WO2011091466A1

WO2011091466A1 - Artificial joint component having hollow interior and means for load dissipation

Info

Publication number: WO2011091466A1
Application number: PCT/AU2010/000094
Authority: WO
Inventors: Ronald Sekel
Original assignee: Maxx Health Inc
Priority date: 2010-02-01
Filing date: 2010-02-01
Publication date: 2011-08-04

Abstract

An artificial joint assembly implantable in bone, the joint assembly including a first component which is at least partially hollow and at least one other component matable with the first component and which in use co operates with the first component, the first component comprising; a body having first and second ends; one said ends including a hollow part and which engages said at least one other component for load transfer between the components, the hollow part of the first component including means to enable dissipation of load through the hollow component when a load is applied to the first component, thereby reducing load transmitted via the hollow part on the first component to the other component when the first component is engaged with said other component.

Description

ARTIFICIAL JOINT COMPONENT HAVING HOLLOW INTERIOR AND MEANS

FOR LOAD DISSIPATION BACKGROUND

The present invention relates to surgical prostheses and more particularly, relates to an artificial joint assembly for use in joint replacements and which includes co operating first and second components such as but not limited to a neck and a stem. More particularly the invention relates to improvements in the geometry and structural performance of a first joint component when engaged with a second joint component such that the first component has geometry to enable dissipation of a load applied to the first component through the first component to at least partially relieve loading on the second component when the first component engages the second component. The invention according to one embodiment further relates to a neck component of a hip prosthesis engageable with a stem via a tapered end, the tapered end having a hollow interior at least part of which is formed so that the tapered end which engages the stem is capable of a elastic displacement or deformation to dissipates load applied to the opposing inner surface of the stem via the neck.

PRIOR ART

Hip replacements are a common orthopaedic surgical procedure and are usually necessitated by degenerative disease of the hip joint, hip trauma or disease of the hip creating later hip trauma. In a total hip replacement, the surgical procedure may involve reaming of the patient's acetabulum, reaming of the proximal medullary cavity of the femur and inserting a prosthesis stem into the medullary cavity to replace the natural femoral head. The head of the prosthesis (usually formed by a detachable ceramic ball) mates with the acetabulum to simulate mating of the natural femoral head with the acetabulum in a normal hip joint. A neck component engages the stem via a first end and the ball via a second end forming the femoral head of the prosthesis. There are in existence a number of hip prostheses which have enjoyed widespread use.

Typically, a hip prosthesis comprises a distal shaft having a gradual taper along its full length and terminating proximally in a neck or elbow portion which mates with a head of the prosthesis via a Morse taper. The shaft is inserted into the intra medullary cavity of the femur. The neck is connected to the stem generally via a Morse taper. By necessity the connection between the neck and stem must be strong enough to resist bending moments, torsional and axial loadings induced by patient movements. When a neck is inserted in a corresponding female recess in a stem, rotational loads applied to the neck portion of the prosthesis are transmitted to the stem. Stems may fail due to high local stresses applied at the interface between the stem and neck. This is in part due to the inability of the current neck components to dissipate energy of loads applied at their extremity. Also the stem has a finite load limit which if exceeded will cause a local failure in the stem wall.

There is a need to provide a femoral prosthesis which has increased resistance to failure and specifically resistance to failure in the region of contact between the neck and stem components. There is a long felt want to provide a hip prosthesis assembly which includes a neck component which allows the local stresses generated at the location of contact between the neck and stem and (particularly though not limited to shear and bending stresses) to be dissipated to avoid any concentrations of loadings high enough to cause failure or damage to the stem component in vivo at that location of contact. INVENTION

The present invention seeks to ameliorate or eliminate the attendant disadvantages which have been manifest in use of certain prior art hip prostheses by providing an improved neck component which, while accepting normal dynamic patient loading will dissipate load to reduce applied loading on a femoral stem component and particularly an inner wall of the stem. In addition to providing significant advantages over the prior art, the present invention overcomes the problems associated with material failure under point load as has occurred in some prior art assemblies. The invention combines the benefits of the known prior art prostheses, provides further benefits and eliminates the prior art disadvantages.

