GB2333961A

GB2333961A - Acetabular cup

Info

Publication number: GB2333961A
Application number: GB9821443A
Authority: GB
Inventors: Ian H Paling; Russell Lloyd; Richard Rawlinson
Original assignee: Corin Medical Ltd
Current assignee: Corin Medical Ltd
Priority date: 1998-02-05
Filing date: 1998-10-05
Publication date: 1999-08-11
Anticipated expiration: 2018-10-05
Also published as: GB2333961B; GB9802499D0; GB9821443D0

Abstract

An acetabular cup 10 comprises an articulating inner surface 12, an outer surface 11 and an annular rim 13 therebetween. The articulating surface of the cup is formed of metal or ceramics material and the rim has at least one recess 14, but preferably three recesses 14, engageable by an introducing device for enabling introduction and location of the cup into a patient's acetabulum. There is also disclosed an introducing tool having three pivotable fingers which can be inserted into the three recesses in the annular rim of the cup and then pivoted so as to urge the fingers into frictional engagement with the walls of the recesses.

Description

ACETABULAR CUP The present invention relates to an acetabular cup alone and in combination with means for introducing the cup into a patient's acetabulum.

Hip prostheses commonly consist of an acetabular cup and a femoral stem having a ball joint designed to be held inside a socket in the acetabular cup. Most commonly, the femoral stem is of a metallic design, whereas the acetabular cup, on which the ball joint of the femoral stem bears, is manufactured from Ultra High Molecular Weight Polyethylene, to allow creation of a low friction bearing.

However, polyethylene components abrade and it is now understood that polyethylene debris, caused as a result of abrasion through articulation, is a major cause of osteolysis and resultant failure of the hip prosthesis.

Recently, there has been a move towards the use of all metal acetabular cups. The advantage of metal-on-metal components is that metallic components abrade very much less than polyethylene components.

One known method of introducing the acetabular cup into the patient's acetabulum is the use of an introducing tool which has a pair of spigots at one end.

One of the spigots is fixed and the other is retractable. The spigots are placed into a pair of diametrically opposed holes formed in the articulating surface of the cup.

The introducing tool has an elongate handle which enables a surgeon to insert the cup attached to the spigots of the tool into the patient's acetabulum. The tool is then removed by disengaging the spigots from the holes.

The introduction holes in the acetabular cup cause discontinuities in the articulating surface. Whilst it is not believed that these holes compromise the articulating surface, if metal burrs were to be present, they could cause minor scratching of the articulating surface.

Another known method of introducing the acetabular cup into the patient' s acetabulum is the use of a dome-shaped plastics introducer which bears on the articulating surface of the cup. This method does not cause a defilement of the articulating surface, but does make it difficult to achieve the correct acetabular cup orientation within the acetabulum.

The present invention seeks to provide an acetabular cup alone and in combination with an introducing tool which overcomes the aforementioned disadvantages.

According to a first aspect of the invention there is provided an acetabular cup comprising an articulating inner surface, an outer surface and an annular rim therebetween, the articulating surface of the cup being formed of metal or ceramics material and the rim having at least one recess engageable by an introducing tool for enabling introduction and location of the cup into a patient's acetabulum.

Provision of at least one slot in the rim of the acetabular cup enables engagement of a suitable introducing tool with the slot so that the cup can easily be introduced into the patient's acetabulum without compromising the articulating surface and thus helping to minimise wear between the femoral stem and the acetabular cup.

Preferably, the cup is formed entirely of metal or ceramics material and typically of high carbon cobalt chrome-alloy.

Preferably, the articulating surface of the cup is an uninterrupted hemispherical or substantially hemispherical surface.

Advantageously, the rim has three recesses engageable by the introducing tool. In this case, the recesses are, preferably, equi-angularly spaced around the rim of the cup.

To improve the fixation of acetabular cup in the patient's acetabulum, the outer surface of the cup may be textured or may comprise a plurality of splines.

