ENDOPROSTHESIS

The present invention relates to a socket, in particular an acetabular socket for a hip endopros diesis.

In die case of total hip endoprosthesis, a prosthesis stem having a joint head is inserted into die femur. An acetabular socket, which serves as a bearing shell for d e joint head, is implanted in the pelvic bone. It is known to construct the acetabular socket from a socket shell and a socket insert. In this way die socket shell can be optimized with regard to implantation in the pelvic bone and the socket insert can be optimized with regard to the bearing properties for the joint head. In these cases die socket shell is formed and positioned in the pelvic bone so that it is as stable as possible. The socket insert is then oriented in the socket shell in such a way d at the joint head is accommodated so that d e prosthesis stem and accordingly die femur of the patient are as far as possible in d e correct orthopaedic position. The aforementioned situation does also apply to sockets of odier kinds of endopro thesis, such as shoulder endoprothesis or die like.

In EP 0 663 193 Al is disclosed an acetabular socket wherein die socket insert has a spherical outer surface and is seated in a corresponding spherical accommodating space with the same spherical radius in die socket shell. Therefore, when die socket insert has been inserted into die socket shell, the socket insert can be rotated at will about its axis of rotation and its axis of rotation can be tilted at will widi respect to die axis of rotation of the accommodating space. As a result, it is possible for die socket shell to be positioned in the pelvic bone in accordance with die bone structure and for the socket insert to be oriented in accordance with the orthopaedic position of die prosthesis stem inserted into the femur. In order to fix the socket insert in its position within the socket shell, the inner spherical surface of die accommodating space of die socket shell has pointed projecting teeth which engage in die outer surface of die socket insert. Because the teeth have to dig into die outer surface of the socket insert, tiiere are limitations with respect to the choice of material for the socket insert. Pressing the socket insert onto the teeth of the socket shell makes it difficult for the socket insert to be inserted in a precisely positioned manner.

This problem was overcome as described in WO2005/063148. Here an acetabular socket for a hip endoprosthesis consists of a socket shell and a socket insert wherein the socket shell has an accommodating space having a conical inner surface in which the spherical outer surface of the socket insert is inserted. Because fixing of the optimally oriented socket insert results from its being simply pressed into the accommodating space, this fixing is simple to carry out and does not require any additional instrumentation or additional fixing means. As a result, the socket insert can be clamped in self-retaining manner in any desired position of rotation and tilt in the accommodating space of the socket shell.

In order to bring about reliable clamping of the socket insert in the socket shell, the socket insert and the socket shell are made from a hard material. The socket shell is preferably manufactured from a biocompatible material, for example a titanium alloy. For the socket insert there can be selected a material corresponding to the sliding characteristics pairing of socket shell and joint head, for example a metallic or ceramic material or a plastics material.

However, it has been found that when a socket insert is first inserted into a socket shell it tends to lock in position. This may be caused d e conical/spherical interface between die two components. The surgeon is then required to use surgical instruments to free d e socket insert so that it can be orientated correctly in a precisely positioned manner before being pressed into the desired position in the socket shell.

It is an object of the present invention to provide a socket, in particular an acetabular socket for a hip endoprosthesis that overcomes die aforementioned problem by preventing a socket insert from locking into an undesirable position in a socket shell prior to correct positioning and orientation.

According to the present invention there is provided a socket, in particular an acetabular socket for a hip endoprosthesis comprising a socket shell configured for implantation in a pelvic bone of a patient and having an inner surface that defines an accommodating space and

a socket insert coupleable with the socket insert and configured to provide a bearing for a joint head of a prosthesis stem and having an outer surface configured to be seated in the accommodating space of the socket shell;

characterised in that a moveable anti-lock means is provided between the socket shell and the socket insert which in a first position restrains the socket insert from seating within the accommodating space of the socket shell and which is moveable into a second position wherein the socket insert is capable of seating within the socket shell and of coupling therewith.

