WO2016055783A1

WO2016055783A1 - An acetabular cup for a hip replacement joint

Info

Publication number: WO2016055783A1
Application number: PCT/GB2015/052933
Authority: WO
Inventors: Ehsanollah TORABI KACHOUSANGI
Original assignee: The University Of Warwick
Priority date: 2014-10-07
Filing date: 2015-10-07
Publication date: 2016-04-14
Also published as: GB201417694D0

Abstract

An acetabular cup for a hip replacement joint. The cup comprises a main body (4) which has a substantially hollow hemispherical shape with an inner surface for receiving, in use, a ball (2) of a femoral component (1). The main body has an end face (6) at the open end of the hollow hemisphere for which a ring (5) is connected. The ring continues the inner surface of the main body without interruption to narrow the opening of the cup. The inner edge of the ring (5) at the end of the ring furthest from the main body having a convexly curved chamfer (12). The face (20) of the ring furthest from the main body is substantially frustoconical.

Description

AN ACETABULAR CUP FOR A HIP REPLACEMENT JOINT

The present invention relates to an acetabular cup for a hip replacement joint.

Hip replacement surgery is not generally available to everyone under the age of 50 years. This is because of the relatively limited lifetime of the implant which consists of an acetabular cup which is fixed to the pelvis and the femoral component which has a ball fitting into the

acetabular cup at one end and a femoral stem that fits into the femur at the opposite end.

The primary long term failure mode of such an implant is caused by edge loading. This occurs because the ball joint does not stay precisely centred within the acetabular cup.

Instead, it undergoes a small but cyclical displacement in and out of the cup as a person moves their leg. As the ball moves back into the cup, it will often not be precisely aligned and any lateral displacement will cause abutment between the ball and the edge of the socket leading to edge loading .

This is a particular problem immediately after surgery when the damaged muscles do not have sufficient strength to keep the joint in place. It is also a problem when the hip is moved 90° or more from the vertical for example when

climbing stairs or getting up out of a chair. An implant which can reduce this edge loading will prolong the lifetime of the implant and hence make it available to younger patients as well as reducing the likelihood of a wear related failure in older patients thereby avoiding the risks and costs of revision surgery.

Of some superficial similarity to the present invention are a number of proposed acetabular cup designs which address the problem of dislocation. These are disclosed, for example, in US 2003/0191537, CN 2642265, US 2013/0231751, US 6,093,208, US 5,989,293 and DE 3200340. All of the documents propose the addition of some form of ring on the bottom of the acetabular cup which have the effect of narrowing the opening of the cup. This is done to prevent dislocation of the hip joint. Dislocation is the complete removal of the ball joint from the acetabular cup. This should be contrasted with edge loading with which the present invention is concerned which is caused by small displacements (typically more than 80 micrometres) of the ball out of the cup which occur frequently and can cause significant wear of the inner edge of the cup and medial of the ball.

None of these documents address the problem of edge loading. The rings which are proposed all have a well-defined edge which produces an abrupt neck which is beneficial in

preventing dislocation. However, there remains surface-to- edge contact between the ball and the cup. Furthermore these well-defined edges have a smaller radius than the radius of a conventional cup without such a ring. This limits the degree of movement of the femoral component.

Also, when the femoral component is moved to its maximum angular displacement, the part of the component below the ball will engage with this edge in the ring thereby introducing another mode of edge loading for the ring. This can also be a cause of fracture, as the femoral component is impinging on a sharp edge. This will occur more frequently than in a cup without a ring given the limited range of movement caused by the ring.

All attempts that we are aware to solve the dislocation problem thereby actually exacerbate edge loading. As far as we are aware, none of the dislocation preventing rings disclosed in the above documents have ever been widely used. As a result of this, conventional acetabular cups available today have a simple hemispherical configuration. According to a first aspect of the present invention there is provided an acetabular cup for a hip replacement joint, the cup comprising a main body which has a substantially hollow hemispherical shape with an inner surface for

receiving, in use, a ball of a femoral component, the main body having an end face at the open end of the hollow hemisphere; a ring connected to the end face, the ring being configured to continue the inner surface of the main body without interruption to narrow the opening of the cup, the inner edge of the ring at the end of the ring furthest from the main body having a convexly curved chamfer.