The outer surfaces of the cup may be coated with any one or any combination of the following: Hydroxyapatite, porous coating and plasma coating.

This encourages bone ingrowth from the patient's acetabulum into the surface of the acetabular cup enhancing fixation of the cup.

In order to prevent toggling of the acetabular cup within the patient's acetabulum, a supra-medial peg may extend from the outer surface of the cup.

To allow initial fixation in the acetabulum, the equatorial diameter of the outer surface of the cup is, preferably, greater than the polar diameter of the outer surface of the cup.

According to a second aspect of the invention there is provided an acetabular cup according to the first aspect of the invention in combination with an introducing tool comprising an elongate stem and a head, the head having means for releasably engaging the at least one slot of the acetabular cup.

Preferably, the rim of the cup has three recesses and the head includes three pivotable fingers which can be inserted into the three recesses, respectively, and then pivoted so as to urge the fingers into frictional engagement with the walls of the recesses. In this case, preferably, the fingers are urged by spring means towards a position in which they are insertable into the recesses. Also in this case, the introducing tool, preferably, further includes a threaded sleeve which is threadably mounted on the elongate stem and which is movable into contact with the fingers to move the fingers into frictional engagement with the walls of the recesses.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of one embodiment of an acetabular cup according to the first aspect of the present invention, Figure 2 is a side view of the acetabular cup of Figure 1, Figure 3 is a bottom plan view of the acetabular cup of Figure 1, Figure 4 is a section view taken along the line A - A of Figure 3, and Figure 5 is longitudinal, sectional view of one embodiment of an introducing tool which forms part of the second aspect of the invention.

Referring firstly to Figures 1 to 4, the acetabular cup 10 shown therein has a part-circular outer surface 11 of an appropriate diameter to fit into the acetabulum of a patient. The acetabular cup 10 also has a part circular, typical hemispherical or substantially hemispherical, articulating surface 12 having a diameter appropriate for receiving a femoral head (not shown). The diameter of the articulating surface 12 is within the range 40 - 60 mm. The acetabular cup 10 is formed of metal, preferably high carbon cobalt chrome-alloy, although it could be formed of a ceramic material such as zirconia.

Extending between the outer surface 11 and the articulating surface 12 is an annular rim 13. The surface of the rim 13 is planar. The rim 13 is provided with three equi-angularly spaced introducer recesses 14. The recesses 14 open out only into the rim 13 and are spaced from the outer surface 11 and the inner surface 12.

To improve the fixture of the acetabular cup 10 in the acetabulum of the patient, the outer surface of the cup may be textured and/or comprise a plurality of splines 15. These splines are located at approximately 45" to the position of one of the recesses 14. To ensure that the acetabular cup 10 remains within the acetabulum before it becomes fixed by bone growth, the cup is shaped such that the equatorial diameter de of the outer surface 11 is greater than the polar diameter cip of the outer surface 11. The acetabulum is then reamed to the same dimension as the polar diameter cip. The cup 10 can then be jammed into the acetabulum due to the larger equatorial diameter of the cup. This is particularly advantageous in the case of introducing metal cups for metal-on-metal articulation since the larger frictional torque can cause the cup to tear out of the acetabulum before it can become fixed by bone growth.

A supra-medial peg 16 also extends from the outer surface 11 of the cup.

The cup is either formed by forging or investment casting. The investment casting is subjected to hot isostatic pressing (HIP) and solution heat treatment processes. The inner surface 12 of the cup is then highly polished.

The outer surface 11 of the cup 10 may be coated with artificial bone substitute material such as Hydroxyapatite, or the outer surface may have a porous coated or plasma coated surface to enhance fixation of the cup within the acetabulum of the patient. The outer surface 11 may, for example, be coated with Hydroxyapatite and a porous coating over a majority of the area of the outer surface 11. The porous coating is also applied to the splines 15 and the area immediately surrounding the splines having a coating of Hydroxyapatite to enhance fixation of the cup 10.

