WO1999048444A1

WO1999048444A1 - Prosthetic stem and centraliser

Info

Publication number: WO1999048444A1
Application number: PCT/GB1999/000898
Authority: WO
Inventors: Bruno Hiernard
Original assignee: Johnson & Johnson Medical Limited
Priority date: 1998-03-23
Filing date: 1999-03-22
Publication date: 1999-09-30
Also published as: GB9806179D0; AU3043599A

Abstract

A combined prosthetic stem (4) and a centraliser (2) in which the centraliser (2) is of open-ended cylindrical shape with a plurality of semi-circular fins (8) extending radially and longitudinally from the cylinder. The stem (4) is in the form of a femoral hip stem and has a shoulder (16) at its distal end (14) at which the diameter of the stem (4) is suddenly reduced resulting in a distal portion (32) of the stem (4) with a smaller diameter. The centraliser (2), when disposed on the stem (4), has an outer diameter which is flush with the diameter of the stem (4) above the shoulder (16). The stem (4) is cannulated to allow cement to be inserted into the bone cavity through the stem (4). The open-ended nature of the centraliser (2) allows cement to pass through the centraliser (2).

Description

PROSTHETIC STEM AND CENTRALISER

This invention relates to a prosthetic stem and centraliser. In particular, a prosthetic femoral stem for implantation in a patient's femur for hip replacement surgery.

Centralisers or spacers provided at the distal end of a femoral hip replacement stem are known, for example as described in GB2104391B. Centralisers keep the implant stem away from the internal surface of the cavity of a bone in which the stem is being inserted. With stems in which the stem is cemented in the bone cavity, there is an area between the stem and the internal surface of the cavity of the bone and it is important to centralise the stem within the cavity before the cement is added and to ensure that the stem does not move during the insertion of the cement.

Cannulated hip stems are known in which cement is introduced into the cavity through a canal extending through the length of the hip stem. In this way, cement is delivered to the distal end of the stem and this results in a uniform distribution of cemen? around the stem avoiding trapping air in the cavity.

Known centralisers are in the form of a cap which fits over the distal end of the stem or the centralisers are fixed inside the drilled tip of the stem. These forms of centraliser have the disadvantage that they cannot be used with a cannulated stem.

According to a first aspect of the present invention, there is provided in combination a prosthetic stem and a centraliser, wherein the centraliser comprises a ring member with a plurality of fins extending radially from the surface of the ring member, and wherein the internal surface of the ring member fits in a locking relationship on the distal end of the prosthetic stem.

Preferably, the prosthetic stem is a cannulated stem with a through canal for introduction of cement. Preferably, the prosthetic stem has a shoulder at its distal end such that a distal portion of the stem is of reduced diameter.

Preferably, the centraliser is disposed on the distal portion of the prosthetic stem with the reduced diameter such that the external surface of the centraliser is generally flush with the surface of the prosthetic stem.

The locking relationship between the prosthetic stem arid the centraliser may be an interference fit. Alternatively, the locking relationship between the prosthetic stem and the centraliser may be a taper lock. A taper angle of 5 to 20 degrees may be used. Optimally, the taper angle is between 6 and 14 degrees.

Preferably, the fins are arcuate. Preferably, the thickness of the fms is minimised to reduce head losses when cement comes into contact with the fins. Preferably, the centraliser has three or four fins.

Preferably, the centraliser is formed of PMMA (polymethylmethacrelate). A plasticiser may be used in the centraliser to allow slight deformation of the centraliser.

According to a second aspect of the present invention, there is provided a centraliser for use with a prosthetic stem wherein the centraliser comprises an open- ended cylinder with a plurality of fins extending radially from the external surface of the cylinder and wherein the internal surface of the cylinder is adapted to engage in a locking relationship with a distal end of a prosthetic stem.

An embodiment of a prosthetic stem and 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 perspective view of a tip of the prosthetic stem with a centraliser;

Figure 2 is a partially transparent view of the prosthetic stem without the centraliser; Figure 3 is a first perspective view of the centraliser; and

Figure 4 is a second perspective view of the centraliser.

Referring to the drawings, a prosthetic component in the form of a femoral hip stem 4 is provided in combination with a centraliser 2 disposed at the distal end 14 of - the stem 4.

The stem 4, in this embodiment, is a cannulated hip stem although the hip stem could equally be non-cannulated. The cannulation takes the form of a canal 22 which extends from the base 26 of the neck 28 of the hip stem 4 to the distal end 14 of the stem 4 through the longitudinal axis of the stem 4.

The distal end 14 of the stem 4 has a shoulder 16 at which the diameter of the stem 4 decreases suddenly such that a distal portion 32 of the stem 4 has a smaller diameter.

The centraliser 2 has a body portion 6 in the form of an open-ended cylinder or elongated ring. A plurality of fins 8 extend radially from the external surface 34 of the body portion 6. In the illustrated embodiment, four fins 8 are provided equally spaced around the circumference of the centraliser 2. However, three or more than four fins 8 can also be used.