The provision of an anti-lock means as aforesaid prevents a premature locking of the socket insert in the socket shell. This allows the socket shell to be implanted in a pelvic bone in accoidance with the bone anatomy and structure of the pelvic bone so that optimum conditions for ingrowth can be achieved. The socket insert can then be freely oriented in relation to the socket shell in an uncoupled position during surgery to enable a surgeon to follow a standard protocol in adjusting the relative position of the socket insert to the socket shell. In particular the socket insert can be rotated in the socket shell and its axis of rotation can be so tilted in relation to the axis of rotation of the socket shell so that die axis of rotation of the socket insert is aligned with the axis of the shaft neck of the prosthesis stem when the femur with the inserted prosdiesis stem is arranged in the optimum orthopaedic posidon. In addition, the invention allows reduction and a range of motion testing by die surgeon while the socket insert is uncoupled from the socket shell. Only when the surgeon is satisfied that the optimum relative position of the socket insert to the socket shell has been achieved will the anti- lock means be moved to enable the socket insert to be coupled to the socket shell.

In embodiments such as those described below wherein the socket insert is coupleable to the socket shell in a self-locking manner within said accommodating space, then the anti-lock means is moved by the application of pressure to the socket insert in a direction pushing it into accommodating space. This action simultaneously moves the anti-lock means into its second position allowing the socket insert to become clamped in the accommodating space in self-retaining manner. However, it should be appreciated that the invention is also suitable for use with socket inserts and socket shells that have a spherical/spherical interface and will again prevent premature locking of the two components together.

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

Fig. 1 is a diagram showing schematically a total hip endoprosthesis using an acetabular socket in accordance widi a first embodiment of the present invention;

Fig. 2 is an axial section through the acetabular socket shown in Fig, 1 but to an enlarged scale, the acetabular socket being shown in an uncoupled position;

Figs. 3a and 3b are scrap diagrammatic views showing an anti-lock means forming part of the acetabular socket shown in Fig. 2 respectively in an anti- lock position and in a position adopted after locking of the acetabular socket;

Fig. 4 is a scrap diagrammatic view showing a modified arrangement of an anti-lock means forming part of the acetabular socket ;

Figs. 5a and 5b are diagrams similar to Fig. 2 but of a second embodiment of acetabular socket in accordance with the invention when respectively in an uncoupled state and in a cotipled state;

Figs. 6a and 6b are plan and side views of an annulus forming part of the second embodiment of acetabular socket shown in Figs. 5a and 5b; is a diagram similar to Fig. 2 but of a tliird embodiment of acetabular socket in accordance with the invention. A total hip endoprosthesis comprises an acetabular socket 1 for implantation in tlie pelvic bone 2 of a patient and a prosthesis stem 3 for implantation into the femur 4. The prosthesis stem 3 has a shaft neck 5 diat carries a joint head 6 which is inserted into the acetabular socket 1. The acetabular socket 1 comprises a socket shell 7 and a socket insert 8. The socket shell 7 is adapted to be inserted in die pelvic bone 2 in a conventional manner, for example by being pressed in with appropriate gri ing structures 9 on its outer surface, by screwing or by the use of appropriate fasteners (not shown). The socket shell 7 is substantially hemispherical and is recessed to provide an accommodating space 10 that is open to the equatorial plane and d at at least in the region in which it comes into contact widi the outer surface of the socket insert has rotational symmetry with respect to the mid-axis 11 of the socket shell 7.

In all of the illustrated embodiments of tlie present invention, die accommodating space 10 has an inner surface 12 in tl e form of a straight circular cone which becomes naiiower from the opening located in the equatorial plane towards d e pole of the socket shell 7. A base 13 of tlie accommodating space 10 is flattened off in the pole region. The angle between the conical inner surface 12 and the axis of rotation 11 is selected dependent on the material pairing of die socket shell 7 and the socket insert 8 so diat self-retention will come about when the socket insert 8 is pushed into the socket shell 7. This angle is preferably between 4° and 10°.