The present invention provides two improvements over the conventional design. The presence of the ring which narrows the opening of the cup means that the opening of the cup is smaller than the diameter of the ball of the femoral

component thereby limiting the degree to which the ball can move out of the cup. More significantly, the presence of the convex curved chamfer provides a dramatic improvement in the edge loading characteristics. In fact, the term "edge loading" is no longer appropriate for the present invention. The presence of the curved chamfer has effectively eliminated the edge against which the ball repeatedly impinges. Thus, in place of a surface-to-edge loading as in all of the prior art, the present invention has a surface-to-surface engagement. This spreads the load over a much wider area. Initial finite element analysis has shown that the combination of reducing the displacement of the ball and then dispersing any contact which does occur over a much wider area to have produced dramatic decreases in the peak stress on the wall of the cup of up to a factor of 20.

The ring may be formed in a number of sections which are separately attached to the main body. However, preferably, the ring is a single component.

The ring may have an undulating profile such that it has different cross-sections in certain parts of the ring.

However, preferably, the ring has a uniform cross-section all the way around the ring. This provides a simpler design which provides more uniform loading.

The ring may be a "split-ring" design, but is preferably a continuous ring which extends for 360°. Again, this gives uniform loading characteristics in all directions.

The ring may be attached to the main body via an external fastener such as a clamp lock. However, preferably, the ring is connected directly to the end face of the main body. This provides a simple low profile design. Such a direct connection is preferably one which is made in the absence of any separate fasteners. Using such fasteners is fiddly for the surgeon, in use. It also generates potential areas of weakness in the material of the ring and cup, particularly in the case of brittle materials such as ceramics.

Preferably the end face of the main body and the

corresponding face of the ring are provided with

complimentary alignment features to align the ring with respect to the main body. This alignment feature is

preferably in the form of a circumferential rib extending partially or wholly around the circumference of one of the ring and body and a complimentary groove extending around the other of the ring and the body.

The joint between the main body and the ring is preferably secured using a biocompatible adhesive. Such a joint avoids the need for fasteners and is easy to apply in practice. A biocompatible adhesive works particularly well in

combination with the complimentary rib and groove

arrangement as set out above as the biocompatible adhesive can be applied within the groove to ensure that it is contained during the assembly process. Also, the rib and groove provide a much increased bonding surface thereby significantly increasing the bonding strength. An adhesive is particularly suitable in the joint for a ceramic cup, although it can also be used for a metal cup.

Particularly in the case of a metal cup, the rib may be larger than the complimentary groove and arranged to undergo plastic deformation as it is inserted into the groove to ensure a secure fitting. This can be done with or without an adhesive. The rib may be provided with one or more barbs to improve the joint.

Preferably the outer face of the ring is also continuous with the outer surface of the main body. This makes for a low profile design with no protruding features in the vicinity of the ring. The ring is preferably contained within a space defined as by the volume enclosed between an inner sphere, the top half of which forms the inner surface of the main body and an outer sphere larger than the inner sphere the top half of which forms the outer surface of the main body. The low profile nature of the design allows the size of the ball of the femoral component to be maximised for any given anatomy. This is beneficial as, the larger the ball, the larger the range of movement. A larger ball also spreads the load over a wider area thereby reducing stress .

Ideally, the ring will only reduce the opening of the cup to a limited degree. The maximum width across the narrowest part of the opening of the ring is preferably at least 80%, more preferably at least 90% and most preferably at least 99% of the inner diameter of the hemisphere. This is sufficient to provide a reasonable contact surface between the ring and the ball to spread the load as mentioned above as well as to hold the ball within the cup. However, it also allows the greatest possible freedom of movement for the joint as it can move through a large angle before the femoral component meets the ring. The relatively low profile nature of the ring may alternatively be defined in that the angle subtended by the ring at the centre of the hemisphere is preferably less than 38°, more preferably less than 20° and most preferably less than 10°. Again, this creates a relatively low profile device .

