WO1993008770A1

WO1993008770A1 - Assembly of components of an osteoprosthesis

Info

Publication number: WO1993008770A1
Application number: PCT/GB1992/001991
Authority: WO
Inventors: Martin Arthur Elloy
Original assignee: Depuy International Limited
Priority date: 1991-10-30
Filing date: 1992-10-30
Publication date: 1993-05-13
Also published as: AU2805192A; GB9122936D0

Abstract

An assembly of components of an osteoprosthesis, such as a hip implant, comprises a primary prosthesis component (2, 4) for connection to a bone, a secondary component for location on a surface of the primary component by which the configuration of that surface can be altered. One of the components bears, on the surface of that component which contacts the other component, a rib (22), and the other component has formed in its contacting surface a groove (24). The components can be attached to one another by sliding the rib into the groove. The depth of the rib or of the groove is less at one end than at its other end so that the space between the rib and the groove decreases as the rib is slid into the groove. A cotter (29) is provided for location between the rib and the groove, to engage the opposed surfaces of the rib and the groove.

Description

ASSEMBLY OF COMPONENTS OF AN OSTEOPROSTHESIS

This invention relates to an assembly of components of an osteoprosthesis, and to a method of making an osteoprosthesis.

The use of prostheses to address skeletal complaints, such as in bones which have been damaged for example by fracture, and in joints for example arising from an arthritic condition, is becoming increasing common. Such use of prostheses can involve replacement of part or all of a bone or of one or more components of a joint between bones. For example, it is known to treat hip arthrosis by relining the acetabulum with a socket formed of a polymeric material, and by replacing the natural head of the femur with a metal component.

The selection of a prosthesis can depend on for example the geometry of the original natural joint and on the technique selected by the surgeon for locating the component .in situ. For example, in the case of a hip joint prosthesis, different stems must be selected by the surgeon depending on whether the trochanter is to be removed prior to insertion of the stem of the component into the medullary cavity of the femur and, if so, on the technique for its reattachment, or whether the connection between the stem of the stem and the femur is to involve the use of a cement. In particular, it is necessary to select a stem which has an appropriate geometry for the joint of which components are being replaced.

It is known from US-4904269 to incorporate wedges on the anterior and posterior surfaces of the stem of a hip joint prosthesis, each of which contacts the stem on one surface, and the surface of the medullary cavity on the other surface, to locate the stem in the cavity between the surfaces thereof. The wedges are attached to the stem by means of ribs on the anterior and posterior surfaces, which engage corresponding grooves formed in the surfaces of the wedges which engage the component. Each of the ribs and of the grooves is dove-tailed, so that it has a cross-section such that it is narrower at its base than at a point above its base, to ensure positive mating engagement.

It is essential that there is no movement between components of a prosthesis since such movement can cause fretting, leading to the creation of small particles which can give rise to an adverse tissue reaction, or cause wear of the joint and therefore interfere significantly with movement of the joint, or both. The present invention provides an assembly of components of an osteoprosthesis, which relies on a cotter positioned between the components when they are assembled, in order to minimise movement between the components.

Accordingly, in one aspect, the invention provides an assembly of components of an osteoprosthesis, which comprises:

(a) a primary prosthesis component for connection to a bone;

(b) a secondary component for location on a surface of the primary prosthesis component by which a characteristic of that surface of the primary prosthesis component can be altered,

one of the primary prosthesis component and the secondary component bearing, on the surface of that component which contacts the other component, a rib which is narrower at its base than at a point above its base when viewed in cross- section, and the other of the primary prosthesis component and the secondary component having formed in its contacting surface a groove which has a cross-sectional configuration corresponding to that of the rib so that the primary prosthesis component and the secondary component can be attached to one another by sliding the rib and the groove relative to one another to locate the rib in the groove; the depth of the rib or of the groove being less at one end than at its other end so that the space between the rib and the groove decreases as the primary prosthesis component and the secondary component are moved relative to one another to attach the secondary component to the primary prosthesis component; and

(c) a cotter which can be located between the primary prosthesis component and the secondary component in the said groove, to engage the opposed surfaces of the rib and the groove.

