WO1997036708A1

WO1997036708A1 - Spark erosion electrode

Info

Publication number: WO1997036708A1
Application number: PCT/NZ1997/000044
Authority: WO
Inventors: Ian Alexander Anderson; Harry Christopher Clark; John Martin Patrick Sullivan; Timothy Peter Wyatt
Original assignee: Industrial Research Limited
Priority date: 1996-04-03
Filing date: 1997-04-02
Publication date: 1997-10-09
Also published as: AU2523797A

Abstract

A spark erosion electrode (1) can create a contoured surface to facilitate the growth and adhesion of bone tissue on the surface. The electrode (1) includes a collection of parallel electrode members (10), which are preferably tubular. These members (10) are held by a releasable clamp (2) such that they can be moved axially whereby ends of the members (10) can form a negative pattern of a required contour which can then be replicated on the surface of an object such as that of a bone implant.

Description

SPARK EROSION ELECTRODE

FIELD OF THE INVENTION

This invention relates to a spark erosion electrode, and in particular, but not limited to, spark erosion electrodes for creating surfaces which will facilitate the growth and adhesion of bone tissue on the surfaces.

BACKGROUND OF THE INVENTION

Conventional methods for the preparation of the surface of bone implants such that an irregular surface might be provided to permit ingrowth of the bone, utilise various techniques such as laser drilling, application of metallic powder to provide a sintered surface, plasma spray of metallic material onto the surface of an implant and the formation of an irregular pattern through a repetitive masking and chemical milling process.

At least some of these techniques are prone to the disadvantage that the method of formation provides an irregular surface that is subject to breakage and dislocation under stress. Further disadvantages include the formation of random pore sizes and narrow entry channels into the resulting pores and also the methods themselves may leave behind residues antagonistic to the growth of biological tissue. Such residues may be further created by a process of fretting wherein beads or projections in contact with each other on the surface of the implant in use work against each other to create debris or residue. Therefore, it is an object of a preferred embodiment of the present invention to provide an apparatus by which the creation of a surface of an implant may be performed which overcomes at least some of the disadvantages of the prior art and/or at least provides the public with a useful choice.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the invention may broadly be said to consist in a spark erosion electrode for use in the creation of a surface comprising:

a plurality of substantially parallel electrode members;

holding means for said electrode members which in use allows substantially axial movement of said electrode members relative to each other to allow an end face of said electrode to follow a contour of an object; and

clamping means associated with said holding means such that said electrode members may be clamped in said holding means to retain a negative pattern of said contour on the end face.

Preferably said clamping means allows variable clamping pressure on said electrode members in said holding means.

Preferably said electrode members are formed from copper or graphite.

Preferably said electrode members are tubes. Preferably said electrode members allow flushing by electrode discharge machining (EDM) electrolyte.

Preferably the said electrolyte is a hydrocarbon, or synthetic oil

Preferably the electrode members are machined and cleaned prior to incorporation in the electrode thereby, in use, providing for a clean surface on the object eroded.

In a second aspect of the invention, there is provided a method of creating a surface suitable for ingrowth of bone tissue comprising:

providing a plurality of electrode members in a holding means to form a spark erosion electrode;

applying an end of said spark erosion electrode to a surface of a required contour;

allowing the respective ends of said electrode members of said spark erosion electrode to move relative to each other substantially axially thereby following the required contour;

clamping said electrode members in position with said clamping means to retain a negative pattern of the required contour on the end of the spark erosion electrode, fitting said spark erosion electrode to an electrode carrier of a spark erosion device;

bringing said spark electrode into proximity with a surface of a bone implant; and

applying operating current to said electrode to erode the surface of the bone implant to replicate the required contour.

Further aspects of this invention, which should be considered in all its novel aspects will become apparent from the following description given by way of example of possible embodiments thereof and in which reference is made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 : Shows very diagrammatically a side view of a spark erosion electrode according to one possible embodiment of the invention adjacent an irregular surface.

FIGURE 2: Shows diagrammatically a side view of a spark erosion electrode applied to the irregular surface.

FIGURE 3: Shows a side view of the spark erosion electrode reflecting contours of the irregular surface. FIGURE 4: Shows a perspective view of the end of the spark erosion electrode before application to an irregular surface.

FIGURE 5: Shows a perspective view of a cut section of an object with a planar surface subjected to spark erosion.

DETAILED DESCRIPTION OF A

PREFERRED EMBODIMENT OF THE INVENTION

It will be appreciated that the present invention is particularly for use in the creation of irregular surfaces for bone implants. However, it will also be appreciated that the present invention has applications wherever there is a requirement to erode a planar surface or surface of variable contour with said erosion being performed in a predetermined pattern.

A preferred embodiment of the invention will now be described.

This invention involves a spark erosion electrode. Referring firstly to Figure 1 , it shows diagrammatically a side view of a spark erosion electrode 1 according to one possible embodiment of the invention. The electrode members 10 of the electrode 1 are shown held together in a bundle by holding means 2, said holding means having a clamping means 1 1 which may be adjustable such as by screw as to the degree of firmness with which the electrode members 1 0 are held in the holding means 2. The invention provides that individual members 1 0 of the spark erosion electrode 1 may be subjected to substantially axial movement of the electrode members 1 0 relative to each other such that when the end of the spark erosion electrode is imposed on a surface 4 of a variable contour the individual electrode members 1 0 are displaced laterally relative to each other.

Figure 2 shows a side view in which the electrode members 1 0 are displaced relative to each other thereby forming a negative image of the contoured surface of the object 4 in the end face of the electrode.

