WO1992018067A1

WO1992018067A1 - Acetabular cup

Info

Publication number: WO1992018067A1
Application number: PCT/GB1992/000716
Authority: WO
Inventors: Martin Arthur Elloy; Paolo Gallinaro; Giacomo Masse
Original assignee: Depuy International Limited
Priority date: 1991-04-16
Filing date: 1992-04-16
Publication date: 1992-10-29
Also published as: AU1691292A; GB9108067D0

Abstract

An acetabular cup (16) is provided with improved means for giving at least temporary mechanical anchoring in position of cup (16) in an acetabular socket. The cup (16) comprises a plurality of anchors (19) mounted to the sidewall (18) of the cup. Each anchor is moveable between a retracted position and an outwardly projecting position, in which the anchor is engageable with the wall of the acetabular socket in order to locate the cup in the socket. Each anchor takes the form of a blade (19).

Description

ACETABULAR CUP

This invention relates to an acetabular cup which is a prosthetic device intended to be implanted in a reamed-out socket of the acetabulum. and into which a liner can be fitted which is shaped so as to receive a spherical head of a prosthetic hip joint.

It is known to provide a metal acetabular cup in a part spherical shape, and with a central hole formed in the base of the cup into which can be received the head of an introducer tool used to insert the cup into the acetabular socket. The tool must be used in order to force fit the cup into the socket, and then after removal of the tool it is usual to insert a plastics inner liner into the cup, which serves to receive the head of the prosthetic hip joint.

An acetabular cup can be implanted with use of cement to fill any space between the inner wall of the socket and the outer surface of the cup, but acetabular cups can also be implanted on a cementless basis, as this is considered in some circles to be able to provide a more durable implant over a period of time.

When an acetabular cup is implanted without use of cement, it is necessary for the cup to be capable of anchoring itself reliably into the socket straight away, and with the surfaces of the cup and the socket being closely adjacent to each other so that tissue growth can take place which adheres to the surface of the cup over a period of time in order to further anchor the cup in position.

However, in the period while tissue growth takes place, it is important that the cup should be securely and reliably anchored in position by mechanical means. It is therefore usual to use screws passing through the wall of the cup into bone. Also, it is known to mount some form of anchoring or locking elements in the wall of the cup, and which can be driven out into gripping engagement with the surface of the socket after, or preferably during the course of implantation of the cup into the socket.

It is known from EP249594 to provide an arrangement of pivoted L-shaped anchors in the walls of a conical socket type basket structure, such anchors being capable of being driven out into the surface of a glenoidal cavity by an introducer tool during the use of the tool to implant the cup in the cavity. The anchors have prongs at one end which are located close to the base of the basket in the retracted position and the opposite ends of the anchors are pivotally mounted in the wall of the basket. Upon the introduction of a threaded head portion of the tool into the basket, to engage with a threaded socket in the base of the basket, the tool comes into engagement with the anchors and rotation of the head portion in the threaded socket then causes the tool to apply progressive outward displacement of the prongs of the anchors and into engagement with the wall of the cavity. The intention is that the pronged ends should engage with the side wall of the cavity and thereby draw the basket type device into firm seating engagement in the cavity. However, the L-shape of the anchors, plus the prong shaped ends do not guarantee a reliable anchoring in the wall of the cavity.

Thus, the prong shaped ends have to be thin and pointed, to be able to drive into the bone, and therefore necessarily are somewhat flexible. The pointed tips therefore have a cantilever effect, and with the inherent flexibility of the outwardly projecting prongs, they are liable to become dislodged in time, or to yield and allow the cup to become loosened on its seating.

The present invention has therefore been developed with a view to providing an acetabular cup having improved means for providing at least temporary mechanical anchoring in position of the cup in an acetabular socket. According to the invention there is provided an acetabular cup for implantation in an acetabular socket in order to receive a spherical head of a prosthetic hip joint and which comprises a base, a side wall, and a plurality of anchors mounted in the side wall of the cup and each moveable between a retracted position in which the cup can be positioned in the socket and an outwardly projected position in which the anchor is engageable with the wall of the socket in order to locate the cup in the socket, in which: each anchor comprises a blade housed in a respective axially extending recess in the side wall of the cup and having a first end located nearer to the base and a second end remote from the base, the blade being pivotable about said first end to move between the retracted position and the extended position: and the second end of the blade has a profile such that, with respect to a reference radial plane passing through the pivot axis, the radial separation of the profile from the pivot axis of the blade increases as the blade is pivoted from the retracted position to the extended position.

The shape of the profile of the second end of each blade can take any suitable form, provided that the radial extent of the profile from the pivot axis increases, so that as the profile moves into the bone material defining the acetabular socket, the profile exerts an increasing reaction force from the bone material through the blade and onto the side wall of the cup via the pivotal mounting of the blade in its wall recess, thereby pressing the cup into firm seating engagement in the socket.