In its broadest form the present invention comprises:

an artificial joint assembly implantable in bone, the joint assembly including a first component which is at least partially hollow and at least one other component matable with the first component and which in use co operates with the first component, the first component comprising;

a body having first and second ends;

one said ends including a hollow part and which engages said at least one other component for load transfer between the components, the hollow part of the first component including means to enable dissipation of load through the hollow component when a load is applied to the first component, thereby reducing load transmitted via the hollow part on the first component to the other component when the first component is engaged with said other component. According to a preferred embodiment, the means to enable dissipation of energy through the first component is a formation in the first component which allows elastic deformation of at least a part of the hollow part. Preferably the first end of the first component includes a hollow passage defined by a wall extending from the first end at least partially along the body of the first component. The first end of the first component engages the second component via a tapered connection, wherein the load is transmitted from the first component to the second component via the tapered connection. According to one embodiment, the first component formation in first end of the first component is a tapered chamfer which provides a distal end thinning of the wall. The thinning in the wall chamfer allows the wall to deflect when load is transferred from the first component to the second component. According to one embodiment, the first component forms a neck of a prosthesis and the second component forms a stem which provides bone anchorage in a hip prosthesis.

Alternatively, the artificial joint assembly combine to form a shoulder or knee or elbow or ankle prosthesis. The first end of the neck has an external taper which engages an internal taper in the second component. The chamfer is the wall may be any one of a variety of shapes. For example the chamfer can be linear, elliptical, concave or any one of a variety of geometries.

In another broad form the present invention comprises:

an artificial joint component for use in an artificial joint assembly implantable in bone, the joint assembly formed by the co operation of the first component and a second component; the first component comprising;

a component body having first and second ends;

one said ends including the hollow portion which engages said second component, the hollow portion including a formation which enables dissipation of load through the hollow component when a load is applied to the hollow component, thereby reducing loading transmitted via the first component to the second component.

Preferably the load dissipation enabled by an elastic deflection in a wall of the hollow portion of the first component.

In another broad form the present invention comprises:

a femoral prosthesis of the type comprising a stem, neck and head and adapted for insertion into the medullary cavity of a femur to thereby form a replacement for the natural femoral head; characterised in that the stem has which receives a distal end of the neck, a proximal end of the neck adapted to mate with an artificial head, wherein the neck component includes a hollow interior at least a part of which includes a formation which allows a deflection in a wall of the hollow portion so that when the neck component is placed under load by loading the femoral head, loading transmitted to the stem via the neck is partially dissipated through the neck by elastic deflection of the wall at the formation. When the neck when is under load at an interface between the neck and stem, this prevents the stem from point load failure by increasing the capacity of the neck to dissipate loads by up to 30%-40% away from the stem compared to the resistance provided by the known neck and stem components. Depending upon the geometry of the formation in the hollow region of the neck load dissipation to sacrifice the stem component can exceed the above range.

The neck component includes a hollow passage and a formation which contributes to load dissipation through the neck at the point of connection between the neck and stem when the neck is under load. The formation is in the hollow passage and creates a thin wall region at a distal end of the neck component, thereby allowing elastic deflection therein to dissipate loads through the neck and reduce point loading on the stem.

In another broad form the present invention comprises:

a femoral prosthesis adapted for insertion into the medullary cavity of a femur said prosthesis comprising, a distal stem, a neck component detachable from said distal stem and a head detachable from said neck component; characterised in that the neck portion comprises an elbow having means at either end to enable male female or female male mating with said distal stem and also with said head to create tight interfitting therebetween, said neck being rotatable relative to said stem and head prior to effecting said tight interfitting and while said distal stem is fixed in situ; characterised in that at least part of the neck includes a hollow interior and a formation which allows at least partial elastic deflection of the neck at a point of load transfer between the neck and stem. Interfitting between the stem and neck components is effected by a Morse taper at either end of the mating member, thereby creating a double Morse taper, allowing inter engagement between the distal stem and the neck. The neck component comprises two legs disposed at an obtuse angle to each other, each of said legs terminating at its extremity in a tapered portion wherein at least one said legs of said neck has the hollow interior and includes the formation which allows the at least partial elastic deflection of the wall of the neck under load transfer to the stem thereby reducing applied stress on an inner wall of the stem.