A side view of the spark erosion electrode 1 when removed from the surface 4 is shown in Figure 3. The electrode members 1 0 of the spark erosion electrode 1 are seen to have been displaced laterally with respect to each other. In this embodiment the ends of the electrode member 1 0 are displaced relatively to the other electrode members 1 0 such that the contours of the surface of the object 4 are represented in the corresponding face of the spark erosion electrode 1 formed by the ends of the electrode members 10.

A perspective view of one end of the spark erosion electrode 1 according to one possible embodiment is represented diagrammatically in Figure 4 showing the electrode members 10 in the form of hollow tubes stacked together to form an array, in which the tube ends form a common face 1 2. Figure 5 shows very diagrammatically a perspective view of a cut away section of a planar surface 6 which has been eroded by the action of a spark erosion electrode according to the present invention. The surface 6 of the material as represented diagrammatically shows a characteristic pattern of circular columns 7 and triangular columns 8 formed by the effect of the erosion of the surface corresponding to the pattern on the face 1 2 of the spark erosion electrode 1 such as shown, prior to contouring, in Figure 4.

It will be appreciated that a spark erosion electrode in the form of a bundle of electrode members held together by a holding device in a manner as to allow lateral displacement of the electrode members relative to each other provides a system whereby the face of the electrode can be made to reflect a negative image of a contoured surface.

A clamping means can retain the negative image of the surface contours on the end of the spark erosion electrode. When the electrode is subsequently fitted into an electrode carrier of a spark erosion device, such that the end of the electrode may be brought into close proximity with the surface of the bone implant prior to initiating electrolyte flow through the electrode and applying the electric current, an efficient and highly controllable method for eroding the surface of a bone implant can be provided.

It will also be appreciated that a system which allows the retention of the negative image of a contoured surface in the face of a spark erosion electrode in use provides a system by which the surface of various objects such as a bone implant can be eroded in a defined pattern and to a defined depth over the face of the contours. The application of the spark erosion method involving the formation of a spark erosion electrode consisting of electrode members wherein the end face of the electrode further provides a negative image of the surface to be eroded and provides a method whereby the erosion pattern can be arranged to allow for optimum physical conditions for bone ingrowth into the surface of an implant.

Furthermore because of the nature of the spark erosion technique allowing for conditions of erosion which do not weaken the projections formed by the spark erosion process an irregular surface can facilitate the ingrowth of bone in which the surface is resistant to shear forces at the implant/bone interface.

The embodiments of the invention as described provide a method of eroding a surface in which the irregularities or projections formed by spark erosion may not in use fret or work against each other to create debris or residue.

It will be further appreciated that the arrangement and geometry of the members used in the electrode is not to be limited in any manner. Depending on the pattern required for erosion, vaπous combinations of round, triangular, square, or oblong tubes, channels or metal rods could be selected to constitute the electrode and provide any required erosion pattern. The electrodes p_er se., such as being hollow, and/or their arrangement, preferably facilitates substantially free flow of an electrode discharge machining (EDM) electrolyte which could be a hydrocarbon or synthetic oil for example While copper and graphite are suitable materials for the electrode members, other materials may also be suitable.

The parameters for operating the spark erosion electrode machine including operating current, voltage, spark gap and the rate of flushing of the electrode by electrolyte may be varied according to normal operating principles of spark erosion electrodes. Such variation in control parameters allow for variation in the degree of overhang of the projections and the texture of the surface of the projections. Such variations in the normal settings of spark erosion electrode machines allow the opportunity to provide specifically designed surfaces for bone ingrowth.

It will be appreciated that while the spark erosion electrode is directed principally towards eroding surfaces to facilitate the growth and adhesion of bone tissue on the surface it is not restricted to this application in any way. For example, the spark erosion electrode could be adapted for use in the erosion of surfaces in jewellery manufacture or other situations where an eroded surface in which the erosion is required to form a definite pattern is required.

Where in the foregoing description reference has been made to specific components or integers of the invention having known equivalents then such equivalents are herein incorporated as if individually set forth

Although this invention has been described by way of example and with reference to possible embodiment thereof it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention as defined in the appended claims

Claims

CLAIMS:

1 . A spark erosion electrode for use in the creation of a surface comprising: a plurality of substantially parallel electrode members;

2. A spark erosion electrode as claimed in claim 1 , wherein the clamping means allows variable clamping pressure on said electrode members in said holding means.

3. A spark erosion electrode as claimed in claim 2, whereby a screw member provides said variable clamping pressure.

4. A spark erosion electrode as claimed in any one of the preceding claims, wherein said electrode members are tubes.

5 A spark erosion electrode as claimed in any one of the preceding claims, wherein said electrode members and/or their arrangement allow flushing by an electrode discharge machining (EDM) electrolyte.

6. A spark erosion electrode as claimed in any one of the preceding claims, wherein said electrode members are formed from copper or graphite.

7. A spark erosion electrode as claimed in any one of the preceding claims, wherein said surface comprises that of a bone implant.

8. A spark erosion electrode substantially as herein described with reference to the accompanying drawings.

9. A method of creating a surface suitable for ingrowth of bone tissue comprising:

allowing the respective ends of said electrode members of said spark erosion electrode to move relative to each other substantially axially thereby following the required contour; clamping said electrode members in position with said clamping means to retain a negative pattern of the required contour on the end of the spark erosion electrode;

fitting said spark erosion electrode to an electrode carrier of a spark erosion device;

10. A method of creating an irregular surface suitable for ingrowth of bone tissue substantially as herein described with reference to the accompanying drawings.

1 1 . A bone implant having a surface created by the electrode or method of any one of the preceding claims.