The increase in radial extent of the profile may vary uniformly, or non-uniformly.

Preferably, each blade has a thin outwardly facing leading edge which is operable to cut into the wall of the socket with only a small amount of displacement of the bone material, and as the leading edge cuts into the material, the second end profile follows this movement and applies a progressive compressive force on the adjacent bone material which results in the reaction force which causes seating of the cup in its socket.

In order to ease the entry of the blade into the wall of the socket, it is preferred that the leading edge of the blade comprises two laterally spaced blade elements which define a recess therebetween in which bone material can be received as the blade elements move into the bone material. This recess therefore allows movement of the bone material as the blade elements cut into the wall of the socket, and this bone material will be likely to remain joined to the underlying bone material and therefore acts as a "key" or "spline" which provides additional resistance to any tendency of the cup to rotate in its socket.

Conveniently, the profile of the second end of the blade forms a curve centred on a point which is located close to, but inwardly of the pivot axis, so that the profile functions a "cam" as the blade is pivoted outwardly.

This curved profile preferably is defined by a toroidal bearing surface which has a curved cross section, when taken on a radial plane passing through the centre of curvature of the profile. This bearing surface therefore reduces local bearing stress, and thereby avoids damage to the bone material.

The number of anchors mounted in the side wall of the cup is not critical, provided that it is at least two in number. A preferred arrangement is three blades, which are angularly spaced from each other by approximately 90°. This can enable the cup to be lined-up in the acetabulum, with one blade lined-up with the iliac spine, another with the ischium and the third with the pubiε ramus. These lie on approximately orthogonal axes. If a fourth blade is provided, this can be located opposite to the pubis ramus, thereby giving a symmetrical and balanced fixation.

The axially extending recess in the side wall of the cup in which each blade is receive will comprise an axial plane passing through the longitudinal axis of symmetry of the cup.

Preferably one or both sides of each blade and/or the mating surface of the corresponding axial recess are "upset" (knurled) to give an interference fit, whereby each blade tends to be maintained in any given angular position to which it is displaced.

The invention is also concerned with a novel design of introducer tool for use with an acetabular cup according to the invention.

Accordingly, in a further aspect of the invention there is provided an introducer tool for use in implanting an acetabular cup which comprises a base having a threaded tool- engaging portion, a side wall and a plurality of anchors mounted in the wall of the cup and each moveable between a retracted position in which the cup can be positioned in the socket and an outwardly projected position in which the anchor is engageable with the wall of the socket in order to locate the cup in the socket, and said tool comprising: a shank; an operating handle connected to one end of the shank; and, a head connected to the opposite end of the shank and having a threaded member to engage the threaded portion on the base and a pusher member operative to engage the anchors and to displace the anchors outwardly after the threaded member of the head has been tightened on the threaded portion of the base. The pusher member will usually be mounted on the shank after positioning of the cup, to provide improved visual access during the seating of the cup in the acetabular socket.

The introducer tool therefore has a dual function which can be carried out during use with the acetabular cup, in that first the cup can be positioned in the acetabular socket when the threaded member and the threaded portion are first brought into engagement, and then with the cup in this position the anchors are caused to move outwardly into engagement with the wall of the socket.

To assist in giving firm seating of the cup in the socket, a slide hammer may be mounted on the shank and which can be driven downwardly in order to press the cup firmly into the socket, and to drive the anchors outwardly.

The invention will now be described in detail, by way of example only, with reference to preferred embodiments thereof, and in which:

Figure 1 is an illustration of the acetabulum in which an acetabular cup according to the invention can be implanted, and using an introducer tool according to the invention;

Figure 2 is a view in vertical section of an acetabular cup according to the invention;

Figure 3 is a detail view to an enlarged scale of part of the cup shown in Figure 2.

Figure 4 is a side view illustrating the geometry of an outwardly displaceable anchor of the cup;

Figure 5 is a side view to an enlarged scale, showing a first practical construction of anchor in detail; Figure 6 is an end view of the anchor of Figure 5, showing the outer cutting edge thereof;

Figure 7 is a side view similar to Figure 5 of a further detailed construction of anchor;

Figure 8 is an end view of the anchor construction of Figure 7;

Figure 9 is a side view of an introducer tool according to the invention, coupled-up with an acetabular cup to implant the latter in the acetabulum; and

Figure 10 is a detailed view to enlarged scale, of a sliding hammer used during the operation of the tool.