The assembly of •^■ the invention has the advantage that the primary prosthesis component and the secondary component can be located relative to one another and held in that location during use of the prosthesis, so that the possibility of fretting of the component during use leading to the formation of small particles of the material of one of the components is minimised. By using an accurately engineered cotter, a secure connection between the components can be formed relying on the frictional forces between the cotter and the opposed surfaces of the rib and the groove respectively, and the reliability of the connection can be controlled by applying a force on assembly which is greater than any force exerted on the assembly during that use.

The connection can be enhanced when suitable materials are selected for the cotter and for the first and second components of the assembly as a result of the metallurgical phenomenon of galling.

The connection can be enhanced by forming at least one of the components or the cotter at least partially from a shape memory alloy which exhibits a shape memory effect. A shape memory alloy article can be supplied in a first configuration in which it remains until the temperature of the article exceeds that at which the alloy begins to transform from the martensite phase to the austenite phase. As a result of appropriate working of the article, the phase change can be accompanied by a change in configuration of the article. Shape memory alloys can be based on, for example, alloys of nickel and titanium, optionally with other elements such as copper, iron, niobium and vanadium.

In another aspect, the invention provides a method of making an osteoprosthesis, which comprises:

(a) providing an assembly as defined above;

(b) positioning the cotter in the said groove; and

(c) moving the primary prosthesis component and the secondary component relative to one another with the rib located in the groove to a position in which the cotter can contact the opposed surfaces of the rib and the groove to attach the components to one another.

In the method of the invention, it can be advantageous for the cotter to be positioned in the groove before the rib is located in the groove and the components are assembled together. In this way, the number of components to be assembled can be reduced, thereby simplifying assembly of the prosthesis. Alternatively, however, the primary and secondary components may be assembled together, with the rib located in the groove, prior to positioning the cotter in the groove. In this way the force required to position the components properly relative to one another can be kept low, and they can then be maintained in that position by inserting the cotter into the groove.

If the cotter or one or both of the components of the prosthesis is formed from a shape memory, the method of the 'invention will include a step of causing the temperature of the component to increase so that the configuration of the cotter or the component changes to tighten the connection between the primary and secondary components.

The osteoprosthesis provided by the present invention can be used to replace a bone or a part of a bone or in a procedure performed on a bone, for example in the fixation of a fracture. It can also be used in the repair or replacement of a joint, for example involving replacement of one or more components of a joint between bones or in a procedure performed on such a component. The osteoprosthesis finds particular application in the treatment or replacement of a skeletal joint which is affected by a condition, for example by arthrosis, such as in a hip or knee joint.

Generally the thickness of the cotter is greater at one end than at its other end. It is particularly preferred that the reduction in thickness of the cotter from one end to the other corresponds to the reduction in the distance between the rib and the groove. This has the advantage that opposite surfaces of the cotter can contact opposed surfaces of the rib and the groove respectively at spaced apart portions along the length of the cotter, allowing appreciable frictional forces to act between the primary prosthesis component and the secondary component through the cotter, to hold the primary prosthesis component and the secondary component together.

It can be preferred to minimise frictional forces other than those between the cotter and the primary and secondary components, to minimise the force required to assembly the components. In this way, the force which is applied to the components on assembly, and therefore also the force required to separate the components from one another, can be controlled more accurately. The frictional forces can be minimised by use of a lubricant on the relevant surfaces of the components, particularly on the rib and the groove. For example, the relevant surfaces may be coated with a layer of a material such as polytetrafluoroethylene.

The cotter is preferably arranged to be positioned and retained in position in the groove in one of the primary prosthesis component and the secondary component as the case may be. This can allow a cotter to be assembled in the groove ready for the two components to be assembled with the rib fitting into the groove. On such assembly, the cotter can be released for movement along the groove.