The invention allows the local stresses generated at the location of contact between the neck and stem and (particularly though not limited to shear and bending stresses) to be dissipated to avoid any concentrations of loadings high enough to cause failure or damage to the stem component in vivo at that location of contact. Preferably the formation is a thinner wall region which allows deflection therein to dissipate loads which would otherwise be received in greater magnitude in a wall of the stem.

In another broad form the present invention comprises:

a femoral prosthesis adapted for insertion into the medullary cavity of a femur said prosthesis comprising, a distal stem, a neck component detachable from said distal stem and a head detachable from said neck component; characterised in that the neck portion comprises an elbow having means at either end to enable male female or female male mating with said distal stem and also with said head to create tight interfitting therebetween, said neck being rotatable relative to said stem and head prior to effecting said tight interfitting and while said distal stem is fixed in situ; characterised in that at least part of the neck includes a hollow interior and a formation which allows at least partial elastic deflection of the neck at a point of load transfer between the neck and stem. Preferably the formation is a chamfered contour inside the hollow interior and is arranged to dissipate applied loading to an inner wall of the stem by allowing elastic deflection of the neck at or near a point of load transfer between the stem and neck.

In a further broad form the invention comprises: a neck component for use in a femoral prosthesis of the type comprising a stem insertable in a medullary cavity of a femur and a neck insertable in the stem; said neck component comprising two legs disposed at an obtuse angle to each other, each of said legs terminating at its extremity in a tapered portion wherein at least one said legs of said neck has a hollow interior and includes a formation which allows at least partial elastic deflection of the neck under load transfer to the stem thereby reducing applied stress on an inner wall of the stem.

The present invention provides an alternative to the known prior art and the shortcomings identified. The foregoing and other objects and advantages will appear from the description to follow. In the description reference is made to the accompanying representations, which forms a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. In the accompanying illustrations, like reference characters designate the same or similar parts throughout the several views.

The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.

BRIEF DESCRIPTION OF DRAWINGS:

The present invention will now be described in more detail according to a preferred but non limiting embodiment and with reference to the accompanying illustrations wherein;

FIG 1 shows a perspective elevation view of a neck component of a prosthesis according to a preferred embodiment.

FIG. 2 shows a long sectional view of a distal leg of the neck component of figure

1 showing the hollow interior and chamfered formation

FIG. 3 shows with corresponding numbering a rear elevation of the neck shown in figure 1 shows a cross sectional view of a distal leg of the neck component of figure 1 showing the hollow interior and chamfered formation, shows a top view of the neck component of figure 1. shows a side elevation of a neck of the type shown in figure I with dimensions showing a typical geometry and proportionality according to one embodiment.

shows a long section through a neck component according to an alternative embodiment.

shows a long section through a fully assembled hip prosthesis including the neck embodiment of figure 2

shows a stress concentration diagram with maximum concentration when a conventional neck component is inserted in the stem,

shows a stress concentration diagram with maximum concentration shown when a neck component with chamfered formation is inserted in the stem.

DETAILED DESCRIPTION

An embodiment of the invention to be described below, with reference to its application in an artificial hip joint. It will be appreciated that although the invention will be described with reference to a hip prosthesis, this is not to be construed as limiting of the joint applications to which the invention may be adapted. The invention improves the load transfer regime including the geometry and structural performance of a load bearing first joint component when engaged with a load bearing second joint component such that the first component has geometry which enables dissipation of a load applied to the first component and dissipated to the second component when the first component engages the second component. The invention according to one embodiment further relates to a neck component of a hip prosthesis engageable with a stem via a tapered end, the tapered end having a hollow interior at least part of which is formed so that a distal end of the tapered end which engages the stem is capable of an elastic displacement to dissipate load transferred through the neck to the stem to relieve the loading transferred to the stem.

