GB2201972A

GB2201972A - Improvements in prostheses

Info

Publication number: GB2201972A
Application number: GB08705637A
Authority: GB
Inventors: Gary Cooper; Shirley Ann Hamilton
Original assignee: TECH NI PLANT Ltd
Current assignee: TECH NI PLANT Ltd
Priority date: 1987-03-10
Filing date: 1987-03-10
Publication date: 1988-09-14
Also published as: GB8705637D0

Abstract

The wear resistance of a batch of objects, e.g. prostheses made of titanium alloy, is improved by mounting the objects on a platform of a jig, the platform being movable, eg. tiltable, and moving the platform relative to a beam of light ions, eg. nitrogen, so that the objects are ion-bombarded to the same degree.

Description

WEAR RESISTANT OBJECTS It is known to make a medical object such as a hip prosthesis or a knee prosthesis of a lightweight alloy such as a titanium alloy. The titanium alloy can have alloying amounts of elements such as aluminium and vanadium. A bearing cup e.g. an acetabular cup formed of high density polyethylene, is placed over the head of the prosthesis.

Unfortunately it has been observed that in use there is a tendency for wear to occur between the components. This wear can lead to problems.

It has been proposed to increase the wear resistance of the alloy item by ion implantation using nitrogen. This has not yet become a practical proposition because the process gives a mottled and discoloured appearance to the prosthesis and this has deterred surgeons from installing treated titanium alloy items in the body of a patient.

It is one aim of this invention to provide a method of treating an object made of titanium alloy so as to provide the object with improved wear resistance and at the same time with a uniform appearance.

According to one aspect of this invention there is provided a method of increasing the wear resistance of an article made of titanium alloy, characterised by bombarding the surface of the article with a beam of light ions having energies in the levels of from about 10 KeV to about 200 KeV so as to form a uniform concentration of reaction products therein.

Preferably the ions are of nitrogen.

The amount of nitrogen introduced into the article determines the extent to which the nitrogen reacts with the elements present in the titanium alloy and this in turn determines the colour of the treated article. According to a preferred feature of the invention the treatment is arranged so that whatever concentration of nitrogen is introduced, that concentration is substantially uniformly distributed throughout the surface of the article, so giving the article a uniform appearance. With increasing concentration of surface fluence above 1015ions/cm2, the colour tends to go from metallic to straw to gold to blue.

Which colour is adopted is a matter of choice, often that of the surgeon; all that matters is that a treated object will be of a different colour from an untreated object.

In a much preferred feature of the invention, a number of prosthesis heads are treated together in a suitable jig.

The jig preferably comprises a platform movable towards and away from a station housing a single source of bombarding ions, the platform having a plurality of supports on each of which may be mounted an article to be treated. Means are provided for rotating the supports in unison and for moving the platform with respect to the horizon, e.g. to tilt it.

In this way, all the articles may be treated together, and the same surface of each may be subjected to a predetermined level of treatment. In use, there is a tendency for the object being bombarded to heat up, and it is preferred that this be controlled to be below about 1500C.

The article may be of any shape and size and is preferably generally ball shape. It is a preferred feature of the invention that the method is applied to a ball shaped head of a prosthesis comprising the head and a separate shank.

In order that the invention may be well understood it will now be described, by way of example only, with reference to the accompanying drawings, in which Figure 1 is an elevation of apparatus according to the invention, Figure 2 is a plan view of the platform of the jig, Figure 3 is a sectional view of the jig, Figure 4 is a sectional view taken on lines IV - IV on Figure 3, and Figure 5 is a sectional view taken on lines V - V on Figure 3.

The apparatus comprises a chamber 1 having a roof 2 and a door 3. A single source of nitrogen ions, not shown, is mounted in the chamber facing the door. A jig 4 is mounted at the forward end of a trolley 5. The jig 4 comprises a platform 6 carrying a number of supports 7, (16 are shown).

On top of the platform are mounted two stepper motors M1,M2, arranged respectively to tilt the platform and to rotate the supports 7, in manner to be explained later.

The supports 7 each comprise an upright stem 8. The base of each stem 8 has a spindle 9 which meshes with an idler gear 10 joined by a linkage 11 to a gear shaft 12. The shaft 12 is connected via bearings to the gear rotation drive 13 connected to motor M2. When motor M2 is actuated, all the stems 8 are rotated to the same extent. A gear train 20 connects motor M1 to the platform 6 to cause the platform to be tilted. The top of the stem 8 has spring fingers 14 adapted to be received in the socket 15 of a ball shaped prosthesis head H.

In use, the heads H are engaged on the stems 8 and the trolley is advanced to cause the jig to enter the chamber 1.

The stems 8 are rotated by power from the motor M2. Ions of nitrogen are caused to bombard the heads H mounted on the supports 7, and the platform 6 is also tilted using motor M1, as required. It is preferred to tilt the platform 6 for a short time duration so that the head portion of the article is directly exposed to the radiation to treat that portion to the same extent as the rest of the article.

The apparatus is water cooled so that the objects are kept below 1500C. In one example, the titanium alloy objects were bombarded with nitrogen ions at about 100 KeV until they were implanted with a surface fluence of about 2 x 1017 ions/cm2 to give the objects an attractive uniform gold coloration. The objects were tested and found to have good wear resistance. Calculations showed that in addition to being lightweight and attractive the prosthesis would have a much prolonged life.

Although the invention has been described in relation to objects for medical use, the invention is applicable to other titanium alloy objects, e.g. for use in aerospace applications or for decoration or ornamentation.

Claims

1. A method of increasing the wear resistance of an article made of titanium alloy, characterised by bombarding the surface of the article with a beam of light ions having energies in the levels of from about 10 KeV to about 200 KeV so as to form a uniform concentration of reaction products therein.

2. A method according to Claim 1 characterised in that the light ions are of nitrogen.

3. A method according to Claim 1 or 2 characterised in that the concentration of light ions is selected to give the treated surface a uniform coloration.

4. A method according to any preceding Claim characterised in that the object is the head of a prosthesis comprising a separable shank and head.

5. A method according to any preceding Claim characterised in that a plurality of objects is treated together on a jig therefor.

6. An article made of titanium alloy characterised in that it has been treated by a method according to any preceding Claim.

7. A prosthesis comprising a head and a separable shank characterised in that the head has been treated by a method according to any of Claims 1 to 6.

8. A prosthesis according to Claim 7 characterised in that the head includes a socket to receive a portion of the shank.

9. A method of increasing the wear resistance of an article made of titanium alloy, substantially as described herein.

10. A jig for use in exposing a number of articles to substantially the same level of radiation simultaneously from a source disposed to one side of the jig, comprising a platform having a plurality of supports each to receive one of the articles, means for rotating the supports at substantially the same rate and means for tilting the horizon of the platform so as to orient end portions of the articles to the radiation.

11. A jig according to Claim 10 characterised in that the articles each have a substantially ball shape, and have a socket portion, and the support includes engaging means which is received in the socket portion to engage each article and the respective support.

12. A jig according to Claim 10 or 11 characterised in that the supports are linked by a common gear mechanism to a motor arranged to rotate the supports.

13. A jig according to any of Claims 10 to 12 characterised in that a motor is present to cause the platform to be tilted for a short time duration and is arranged to tilt the platform while the supports are being rotated.

14. A jig according to any of Claims 10 to 13 characterised in that the radiation is a beam of light ions and the jig is transportable to move relative to a chamber in which the radiation is present.

15. A jig for use in exposing a number of articles to radiation, substantially as described with reference to the drawings.