Referring now to Figure 5 of the drawings, the introducing tool shown therein comprises an elongate stem 20 and a head 21. The stem 20 comprises an externally threaded part 22 and a handle 23 pivotably connected to the threaded part 22. An internally threaded sleeve 24 is mounted on the threaded part 22 of the stem 20 for a purpose which will become apparent.

The head 21 comprises a plate 25 having three upstanding lugs 26. A generally L-shaped finger 27 is pivotably mounted on each of the lugs 26. Springs (not shown) urge the free ends of the fingers radially inwards towards the axis of the stem 20 and into a position in which they are insertable into the recesses 14 in the rim 13 of the cup 10. Once the fingers 27 have been inserted in the recesses 14, the threaded sleeve 24 can be rotated to bring one end of the sleeve into contact with the fingers 27. This causes the free ends of the fingers 27 to pivot radially outwards and to be urged into frictional engagement with the walls of the recesses 14. The surgeon then uses the introducing tool to insert the acetabular cup 10 into the acetabulum of the patient. Once inserted into the acetabulum, the surgeon can rotate the sleeve 24 in the opposite direction so as to release the fingers from frictional engagement with the recesses. The introducing tool is then removed from the cup.

In providing the recesses in the rim of the cup, the articulating surface of the cup is not compromised and this helps to minimise the wear between the femoral head and the interior surface of the cup. Furthermore, the introducing tool enables the cup to be easily inserted into the acetabulum of the patient.

Although the cup is described as having three recesses, it may have more or less recesses.

The acetabular cup may be used with a re-surfaced patient's femoral head or with a stemmed femoral hip replacement. The femoral head of the hip replacement may be of metal to provide metal-on-metal articulation.

Claims

CLAIMS 1. An acetabular cup comprising an articulating inner surface, an outer surface and an annular rim therebetween, the articulating surface of the cup being formed of metal or ceramics material and the rim having at least one recess engageable by an introducing device for enabling introduction and location of the cup into a patient's acetabulum.
2. An acetabular cup as claimed in claim 1, wherein the cup is formed entirely of metal or ceramics material.
3. An acetabular cup as claimed in claim 2, wherein the cup is formed of high carbon cobalt chrome-alloy.
4. An acetabular cup as claimed in any one of the preceding claims, wherein the articulating surface of the cup is an uninterrupted hemispherical or substantially hemispherical surface.
5. An acetabular cup as claimed in any one of the preceding claims, wherein the rim has three recesses engageable by the introducing device.
6. An acetabular cup as claimed in claim 5, wherein the recesses are equiangularly spaced around the rim of the cup.
7. An acetabular cup as claimed in any one of the preceding claims, wherein the outer surface of the cup has a plurality of splines.
8. An acetabular cup as claimed in any one of the preceding claims, wherein the cup further comprises a supra-medial peg extending from the outer surface.
9. An acetabular cup as claimed in any one of the preceding claims, wherein the equatorial diameter of the outer surface of the cup is larger than the polar diameter of the outer surface of the cup to allow initial fixation in the acetabulum.
10. An acetabular cup as claimed in any one of the preceding claims, wherein the outer surface of the cup is coated with any one of or any combination of the following: Hydroxyapatite, porous coating or plasma coating.
11. An acetabular cup substantially as hereinbefore described with reference to the accompanying drawings.
12. An acetabular cup as claimed in any one of the preceding claims in combination with an introducing tool comprising an elongate stem and a head, the head having means for releasably engaging the at least one slot of the acetabular cup.
13. The combination of claim 12, wherein the rim of the cup has three recesses and the head includes three pivotable fingers which can be inserted into the three recesses, respectively, and then pivoted so as to urge the fingers into frictional engagement with the walls of the recesses.
14. The combination of claim 13, wherein the fingers are urged by spring means towards a position in which they are insertable into the recesses.
15. The combination of claim 13 or claim 14, wherein the introducing tool further includes a threaded sleeve which is threadably mounted on the elongate stem and which is movable into contact with the fingers to move the fingers into frictional engagement with the walls of the recesses.