The invention will now be described in more detail with reference to a neck which typically engages a femoral stem component to form in conjunction with a femoral head component which engages the neck, a hip prosthesis. Although the neck component is totally dependent upon its use with the stem component, the description below concentrates on the neck and its geometry which embodies the invention described herein. Since the invention resides on a first of two components the second component which according to the embodiment to be described is a stem will not be described in detail. It will be appreciated by persons skilled in the art that the neck component according to the invention may be adapted to a variety of stem types.

Typically at the upper end of a femoral stem component there exists a Morse tapered recess which receives the neck. The neck which typically has a tapered male profile part which co operates with the female taper in the stem recess to facilitate upon coupling a tight male/female interfitting.

In a femoral prosthesis, a stem is inserted in the femur followed by insertion of the neck into the stem and rotated by the surgeon to the correct position of alignment with the acetabulum (not shown). Once this position is determined, the elbow is hammered to effect tight fitting with the stem. The Morse taper prevents unwanted rotational and axial movement, once the neck is aligned and set. The neck will typically include a collar or flange which provides a levering point to enable the surgeon to remove the neck as required. Preferably, the neck is made from Titanium or chrome-cobalt alloy.

Referring to figure 1 there is shown a side perspective elevation view of a neck component 1 of a prosthesis according to a preferred embodiment. Neck 1 comprises a body 2 having a first leg 3 terminating in end 4 and a second leg 5 terminating in end 6. Longitudinal axes A and B through legs 3 and 5 intersect through leg 3 which is disposed at an oblique angle to leg 5. Leg 3 is adapted to engage a ball ( not shown) which may be manufactured from metal or ceramics and which provides an artificial femoral head. End 4 is adapted with a Morse taper 3a for engagement with an artificial femoral head ( not shown). Likewise end 6 is adapted with a Morse taper 5a for engagement with a femoral stem ( not shown). Neck component 1 further comprises intermediate the ends 4 and 6 is a transition elbow 7 which joins legs 3 and 5. End 6 includes a hollow interior 8 which is defined by a wall 10 having external surface 11 and internal formation 9 which may be a chamfer. Other formations will suit the objective of load relief or dissipation.

FIG. 2 shows a long sectional view of a distal leg of the neck component of figure 1 showing the hollow interior and chamfered formation. The long sectional view shows distal leg 5 of the neck component 1 of figure 1 with the hollow interior 8 and chamfered formation 9. Hollow interior 8 comprises a through passage 12 which may be used for insertion of a fastener which secures neck 1 to a stem component (not shown). Leg 5 as shown in long section includes wall 10 having a region 13 of relatively constant thickness and a second region 14 of varying thickness near end 4. Region 14 includes formation 9. Formation 9 commences at location 14 and extends to end 6. At location 14 the width or diameter of formation 9 commences enlargement transitioning from the diameter of through passage 12 to a larger diameter. As formation 9 moves distally the diameter of the formation 9 gradually increases with a corresponding reduction in the thickness of wall 1 1 at region 13. The rate of change of the formation may be linear, parabolic, circular, curved or any other suitable geometry. The reduction in thickness of wall 1 1 allows wall 10 to absorb load through leg 5 and if required, wall 10 can undergo deflection under point load when a dynamic or static load is applied to neck 1. Figure 3 shows with corresponding numbering a rear elevation of the neck 1 shown in figure 1 , comprising a body 2 having a first leg 3 terminating in end 4 and second leg 5 terminating in end 6. Leg 3 is adapted to engage a device such as a ceramic ball which provides an artificial femoral head. Ends 4 and 6 are adapted with a Morse tapers 3a and 5a for engagement with counter components not shown. End 6 is adapted with a Morse taper 5a for engagement with a femoral stem ( not shown). FIG. 4 shows a cross sectional view of a distal leg 5 of the neck component of figure 1 showing wall 10 and the hollow interior passage 12 and chamfered formation 9.