Referring first to Figure 1, this shows a typical illustration of the innominate bone designated generally by reference 10 and which comprises an acetabulum 11 which will be reamed-out to form an acetabular socket in which the acetabular cup can be implanted, an iliac spine 12 and ischium 13 approximately diametrically opposed to each other, the pubis ramus 14 arranged at one end of an orthogonal axis with respect to the axis passing through iliac spine 12 and ischium 13, and there will be a fourth point 15 opposed to the pubis ramus 14, which can provide a symmetrical equi- circumferentially spaced support for an acetabular cup, when the cup is provided with four similarly spaced outwardly displaceable anchors, as discussed in more detail below. However, the number of anchors provided on an acetabular cup according to the invention is not critical, provided that it is at least two.

An acetabular cup according to the invention is designated generally by reference 16 in Figure 2, and is provided with improved means for giving at least temporary mechanical anchoring in position of cup 16 in an acetabular socket. The cup 16 serves to receive a liner (usually plastics) which in turn takes a spherical head (not shown) of a prosthetic hip joint, and comprises a base 17, a part spherical side wall 18 joined to the base 17, and a plurality of anchors 19 mounted in the side wall 18 of the cup and each moveable between a retracted position shown in dotted outline, and an outwardly projected position (shown in full line) in which the anchor is engageable with the wall of the acetabular socket in order to locate the cup in the socket.

Different possible designs of anchor 19 are shown in Figures 5 to 8, by way of example only, but essentially each anchor takes the form of a blade which is housed in a respective axially extending recess 20 formed in the side wall 18 of the cup. and located in an axial plane containing the longitudinal central axis 21 of the cup. Each blade 19 has a first end 22 located nearer to the base 17 than a second end 23, with the blade being mounted in axial recess or slot 20 in such a way as to be pivotable about first end 22 to move between the retracted position shown in dotted outline in Figure 2 and 3, and the outwardly extended position shown in full lines.

The second end 23 of the blade has a profile such that, with respect to a reference radial plane passing through the pivot axis of the first end 22, the radial separation of the profile from the pivot axis of the blade increases as the blade is pivoted from the retracted position to the extended position.

The shape of the profile of the second end 23 of each blade can take any suitable form, provided that the radial extent of the profile moves into the bone material defining the acetabular socket, the profile exerts an increasing reaction force from the material through the blade and onto the side wall 18 of the cup via the pivotal mounting of the blade in its recess 20, thereby pressing the cup into firm seating engagement in the socket.

As shown in Figure 4, the centre of curvature 24 of the curved profile 25 of the second end 23 is laterally and inwardly off-set from the pivot axis of the first end 22. Therefore, as the blade pivots in a clockwise direction about the pivot axis, the radial separation of the profile 25 from the pivot axis increases.

Referring now to Figures 5 and 6, this shows one detailed construction of an example of anchor 19, and Figure 5 is a side view of the wing, and Figure 6 is a view looking in an inward direction onto the outwardly facing edge 26. As can be seen in Figure 6, the edge 26 is formed by a tapering blade edge, which can cut into the wall of the acetabular socket as the wing is displaced outwardly.

The free end 27 of the cutting edge 26 forms the leading cutting edge also for the curved profile 25 of the second end 23, and therefore is followed by the curved profile 25 as the wing is pivoted outwardly.

Figure 5 shows reference plane P-P passing through the pivot axis, and evidently clockwise pivoting of the wing will cause movement of the curved profile 25 through this reference plane, and with an increasing radial extent of any portion of the profile instantaneously as it passes through the reference plane, whereby a compressive force is applied to the bone material which is first cut by the leading edge 27.

As can be seen in Figure 6, the curved profile 25 is formed by a part toroidal surface.

A further detailed example is shown in Figures 7 and 8, but which differs in that the leading edge 26 of the wing or blade comprises two laterally spaced thin and sharp blade elements 28 joined by curved blade edge 28a and which define a recess 29 therebetween in which bone material can be received as the blade elements 28 move into the bone material. Recess 29 allows movement of the bone material as the blade elements 28 cut into the wall of the acetabular socket, and this bone material will tend to remain joined to the underlying bone material and therefore acts as a "key" or "spline" which provides additional resistance to any tendency of the cup to rotate in its socket.

The curved profile 25 may achieve the desired increase in radial extent as in the previously described constructions, by having a laterally off-set centre of curvature as shown in Figure 4. Also, curved profile 25 is formed by a bearing surface which has a curved cross section, when taken on a radial plane passing through the centre of curvature of the profile, and which thereby reduces local bearing stress, and avoids damage being done to the bone material.

The arrangement of the blades in each axial recess is such that each blade is locked when it reaches its fully outwardly deployed position, by reaction between the trailing end of the curved profile 25 of second end 23 and the wall of the recess, and which then passes through the blade to its pivot. This prevents over deployment of the blade, and also defines the outwardly deployed position.

