Patent Application
20070038304
This invention relates to a flexible acetabular cup and a method of manufacture thereof.
Flexible acetabular cups are known, the inner bearing surfaces of which comprise a substantially part-spherical portion and two independent arms projecting therefrom and formed by a separation or opening in the rim of the cup.
Flexible acetabular cups of this type are shown in U.S. Pat. Nos. 5,609,646 and 6,758,864, the disclosures of which are incorporated herein by reference.
Cups of this type have a polar area in a part-spherical portion which carries the main load and the part-spherical portion can be relieved over part of its surface and the arms.
With flexible cups of this type the flexibility can cause the risk of deformation at the polar contact area so that the cup deforms and creates only ring contact which is undesirable.
Prior to fitting of the cup the acetabular socket is reamed and the tolerances can leave a difference of 0 mm to 0.6 mm between the socket and the cup so it may not be a perfect fit.
It has been found that the cup can be arranged to be inserted with an interference fit of up to about 0.6 mm but above that it as been found that the interference causes too much deformation and again creates a ring contact.
The present invention is intended to provide a construction and method of making a flexible acetabular cup which overcomes the difficulties concerned.
According to the present invention a flexible acetabular cup, the inner bearing surface of which comprises a substantially part-spherical portion and two independent arms projecting therefrom and which are formed by a separation or opening in the rim of the cup, and in which there is a polar bearing area in the part-spherical portion and part of the part-spherical portion and the arms is relieved, characterized in that the external surface of the cup is shaped to cause the independent arms to be pre-stressed towards each other when the cup is inserted in place in a pre-prepared acetabular socket.
Thus, the external cordal-dimensions of the cup across the separation or opening between the arms can be greater than the external diameter of the part-spherical portion so that when the cup is placed in position in a pre-prepared part-spherical socket the arms are pre-stressed inwardly towards the separation between them.
The separation between the arms can be greater by 0.3 mm to 0.5 mm than the external diameter of the part-spherical portion.
It has been found that the pre-stressing of the arms reduces deformation and acts to prevent the polar area of contact degenerating into ring contact.
The inventions also includes a process for making the flexible acetabular cup as set forth above by including the steps of:
Thus the separator can distort the arms by between 0.1 mm and 1 mm and preferably between 0.3 mm to 0.5 mm from the original molded shape so that the arms pre-stress when the cup is placed in the prepared acetabulum.
The invention can also include a method of fitting a flexible acetabular cup into an acetabular socket which includes machining the socket to provide a part-spherical recess and inserting a flexible cup as set forth above under pressure to cause the arms to deform towards each other so that they are pre-stressed toward each other when in place.
The invention can be performed in various ways and one embodiment will now be described by way of example and with reference to and as shown in the accompanying drawings.
The cup can be made from a single material, for example (CFR PEEK) as described in U.S. Pat. No. 6,758,864, or from two or more materials as set forth in U.S. Pat. No. 5,609,646.
The cup shown in
The overall shape and construction of the cup is similar to that described in U.S. Pat. No. 5,609,646, that is the main portion 4 of the backing is substantially part-spherical and there are two independent arms 5 and 6 which extend from the main part 4. The external shape of the inner bearing liner is also hemispherical and fits within the backing 1 but the inner surface of the bearing component is only hemispherical over its main portion and is relieved over the inner surfaces 7 and 8 of its arms 9 and 10, best shown in
If desired the inner bearing liner can be substantially hemispherical and merely carry a thin splitting line to provide the two arms but in the construction shown in the drawings the arms 5 and 6 and 9 and 10 are spaced apart to provide a gap or opening 12 between them. The arms are spaced apart about an arc on the part-spherical main portion of the backing and the liner breaking out on the rim 13 and the outer surface of the arms themselves and the main portion are together substantially part-spherical.
Backing 1 thus comprises a substantially part-spherical wall having a rim 13 which is interrupted by a shaped opening to provide two spaced apart arms 5 and 6.
The main part 14 of the opening 12 is substantially semi-circular and has a mouth 15 which provides the interruption in the rim 13 and which is of smaller width than the remainder 14 of the opening. The backing 1 is therefore substantially horseshoe-shaped.
Because of its construction and the materials used the backing is sufficiently flexible to accept deformation of the acetabulum of the patient but is stiffer than the inner bearing component.
In the preferred construction shown in the drawings the outer surface of the backing 1 does not include the projecting strakes of U.S. Pat. No. 6,758,864 and the surface is substantially part-spherical apart from a pair of projecting fins 19 which extend in spaced apart parallel cordal directions.
The outer surface of the backing is preferably plasma sprayed with hydroxyapatite (HA) coating which is osteo-conductive and stimulates bone growth. The cup can also be made as set forth in UK Patent Application No. 04 22066.8 and have a metal backing layer, for example titanium, tantalum or niobium or even pure PEEK, to produce a barrier between composite material and the bone cells.
In order to insert the acetabular cup into an acetabulum, the bone is first prepared to the appropriate shape and a series of triangular grooves are carved into it by the surgeon which will line up with and accept fins 19.
The cup is now placed in position and tapped, the fins 19 holding it accurately as required. It has been found that the pressure of the ball of the femoral prosthesis or the natural ball shape of the bone acts to pressure the cup to hold it in position while bone growth takes place.
With this arrangement it is desirable to provide certain tolerances but it has been found that there is a risk of deformation at the polar contact area (indicated by reference numeral 20 in
The effect of the deformation is shown diagrammatically in
The cup is molded initially to the shape shown in full lines in
After the cup has been initially molded, but whilst it is still warm, it is placed in the jig 30. The width of the block 34 is greater than the width of the opening 12 of the molded cup by between 0.1 mm and 1 mm and preferably between 0.3 mm and 0.5 mm. With the cup placed on the block 34 pressure is applied to the pressure pad 35 which forces the cup over the block thus distorting the arms 5 and 6 so that they are pressured outwardly to the position shown by broken lines 24 in
With the cup held in this position it is then allowed to cool assisted by the water cooling in the channels 32.
The removed cup is of the shape shown in chain lines in
In order to install the cup in a prosthetic socket the socket is machined to a hemispherical shape it will be appreciated that the distortion has caused the arms to expand slightly in relation to the remaining portion of the cup so that it is no longer part-spherical but of a slightly deformed shape. When the cup is now placed in the part-spherical machined socket the arms are pressed towards each other so that the cup is pre-pressurized.
It has been found that with this pre-pressurization and with an interference fit effect of up to 0.6 mm deformation of the cup in the polar area is prevented as is the creation of ring contact.
It will be appreciated that in the description of the device and apparatus shown above a particular embodiment of cup is referred to but the invention can be applied to any cup of the type which has the two separated arms and which can have a variety of types of construction or shape.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0507884.5 | Apr 2005 | GB | national |