FIG 5 shows with corresponding numbering a top view of the neck component of figure 1. When neck component 1 is located in a femoral stem component, leg 3 engages a female opening in the stem. Patient loads are transmitted from leg 3 to an inner wall of the stem The loads are distributed along the Morse taper on leg 3. Since the applied loads include rotational loads, the applied moments generate a point load transmitted at the extremity of leg 3 to inner wall of the stem. In the past, prostheses have failed when the load transmitted by an extremity of the neck exceeds the elastic limit of the stem material. The present invention seeks to dissipate rotational energy applied to the stem by inducing a capacity for deflection in the elbow as the loading is applied to the stem. An elastic deflection is introduced into wall 11 of leg 3 of neck 1 and this reduces the point load on the stem as load is dissipated by the elasticity of the wall 1. The elasticity is introduced into the wall 1 1 by the chamfered region created by formation 9.

Structural testing demonstrates that the introduction of the chamfer into the leg 3 of neck 1 reduces the point loading at an interface between the extremity of leg 3 and an inner surface of a stem in which neck 1 is fitted. As shown in the comparative stress loading diagrams of figures 6 and 7 the loading concentration differs when a chamfer 9 is introduced into the neck 1.

Figure 6 shows a long section through a neck component 30 according to an alternative embodiment. The long sectional view shows distal leg 31 of the neck component 30 with the hollow interior 32 and formation 33. Hollow interior 32 comprises a through passage 34 which may be used for insertion of a fastener (not shown) which secures neck 30 to a stem component. Leg 31 as shown in long section includes wall 35 having a region 36 of generally constant thickness and a second region 37 of varying thickness near end 38. Region 37 includes formation 33. Formation 33 commences at location 40 and extends to end 38. At location 40 the diameter of formation 33 commences enlargement transitioning from the diameter of through passage 34. As formation 33 moves distally the diameter of the formation 33 gradually increases with a corresponding reduction in the thickness of wall 35 at region 37. At location 41 formation 33 transitions into a more constant wall thickness The reduction in thickness allows wall 35 to deflect under point load when a dynamic load is applied to neck 30. The geometry of the formation 33 is one of many suitable geometries which can assist in load dissipation. As wall 35 reduces in thickness, deflection response increases. The formation may be designed to enable abrupt load dissipation at a particular location or gradual dissipation depending upon design requirements and the extent of load dissipation or reduction on the counter component against which wall 35 acts when neck 30 is under load. .

Figure 7 shows a long section through a neck component 50 according to an alternative embodiment. The long sectional view shows distal leg 51 of the neck component 50 with the hollow interior 52 and chamfered formation 53. Hollow interior 52 comprises a through passage 54 which may be used for insertion of a fastener (not shown) which secures neck 50 to a stem component. Leg 51 as shown in long section includes wall 55 having a region 56 of generally constant thickness and a second region 57 of a second thickness near end 58. Region 57 includes formation 53. Formation 53 commences at location 59 and extends to end 58. As formation 53 moves distally the width/ diameter of the formation 53 is comparatively constant with a corresponding reduction in the thickness of wall 55 at transition region 59. The reduction in thickness allows wall 55 to deflect under point load when a dynamic load is applied to neck 50.

Figure 8 shows a long section through a fully assembled hip prosthesis 70 including the neck embodiment of figure 2 . The long sectional view shows distal leg 5 of the neck component 1 with the hollow interior 8 and chamfered contoured formation 9. Hollow interior 8 comprises a through passage 12 which may be used for insertion of a fastener or access to a fastener which secures neck 1 to a stem component 71. leg 3 receives and retains a head component 72 which acts as an artificial femoral head. Leg 5 as shown in long section includes wall 10 having a region 13 of relatively constant thickness and a second region 14 of varying thickness near end 4. Formation 9 commences at location 14 and extends to end 6. At location 14 the width or diameter of formation 9 commences 00094

