WO2003047471A1

WO2003047471A1 - Femoral component of hip joint prosthesis

Info

Publication number: WO2003047471A1
Application number: PCT/GB2002/005460
Authority: WO
Inventors: Matthias Honl; James Brooks; Stefano Alfonsi; Gary Moore
Original assignee: Depuy International Limited
Priority date: 2001-11-30
Filing date: 2002-12-02
Publication date: 2003-06-12
Also published as: AU2002350899A1; GB0128729D0

Abstract

A femoral component of a hip joint prosthesis has a stem (2) which can be implanted without cement in the metaphysial region (42) of a femur below the greater trochanter (38) while partially conserving the femoral neck (30). The portion of the stem (2) which is intended to be fitted into the cavity within the femur is rotationally symmetrical. This portion has (a) a proximal portion (4) which is tapered inwardly towards the distal end (8), and which has a surface which promotes ingrowth of bone tissue, and (b) a distal end portion (14) which has a continuously rounded surface which is smooth so as to minimise bonding between the femoral component and adjacent bone tissue at the distal end (8).

Description

FEMORAL COMPONENT OF HIP JOINT PROSTHESIS

This invention relates to a femoral component of a hip joint prosthesis.

US-5766263 and US-6120544 relate to a femoral components of artificial hip joints which are mounted in the upper region of the femur below the greater trochanter, within and aligned with the femoral neck axis. These components can be contrasted with widely used hip joint femoral components which extend into the intramedullary cavity extending along the femur axis. Use of the components with which US-5766263 and US-6120544 are concerned has the advantage that less of the patient's natural bone tissue is removed during surgery, and associated soft tissue can also be preserved: the bone is resected on the femoral neck, just below the spherical head, rather than by removal of the entire head section of the femur, including the greater trochanter by resection in the metaphysial region. In addition, the length of the incision that is required is smaller than with conventional implants. The procedure for implanting a prosthesis of the kind disclosed in US-5766263 and US-6120544 is therefore less traumatic for the patient. The resection of less bone allows better for the possibility of revision surgery at a later date in the event that the original prosthesis implantation fails.

The prostheses disclosed in US-5766263 and US-6120544 are implanted on or close to the axis of the femoral neck. Preparing the bone for the implantation requires accurate location of the neck axis. This is also important because the resection plane should generally be orthogonal to this axis. The axis can be defined by locating a point on the lateral cortex which lies opposite to the centre of the femoral head, where the axis intersects the cortex. This can be identified during pre-operative planning, for example using X-ray or other images of the femur, in particular relative to prominent landmarks on the bone surface. In the case of the prosthesis disclosed in US-6120544, the femur remains intact in the region of the lateral cortex opposite to the resected femoral neck. The prosthesis is bent caudally at its distal end, and is covered there with an open-meshed lattice structure. The prosthesis is inserted into a prepared cavity in the bone so that the bent end region acts against the lateral cortical bone tissue with the lattice structure in contact with the bone tissue. In the case of the prosthesis disclosed in US-5766263, a hole is formed in the lateral cortical tissue for a draw-in screw, by which a shell part introduced into the femur through the resected femoral neck is drawn into the bone. The hole can be formed by a drill after resection of the bone, extending through the femur, along the neck axis. Drill guides which enable accurate location of the neck axis for drilling from the lateral surface to the medial surface are known. They have the disadvantage that they require exposure of the point on the lateral cortical bone at which the axis intersects that bone, involving in particular the displacement of muscle and other soft tissue. Forming an opening in the cortical bone also has the effect of weakening the bone, making it less able to withstand the stresses that are imposed when the joint is under load.

The present invention provides a femoral component of a hip joint prosthesis which is rotationally symmetrical (at least in the portion of the component which is intended to be fitted into the cavity within the femur), and in which the proximal region is configured to promote ingrowth of bone tissue, and in which the distal region has a continuously rounded surface which is smooth so as to minimise bonding between the femoral component and adjacent bone tissue at the distal end.

Accordingly, in one aspect, the invention provides a femoral component of a hip joint prosthesis, having a stem which can be implanted without cement in the metaphysial region of a femur below the greater trochanter while partially conserving the femoral neck, the portion of the stem which is intended to be fitted into the cavity within the femur being rotationally symmetrical and having (a) a proximal portion which is tapered inwardly towards the distal end, and which has a surface which promotes ingrowth of bone tissue, and (b) a distal end portion which has a continuously rounded surface which is smooth so as to minimise bonding between the femoral component and adjacent bone tissue at the distal end.

The component of the present invention can be implanted in a bone cavity in such a way that stress directed on to the prosthesis when the joint is under load are transmitted on to the mantle of bone tissue which surrounds prosthesis in the proximal region. In contrast to the prostheses that are the subjects of US-5766263 and US-6120544, the component of the present invention seeks to ensure that the stresses that are transmitted on to the lateral cortical bone tissue are minimised or even eliminated altogether. This can provide the advantage of a more secure and stable implantation of the prosthesis. It also represents a more natural mode of load transfer from the joint onto the bone tissue.

