This invention relates to a centraliser for a prosthetic implant, in particular a prosthetic femoral stem for implantation in a patient's femur for hip replacement surgery. _ Centralisers or spacers are provided for fitting to the distal end of a femoral hip replacement stem in order to keep the implant stem away from the internal surface of the cavity of the bone in which the stem is being inserted. In the case of stems which are cemented in the bone cavity, there is a space between the stem and the internal surface of the cavity of the bone in which bone cement is placed. Controlling the position of the stem within the surrounding bone cement mantle is vital to the longterm survivability of the replacement joint. Cement can be deposited in the bone cavity and the stem with attached centraliser can be inserted. It is important to try to obtain an even and intact cement mantle around the stem.

Known centralisers are in the form of a cap which fits over the distal end of the stem or are in the form of centralisers which are fixed inside the drilled end of the stem. Centralisers are also known, for example as described in US patent no. 5,658,351 of ring form which can have a tapered inner surface corresponding to the tapered surface of the distal end of a femoral stem on which the centraliser is located.

Problems can arise in locating a centraliser on the tip of a tapering stem. The location can be achieved by close fitting of the centraliser by shaping the inside of the centraliser to the stem to ensure suitable positioning. The problem associated with shaping the internal surface of the centraliser is that the fit to the femoral stem is uni- directional and incorrect fitting could cause fracture of the centraliser due to the elevation of hoop stresses on the centraliser caused by point or ring loading. The centraliser could be marked to indicate the correct orientation; however, orientation marking is very difficult due to the size of the centraliser.

An object of the present invention is to ensure correct positioning of a centraliser with no orientation problems.

According to a first aspect of the present invention, there is provided a centraliser for use on a prosthetic femoral stem to be inserted in a bone cavity, the centraliser comprising a hollow body having an internal surface and an external surface and spacer means projecting from the body, wherein the internal surface is of convex curvature.

Preferably, the radius of curvature of the internal surface is in the range of 20- 100mm. Most preferably, the radius of curvature of the intemal surface is in the range of 30-SOmm, optimally 40mm Preferably, the spacer means has an outer edge of convex curvature for contacting an internal surface of the bone cavity. The radius of curvature of the outer edge of the spacer means may be in the range of 10-100mm. Preferably, the radius of curvature of the outer edge of the spacer means is in the range of 10-40mm.

The spacer means may be in the form of a plurality of fins extending from the extemal surface of the hollow body. The hollow body may be of ring form with two open ends joined by the internal and external surfaces.

Preferably, a mid-portion of the hollow body is of increased thickness.

Preferably, the external surface of the hollow body of the centraliser is also of convex curvature. The radius of curvature of the external surface may be in the range of 10-100mm. Preferably, the radius of curvature of the external surface is in the range of 1 0-40mm.

Preferably, the radius of curvature of the outer edge of the spacer means and radius of curvature of the extemal surface of the hollow body are approximately equal.

The hollow body of the centraliser may have a generally rectangular or oval cross-section.

In the case of a rectangular cross-section, the internal corners of the generally rectangular hollow body have indented reliefs.

Preferably, the fins extend radially from the outer surface of the hollow body' and longitudinally in the direction of a femoral stem on which the centraliser is disposed. In the case where four fins are provided, one fin corresponds to each of the anterior, posterior, medial and lateral surfaces of the femoral stem.

According to a second aspect of the present invention, a centraliser is provided in combination with a tapered femoral stem wherein taper contact between the femoral stem and the centraliser is assumed to be at a maximum length of 2/3L for stress reduction, where L is half the height of the centraliser, for centreline contact between the femoral stem and the centraliser, 0=0 and Rroo, where R is the radius of curvature of the internal surface of the centraliser, wherein: At 2/3L contact the following equations apply: The taper angle/radius relationship is: Sin6=2L/3R h=R-4 (R- (2/3L)') 2/3L2=h (2R-h) Wherein h is the distance between the surface of the centraliser at the centre-line and at 2/3L.

Preferably, the radius of curvature of the internal surface of the centraliser is related to the taper of femoral stem in the same relationship as a curvature of an outer edge of the spacer means is related to the taper of an internal surface of the bone cavity.

An embodiment of a centraliser in accordance with the present invention will now be described, by means of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of the centraliser in position on a distal end of a femoral stem prosthesis with variations in stem size shown in broken lines; Figure 2 is a cross-section through line A-A of the centraliser and femoral stem of Figure 1; Figure 3 is a cross-section of a prior art centraliser located on a distal end of a femoral stem illustrating the instability of the prior art centraliser; Figure 4 is a cross-section of the centraliser of Figure 1 disposed at an angle on the femoral stem; and Figure 5 is a diagrammatic representation of a cross-section of the centraliser of Figure 1 disposed on a femoral stem showing the relationship between the taper angle of the femoral stem, the radius of curvature R of the internal surface of the centraliser and the dimensions L, T of the centraliser.

Referring to the drawings, a centraliser 1 is provided for use with a prosthetic femoral stem 2 for implantation in a patient's femur for hip replacement surgery. The centraliser 1 is located on a distal end 4 of the femoral stem 2.

In the described embodiment, the centraliser 1 is used with a tapered femoral stem 2 with anterior 6, posterior 8, medial 10 and lateral 12 surfaces which are generally flat resulting in a tapered rectangular form of cross-section of the stem 2 with the cross- sectional area decreasing towards the distal end 4 of the femoral stem 2. The centraliser of the present invention can be used with other forms of femoral stem and is not restricted to use with the tapered femoral stem 2 of the described embodiment.