enlargement transitioning from the diameter of through passage 12 to a larger diameter. As formation 9 moves distally the diameter of the formation 9 gradually increases with a corresponding reduction in the thickness of wall 1 1 at region 13. The rate of change of the formation may be linear, parabolic, circular, curved or any other suitable geometry. In use when head 72 takes loading transmitted from an acetabular cup that load is transferred along leg 3 and then via elbow 7 to leg 5. Loading on head 72 induces at least an axial and bending load in leg 5 which in turn is transmitted to region 73 of stem 71. This places a point loading at a junction between region 73 and wall 10 of leg 5. In a case where wall 10 of leg 5 is fully rigid and has no 'give', all of the loading will be transmitted to stem 71 via region 73. Although the point loading may vary about the periphery of wall 10, there are normally locations of high load transfer and it is likely that if a stem failure occurs, it will occur in that region. The reduction in thickness of wall 1 1 due to the introduction of formation 9 in leg 5, allows wall 10 to absorb load through leg 5 and if required, wall 10 can undergo deflection under point load when a dynamic or static load is applied to neck 1. This sacrifices the loading at region 73 and reduces or eliminates the likelihood of a stem failure. Thus a stem will be able to withstand higher loadings than occurred in the past and stem failures will recue or be eliminated.

Figure 9 shows a stress concentration diagram with maximum concentration shown as 552.754MPa when a conventional neck component is inserted in the stem. The stress regions are shown in the wall of the stem graduating from low stress regions around 61.1 MPa to high stress regions around 552.754MPa. The high stress regions will be at the location of engagement between the extremity of that part of the neck inserted in a stem. Figure 10 shows a stress concentration diagram with maximum concentration shown as 504.332MPa when a neck component with chamfered formation is inserted in the stem. The stress regions are shown in the wall of the stem graduating from low stress regions around 55MPa to high stress regions around 504.332MPa. The higher stress regions will be at the location of engagement between the extremity of the neck inserted in a stem. However, when a chamfered formation is introduced into the distal leg of the neck component the minimum and maximum measured stresses at the contact point in the stem wall reduce. In fact the full range of measured stresses reduce compared to a neck without the chamfer formation. It is believed that this change in force regime is facilitated by an elastic load dissipating deflection in the wall of the neck due to reduced in thickness created by the chamfer.

It will be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention as broadly described herein without departing from the overall spirit and scope of the invention.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 An artificial joint assembly implantable in bone, the joint assembly including a first component which is at least partially hollow and at least one other component matable with the first component and which in use co operates with the first component, the first component comprising;

a body having first and second ends;

one said ends including a hollow part and which engages said at least one other component for load transfer between the components, the hollow part of the first component including means to enable dissipation of load through the hollow component when a load is applied to the first component, thereby reducing load transmitted via the hollow part on the first component to the other component when the first component is engaged with said other component.

2 An artificial joint assembly according to claim 1 wherein the means to enable dissipation of energy through the first component is a formation which allows elastic deformation of at least a part of the hollow part.

3 An artificial joint assembly according to claim 2 wherein the first end of the first component includes a hollow passage defined by a wall extending from the first end at least partially along the body of the first component.

4 An artificial joint assembly according to claim 3 wherein the first end of the first component engages the second component via a tapered connection.

5 An artificial joint assembly according to claim 4 wherein the load is transmitted from the first component to the second component via the tapered connection.

6 An artificial joint assembly according to claim 5 wherein the formation in first end of the first component is a chamfer which provides a distal end thinning of the wall 7 An artificial joint assembly according to claim 6 wherein the thinning in the wall chamfer allows the wall to deflect when load is transferred from the first component to the second component. 8 An artificial joint assembly according to claim 7 wherein, the first component forms a neck of a prosthesis.

9 An artificial joint assembly according to claim 8 wherein the second component is a stem which provides bone anchorage.