The use of a component that is rotationally symmetrical, especially when it is configured in such a way that the stresses transmitted on to the lateral cortical bone tissue are minimised, has the significant advantage that it facilitates accurate preparation of the bone cavity for receiving the prosthesis component. In particular, the cavity can be prepared using a rotating tool such as a reamer. This is in contrast with the instruments that are used to prepare a cavity for receiving a prosthesis of the type disclosed in US-6120544, which requires use of a broach. Accurate preparation of the bone cavity is important to ensure that loads are accurately transmitted on to the mantle of bone tissue which surrounds prosthesis in the proximal region.

Preferably, the surface of the proximal portion of the stem is porous. The use of porous coatings to promote bone ingrowth on orthopaedic prosthesis components, which are to be implanted without the use of bone cement, is well established.

Preferably, the taper angle between the surface of the component in the proximal portion and the axis of the component is substantially constant over the length of the proximal portion. The side walls of the proximal portion of the stem are therefore defined by essentially straight lines when the stem is viewed in cross-section from one side. This can facilitate securing of the stem into a prepared bone cavity, and maintenance of a secure fit of the stem in the cavity even when the stem subsides slightly under load.

Preferably, the taper angle between the surface of the component in the proximal portion and the axis of the component is not more than about 5°, more preferably not more than about 4°. Preferably, the taper angle is at least about 1.5°, more preferably at least about 2.5°. It is particularly preferred that the taper angle is about 3°. Preferably, the femoral component includes a neck on the proximal end of the stem, for mounting a head which can articulate with an acetabular component. The neck need not be rotationally symmetrical; for example, the neck might be offset so that the axis of the head is not aligned with the axis of the rotationally symmetrical portion of the component. Preferably, the stem is tapered inwardly continuously from the base of the neck to the rounded end portion at the distal end (that is along the portion of the stem which is intended to be fitted into the cavity within the femur). Preferably, the surface of the stem is arranged to promote ingrowth of bone tissue over the entire surface from the base of the neck to (but not including) the rounded end portion at the distal end.

The length of the stem will be selected according to the anatomic requirements of the patient, in terms of the transverse dimension of the neck region of the femur and the distance between the resection plane on the femoral neck and the lateral cortical bone tissue opposite to the femoral neck. The implant should be selected so that there is a space between the distal end of the implant and the cortical bone tissue, and so that the proximal portion of the stem protrudes from the bone cavity above the resection plane. The distance between the distal end of the implant and the cortical bone tissue should be about the same as the height of the proximal portion of the stem which protrudes from the bone cavity above the resection plane. Preferably, these distances are at least about 2 mm, for example at least about 3 mm. The component of the invention has the advantage that it can accommodate subsidence of the component due to changes in the structure of the bone in the femoral neck, without significant loosening of the component. In contrast, the prostheses that are disclosed in US-5766263 and US-6120544 can in some circumstances tend to become loose if there are significant changes to the structure of the bone in the femoral neck because the contact between the prosthesis and the lateral cortical bone tissue.

Generally, the length of the stem, measured from the base of the neck, well be not more than about 80 mm, for example not more than about 75 mm. The length will generally be at least about 45 mm. The component can be made with a head portion connected to the stem, for example from a single piece of material. Preferably however the head portion and the stem portion are made as separate parts. They can be fitted together by means of cooperating tapers as is well established in the field of orthopaedic prostheses, especially femoral components of hip joint prostheses.

The surface of the prosthesis component at the distal end is continuously rounded in the sense that it does not have holes formed in it to engage other components, for example in contrast with the device that is disclosed in US-5766263 in which the prosthesis has a bore at its distal end to engage a fastener. Making the surface rounded has the advantage that, when there is contact between the prosthesis component and the lateral cortical bone tissue (for example as a result of subsidence of the component into the bone cavity), the transfer of load on to the cortical bone tissue is over an area rather than at a point. The surface is smooth so as to minimise bonding between the femoral component and adjacent bone tissue at the distal end, in particular when there is contact between prosthesis component. For example, when the proximal portion of the stem has a porous coating, the distal end portion will be smooth, without the porous coating. Preferably, the smooth end portion is polished, for example so that its surface roughness is less than about 0.8 R^. A smooth surface of the component at the distal end means that the surface will be resistant to ingrowth of bone tissue, even in the event that the distal end comes into contact with bone tissues (for example after subsidence of the component).

Preferably, the surface of the stem at the distal end is substantially spherical. The rounded surface at the distal end can include a spherical portion at the distal end and another portion between the spherical portion and the proximal portion of the stem. That other portion need not have a constant radius: for example, it could form a part of a parabola (when the stem is viewed in cross-section from one side. When the surface of the stem is substantially spherical at the distal end, it is preferred that the radius of the spherical surface is at least about 4 mm, more preferably at least about 5 mm. The stem will generally be made from an alloy. Suitable alloys include cobalt chrome based alloys, titanium alloys, and certain stainless steels, especially a titanium-aluminium- vanadium alloy.

