WO1994028827A1

WO1994028827A1 - A prosthesis component

Info

Publication number: WO1994028827A1
Application number: PCT/GB1994/001212
Authority: WO
Inventors: Boguslaw Michael Wroblewski
Original assignee: Depuy International Ltd.
Priority date: 1993-06-05
Filing date: 1994-06-03
Publication date: 1994-12-22
Also published as: ZA943885B; GB9311685D0; AU6852794A

Abstract

A prosthesis component which is to be implanted in a patient and retained in situ by means of an adhesive bond between a bonding surface of the component and another surface, in which the bonding surface of the component has an array of discrete formations (3) formed on it. The configuration of the formations (3) can be generally part-spherical. The formations (3) enhance the connnection between the bonding surface of the component and the surface to which the surface is to be bonded.

Description

A PROSTHESIS COMPONENT

This invention relates to a prosthesis component which is to be implanted in a patient, and retained in situ by means of an adhesive bond.

Prosthesis components which are implanted in a patient and retained in situ by means of an adhesive bond include a prosthesis acetabular cup, which is used in hip replacement to line the pelvis to receive the head of a prosthetic femur head. The cup is generally made from a polymeric material, such as ultra high molecular weight polyethylene.

The present invention provides a prosthesis component which is to be implanted in a patient, and retained jLn situ by means of an adhesive bond, which has a plurality of formations formed on its bonding surface.

Accordingly, in one aspect, the invention provides a prosthesis component which is to be implanted in a patient, and retained in situ by means of an adhesive bond between a bonding surface of the component and another surface, in which the bonding surface of the component has an array of discrete formations formed on it.

The provision of discrete formations on the surface of the component enables a secure connection to be made between the component and the said other surface, for example a surface of a bone or of another implanted component. This has the advantage of greater stability in the connection, which can be of particular significance when the connection has to bear loads, as in the case of, for example, a hip joint, when the component in question might be the acetabular cup component.

The bonding surface will generally be that part of the surface of the component which is required to be bonded to the surface of the other component for the connection to be stable. Preferably, the formations will be provided in an array over substantially the entire area of the bonding surface. It can be contrasted with the provision of a groove for receiving marker wires in acetabular cups which, while possibly enhancing a bond to some degree, extends only across the surface along one axis, or possibly along two perpendicular axes.

The formations can be recessed below the bonding surface of the component, in the manner of depressions, or they can stand proud of the bonding surface. Depressions are generally preferred because of the characteristics of the bond between the bonding surface and another surface that can result.

The bonding surface on which the formations are provided may be provided by a portion only of the entire surface of the component which is in contact with the other surface to which the bond is to be made. For example, formations may be provided in a band which extends across the surface of the component. When the bond between the component and the other surface is subject to a torque, the band may extend around the component, in a plane that does not include the axis of the torque, especially a plane that is substantially perpendicular to the axis, for example in an equatorial region of a spherical component.

The provision of discrete formations on the bonding surface of the component has the advantage that the surface of the component to which the bond is formed is increased compared with a surface without formations, providing a stronger bond than would otherwise be possible.

The provision of a plurality of discrete formations on the bonding surface of the component has the advantage that the portion of the surface involved in securing the connection to the other surface is discontinuous, in contrast with a groove provided in the surface. The formations may be substantially discrete in the sense that they are not entirely unconnected; for example formations may be connected by means of a groove, or by a rib. Generally, the groove or rib will be less deep or narrower or both than the formations which it connects.

Formations which are discrete provide good resistance to crack propagation in the component, or in the adhesive by which the component is bonded to the surface of another component, for example as a result of movement giving rise to localised failure of the adhesive.

The provision of discrete formations to enhance the strength of the bond between a surface of a prosthesis component and another surface, instead of one or more grooves, has the advantage that the depth of the formations can be rather less than the depth of the continuous groove(s), while still allowing significant enhancement of the bond strength. This has the particularly significant advantage of allowing the thickness of the material in the component to be kept high. This in turn can allow the component to be made stronger, or to withstand greater wear, or to be machined internally to change its configuration without reducing the thickness of the material of the component unacceptably. Furthermore, the use of relatively shallow depressions has the advantage that adhesive supplied to the depressions can fill the depressions more readily than would be the case of rather a deeper groove.

The depressions may be connected, for example by shallow channels which aid flow of adhesive between adjacent depressions, as long as the channels by which the depressions are connected are significantly less deep than the depressions themselves.

Preferably, the formations on the bonding surface are generally rounded when viewed from above, for example, generally oval or, more preferably, circular. The provision of rounded formations has the advantage of reducing the build up of stress, both within the material of the component and within adhesive used to form the bond between the component and the other surface to which the bond is made.

Preferably, the formations are generally rounded when viewed in cross-section. For example, the formations may define approximately part of the surface of a sphere, although deviations from true sphericity will of course be tolerated for many applications, so that the surface may define for example part of an ovoid. The provision of formations which are rounded when viewed in cross-section has the advantage of reducing build-up of stress, both within the material of the component and within adhesive used to form the bond between the component and the other surface to which the bond is made.

When the formations are defined by part of the surface of a sphere, it is preferred that the value of the ratio of the depth of the formations to the radius of the sphere is less than about 1, more preferably less than about 0.8.