The cotter may be retained in the groove by being wider at one end than at its other end. This is preferably achieved by means of a longitudinal slit in the cotter. Alternatively or in addition, a retaining plug may be provided in the groove to act between the cotter and the groove. The plug may take the form of a post of a plastic such as polyethylene which is inserted into the groove through a hole in its wall to contact the cotter. The plug may then be deformed by the cotter when it is caused to move on assembly of the components.

It is preferred that location of the cotter within the groove of one of the components^"be arranged by use of a cotter which is bent, generally about an axis parallel to the surfaces which contact the surfaces of the rib and the groove. The cotter may be provided with a point of weakness, such as a groove or a slit, partway along its length to facilitate the formation of a bend. A bent cotter can be fitted into the groove towards its base and retained there by contact with the base and internal walls of the groove. When the components of the assembly are assembled, the cotter can be deformed from its bent configuration towards a straight configuration, so that it is able to slide along the space between, the rib and the groove, and to contact the surfaces of the rib and the groove along more of its length.

The cotter may be formed at least in part from a shape memory allow, and may be retained in the groove, and then released to slide along the groove, as a result of a change in configuration as a result of a change in phase of the alloy. For example, the cotter may be supplied in the martensite phase of the alloy with a bend along its length. The cotter may be made to straighten as a result of an increase in temperature of the cotter which causes the alloy to transform to its austenite phase.

The cotter may be formed at least in part from a shape memory alloy, made to engage the rib and the groove, or to engage the rib and the groove more securely, as a result of a change in configuration as a result of a change in phase of the alloy. For example, the cotter may be supplied in the martensite phase of the alloy in a stretched, and thinned configuration. The length of the cotter may be made to decrease, and its thickness to increase to contact the surfaces of the rib and the groove, as a result of an increase in temperature of the cotter which causes the alloy to transform to its austenite phase.

The cross-section of the rib and the groove will generally correspond to the extent that the rib can fit into the groove by sliding the rib into the groove, so that the rib cannot be removed from the groove other than by reversing the sliding action, at least when fully inserted into the groove. To this end, the rib is narrower at its base than at a point above its base when viewed in cross-section and the groove has a corresponding cross-section which may be substantially identical. A preferred example of such a cross-section is a dove-tail cross-section, in which the rib (and preferably also the groove) flares gradually outwardly from a point at or towards its base towards a point above its base. Another suitable rib might be one with a T-shaped cross-section. The cross-section such a rib can be considered in terms of the included angle between the surfaces of the rib which engage the corresponding surfaces of the groove. A rib with a T-shaped cross-section can be considered to have an included angle of 180°. The forces by which the rib and the groove engage one another between the opposed surfaces, and the forces acting between the rib and the groove through the cotter, can be adapted by use of a rib (with corresponding groove) with an appropriate included angle. Frictional forces between the cotter, and the rib and the groove can be maximised by selecting rib with a large included angle, for example, an included angle as close as possible to 180°, for example at least about 120°, preferably at least about 145°, especially at least about 160°.

Frictional forces between the opposed surfaces of the rib and the groove can be maximised by use of a rib with a relatively small included angle. It can therefore be preferred for some applications to use a rib which has a dove-tailed cross-section with an included angle which is less than about 150°, preferably less than about 120°, especially less than about 95°. Included angles as small as 40° or less can also find application.

It is preferred that the space between the rib and the groove be arranged to decrease by virtue of the depth of the rib being less at one end than at the other end. This has the advantage that of ease of manufacture.

The reduction in depth of the rib or the groove may be gradual so that it is tapered uniformly. The angle between the surfaces of the cotter which contact the opposed surfaces of the rib and the groove will be selected to balance the requirements that the cotter be capable of accommodating the tolerances to which the rib and groove are manufactured, and that the forces between it and the surfaces are as close as possible to perpendicular to the direction along which it moves along the groove. The angle will commonly be less than 10°, for example in the range 1 to 5°, preferably about 3° .

The materials of the assembly will, or course, be selected according to requirements of biocompatibility placed upon them in the application for which the prosthesis is intended to be used. Frequently, the materials of the 'primary prosthesis component and the secondary component will be the same; the material of the cotter may be the same as the material of one or each of the first and second materials.

