface C of the retaining elements 11, 12 provides that when the hip prosthesis is subjected to a normal load, the fastening assembly is pressed into the femur F more and more tightly.

In the exemplary embodiment shown in Fig. 6, the means for connecting the retaining elements 31, 32 and the pin 33 are exclusively formed by a screw thread 35 provided on the pin 33 and mating with internal screw thread provided on the inside of the retaining elements 31, 32.

Unlike the previous exemplary embodiments, the exemplary embodiment shown in Fig. 7 has a pin 43 tapering over at least a part of the length thereof. The tapered pin part tapers to the lateral end 43L, viewed from the medial end 43M. Viewed from the medial to the lateral end of the hole, the hole bounded by the two retaining elements 41, 42 has a tapering sectional profile in which the tapering pin part 43 is at least partly received. Here, the connection between the retaining elements 41, 42 and the pin 43 is effected by drawing the pin 43 continuously to the lateral side. This is effected in that the means for connecting the retaining elements 41, 42 to the pin 43 comprise a drawing plate 46 and a wire of memory metal 45. The wire of memory metal 45 is by a first end thereof connected to a medial end 43M of the pin 43. The second end of the wire 45 is connected to a drawing plate 46. In mounted condition, the drawing plate 46 abuts against the outer side F₀ of the femur F at the location of the lateral end of the femoral bore . The wire of memory metal 45 is designed such that at body temperature it tends to shorten, causing a tensile stress in the wire. During positioning of the wire 45, it is in a stretched condition, so that during positioning some play is present for fitting the pin 43 and the retaining elements 41, 42. Although in the Figure the retaining elements 41, 42 have a slightly tapered configuration, this is not required, because the tapered configuration of the anchoring means may be entirely provided by the pin 43. In the exemplary embodiment shown in Fig. 7, the retaining elements 41, 42 are moreover further provided, at their medial ends, with a collar 47, 48 intended for engagement with the cortical bone edges bounding the medial opening of the femoral bore. If necessary, such collar could also be provided in the other exemplary embodiments .

Figs. 8a-8e show the successive steps for mounting the anchoring means shown in Fig. 1. First, the retaining element 1 is positioned (Fig. 8a) ; then, the retaining element 2 is positioned (Fig. 8b); after that, the pin 3 is provided in the hole bounded by the retaining elements 1, 2 (Fig. 8c) ; then, the sleeve 6 is slid over the lateral ends of the retaining elements 1, 2 (Fig. 8d) . Finally, the bolt 7 is tightened, after which the anchoring means has been mounted in the femur F (Fig. 8e) .

It is observed that before the pin is positioned, the retaining elements may temporarily be slid slightly deeper into the joint, as a result of which the hole bounded by the retaining elements 1, 2 is slightly wider for inserting the pin 3 therein. The retaining elements 1, 2 may subsequently be pulled back in lateral direction into their end positions, after which the connection between the pin 3 and the retaining elements 1, 2 is effected.

The above embodiments all have two retaining elements 1, 2; 11, 12; 31, 32; 41, 42. However, all of these embodiments may be modified by providing only a single retaining element 1, 2; 11, 12; 31, 32; 41, 42. For example, referring to Fig. 1, the retaining means 2 may be formed integrally with the pin 3 , such that the anchoring means substantially comprise only two elongate components; the pin 3 and retaining means 1. Alternatively, the retaining means 2 can be omitted and the bore made to different dimensions such that the upper surface of the pin 3 (as shown in Fig. 1) contacts the bone directly.

Referring to Fig. 9, in a fifth embodiment of the present invention a pin 53 has a substantially conical shape, tapering from a medial end, proximal to a joint articulation 54 to a lateral end, distal to the joint articulation 54. The pin 53 may be arranged to fit directly into a prepared bore in a femur from the location of the prosthesis, i.e. using a conventional (invasive) surgical technique. In that case, the pin 53 is shorter than shown in Figure 9 in order that it extends only part way through the femur F and the cortical bone wall opposite the joint articulation 54 remains intact, i.e. the bore does not extend all the way through the femur F . However, the example shown in Figure 9 is suitable for use in a minimally invasive surgical technique such as that described in W098/34567 and has a medial seal 51 is provided at its proximal end around the circumference of the pin 53. The medial seal 51 is generally annular. A lateral seal 52 is provided at the lateral end of the pin 53 distal from the joint articulation 54, which is again annular and disposed around the circumference of the pin 53.