Tlie socket insert 8 also has a substantially hemispherical shape. Its outer surface 14 is spherically shaped, at least in the region in which it comes into contact with the inner surface 12 of die accommodating space 10. It also has a recessed spherical bearing surface 15, which serves to accommodate, and provide a bearing for, die joint head 6. The spherical outer surface 14 and die spherical bearing surface 15 at least in die region of contact are lOtationally symmetrical with respect to an axis of rotation 16 of d e socket insert 8.

In order to restrain the socket insert 8 from prematurely seating within the accommodating space 10 of die socket shell 7 during surgery, a moveable anti-lock means 17 is provided between die socket shell 7 and the socket insert 8. This means 17 holds the socket insert 8 in an elevated position relative to d e socket shell 7 until die surgeon is ready couple die socket insert 8 to die socket shell 7.

In the first embodiment shown in Fig. 2, die and-lock means 17 comprises a pin 18 which is push-fitted witliin a central channel 1 of a screw fitting 20 or similar located at the pole of die socket shell 7. The length of die pin 18 is commensurate widi the lengtii of the channel 19 so that it can be accommodated at least pardy, and preferably completely within die channel 19, as shown in Fig.3b. However, initially, it is left projecting out of the channel 19 as shown in Fig. 3a. The projecting end of the pin 18 therefore prevents die socket insert 8 from seating witliin the socket shell 7, as shown in Fig. 2. However, when d e socket insert 8 has been optimally oriented and it is desired to couple die socket insert 8 to die socket shell 7, pressing of the socket insert 8 axially into the accommodating space 10 will also push the pin 18 into the channel 19 so that it no longer projects this enables d e insert 8 to be coupled to die shell 7 in a self-retaining manner in diat orientation position.

It will be appreciated that the pin 18 and channel 19 arrangement may be modified and still perform the same function. For example, as shown in Fig. 4, the pin 18 may be replaced by a spring 21 or an equivalent elastic means, for example a rubber or plastics plug or similar, tiiat is compressed into the channel 19 when the socket insert 8 is pressed into the socket shell 7. Alternatively, die pin 18 may be made freely movable witliin the channel 19 but retained iti a projecting position by a spring or an equivalent elastic means (not shown) located beneath it in the channel 19. In these cases, die strength of the spring or elastic means needs to be sufficient to support the socket insert 8 but not so strong that it significantly impedes coupling of the socket insert 8 in the socket shell 7 when the former is pushed into place.

In addition, it will be appreciated diat it may also be possible for the pin 18, spring or elastic member to project from a channel defined in die outer surface 14 of the socket insert 8 and to be pushed into this channel when the socket insert 8 is pushed into the socket shell 7 to couple therewith. Prefer bly but not necessaiily, the pin 18, spring or elastic member is aligned with the mid-axis 11 at the pole of the socket shell 7. This position is that which tends to receive the greatest force when the socket insert 8 is pressed into the socket shell 7.

A second embodiment of the invention is shown in Figs. 5a and 5b. Here the anti-lock means 17 comprises a thin, flexible annular disc 22, as shown in Figs. 6a and 6b, that is located within the socket shell 7 beneath the socket insert 8. Such a disc 22 may comprise a disc spring, a springy, flexible washer or similar means that is capable of being flexed out of a planar position. Initially, the disc 22 is planar and therefore holds the socket insert 8 in an elevated position relative to the socket shell 7, as shown in Fig. 5a, thereby preventing premature locking of the insert 7 in die shell 7. However, when it is desired to couple and thereby to lock the socket insert 8 into the socket shell 7, pressing of the socket insert 8 axially into the accommodating space 10 will cause the disc 22 to flex so that it is clamped between d e inner surface 12 of the accommodating space 10 and the outer surface 14 of the socket insert 8. Again, the strength of the springiness of the disc 22 should only be sufficient to support die socket insert 8 but not so strong that it impedes coupling of the socket insert 8 in the socket shell 7.

In a modification., the anti-lock means 17 may comprise a compressible ring rather than an annular disc 22 so that rather than flexing into a second position when the socket insert 8 is pressed into the socket shell 7, it is compressed between the inner surface 12 of d e accommodating space 10 and die outer surface 14 of d e socket insert 8.

