GB2315222A

GB2315222A - Joint for use in total hip replacement

Info

Publication number: GB2315222A
Application number: GB9712506A
Authority: GB
Inventors: Kevin Hardinge
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-07-15
Filing date: 1997-06-16
Publication date: 1998-01-28
Anticipated expiration: 2017-06-16
Also published as: GB9712506D0; GB2315222B

Abstract

The joint is provided as a set of components including a stem component (10) and a separate head component (12). The stem component (10) comprises a shaft (14) adapted to be secured within a patient's femur (52), in accordance with known practice, and a spigot (16) adapted to project from the femur (52). The head component (12) comprises an inner body (22) and an outer body (20), the inner body (22) being pre-assembled to be pivotally retained within the outer body (20). The outer body (20) is adapted to be fixed within an acaetabulum of a patient's pelvis (58). The inner body (22) has a recess (40) into which the spigot (16) of the stem component (10) can be securely received through an aperture (32) in the outer body (20).

Description

JOINT FOR USE IN TOTAL HIP REPLACEMENT The present invention relates to a joint for use in total hip replacement.

Total hip replacement surgery for treatment of degenerative arthrosis is considered to be one of the most highly successful elective procedures in surgery.

A successful outcome can result in increased function and mobility for the patient.

The success of such surgery in increasing mobility of the patient can give rise to problems in some cases.

Increased movement is then accompanied by increased muscle function with a consequence that leverage on the artificial joint increases with time. Occasionally, leverage on the joint may exceed its design limitations and this can result in dislocation of the joint. Such dislocation can typically occur in everyday activities such as getting out of bed, moving between a seated and a standing position, and getting into and out of a motor car.

In a replacement hip joint, mobility is dependent upon smooth movement of a ball in a socket. Dislocation typically occurs as a result of excessive leverage which causes the ball to be levered from the socket. A consequence of dislocation is that the capsule of the hip joint is stretched and muscle tone is reduced. This can further reduce the stability of the joint, so permitting dislocation to occur with lesser amounts of leverage on the joint. The effect of this can be to cause recurrent dislocations.

Recurrent dislocation in situations such as this is a well recognised complication in an otherwise successful surgical procedure. Patients suffering from this condition frequently experience a loss of confidence, with the result that their movement is once again impaired.

One known proposal attempts to solve this problem by providing a socket of the joint of extent slightly greater than hemispherical. This results in the ball being a "snap-fit" within the socket, with the effect of increasing stability of the joint. However, this has been only partially successful. The snap-fit cannot be made particularly strong since this would make it very difficult, if not impossible, for the surgeon to effect reduction of the joint during surgical installation of it.

It is an aim of the present invention to provide a joint for use in total hip replacement surgery which results in increased stability without substantially increasing the difficulty of its surgical application.

According to a first of its aspects, the invention provides a joint for use in total hip replacement surgery comprising a stem component and a head component, the stem component comprising a shaft adapted to be secured within a patient's femur and a spigot adapted to project from the femur; and a head component comprising an outer body adapted to be fixed within an acaetabulum of a patient's pelvis and an inner body pivotally retained within the outer body, the inner body having a recess into which the spigot of the stem component can be securely received through a suitably positioned aperture in the outer body.

In the present invention, the inner and outer body of the head component respectively constitute ball and socket joint parts. These are assembled prior to surgical fitting, so can be made a particularly secure fit. Reduction of the joint in surgery is made by insertion of the spigot into the recess. It will be understood that the spigot will not be dislodged by the levering forces which are typically encountered during the day to day life of the patient.

Preferably, the inner body is a tight fit within the outer body, such that it will rotate smoothly but not freely. This allows the inner and outer bodies to be aligned prior to fitting while minimising the risk that the inner body will accidentally be rotated out of proper alignment.

The aperture within the outer body is preferably closely similar in size and shape to the recess in the inner body. By this arrangement, during reduction of the joint, the spigot can be displaced transversely across the head component in order that it may drop into the recess without causing accidental displacement of the inner body.

The spigot is advantageously formed as a gentle taper, with the recess being complimentarily tapered.

This helps to ensure that a close, high quality fit is maintained between the stem component and the inner body.

Typically, the spigot and the recess are of circular cross-section so as to avoid the need to align these components axially before reduction.

It will be seen that movement of the joint is limited by the spigot coming into contact with the periphery of the aperture in the outer body. By suitably selecting the size and shape of this aperture, the amount of articulation provided by the joint can be restricted to a desired range. This can be employed to inhibit those movements of the patient's limb which are most likely to give rise to the forces which cause dislocation.

An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a head component of a first embodiment of the invention; Figure 2 shows a stem component of a first embodiment of the invention; and Figure 3 is a cross-sectional view of an embodiment of the invention while in use.

With reference to the drawings, a joint for use in total hip replacement comprises a stem component 10 and a head component 12.

The stem component 10 comprises a shaft 14 which may be one of the many variant designs well known to those skilled in the art.

The stem component 10 further comprises a spigot 16, which projects from an upper end portion of it. The spigot 16 has a circular cross-section which tapers away from the shaft 14. (The angle of the taper is exaggerated in the drawings in the interest of clarity.) The head component 12 comprises an outer body 20 and an inner body 22.

The outer body 20 is formed as a unitary moulding of high density polyethylene. Outwardly, the outer body is generally hemispherical. It may have formations on its outer surface (not shown) of various forms to enhance its retention when fitted to a patient. A cavity 30 is formed within the outer body 20. The cavity 30 is shaped as a part of a sphere which is somewhat greater than hemispherical. An aperture 32 extends into the outer body 20 through its outer flat surface 34, to communicate with the cavity 30.