The pin 53 is fitted in a bore 55, in this example in a femur F. The medial seal 51 seals the medial end of the bore and helps to locate the pin 51 centrally in the bore . The lateral seal 52 is then fitted at the distal end of the pin, and seals the lateral end of the bore. There is a cement delivery orifice 58 in the lateral seal 52 through which cement is injected into an elongate annular space 57 between the pin 53 and the bore. The cement is delivered at a pressure generally higher than normal blood pressure in order that any fluid is driven out of the space 57 and the cement abuts the bone over the whole exposed inside surface of the bore. Preferably, the cement also permeates the Spongiform bone of the inside of the femur surrounding the bore to some degree dependent on the type of cement used and the pressure it is delivered at . This provides a more secure interface between the cement and bone F when the cement has set . When the cement sets, the pin 53 is held rigidly in the bore. Any force exerted from the joint articulation end of the pin 53, for example through weight put on the hip prosthesis in use, forces the pin 53 towards its lateral or distal end, and due to the taper of the pin 53 forces the cement radially outwards. Thus, if the pin 53 moves at all, it pushes more and more tightly into the femur.

In Fig. 9, the bore is shown as uniformly cylindrical. The bore may, however, be generally conical, tapering in the same direction as the pin 53. Referring to Fig. 10, a sixth embodiment of the present invention is largely similar to the fifth embodiment shown in Fig. 10. However, in this example, a cap 60 is provided to seal the end of the bore proximal to the joint articulation 64.

The cap 60 has an annular recess 61 which holds a gasket or seal 62. The seal engages a rim of cortical bone around a resected femoral neck to seal the bore and hold the cap 60 in place. An annular resilient means 65 is provided inward of the recess 61 which presses against the inside surface of the cortical bone wall of the femoral neck to fit the cap 60 more securely. The resilient means 65 need not be annular and plural separate resilient means can be provided around the cap 60 instead.

The pin 63 extends through the cap 60 and is attached to the cap 60 by an external screw thread 66 on the pin 63. The cap is put in place first, and the pin 63 is then located and screwed into the cap 60. Due to the dimensions of the cap 60, if it is required to pass the cap 60 through the femoral bore to put the cap 60 in place for example when using the minimally invasive technique described above, the cap 60 is collapsable or made from deformable material to fit through the bore. The screw thread 66 may self tap into soft material disposed on the cap 60 or engage a screw thread on the cap 60.

Referring to Fig. 11, a seventh embodiment of the present invention is again largely similar to the fifth embodiment shown in Fig. 9. However, in this embodiment the medial seal 55 shown in Fig. 9, is an annular inflatable seal 70. The inflatable seal is connected to an air passage 71 inside the pin 73.

Thus, when the pin 73 is fitted, the seal is inflated such that it locates the pin 73 centrally in the bore and seals the medial or proximal end of the bore.

It may be understood that the invention is not limited to the exemplary embodiments described, but that various modifications are possible within the scope of the invention defined in the claims.

Claims

1. A hip prosthesis comprising a first fastening assembly intended for being mounted in the hip bone and an anchoring means intended for being mounted in the top end of the femur (F) , wherein the first fastening assembly and the anchoring means are interconnected by means of a pivotable connection, wherein all parts of the hip prosthesis are so small and/or slender that they can each be arranged in the intended end position thereof via a bore in the femur (F) , which bore extends from the lateral outer side (F₀) of the femur (F) through the femoral neck (F_n) substantially in the direction of the imaginary longitudinal centre line of the femoral neck (F_n) to the femoral head, wherein the anchoring means of the hip prosthesis in mounted condition comprises a tapering part (C) , with a relatively wide, medial side (CM) of the tapering part (C) being located adjacent the femoral neck (F_n) , while a relatively narrow, lateral side (CL) of the tapering part (C) is located adjacent the outer side (F₀)of the femur (F) .

2. A joint prosthesis (4) having components which are sufficiently small and slender that the joint prosthesis is mountable through an extra-medullary bore in a bone (F) from a position distal to the intended position of the prosthesis, the joint prosthesis having a tapered anchoring means (C) which is mountable in the bore such that its narrow end is remote from the joint (4) when mounted.