The number of anchors mounted in the side wall 18 of the acetabular cup is not critical, provided that it is at least two in number. However, a preferred arrangement is three anchors or blades, which are angularly spaced from each other by 90°, which then enables the cup to be lined-up in the acetabulum, with one blade lined-up with the iliac spine, another with the ischium and the third with the pubis ramus. However, if a fourth blade is provided, this can be located opposite to the pubis ramus, thereby giving a symmetrical and balanced fixation. Figure 3 shows by reference 30 the bone compression which can be achieved, by virtue of the cam-like or eccentric manner of operation of the wings or blades.

The construction of acetabular cup described above with reference to the drawings provides a technically advantageous and easily manipulated cup which can be implanted firmly in position in the acetabulum, and which can have sufficient anchoring in position to hold the cup during the period while tissue growth takes place in order to further anchor the cup in position.

Figure 9 and 10 of the drawings show a novel design of introducer tool for use in implanting the acetabular cup. The introducer tool is designated generally by reference 31 and comprises a shank 32, a detachable operating handle 33 connected to one end of the shank 32, and a head designated generally by reference 34 in Figure 9, and which comprises a threaded member 35 to engage an internally threaded portion

36 in the base 17 of the cup, and a pusher member 37 operative to engage the anchor blades 19 after the threaded member 35 has been fully tightened on the threaded portion 36.

The acetabular cup therefore is positioned in the acetabulum by use of the introducer tool 31, and then when in this position (during which time slide hammer 38 and pusher member

37 may be detached to provide improved visual access and greater positional control). Slide hammer 38 is then mounted on the shank and the anchor blades are displaced outwardly to engage the wall of the socket and thereby drive the anchors firmly into the socket.

As can be seen in Figure 10, slide hammer 38 comprises part of the introducer tool, and is slidably mounted on shank 32, and carries pusher member 37 at its lower end, which can be driven downwardly into engagement with the inwardly projecting edges of the anchor blades, in order to pivot them outwardly into firm seating engagement with the wall of the acetabular socket. The members 37 can be provided in a variety of sizes to suit different size cups.

Claims

CLA IMS

1. An acetabular cup for implantation in an acetabular socket and adapted to receive a spherical head of a prosthetic hip joint, comprising a base, a side wall and a plurality of anchors mounted in the side wall of the cup, each of which anchors is moveable between a retracted position in which the cup can be positioned in the socket, and an outwardly projected position in which the anchor is engageable with the wall of the socket, in order to locate the cup in the socket in which: each anchor comprises a blade housed in a respective axially extending recess in the side wall of the cup, and having a first end located nearer to the base and a second end remote from the base, the blade being pivotable about said first end to move between the retracted position and the extended position; and the second end of the blade has a profile such that with respect to a reference radial plane passing through the pivot axis, the radial separation of the profile from the pivot axis of the blade increases as the blade is pivoted from the retracted position to the extended position.

2. An acetabular cup according to claim 1, wherein each blade has a thin outwardly facing leading edge which is operable to cut into the wall of the socket, with only a small amount of displacement to the bone material.

3. An acetabular cup according to claim 1 or claim 2, wherein the leading edge of the blade comprises two laterally spaced blade elements, which define a recess therebetween in which bone material can be received, as the blade elements move into the bone material.

4. An acetabular cup according to any one of the preceding claims, wherein the profile of the second end of the blade forms a curve centred on a point, which is located close to. but inwardly of the pivot axis, whereby the profile functions as a "cam" as the blade is pivoted outwardly.

5. An acetabular cup according to claim 4, wherein the curved profile is defined by a toroidal bearing surface which has a curved cross section, when taken on a radial plane passing through the centre of curvature of the profile.

6. An acetabular cup according to any one of the preceding claims, comprising three anchors, each of which comprises a blade which blades are angularly spaced from each other by approximately 90°.

7. An acetabular cup according to any one of the preceding claims, wherein one or both sides of each blade and/or the mating surface of the corresponding axial recess, are "upset" (knurled), to give an interference fit, whereby each blade tends to be maintained in any given angular position to which it is displaced.

8. An introducer tool for use in implanting an acetabular cup, comprising a base having a threaded tool engaging portion, a side wall and a plurality of anchors mounted in the wall of the cup, each of which anchors is moveable between a retractive position in which the cup can be positioned in the socket, and an outwardly projected position in which the anchor is engageable with the wall of the socket, in order to locate the cup in the socket, the tool comprising: a shank; an operating handle connected to one end of the shank; and a head connected to the opposite end of the shank and having a threaded member to engage the threaded portion on the base, and a pusher member operative to engage the anchors and to displace the anchors outwardly after the threaded member of the head has been tightened on the threaded portion of the base .

9. An introducer tool according to claim 8, wherein the pusher member is mounted on the shank after positioning of the cup, to provide improved visual access during the seating of the cup in the acetabular socket.

10. An introducer tool according to claim 8 or claim 9, further comprising a slide hammer mounted on the shank, which hammer can be driven downwardly in order to press the cup firmly into the socket, and to drive the anchors outwardly.