Commonly used materials for certain prostheses include certain stainless steels, titanium alloys and cobalt-chrome alloys.

It can be preferred for many applications to select a material for the cotter which is prone to exhibit galling when the cotter is rubbed against a surface. Suitable materials include titanium alloys.

The primary prosthesis component of the assembly may comprise a part which is arranged for introduction into a bone cavity. For example, the primary prosthesis component may be the stem of a hip joint prosthesis, of which a tapered end part is for insertion into the medullary cavity in the femur. The secondary component may be a wedge member for location between a surface of the primary prosthesis component and an adjacent surface of the bone cavity to contact those surfaces so as to locate the primary prosthesis component in the cavity. For example, when the primary prosthesis component is the stem of a hip joint prosthesis, secondary components in the form of wedges may be provided on the anterior and posterior surfaces of the stem to locate the stem against the surfaces of the medullary cavity, and to support the stem in the cavity anteriorly and posteriorly. The wedges may be the same as one another or they may be different, to accommodate the structure of the cavity of the patient to whom the prosthesis is to be fitted.

The secondary component may be one which is fitted to the primary prosthesis component in order to adapt the primary component for use in a selected surgical technique. For example, it may have on its outer surface a porous material which facilitates the ingrowth of tissue to form a connection between the tissue and the secondary component. For example, the porous material may be provided by a layer of a porous polymeric material. The secondary component may have a plurality of openings extending through it for attachment to it by means of sutures of soft tissue. The secondary component may have a formation such as a flange by which flow of a fluid such as a cement or other bonding agent can be controlled. Secondary components which can be attached to the primary prosthesis component by the technique of the present invention are disclosed in the application which claims priority from UK patent application no. 9122937.7, filed with the present application, bearing agents' reference P21250. In that document, a femoral prosthesis is disclosed in which a rib or a groove is provided on a primary component, extending in a direction from a distal portion for insertion into the medullary cavity of a femur towards a proximal portion for engaging an acetabulum. Subject matter disclosed in that document is incorporated in the specification of the present application by this reference.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 (a) and (b) are elevations of a stem of a hip joint prosthesis viewed anteriorly and laterally, and Figure 1 (c) is a cross-section through the stem shown in Figure 1 (a) on the line A-A;

Figure 2 is a part sectional view of a rib formed on the surface of the stem shown in Figure 1 and of a mating groove in which the rib can be received;

Figures 3 (a) , (b) and (c) are part sectional views of other forms of ribs and cooperating grooves;

Figures 4 (a) , (b) and (c) are isometric views of cotters for use in the assembly of the invention;

Figure 5 (a) is a plan view of a secondary component which has a groove formed in one surface, and Figure 5 (b) is a sectional elevation through the secondary component on the line defined by the opening of the groove; and

Figures 6 (a) to (d) are cross-sections through first and second components of a prosthesis during assembly of the prosthesis.

Referring to the drawings, Figures 1 (a) to (c) show a stem for a hip joint prosthesis, which has a head 2 on which a part can be fitted to be received in the acetabulum, and a tapered end part 4 for insertion into the medullary cavity of the femur.

The stem has two ribs 6 formed on its anterior and posterior surfaces. Each of the ribs is deeper at one end than at its opposite end. The ribs 6 can each receive a secondary component in the form of a wedge member for location between the surface (anterior or posterior as the case may be) of the stem and an adjacent surface of the medullary cavity to contact those surfaces so as to locate the stem anteriorly and posteriorly in the cavity. In another embodiment, an additional rib may also be provided on the superior shoulder 10 of the ste , between the head 2 and the tapered end part 4. The additional rib can receive a secondary component which might adapt the stem for use in a selected surgical technique; for example, it might have on its outer surface a porous material which facilitates the ingrowth of tissue to form a connection between the tissue and the secondary component, or it might have a plurality of openings extending through it for attachment to it by means of sutures of soft tissue.