Preparation of the femur for implantation of the femoral component of the invention requires accurate location of the axis on which the component will be implanted. This can be achieved using the instrument which is disclosed in International patent applications nos. GB02/04334 and '337. Subject matter disclosed in those applications is incorporated in the specification of the present application by this reference.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a side view of the prosthesis component of the invention.

Figure 2 is a side view partially in cross-section, of a prosthesis component of the invention implanted in a patient's femur.

Referring to the drawings, Figure 1 shows a prosthesis component having a stem 2. The stem is formed from an alloy of titanium, aluminium and vanadium. The stem has a proximal portion 4 which is tapered inwardly from the proximal end 6 towards the distal end 8. The angle between the external surface of the proximal portion and the axis 12 of the stem is 3 °. The external surface of the proximal portion is porous by virtue of the application to it of one or more layers 10 of small metal beads; such porous coatings are known, for example in products that are marketed by DePuy Orthopaedics Inc of Warsaw Indiana USA under the trade mark POROCOAT, in which layers of metal beads are affixed in place by means of a sintering process.

The stem is generally rounded between the proximal portion 4 of the stem and the distal end 8. The rounded distal end portion 14 has a smooth surface which is highly polished so that the surface is resistant to the formation of bonds between adjacent bone tissue and the stem. In the immediate vicinity 16 of the end 8 of the stem, the surface of distal end portion has a spherical configuration. The radius of the sphere is 6 mm. In the region 18 between the vicinity 16 of the end 8 of the stem and the distal end of the proximal portion, the surface is generally rounded so that there is a continuous rounded surface extending over the entire end portion 14 of the stem.

A neck 20 is provided at the proximal end of the stem. The neck 20 has a frusto-conical portion with a taper angle of 5.667°, which can be received in a corresponding frusto- conical bore in a spherical head.

The length of the proximal portion of the component can be varied according to the dimensions of the bone in which the component is to be implanted. Examples of suitable dimensions (in millimetres) for a range of prostheses include:

Figure 2 shows the component shown in Figure 1 which has been implanted into the neck region 30 of a femur. The component has a head 34 on the neck. The head is spherical and has a highly polished surface for articulation with the bearing surface of an acetabular component, for example made from ultrahigh molecular weight polyethylene.

The femur has been resected in the neck region on a plane 32 which is approximately perpendicular to the axis of the femoral neck, below the greater trochanter 38 and above the lesser trochanter 40, generally in the metaphysial region 42 of the femur. Location of the axis of the femoral neck can be achieved using an instrument as disclosed in International patent applications nos. GB02/04334 and '337. After resection, the cavity is prepared using a rotating cutting tool such as a reamer which is able to prepare the internal bone surfaces within the femoral neck so that the cavity has dimensions (including a taper angle) appropriate to receive the component in a tight fit, and so that (a) there is a gap of about 3 mm between the end 8 of the stem and the lateral cortical bone 36, and (b) there is about 3 mm between the resection plane 32 and the proximal end of the proximal portion of the stem.

The component can be implanted in a femur after preparation of the bone cavity by resection and reaming (as referred to above). The component becomes fixed in place by means of ingrowth of bone tissue into the porous structure of the surface 10 in the proximal portion 4 of the component. Subsidence of the component into the bone cavity is possible because of the gap between the end 8 of the stem and the lateral cortical bone 36. After any such subsidence has taken place, the part of the porous surface of the proximal portion of the component which was previously exposed comes into contact with the internal surface of the bone cavity so that bone tissue can grow into that part of the porous surface.

Claims

CLAIMS:

1. A femoral component of a hip joint prosthesis, having a stem which can be implanted without cement in the metaphysial region of a femur below the greater trochanter while partially conserving the femoral neck, the portion of the stem which is intended to be fitted into the cavity within the femur being rotationally symmetrical and having (a) a proximal portion which is tapered inwardly towards the distal end, and which has a surface which promotes ingrowth of bone tissue, and (b) a distal end portion which has a continuously rounded surface which is smooth so as to minimise bonding between the femoral component and adjacent bone tissue at the distal end.

2. A femoral component as claimed in claim 1, in which the surface of the proximal portion of the stem is porous.

3. A femoral component as claimed in claim 1, in which the taper angle between the surface of the component in the proximal portion and the axis of the component is substantially constant over the length of the proximal portion.

4. A femoral component as claimed in claim 1 , in which the taper angle between the surface of the component in the proximal portion and the axis of the component is not more than about 5°.

5. A femoral component as claimed in claim 1, which includes a neck on the proximal end of the stem, for mounting a head which can articulate with an acetabular component.

6. A femoral component as claimed in claim 5, in which the length of the stem, measured from the base of the neck, is not more than about 80 mm.

7. A femoral component as claimed in claim 1, in which the surface of the stem at the distal end is substantially spherical.

8. A femoral component as claimed in claim 7, in which the radius of the spherical surface at the distal end is at least about 4 mm.