3. The prosthesis of claim 1 or claim 2, wherein the anchoring means is radially expandable in the bore.

4. The prosthesis of any one of the preceding claims, the anchoring means comprising a pin (3) and one or more retaining elements (1, 2) , at least one of the pin (3) and/or retaining element (s) (1, 2) being tapered.

5. The prosthesis of claim 4, the anchoring means having two retaining elements (1, 2) sandwiching the pin (3) .

6. The prosthesis of any one of the preceding claims, the anchoring means having a medial seal (51) for sealing the medial end of the anchoring means in the bore when the anchoring means is in place.

7. An anchoring means for a component for fitting to or in a bone, the anchoring means insertable in a bore in a bone and having a seal (51) for sealing the anchoring means in the bore at an end of the bore distal to the end through which cement is injected for cementing the anchoring means in place.

8. The anchoring means of claim 7, comprising a single part (53) .

9. The anchoring means of claim 7 or claim 8, the seal (51) comprising an annular ring around the anchoring means .

10. The anchoring means of claim 7 or claim 8, the seal (51) comprising a cap (60) having an annular recess (61) for engaging a resected end surface of the cortical bone wall of the bone.

11. The anchoring means of claim 7 or claim 8, the seal (51) comprising an inflatable "0"-ring (70) .

12. The anchoring means of claim 11, the anchoring means further comprising an internal passage (71) connected to the "0"-ring (70).

13. The anchoring means of any of claims 7 to 12, in which an additional seal is provided at the end of the anchoring means where cement is delivered, this seal having one or more cement delivery orifices.

14. The prosthesis of claim 4, further comprising means (5-10; 15-20; 35; 45, 46) for connecting the retaining elements (1, 2) and the pin (3) comprising a screw thread, a tapered clamping connection, or a click and/or snap connection.

15. The prosthesis of claim 4, further comprising means (45, 46) for connecting the retaining elements (41, 42) to the pin (43), the means (45, 46) comprising a drawing plate (46) and a wire of memory metal, the wire of memory metal (45) having a first end thereof connected to a medial end of the pin (43) and a second end thereof connected to the drawing plate (46) , the drawing plate (46) in mounted condition abutting against the outer side (f₀) of the femur (F) at the location of the lateral end of the femoral bore, the wire of memory metal (45) tending to shorten at body temperature to effect a tensile stress in the thread (45) .

16. A method of fitting a joint prosthesis comprising inserting the joint prosthesis through an extra- medullary bore in a bone (F) from a position distal to the intended position of the prosthesis, and mounting a tapered anchoring means for the joint prosthesis in the bore such that its narrow end is remote from the joint when mounted.

17. A component for fitting to or in a bone, the component having an anchoring means insertable into a bone cavity and radially expandable in the cavity such that the anchoring means has a tapered configuration, with the narrow end of the taper furthest from the component .

18. A method of anchoring a component to or in a bone, the method comprising inserting an anchoring means into the bone cavity and radially expanding the anchoring means in the cavity such that the anchoring means has a tapered configuration, with the narrow end of the taper furthest from the component .

19. The prosthesis of claim 17 or the method of claim 18, wherein the anchoring means comprises a pin (3) and one or more retaining elements (1, 2) , at least one of the pin (3) and/or retaining element (s) (1, 2) being tapered, the pin (3) and retaining element (s) (1, 2) being inserted in the bore and moved relative to one another, substantially in the direction of their major axes, such that the diameter of the anchoring means is increased.

20. A method of cementing an anchoring means in a bore in a bone (F) comprising inserting the anchoring means in the bore, sealing one end of the bore in order to prevent cement from escaping from the bore via that end, and injecting cement into the other end of the bore.

21. A prosthesis having an anchoring means configured for mounting in an extra-medullary bore in a bone (F) , one end of the anchoring means being provided with an annular seal (51) through which cement may be forced into the bore in use.

22. A method of anchoring a prosthesis in an extra- medullary bore in a bone (F) , comprising locating a tapered anchoring component in the bore, sealing the bore opening, and forcing cement under pressure into the bore with the anchoring means positioned therein.

23. A hip prosthesis having an anchoring means for supporting the prosthesis in an extra-medullary bore in a femur, the anchoring means being tapered away from the prosthesis and extending only part way through the femur .