While Figure 1 shows a primary prosthesis component in the form of a stem of a hip joint prosthesis bearing ribs, for receipt in grooves in a secondary component, the grooves could of course be provided in the primary component, which might receive a rib provided on the secondary component. In each of the examples of suitable ribs 22, shown in Figures 2 and 3 of the drawings with their cooperating grooves 24, which might be provided on the first and second components of the assembly, each is narrower at its base 26 than at a point 28 above its base. Such a configuration ensures that the components of the assembly can be attached to one another by sliding the rib into the groove, so that the components cannot be separated from one another other by reversing the sliding action. In particular, the components cannot be separated by lifting one component from the other. In the embodiment shown in Figure 2, the rib (and the corresponding groove flares gradually outwardly from a point at or towards its base towards a point above its base, so that it has a dove-tail shape when viewed in cross-section.

In each of the embodiments shown in Figure 3, a cotter 29 is provided between the rib and the groove, by which the primary and secondary components are connected to one another. In the embodiment shown in Figure 3 (a) , the transition from a narrow cross-section to a broad cross-section is essentially step¬ like, and in the embodiment shown in Figure 3 (b) , the transition is generally rounded. In the embodiment shown in Figure 3 (c) , each of the rib and the groove has a rounded cross-section.

Figure 4 (a) shows a cotter 32 which is suitable for use in the assembly of the invention. It is generally tapered along its length, from one end 33 towards its other end 34. The taper applied to the cotter corresponds to the reduction in space between the rib and the groove from one end towards the other, as a result of the reduction of the depth of the rib or the groove or both.

The cotter 35 shown in Figure 4 (b) is tapered, and has a transverse groove 36 formed in one of its. surfaces 37. The groove defines a point of weakness, about which the cotter is bent. The cotter 38 shown in Figure 4 (c) has an opening 39 at its shallow end 40, which defines two longitudinally extending fingers 41, 42. The fingers are splayed outwardly, so that the width of the cotter is greater at the narrow end 40 than at the wider end 44.

Each of the embodiments of cotters shown in Figures 4 (b) and (c) can be positioned in a groove in one of the primary prosthesis component and the secondary component, and retained in position temporarily be contact between the cotter and the internal surfaces of the groove. In the case of the cotter shown in Figure 4 (b) , contact is between the base and opposite surface of the groove on the one hand, and the grooved surface 37 and the opposite surface of the cotter. In the case of the cotter shown in Figure 4 (c) , contact is between the fingers 41, 42 and the side surfaces of the groove.

Figure 5 shows a secondary component in the form of a wedge 52 for mounting on the side surface of a stem of a hip joint prosthesis. The wedge is thinner at one end 54 than at its other end 56, and has formed in the surface 57 which contacts the stem of the prosthesis a groove 58 which extends between the ends 54, 56 of the wedge. The groove is wider at its base 58 than at the opening on the surface 57, and has a generally dove-tail shape when viewed in cross-section. A hole is provided in the wedge, on the line defining one edge of the groove. A quantity 62 of a polymeric material is inserted in the hole, which can contact a cotter inserted into the groove, to retain the cotter in position in the groove prior to assembly of the wedge on the stem. This arrangement can be used with a cotter such as that shown in Figure 1 (a) referred to above.

Figure 6 shows a primary prosthesis component 72, which might be for example a stem of a hip joint prosthesis, on which is formed a rib 74. The depth of the rib is greater at one end 76 than at its other end 78. A secondary component 80, which might be for example a wedge for mounting on the side of the stem to locate the stem anteriorly or posteriorly in the medullary cavity of a femur, has a groove 82 formed in it, the depth of the groove being substantially constant along its length. Each of the rib and the groove has a cross-section which is dove-tailed, so that the groove can be fitted onto the rib by sliding the primary and secondary components relative to one another. In the absence of a cotter, full insertion of the rib into the groove would result in a space between the two, whose depth would reduce gradually from one end of the groove towards its other end. The cotter obstructs insertion of the rib into the groove, not because of the material which is present since its taper corresponds to the reduction in depth of the space, but because of the bend which is imparted to the cotter.