Alternatively, the low profile may be defined as the cup having an opening that subtends an angle at the centre of the hemisphere preferably of at least 104°, more preferably at least 120° and most preferably at least 160°.

Alternatively, the low profile may be defined by the

thickness of the ring, namely the dimension of the ring in the direction of the axis of the hemisphere. Preferably, the thickness of the ring is less than 60%, more preferably less than 40% and most preferably less than 20% of the radius of the inner face of the cup. The chamfer preferably has an average radius of curvature which is preferably between 5 and 25% of the radius of the inner face of the cup, and is more preferably between 10 to 15% of the radius of the inner face of the cup. In a conventional cup design, the ball is able to leave the cup if subjected to enough force in the direction of the cup opening. The discussion above deals with the problems of edge loading associated with engagement between the ball and the sharp end of the cup caused by a small but cyclical displacement in an out of the cup. The present invention addresses this by the combination of the narrow opening and convexly curved chamfer as described above. However, one consequence of the narrow opening is that the ball is incapable, in practice, of leaving the cup. This is, in many ways, positive for the reasons set out above. However, it does introduce a new loading regime on the cup. Thus, with the prior art, if a force is applied to the ball in a direction tending to move it out of the cup, other than localised edge loading as mentioned above, because the ball is able to leave the cup, it is free to do so when subjected to a force in this direction. By

contrast, with the present invention, the ball cannot readily leave the cup and therefore the force exerted by the ball on the ring is greater than it would be in the prior art in this situation as, in the prior art, the ball will have simply left the cup when subjected to such a force. The ring as described above is able to deal with such loading and any negative influence caused by this is more than offset by the benefits highlighted above.

However, preferably, in order to reduce this form of

loading, the face of the ring at the end furthest from the main body is substantially frustoconical . Such a

frustoconical face can be matched to the position of the neck of the femoral component so that the neck engages with the frustoconical face thereby distributing the contact load over a wider area.

In practice, this allows the convexly curved chamfer on the inner edge of the ring to have a smaller radius of curvature as the intended primary contact is between the femoral component and frustoconical face, not the femoral component and the convexly curved chamfer as before. This allows the thickness of the ring to be significantly reduced.

Preferably the outer edge of the ring at the end of the ring furthest from the main body has a convexly curved chamfer. This preferably has a radius of curvature of between 1 to 10% (preferably 2 to 5%) at the radius of the inner face of the cup. This avoids the presence of a sharp edge against which the neck may otherwise abut. This feature forms a second aspect of the invention which can be defined, in the broadest sense, as a combination of an acetubular cup and a femoral component for a hip

replacement joint, the cup comprising a main body which has a substantially hollow hemispherical shape with an inner surface for receiving, in use, a ball of a femoral

component, the main body having an end face at the open end of the hollow hemisphere, a ring connected to the end face, the ring being configured to continue the inner surface of the main body without interruption to narrow the opening of the cup; the femoral component comprising a ball received in the cup and a neck component extending from the ball;

the face of the ring at the end furthest from the main body is substantially frustoconical ; the outer edge of the ring at the end of the ring furthest from the main body has a convexly curved chamfer; the angle of the frustoconical face being such that when the ball rotates in the cup, at the point at which the neck meets the frustoconical end, the respective adjacent surfaces of the neck and frustoconical end are substantially parallel in a cross-sectional plane containing the centre of the ball and a line of contact between the frustoconical end and the neck. The plane defined here is effectively the median plane containing the line of contact. It is also the plane of the cross-section of Fig. 4. Examples of an acetabular cup in accordance with the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1A is a perspective view of the cup and upper portion of the femoral component;

Fig. IB is a cross-section through the main body and ring prior to attachment of the ring to the main body; Fig. 1C is a schematic side view of the cup including the top portion of the femoral component;