Tapered femoral stems 2 can have polished surfaces which co-operate advantageously with the cement mantle resulting in reduced fractures in the cement.

The centraliser 1 has an elongated ring body 14 with a top edge 16 defining a first open end of the elongated ring body 14 and a bottom edge 18 defining the second open end. An internal surface 22 of the ring body 14 extends between the top edge 16 and the bottom edge 18. The internal surface 22 is of convex curvature between the top edge 16 and the bottom edge 18.

Figure 5 illustrates the optimum relationship between the internal radius of curvature R and dimensions of the centraliser 1 and the taper angle 6 of the femoral stem 2.

Taper contact between the femoral stem 2 and the centraliser 1 is assumed to be at a maximum length of 2/3L to ensure the material is of adequate thickness for stress reduction, where L is half the height of the centraliser 1. For centreline contact between the femoral stem 2 and the centraliser 1, A=0 and R=>, where R is the radius of curvature of the internal surface 22 of the centraliser 1.

At 2/3L contact the following equations apply: The taper angle/radius relationship is: SinO=2L/3R h=R 4 (R2 (2l3L) 2) 2/3L2=h (2R-h) Wherein h is the distance between the surface of the centraliser at the centre-line and at 2/3L.

An optimal radius of curvature R of the internal surface 22 of the centraliser 1 for use with the tapered femoral stem 2 of the described embodiment is approximately R=40mm, or in the range of R=30-50mm. Although a radius of curvature of the range R=20-100mm would also work. In the preferred embodiment in which R=40mm, L=5mm, T=2mm for use with a taper angle of 0=3°.

The ring body 14 has an external surface 24 which extends between the top edge 16 and the bottom edge 18. The external surface 24 has a convex curvature. The radius of the external surface 24 of the centraliser 1 is as small as possible (i. e. close to L). In the described embodiment the radius of curvature of the external surface 24 is

approximately 40mm or in the range of 10-40mm, although a radius of curvature in the range of 10-100mm would also be acceptable.

A mid-portion 26 of the ring body 14 has a greater thickness than the top edge 16 and the bottom edge 18.

In the described embodiment. the centraliser 1 has a generally rectangular cross-~ section formed of four sides 30 as shown in Figure 2. The centraliser 1 has corner reliefs 28 at the internal corners of the rectangular cross-section for increased flexibility.

The four sides 30 of the centraliser 1 correspond to and contact the anterior 6, posterior 8, medial 10 and lateral 12 surfaces of the femoral stem 2. Each of the sides 30 of the centraliser 1 has a fin 20 which extends radially from the external surface 24 of the centraliser 1. The fins 20 are located centrally on each side 30 of the centraliser 1 and extend longitudinally between the top edge 16 and the bottom edge 18 of the centraliser 1. A centraliser 1 in accordance with the present invention may have any suitable shape of cross-section to correspond to the type of femoral stem 2 with which it is to be used.

The fins 20 have outside edges 32 which contact the internal surfaces 34 of the cavity in the patient's bone in which the femoral stem 2 and the centraliser 1 are inserted. The edges 32 of the fins 20 have convex curvature. The convex curvature of the fins 20 has a radius of curvature of approximately 40mm or in the range of 10- 40mm although a radius of curvature in the range of 10-100mm would be acceptable. The radius of curvature of the edges 32 of the fins should correspond to the radius of curvature of the external surface 24 of the ring body 14.

In use, the centraliser 1 is disposed on the distal end 4 of the femoral stem 2 before insertion of the femoral stem 2 into a cavity in a patient's femur.

Due to the convex curvature of the internal surface 22 of the centraliser 1, the centraliser 1 can be easily orientated on the distal end 4 of the femoral stem 2. The contact between the internal surface 22 of the centraliser 1 and the surface of the femoral stem 2 is a point or ring contact. The contact is in the mid-portion 26 of the

centraliser 1 where the material thickness reduces the effect of the hoop loading to a level where it is no longer of concern.

The convex curvature of the internal surface 22 of the centraliser allows the centraliser 1 to be placed unevenly on the femoral stem 2 whilst maintaining the correct cement mantle as shown in Figure 4. This was not possible with centralisers known from the prior art which did not have internal surfaces with a convex curvature as- shown in Figure 3.

The convex curvature of the edges 32 of the fins 20 allows the centraliser 1 to be held securely within the bone cavity even when it is misaligned with the femoral stem 2. If the relationship between the curvature of the edges 32 of the fins 20 and the angle of the internal surface of the bone cavity is adapted to correspond to the relationship between the curvature of the internal surface of the centraliser 1 and the angle of the tapered femoral stem, the centraliser 1 can be positioned in a stable manner even if it is misaligned with the femoral stem.

The external surface 24 can have a convex curvature to help direct hoop stresses to the thickest portion of the ring body 14. Corresponding the curvature of the external surface 24 to the curvature of the edges of the fins 20 has the benefit removing any interruption of the cement mantle by the ring body 14 in the event that the centraliser 1 is misaligned as shown in Figure 4.

The centraliser 1 has the added advantage that at can be used on a number of different sizes of femoral stems 2 as shown in Figure 1.

The edges 32 of the fins 20 contact the internal surfaces 34 of the prepared bone cavity thereby positioning the femoral stem 2 centrally in the cavity.

Cement can be introduced into the cavity in which the femoral stem 2 and centraliser 1 have been placed. The fins 20 provide minimal interruption to the cement flow past the fins 20.

It will be appreciated that the centraliser 1 can have a circular, or other shaped, cross-section and any number of fins 20 greater than two.

Modifications and improvements can be made to the foregoing without departing from the scope of the present invention.