The inner body 22 is generally spherical and is formed from finally polished stainless steel. The inner body 22 is retained within the cavity 30 of the outer body 20. A tapered recess 40 of circular section extends radially into the inner body 22. The taper of the recess 40 is closely similar to the taper of the spigot 16. The relative sizes of the inner body 22 and the recess 30 are such that the inner body 22 fits closely therein, such that it can rotate if urged to do so, but such that there is sufficient resistance to rotation for the inner body 22 not to rotate due to its own weight or during normal handling. (It is to be noted that the clearance between the inner and outer bodies 22,20 shown in the Figures is for convenience of illustration only and will not appear in practice.) The inner body 22 is disposed within the outer body 20 such that the recess 40 is open at the aperture 32. For use, the stem component 10 is secured in a canal 50 formed in a head portion of the femur 52 of a patient. It is secured therein by cement 54. The stem component 10 is secured with its spigot 16 projecting from the femur 52. Conventional techniques may be used to achieve this.

The head component 12 is similarly secured in an acaetabulum 56 formed in the pelvis 58 of a patient. The head component 12 is secured by cement 60 in a manner substantially similar to that conventionally used in fixing the socket component of a replacement hip joint.

The head component 12 is secured with its flat surface 34 and, therefore, the aperture 32, facing outwardly.

Reduction of the joint can then be achieved by sliding the spigot 16 into the recess 40. During this operation, the spigot 16 may slide across the flat surface 34 of the outer body 20. Retention of the inner body 22 within the cavity 40 of the outer body 20 ensures that contact between the end of the spigot 16 and the inner body 22 is minimised during reduction of the joint.

The structure of the head component 12 tends to ensure that the inner body 22 will remain aligned ready to receive the spigot 16. However, there is inevitably a risk that the inner body 22 will be moved such that the recess 40 no longer aligns with the aperture 32. If this happens, a suitably shaped tool may be inserted into any part of the recess 40 which is accessible through the aperture 32, and the inner body 22 may then be urged to rotate. The inner body 22 will then remain stationary until the spigot 16 is properly inserted into the recess 40.

It will be seen that the operation of securing the inner body 22 within the outer body 20 is not part of the surgical procedure. Rather, it is carried out beforehand. It is therefore possible to fit these two components together using forces and complex operations which are achievable in a manufacturing environment but which would not be readily possible during a surgical procedure. It is therefore possible to ensure that the inner body 22 is retained within the outer body 20 with a tenacity substantially greater than could be achieved by a simple snap-fit operation during surgical procedure.

Once the joint is in use, the stem component 10 and the head component 12 can be separated only by movement in a direction axial of the spigot 16. This connection is substantially unaffected by levering forces of the type which are normally responsible for hip dislocations. In practice, it is relatively unlikely for large forces to be exerted on the joint in this direction. This leads to far greater stability of the joint.

Additionally, it will be seen that the limits of articulation of the joint occur where the spigot 16 makes contact with the periphery of the aperture 32. Thus, the degree of articulation can be controlled readily by suitably shaping and sizing the aperture 32. It is therefore possible to set up the joint such that the patient is strongly inhibited from moving in such a manner which is likely to cause dislocation.

Claims

CLAIMS:

1. A joint for use in total hip replacement surgery comprising a stem component and a head component, the stem component comprising a shaft adapted to be secured within a patient's femur and a spigot adapted to project from the femur; and a head component comprising an outer body adapted to be fixed within an acaetabulum of a patient's pelvis and an inner body pivotally retained within the outer body, the inner body having a recess into which the spigot of the stem component can be securely received through a suitably positioned aperture in the outer body.

2. A joint for use in total hip replacement surgery according to claim 1 in which the inner and outer body of the head component respectively constitute ball and socket joint parts.

3. A joint for use in total hip replacement surgery according to claim 1 or claim 2 in which the inner body and the outer body of the head component are assembled prior to surgical fitting.

4. A joint for use in total hip replacement surgery according to any preceding claim in which the inner body is a tight fit within the outer body, such that it will rotate smoothly but not freely.

5. A joint for use in total hip replacement surgery according to claim 1 in which the aperture within the outer body is closely similar in size and shape to the recess in the inner body.

6. A joint for use in total hip replacement surgery according to any preceding claim in which the spigot is formed as a gentle taper, with the recess being complimentarily tapered.

7. A joint for use in total hip replacement surgery according to any preceding claim in which the spigot and the recess are of circular cross-section.

8. A joint for use in total hip replacement surgery according to any preceding claim in which the size and shape of the aperture is selected such that the amount of articulation provided by the joint is restricted to a desired range to inhibit dislocation of the joint.

9. A joint according to any preceding claim in which the inner body is located within a recess of the outer body, that recess being shaped as a part of a sphere, that part being greater than a hemisphere.

10. A joint for use in total hip replacement surgery substantially as herein described with reference to the accompanying drawings.

11. A set of components for use in total hip replacement surgery, comprising stem component and a separate head component, in which: the stem component comprises a shaft adapted to be secured within a patient's femur and a spigot adapted to project from the femur; the head component comprises an inner body and an outer body, the inner body being pre-assembled to be pivotally retained within the outer body; the inner body has a recess into which the spigot of the stem component can be securely received through a suitably positioned aperture in the outer body; and the outer body is adapted to be fixed within an acaetabulum of a patient's pelvis.