Preferably, the area of the formations where they open onto the bonding surface of the component is less than about 320 mm², more preferably less than about 250 mm², for example about 100 mm². Preferably, that area is greater than about 7.5 mm², more preferably greater than about 15 mm².

The depth of a depression can be considered by measuring the distance from the deepest point of a depression to a line joining the two lowest opposite edge points. Preferably, the depth of the depressions is less than about 4 mm, more preferably less than about 3 mm, for example about 2.5 mm. Preferably, that depth is greater than about 1 mm, more preferably greater than about 1.5 mm.

Preferably, the formations are provided on the bonding surface of the component in a regular pattern. For example, they may be arranged with the distance between a formation and one of its neighbours being approximately equal to the distance between the formation and the other of its neighbours, but for other constraints on the location of the formations such as the location of other features of the component.

An appropriate size of a depression is a function of the area of the bonding surface of the component, the desired number of depressions and the desired aspect ratio of the depressions. It has been found that an appropriate pattern of depressions can be provided on a hemispherical component provided that the number of depressions (N) takes one of a number of values, set by the expressions:

N = 3n

when n = 2, and

N_n = 5(π-l) + N_a_.

when n is an integer greater than or equal to 3

The maximum radius for circular depressions on the surface of an approximately hemispherical component can be calculated using the expression:

r = [ 2 . R - "→

N 2 . TI . R N 2 . TL . R

where

r = the radius of the depression

A = the area of the bonding surface

Ν = the number of depressions, and

R = the radius of the bonding surface (0 < R ≤ oo)

The radius of the depression may be less than that derived from the expression set out above, which will have a result of reducing the proportion of the area of the bonding surface that is provided by the depressions. An appropriate depth of the depressions can be determined from the calculated radius, by setting an appropriate aspect ratio.

The calculation of the configuration of depressions is exemplified by the following data, for a hemispherical surface with 51 depressions on it, and an aspect ratio of 0.8:

Diameter Surface Depression Max radius Max depth (mm) area (mm²) surface depression depression area (mm²) (mm) (mm)

35 1924 38 3.0 2.4

40 2513 49 3.4 2.8

50 3927 77 4.3 3.5

60 5655 111 5.1 4.2

70 7697 151 6.0 4.9

The component may be formed from a polymeric material such as ultra high molecular weight polyethylene. Other materials for the component include metals and polymeric materials reinforced with fibrous materials. The component may be made by, for example, moulding or machining.

The nature of the adhesive used to form the bond between the bonding surface of the component and the other surface will depend on the materials of the bonding surface and the other surface. When the bonding surface is provided by polyethylene and the other surface is of bone, suitable adhesive materials include polymethylmethacrylate cements.

The bonding surface may be generally rounded, for example it may be generally dome-like. The component which provides the bonding surface may be hollow.

The component can be used in hip prosthesis to provide the acetabular cup into which the head of a femur prosthesis can be received. This type of prosthesis is the subject of GB-1510968 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 schematic plan view of an acetabular cup, showing the external bonding surface on which a plurality of formations are provided;

Figure 2 is a schematic side view of the cup shown in Figure 1.

Referring to the drawings, an acetabular cup 1 is hollow and is provided with a rounded outer surface which is generally dome¬ like. The surface has formed in it a number of discrete formations in the form of depressions 3, each of which is substantially circular when viewed from above, and which form part of a sphere when viewed in cross-section.

A pair of shallow grooves 5, 7, which intersect at the top of the sphere, are provided in the surface of the cup. The grooves contain a marker wire 9 made from a metal, which is opaque to X-rays and can therefore provide an indication of wear of the cup during use. The grooves may connect some of the depressions to one another.

The cup has a plurality of cut-outs 11 provided around its base, to be engaged by a tool when the device is to be held against a surface while a bond to the surface is formed.

Claims

CLAIMS :

1. A prosthesis component which is to be implanted in a patient, and retained in situ by means of an adhesive bond between a bonding surface of the component and another surface, in which the bonding surface of the component has an array of discrete formations formed on it.

2. A component as claimed in claim 1, in which the formations are depressions formed in the said bonding surface.

3. A component as claimed in claim 1 or claim 2, in which the value of the ratio of the depth of the depressions to the square root of the area is less than about 1.0.

. A component as claimed in any one of claims 1 to 3, in which each of at least some of the depressions is generally rounded when it is viewed from above.

5. A component as claimed in claim 4, in which the said depressions are circular when viewed from above.

6. A component as claimed in any one of claims 1 to 5, in which each of at least some of the depressions is generally rounded when viewed in cross-section.

7. A component as claimed in claim 6, in which the said depressions define part of the surface of a sphere when viewed in cross-section.

8. A component as claimed in claim 7, in which the ratio of the depth of the depressions to the radius of the sphere is less than about 1.0.

9. A component as claimed in any one of claims 1 to 8, in which the distance- between a first depression and an adjacent depression is approximately equal to the distance between the said first depression and each other depression that is adjacent to it.

10. A component as claimed in any one of claims 1 to 9, in which the bonding surface is generally rounded.

11. A component as claimed in any one of claims 1 to 10, which is for use as an acetabular cup.

12. A component as claimed in any one of claims 1 to 11, which includes at least one groove which extends across the bonding surface, for receiving a wire.