10 An artificial joint assembly according to claim 9 wherein the neck and stem engage to form a hip prosthesis.

11 An artificial joint assembly according to claim 9 wherein the neck and stem combine to form a shoulder prosthesis.

12 An artificial joint assembly according to claim 9 wherein the neck and stem combine to form a knee prosthesis.

13 An artificial joint assembly according to claim 9 wherein the neck and stem combine to form an elbow prosthesis.

14 An artificial joint assembly according to claim 9 wherein, the neck and stem combine to form an ankle prosthesis

15 An artificial joint assembly according to claim 9 wherein the first end of the neck has an external taper which engages an internal taper in the second component.

16 An artificial joint assembly according to claim 9 wherein, the chamfer in the wall of the first end is linear

17 An artificial joint assembly according to claim 9 wherein the chamfer in the wall of the first end is elliptical

18 An artificial joint assembly according to claim 9 wherein the chamfer in the wall of the first end forms a concave inner wall surface.

19 An artificial joint assembly according to claim 10 wherein the chamfer in the wall of the first end 20 An artificial joint component for use in an artificial joint assembly implantable in bone, the joint assembly formed by the co operation of the first component and a second component; the first component comprising;

a component body having first and second ends;

one said ends including the hollow portion which engages said second component, the hollow portion including a formation which enables dissipation of load through the hollow component when a load is applied to the hollow component, thereby reducing loading transmitted via the first component to the second component, the load dissipation enabled by an elastic deflection in a wall of the hollow portion of the first component.

21 A femoral prosthesis of the type comprising a stem, neck and head and adapted for insertion into the medullary cavity of a femur to thereby form a replacement for^' the natural femoral head; characterised in that the stem has which receives a distal end of the neck, a proximal end of the neck adapted to mate with an artificial head, wherein the neck component includes a hollow interior at least a part of which includes a formation which allows a deflection in a wall of the hollow portion so that when the neck component is placed under load by loading the femoral head, loading transmitted to the stem via the neck is partially dissipated through the neck by elastic deflection of the wall at the formation.

22 A femoral prosthesis according to claim 21 wherein the neck when under load at an interface between the neck and stem prevents the stem from point load failure by increasing the capacity of the neck to dissipate loads by up to 30%-40% compared to the resistance provided by the known neck and stem components.

23 A femoral prosthesis comprising a distal stem and a detachable neck component having a double Morse taper and a head component; the neck component including a hollow passage and a formation which contributes to load dissipation through the neck at the point of connection between the neck and stem when the neck is under load. 24 A femoral prosthesis according to claim 23 wherein the formation is in the hollow passage and creates a thin wall region at a distal end of the neck component, thereby allowing elastic deflection therein to dissipate loads through the neck and reduce point loading on the stem.

25 A femoral prosthesis adapted for insertion into the medullary cavity of a femur said prosthesis comprising, a distal stem, a neck component detachable from said distal stem and a head detachable from said neck component; characterised in that the neck portion comprises an elbow having means at either end to enable male female or female male mating with said distal stem and also with said head to create tight interfitting therebetween, said neck being rotatable relative to said stem and head prior to effecting said tight interfitting and while said distal stem is fixed in situ; characterised in that at least part of the neck includes a hollow interior and a formation which allows at least partial elastic deflection of the neck at a point of load transfer between the neck and stem.

26 A femoral prosthesis according to claim 25 wherein the formation is a chamfered contour inside the hollow interior and is arranged to dissipate applied loading to an inner wall of the stem by allowing elastic deflection of the neck at or near a point of load transfer between the stem and neck.

27 A femoral prosthesis according to claim 26 wherein, the interfitting between the stem and neck components is effected by a Morse taper at either end of the mating member, thereby creating a double Morse taper, allowing inter engagement between the distal stem and the neck.

28 A femoral prosthesis according to claim 27 wherein the neck component comprises two legs disposed at an obtuse angle to each other, each of said legs terminating at its extremity in a tapered portion wherein at least one said legs of said neck has the hollow interior and includes the formation which allows the at least partial elastic deflection of the wall of the neck under load transfer to the stem thereby reducing applied stress on an inner wall of the stem.