Fig. ID shows detail in the region labelled D in Fig. 1C; Fig. IE shows detail in the region denoted by line E in Fig. 1C;

Fig. 2 is a view similar to Fig. IE showing an alternative rib configuration;

Fig. 3 is a view similar to Fig. IB of a second ring

configuration;

Fig. 4 is a cross-sectional view of the cup including the top portion of the femoral component with a third ring configuration; Fig. 5 is a view similar to Fig. 4 with the femoral

component shown in a different position; and

Figs. 5A to 5D show details of the design as marked in Fig. 5.

The femoral component 1 has a ball 2 at its upper end. This ball is conventional in nature and will not be described further other than for its interaction with the cup 3.

The cup 3 comprises two components, namely the main body 4 and a ring 5. The main body 4 is similar to a conventional acetabular cup, with the exception of the nature of its end face 6 as described below. It has a hemispherical

configuration and is concentric with the ball 2. Its outer surface describes a hemisphere 7 and its inner surface describes a hemisphere 8 which abuts the ball 2.

The improvement provided by the present invention is the attachment of the ring 5. The ring 5 is a single piece continuous ring which extends all the way around the ball 2. It has a constant cross-section throughout. The ring has an upwardly extending rib 10 which extends fully around the circumference of the ring although it could alternatively be intermittent. This rib 10 fits in a complementary recess 11 in the end face 6 of the main body 4.

The ring 5 is initially loosely retained on the femoral component 1 before surgery. During surgery, the ball 2 is inserted into the main body such that the ring 5 is

presented to the lower face of the main body. It is then fixed in place as described below. In the case of a ceramic or metal, a biocompatible adhesive such as Loctite made by Henkel may be used. For a metal, its ductility allows the rib 10 to initially be slightly larger than the groove and be arranged to undergo plastic deformation as it is inserted into the groove to securely hold the ring in place without the need for an adhesive. However, an adhesive can also be used. The metal rib may be provided with one or more deformable barbs which assist in clamping the ring to the main body. The areas around the barbs may provide an ideal location for any biocompatible adhesive. Other joining methods, such as a shrink-fit process or a locking taper could be used in addition to or as an alternative to the above.

The ring is designed to have a "low-profile" design. If the hemispheres 7, 8 defining the inner and outer surfaces of the main body 4 are extended to complete their corresponding spheres, the ring 5 is bounded by these two spheres.

Typical dimensions for the cup are an inner radius of curvature of 24.04 mm and a wall thickness of 5 mm with the thickness of the ring (i.e. the distance between the planes defining upper and lower surfaces) being 6.2 mm. However, the dimensions are entirely dependent on patient anatomy such that the inner radius may vary significantly (e.g. from 11mm to 30mm) . Generally, smaller cups will be of

polyethylene, while larger ones will be metal or ceramic. A larger cup is preferred if possible as the larger ball that is can accommodate increases the range of motion and

decreases the stress. The innermost edge of the ring 5 at the end opposite to the main body 4 is provided with a convexly curved chamfer 12 when seen in a cross-sectional plane through the axis of the hemisphere. The chamfer 12 is continuously curved. It may have a constant or a variable radius of curvature. In the present case, the radius of curvature of the chamfered surface is, for example, between 2 and 4 mm (preferably 3mm) but may vary even more, particularly if the size of the cup changes. The important consideration is that the chamfer removes an abrupt edge that the ball can contact.

The effect of the ring is that the narrowest part of the cup occurs just below the centre of the ball. This means that the ball is effectively captive within the cup and this significantly limits the extent to which the ball can move in the direction out of the cup during normal use.

Effectively it can only move a small amount allowed for by any dimensional tolerances. However, given the relatively thin nature of the ring, it will be appreciated from Figs. 1A and IB that the patient retains most of the mobility in the joint that they would have had without the ring being present. For a ball with an inner cap radius of 3mm, the angle Θ subtended by the ring at the centre of the cup is less than 13.5° meaning that the patient has lost just 27° of mobility providing more than enough freedom for regular activities such as climbing stairs and getting in and out of chairs without causing the femoral component to abut against the cup.