In a further modification the anti-lock means 17 may compiise a compressible spring leaf, pin, membrane or die like rather dian an annular means, which may be deformed in a similar way as described above when die insert 8 is pressed into the socket shell 7.

In addition, die disc 22 or ring may be arranged to form part of or be secured to the outer surface 14 of d e socket insert, for example by projecting from an annular groove formed in the outer surface 14, rather dian comprising an separate component or a component secured in a similar way to the inner surface 12. A thkd embodiment of d e invention is shown in Fig. 7. In this acetabular socket, a pin 23 is used to predefine the angulation of die socket insert 8 to die socket shell 7 in addition to providing die and-lock means 17. In the illustrated arrangement shown in Fig. 7, die pin 23 is fixed, moulded or otherwise secured to die outer surface 1 of die socket insert 8 in a predetermined position which in tills case is at its pole although odier predetermined positions are possible. The pin 23 is designed to be a push fit in a channel 24 formed in the base 13 of the accommodating space 10, again at a predetermined position which in this case is at die pole. In use, the pin 23 prevents the socket insert 8 from being prematurely locked into the socket shell 7. However, it also requires the socket insert 8 to be angulated correctly widi the socket shell 7 before coupling can take place as for diis to occur die pin 23 must be pushed into the channel 24.

It will be appreciated d at modifications to the aforementioned arrangement are possible. For example, the pin 23 could be secured to the socket shell 7 and penetrate a channel or cavity formed in die outer surface 14 of die socket insert 8. The pin 23 could also be arranged to project out of a cavity defined in either the socket shell 7 or the socket insert 8 and to penetrate a corresponding channels or cavity in the odier component so as to form a bridge between diem. It will also be appreciated that die pin 23 could be spring-loaded or odiervvise biased and retained in a projecting position by a spring or an equivalent elastic means (not shown) located beneath it in a channel. In such an arrangement die pin 23 will click into position within the channel or cavity in the other component.

In other arrangements, a plurality of channels may be provided in the shell 7 or the insert 8 and the surgeon left to decide which one of these a pin secured to the other part of the acetabular socket is to be pushed during coupling of die two components 7, 8. In yet a furdier modification, this pin arrangement may be combined with the second embodiment described above so that effectively die acetabular socket comprises two separate anti-lock means.

In all of die illustrated embodiments, the accommodating space 10 has an inner surface 12 in the form of a straight circular cone as described above. However, it will be appreciated that it is also possible to make use of tlie invention in acetabular sockets wherein tlie accommodating space is hemi-spherical as described in EP 0 663 193 A 1 or has another profile as in all cases tlie invention can be used to prevent a locking of tl e socket insert 8 in tlie socket shell 7 prior to tl e correct positioning of the socket insert 8 within die socket shell 7.

It may be mentioned diat die anti-locking means 17, e.g. in form of the pin 18 can be aligned with a pole of die socket shell 7, or alternatively with a pole of the inseit 8 in case that die anti-locking means 17 is associated to tl e insert 8 instead of die socket shell 7. It may be preferred that die aforementioned locking-means 17 are placed eccentrically in relation to the axis of rotation of eidier die socket shell or insert respectively. The angle of eccentricity is up to 25° , pieferably between 5° to 15° .

Furdier, it may be mentioned diat die present invention applies to sockets for all kinds of joints, such as hip joints, shoulder joints, or die like, each comprising a socket shell and a socket insert.

Reference Numerals

1 Acetabular socket

2 Pelvic bone

3 Prosthesis stem

4 Femur

5 Shaft neck

6 Joint head

7 Socket shell

8 Socket insert

9 Gripping structures

10 Accommodating space

11 Mid-axis of socket shell

12 Inner surface of socket shell

13 Base of accommodating space

14 Outer surface of socket insert 15 Spherical bearing surface

16 Axis of rotation of socket insert

17 Anti-lock means

18 Pin

19 Channel

20 Screw fitting

21 Spring

22 Annular disc

24 Channel