A cotter 84 is provided in the groove in the secondary component. The cotter is tapered along its length, the taper corresponding to the reduction of the depth of the rib. The cotter might have any of the other features of the cotters shown in any of Figures 4 (a) to (c) referred to above, and may be retained in the groove in the manner described above with reference to those Figures or to Figure 5. However it is retained, it is preferred that the cotter be held in position by frictional forces against movement along the groove during transportation and prior to installation of the secondary component on the primary component. At this stage, as shown in Figure 6 (a) , the cotter is at the end 86 of the groove which first receives the shallow end 78 of the rib 74. As the secondary component is moved towards the primary component and the shallow end 78 of the rib is received in the groove, the cotter 84 engages the rib. The cotter moves with the secondary component relative to the rib until, as shown in Figure 6 (b) , it contacts both of the opposed surfaces of the rib and the groove and the frictional forces between the cotter and those surfaces exceed the frictional forces between the cotter and the internal surfaces of the groove, by which the cotter has been retained in the groove. Continued movement of the secondary component relative to the primary component, towards the position shown in Figure 6 (d) causes the cotter to slide further along the groove. By controlling the force by which the primary and secondary components are moved relative to one another, as shown in Figures 6 (b) to (d) , control is possible of the force which will be required to overcome the frictional forces between the cotter and the opposing surfaces of the rib and the groove, to separate the components. This might be achieved by means of a tool designed specifically for use with the components of the invention.

Claims

CLAIMS :

1. An assembly of components of an osteoprosthesis, which comprises:

(a) a primary prosthesis component for connection to a bone;

2. An assembly as claimed in claim 1, in which the thickness of the cotter is greater at one end than at its other end.

3. An assembly as claimed in claim 2, in which the reduction in thickness of the cotter from one end to the other corresponds to the reduction in the distance between the rib and the groove so that opposite surfaces of the cotter contact opposed surfaces of the rib and the groove respectively at spaced apart portions along the length of the cotter.

4. An assembly as claimed in any one of claims l to 3, in which at least one of the surfaces of the cotter which contacts a surface of the rib or the groove, or of the surfaces of the rib and the groove which contact the cotter, is rounded.

5. An assembly as claimed in any one of claims 1 to 4, in which at least part of the primary prosthesis component is arranged for introduction into a bone cavity.

6. An assembly as claimed in claim 5, in which the secondary component is a wedge member for location between a surface of the primary prosthesis component and an adjacent surface of the bone cavity to contact those surfaces so as to locate the primary prosthesis component in the cavity.

7. An assembly as claimed in any one of claims 1 to 6, in which the secondary component has on its outer surf ce a porous material which facilitates the ingrowth of tissue to form a connection between the tissue and the secondary component.

8. An assembly as claimed in claim 7, in which the porous material on the surface of the secondary component is provided by a layer of a porous polymeric material.

9. An assembly as claimed in any one of claims l to 8, in which more than one rib or groove is provided on the primary prosthesis component for attachment thereto of more than one secondary component.

10. An assembly as claimed in any one of claims 1 to 9, in which the depth of the rib is less at one end than at its other end.

11. An assembly as claimed in anyone of claims 1 to 10, in which at least one of the components or the cotter is formed at least partially from a shape memory alloy.

12. An assembly as claimed in claim 11, in which the cotter is formed from a shape memory alloy, and has been deformed so as to increase its length and to decrease its thickness, while in the martensite phase of the alloy.

13. A hip joint prosthesis, which comprises an assembly of components as claimed in any one of claims 1 to 10, in which the primary prosthesis component comprises a stem for insertion into the medullary cavity of a femur

14. A method of making an osteoprosthesis, which comprises:

(a) providing an assembly as claimed in any one of claims 1 to 12;

(b) positioning the cotter in the said groove; and

(c) moving the primary prosthesis component and the secondary component relative to one another with the rib located in the groove to a position in which the cotter can contact the -opposed surfaces of the rib and the groove to attach the components to one another.

15. A method as claimed in claim 14, in which step (b) is carried out before step (c) .