Any loading caused by forces which tend to pull the ball out of the cup will be spread across the surface of the ring beneath the centre of the ball and above the chamfer. This region is marked as R in Fig. ID. Thus, the load is distributed over a much wider area than any of the prior art leading to a significant reduction in the peak stresses on the cup.

Fig. 2 shows a different type of rib 10' configuration particularly suited for a metal cup. In this case, the rib 10' is provided with a series of sharp edged annular barbs 13 which engage with the wall of the groove 10. The

ductility of the metal causes the barbs 13 to deform to provide an enhanced gripping force. An adhesive may

conveniently be applied in the space above and/or between the barbs 13.

The example shown in Fig. 3 shows a ring 5' which has a much greater thickness. In this case, the ring has a thickness of 19 mm. This provides a much larger surface for the load caused by forces tending to pull the ball out of the cup. However, the mobility here has been reduced by over 45%. Thus, it will be appreciated that there is a trade-off between the distribution of the load between the ball and the cup and the loss of mobility. We believe that the optimum design will be one which tends closer to the example of Fig. 1 as initial testing indicates that this will provide adequate distribution of the load.

A third aspect of the present invention is shown in Figs. 4 and 5.

Most aspects of this example are the same as those described above. The same components have been designated with the same reference numerals. Only the differences are set out below .

While in the previous examples, the bottom face of the ring 5, 5' was shown as being in a plane parallel to the opening of the main body 4, the key difference with the third example of Figs. 4 and 5 is that the lower face 20 of the ring 5'' is frustoconical . In particular, the angle of the frustoconical surface is determined by the position of the neck 21 of the femoral component 1 when it is rotated into a position in which it meets the cup as shown in Fig. 4.

As a result of this change, the radius of curvature of the chamfer 12' can be reduced to the less than 1mm. This means that the depth of the ring can be correspondingly reduced for example to 4.3mm and that there is no need to

accommodate such a large radius of curvature. This, coupled with the frustoconical surface can significantly increase the angular range of movement of the femoral component 1 by as much as 10°. In this case the angle Θ subtended by the ring at the centre of the cup is 9° meaning that the patient has lost just 18° of mobility. As well as being beneficial in providing a greater range of movement for the user, this also serves to reduce the number of incidences of direct contact between the femoral component 1 and the cap 5.

Because any contact between the two components is now distributed along the neck 21, the peak contact pressure can be significantly reduced from around 19.8 GPa conventional ring to 10.9 GPa in the first example to 7.3 GPa in the new example . In the cross-sectional views of Figs. 4 and 5, the

frustoconical surface 20 and the corresponding surface of the neck 21 may have a very slightly convex configuration (such that the top of the neck 21 has a slightly "barrelled" configuration and the surface 20 bulges out slightly) to ensure that there is a progressive loading effect as the two components meet one another. The convex configuration preferably has a maximum height (compared to a plane

surface) of 10-20 pm. Alternatively convex configuration may be defined as a 1-2° radius of curvature.

As best shown in Fig. 5C, the radially outermost edge 22 of the ring 5'' also has a convexly curved chamfer. This may have the same radius of curvature as the inner chamber 12'. This is done in order to avoid the presence of sharp edge which might otherwise engage with the neck 21. The first example may also have a convexly curved chamfer at is outermost edge. Because of the frustoconical surface 20, the depth of the ring at this outer edge is significantly greater than at the inner edge so the radius of curvature of the convexly curved chamfer 22 can be greater than the curvature of the inner edge.

Claims

1. An acetabular cup for a hip replacement joint, the cup comprising a main body which has a substantially hollow hemispherical shape with an inner surface for receiving, in use, a ball of a femoral component, the main body having an end face at the open end of the hollow hemisphere, a ring connected to the end face, the ring being configured to continue the inner surface of the main body without

interruption to narrow the opening of the cup, the inner edge of the ring at the end of the ring furthest from the main body having a convexly curved chamfer.