The fins 8 are arcuate or generally semi-circular such that a curved edge 10 extends away from the body portion 6 of the centraliser 2. The fins 8 have a generally uniform width. The fins 8 extend longitudinally between the open ends of the body portion 6. The thickness of the fins 8 is minimised to reduce head losses when bone cement comes into contact with the fins 8. The thickness of the fins 8 should be as small as possible to prevent any disturbance of the cement as it passes through the fins 8. On the other hand, the fins 8 should be as strong as possible to avoid breaking when the femoral stem 4 and centraliser 2 is implanted in the bone cavity. A thickness of fins 8 of 1mm is acceptable. The internal surface 12 of the centraliser 2 is adapted to provide a locking fit with the external surface 36 of the distal portion 32 of the stem 4 which has the reduced diameter. The locking relationship may be in the form of an interference fit or a taper lock in which the external surface 36 of the distal portion 32 of the hip stem 4 is tapered and the internal surface 12 of the centraliser 2 has a corresponding taper. An edge 24 of the centraliser 2 abuts the shoulder 16 of the stem 4.

In order to have a solid lock between the tapers of the distal portion 32 of the hip stem 4 and the internal surface 12 of the centraliser 2 it has been shown through experiments that the taper angle can be between 5 and 20 degrees with an optimal angle between 6 and 14 degrees. A 6 degree angle will give an excellent lock but it is difficult to separate the centraliser 2 from the hip stem 4. A 14 degree angle gives an acceptable lock and it is easier to separate the centraliser 2 from the stem.

The centraliser 2 is formed of PMMA (polymethylmethacrelate) as this has nearly the same properties as bone cement which is used to cement hip stems in bone. To improve the friction between the centraliser 2 and the stem 4 and to ensure a good locking mechanism, a plasticiser can be added to the PMMA to make it less rigid thus allowing a slight deformation of the body portion 6 of the centraliser 2 when mating with the distal portion 32 of the stem 4.

The outside diameter of the body portion 6 of the centraliser 2 has a maximum diameter equal to or slightly less than the diameter of the stem 4 at the shoulder 16 so as to allow a smooth flow of cement over the centraliser 2.

In use, the centraliser 2 is attached to the distal portion 32 of the stem 4 such that it abuts the shoulder 16 of the stem 4 and the external surface 34 of the centraliser 2 is flush with the stem 4 above the shoulder 16. The locking relationship between the centraliser 2 and the distal portion 32 of the stem 4 holds the centraliser 2 in place on the stem 4.

The combination of the stem 4 with the centraliser 2 attached is inserted in a cavity in a prepared femur such that the edges of the fins 8 contact the intemal surface of the prepared bone cavity thereby positioning the stem 4 centrally in the cavity. Cement can be introduced through the canal 22 in the stem 4 and the cement fills the cavity from the distal end of the stem 4. The reduced width of the fins 8 provides minimal interruption to the cement flow past the fins 8. As the centraliser 2 is open- ended, there is no restriction to the flow of cement from the distal opening of the canal 22 in the stem 4.

It will be appreciated, that the centraliser 2 can be used with a non-cannulated stem and is therefore adaptable for use with both types of stem.

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

Claims

1. In combination a prosthetic stem and a centraliser, wherein the centraliser comprises a ring member with a plurality of fins extending radially from the surface of the ring member, and wherein the internal surface of the ring member fits in a locking relationship on the distal end of the prosthetic stem.

2. A prosthetic stem and a centraliser as claimed in claim 1, wherein the prosthetic stem is a cannulated stem with a through canal for introduction of cement.

3. A prosthetic stem and a centraliser as claimed in claim 1 or claim 2, wherein the prosthetic stem has a shoulder at its distal end such that a distal portion of the stem is of reduced diameter.

4. A prosthetic stem and a centraliser as claimed in claim 3, wherein the centraliser is disposed on the distal portion of the prosthetic stem with the reduced diameter such that the external surface of the centraliser is generally flush with the surface of the prosthetic stem.

5. A prosthetic stem and a centraliser as claimed in any of the preceding claims, wherein the locking relationship between the prosthetic stem and the centraliser is an interference fit.

6. A prosthetic stem and a centraliser as claimed in any of claims 1 to 5, the locking relationship between the prosthetic stem and the centraliser is a taper lock.

7. A prosthetic stem and a centraliser as claimed in claim 6, wherein the taper lock angle is between 5 and 20 degrees.

8. A prosthetic stem and a centraliser as claimed in claim 7, wherein the taper lock angle is between 6 and 14 degrees.

9. A prosthetic stem and a centraliser as claimed in any of the preceding claims, wherein the fins are arcuate.

10. A prosthetic stem and a centraliser as claimed in any of the preceding claims, wherein the thickness of the fins is minimised to reduce head losses when cement comes into contact with the fins. Preferably, the centraliser has three or four fins.

11. A prosthetic stem and a centraliser as claimed in any of the preceding claims, wherein the centraliser is formed of PMMA (polymethylmethacrelate).

12. A prosthetic stem and a centraliser as claimed in claim 10, wherein a plasticiser is used in the centraliser to allow slight deformation of the centraliser.

13. A centraliser for use with a prosthetic stem wherein the centraliser comprises an open-ended cylinder with a plurality of fins extending radially from the external surface of the cylinder and wherein the intemal surface of the cylinder is adapted to engage in a locking relationship with a distal end of a prosthetic stem.