Cementless prosthetic bearing element

Abstract: A cementless prosthetic bearing element comprises a backing (1) which supports a bearing liner (2) having a bearing surface (3), said backing (1) being moulded from a rigid polymeric material and said bearing liner (2) being made from an elastomeric polyurethane material and to which said backing (1) is moulded with an outer surface (3) which includes a number of raised engagement features provided in directions extending away from and/or towards the outer periphery of the outer surface.

Description: This invention relates to a cementless prosthetic bearing element. At the present time wear of the well known polyethylene acetabular components in hips, the tibial component in knees, and other prosthetic bearing inserts limits the life of these artificial joints. Normally orthopædic devices for joint construction and reconstruction comprise polyethylene bearings on metal or ceramic or metal on metal for articulation. The polyethylene bearing is often held in a metal shell, for example in hip constructions or a tray in knee constructions. Conveniently bearing liners made from ultra-high molecular weight polyethylene are fixed via a snap fit into the metal backing. The possible use of elastomers in bearings to encourage fluid film lubrication at low velocity, and with a low viscosity lubricant such as in a reconstructed joint has been published in laboratory studies which show that such "compliant" bearings provide lower coefficient of friction compared with standard polyethylene versus metal bearings. Soft grades of polyurethane have been shown to be suitable materials for compliant bearings in, for example, EP 0 698 382 EP 0 876 820 and EP 0 876 821 Because of the fluid film lubrication, it is far less likely that the two bearing surfaces touch during use and thus wear is lower. It is however difficult to apply this concept for a number of reasons. One difficulty lies in selecting a polyurethane or other elastomer which will not be degraded by the human body so great care must be taken in selecting compliant bearing material. As mentioned above polyethylene bearing are usually held by physical means in, for example, a metal shell but it is not possible to use this approach with a "compliant" bearing because as its stiffness is so much lower than the metal it would be possible for it to extrude from the shell or be easily moved relative to the shell causing damage. Also lower stiffness would lead to greater interface strain. Because of the above difficulties the Applicants have tried several different approaches. Initially, for ease of use and conventional appearance, it was attempted to bond a soft grade of polyurethane onto a stiffer polyurethane backing to form a construct similar to pre-existing polyethylene components. This construct could then be assembled in a metal shell or cemented depending on the surgeon's preference and clinical judgement. The present invention therefore is related to a prosthetic bearing element which uses an elastomeric "compliant" bearing material. EP 0 698 382 describes a bearing element according to the preamble of appended claim 1, which has a backing made from a rigid polymeric material and a soft elastomeric polyurethane material liner. Bearing elements of this type can be made by a moulding process and this is also described. Similarly, EP 0 552 949 also describes a process for the manufacture of a acetabular cup of particular configuration and in which the inner bearing component and the backing are moulded together to form a single component, the backing being stiffer than the inner bearing component. The backing comprises a part-spherical main portion and two independent arms projecting therefrom and which are separated from each other. It has been found that the use of a moulding technique to make such prosthetic bearing elements can be used to advantage to produce elements which have superior qualities and which do not require machining on the external surface of the backing. It is known, for example from EP 0 297 789 to provide bearing elements of this type which are to be held in place by cement with concentric circumferential and radial grooves to assist in stabilising an securing the shell stiffener to the layer of cement when the cup is placed in the acetabular socket. It is relatively easy to machine such grooves. WO 99/03429 shows an acetabular cup provided with radially extending and projecting asymmetric support ribs are provided and in the preferred arrangements are initially separate from the cup and are subsequently fastened to it. The intention of this construction is that the ribs can be located for adaption to the particular bone defects. The ribs project beyond the outer rim of the cup and are therefore vulnerable to damage whilst the cup is being inserted. Moreover, with this construction the preparation of the acetabulum can be complicated. Due to the ribs projecting beyond the outer rim of the cup they are vulnerable and, under stress, are liable to detach from the cup, especially as according to this construction they can be made so that they release under impaction instruments. The labour and skill required in attaching the ribs to the cup also renders this construction expensive to manufacture and operate. The present invention is intended to provide a cementless prosthetic bearing element which has been moulded and which is possible to place in an acetabular socket without cement, the outer surface of the backing being provided with means to assist stabilization and the ingrowth of bone. Thus, the bearing element according to the present invention is cheaper and easier to produce and provides superior location and installation in the bone. According to the present invention a cementless prosthetic bearing element comprises a backing moulded from a rigid polymeric material which is