2. A cup according to claim 1, wherein the ring is a single component.

3. A cup according to claim 1 or claim 2, wherein the ring has a uniform cross-section all the way around the ring.

4. A cup according to any one of the preceding claims, wherein the ring is a continuous ring which extends for 360° .

5. A cup according to any one of the preceding claims, wherein the ring is connected directly to the end face of the main body.

6. A cup according to any one of the preceding claims, wherein the connection to the main body is made in the absence of any separate fasteners.

7. A cup according to any one of the preceding claims, wherein the end face of the main body and the corresponding face of the ring are provided with complementary alignment features to align the ring with respect to the main body.

8. A cup according to claim 7, wherein alignment features are in the form of a circumferential rib extending partially or wholly around the circumference of one of the ring and main body and a complimentary groove extending around the other of the ring and main body.

9. A cup according to any one of the preceding claims, wherein the joint between the main body and the ring is secured using a biocompatible adhesive.

10. A cup according to claim 8 or claim 9, wherein the ring and main body are metal and wherein the rib is larger than the complimentary groove and arranged to undergo plastic deformation as it is inserted into the groove to ensure a secure fitting.

11. A cup according to any one of the preceding claims, wherein the outer face of the ring is also continuous with the outer surface of the main body.

12. A cup according to any one of the preceding claims, wherein the ring is preferably contained within a space defined as the volume enclosed between an inner sphere, the top half of which forms the inner surface of the main body and an outer sphere larger than the inner sphere the top half of which forms the outer surface of the main body.

13. A cup according any one of the preceding claims, wherein the maximum width across the narrowest part of the opening of the ring is at least 80%, preferably at least 90% and more preferably at least 99% of the inner diameter of the hemisphere.

14. A cup according to any one of the preceding claims, wherein the cup has an opening that subtends an angle at the centre of the hemisphere of at least 104°, preferably at least 120° and more preferably at least 160°.

15. A cup according to any one of the preceding claims, wherein the angle subtended by the ring at the centre of the hemisphere is less than 38°, preferably less than 30° and more preferably less than 15°.

16. A cup according to any one of the preceding claims, wherein the thickness of the ring, namely the dimension of the ring in the direction of the axis of the hemisphere, is less than 60%, more preferably less than 40% and most preferably less than 20% of the radius of the inner face of the cup.

17. A cup according to any one of the preceding claims, wherein the chamfer has an average radius of curvature which is between 5 and 25% of the radius of the inner face of the cup, and is preferably between 10 to 15% of the radius of the inner face of the cup.

18. A cup according to any one of the preceding claims, wherein the face of the ring at the end furthest from the main body is substantially frustoconical .

19. A cup according to any previous claim, wherein the outer edge of the ring at the end of the ring furthest from the main body has a convexly curved chamfer.

20. A combination of an acetubular cup and a femoral component for a hip replacement joint, the cup comprising a main body which has a substantially hollow hemispherical shape with an inner surface for receiving, in use, a ball of a femoral component, the main body having an end face at the open end of the hollow hemisphere, a ring connected to the end face, the ring being configured to continue the inner surface of the main body without interruption to narrow the opening of the cup;

the femoral component comprising a ball received in the cup and a neck component extending from the ball;

the face of the ring at the end furthest from the main body is substantially frustoconical ;

the outer edge of the ring at the end of the ring furthest from the main body has a convexly curved chamfer; the angle of the frustoconical face being such that when the ball rotates in the cup, at the point at which the neck meets the frustoconical end, the respective adjacent surfaces of the neck and frustoconical end are substantially parallel in a cross-sectional plane containing the centre of the ball and a line of contact between the frustoconical end and the neck.

21. A combination according to claim 20, wherein the convexly curved chamfer has a radius of curvature of less than 1.5mm .

22. A combination according to claim 20 or 21, wherein the thickness of the ring is less than 5mm.