moulded to and supports a bearing liner made from an elastomeric polyurethane material and which has a bearing surface, said backing comprising a part-spherical main portion and two independent arms projecting therefrom and which are separated from each other, wherein said backing is moulded with an outer surface having a rim and moulded raised engagement features which include at least two spaced apart upwardly projecting engagement fins normal to the surface and which extend inwardly away from the rim of the outer surface of the part-spherical main portion in parallel chordal directions towards the independent arms. In one preferred embodiment the engagement features can also include projecting strakes. The engagement features can be of substantially triangular cross-section. The invention can be applied to bearing elements, the outer surface of the backing of which can be of any desired shape, for engagement with the bone concerned and if the element is in the form of an acetabular cup or similar device the outer surface of the backing can be substantially part-spherical and the projecting strakes can extend in radial directions. It will be appreciated that the time and expense involved in attempting to machine such a surface having the kind of features referred to and such fins is extremely costly and time consuming whereas by applying the Applicant's moulding technique they can be formed relatively easily. The information set forth in EP 0 698 382 can be applied to the present invention and thus the backing can be made from a "rigid" polymeric material having a minimum hardness value of 65 N/mm 2 and the bearing liner can be made from a "soft" elastomeric polyurethane material having a hardness value of 3.0 to 9.0 N/mm 2 using hardness testing method BS 2782;PT3 Method 365D. The bearing liner is preferably moulded to the backing to form a single component and the backing can be made from polyurethane, for example Corothane 75D. Alternatively the backing can be made from carbon fibre reinforced plastics material, for example polybutyleneterphthlate (CFR-PBT), or an alloy of CFR-PBT with polyurethane. Again, the backing material can be made from polyetheretherketone (CFR-PEEK). With yet another construction the backing material can be made from polymethylmethacrylate, all these materials being suitable for moulding. Preferably the bearing liner is made from Corothane 80A. The inner bearing surface of the bearing liner can be substantially part-spherical over a portion thereof spaced substantially opposite a separation or opening between the arms of the backing. Preferably the bearing liner has independent arms similar to the backing with a separation or opening between them and preferably the bearing liner is of substantially the same configuration as the backing. Alternatively the bearing liner can be substantially hemispherical and extend across the separation or opening between the arms of the backing. The invention can be performed in various ways but one embodiment, as applied to an acetabular cup, will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a side elevation of a prosthetic bearing element according to the invention; Figure 2 is a front elevation of the element shown in Figure 1; Figure 3 is a plan view from above of the element shown in Figures 1 and 2; Figure 4 is a plan view from below of the element shown in the preceding Figures; Figure 5 is a cross-sectional side elevation on the line V-V of Figure 3; Figure 6 is a cross-sectional view on the line VI-VI of Figure 3; Figure 7 is an isometric view from above of the element shown in the preceding Figures; and Figure 8 is a cross-sectional view through part of the element. As shown in the drawing the cementless prosthetic bearing element according to the invention comprises a backing 1 which supports a bearing liner 2 (most clearly shown in Figures 5 and 6). The bearing liner 2 has a bearing surface 3. The backing is moulded from a relatively rigid polymeric material, for example polyurethane in the form of Corothane 750D, CFR-PBT, CFR-PEEK or polymethylmethacrylate. The liner can be made from Corothane 80A and the liner and backing can be moulded together by the technique described in EP 0 698 382, the liner being bonded to the backing by the moulding process. The general overall shape and construction of the prosthesis is similar to that described in EP 0 552 949, 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 3 is also hemispherical and fits within the backing 1 but the inner surface of the bearing component is only hemispherical over its main portion only it is relieved over inner surfaces 7 and 8 of its arms 9 and 10. If desired the inner bearing liner 3 could be substantially hemispherical and merely carry a thin splitting line to provide the two arms. 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 arms themselves and the main portion are together substantially part-spherical. The 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 semicircular 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. It has been found that this particular shape of opening is convenient and successful and the loading is transferred into the pelvis as required, in particular this shape of opening ensures efficiently that no load is transferred into the bone at an undesired location. The outer surface of the backing 1 includes a number of raised engagement features which are provided in directions extending away from and/or towards the outer periphery 13 of the outer surface. Thus, the raised engagement features are in the form of projecting strakes 18 which extend radially from the outer periphery 13 and are of triangular cross-section, as is most clearly shown in Figure 8. Between the projecting strakes the surface is substantially part-spherical. The outer surface of the backing 1 also carries a pair of projecting fins 19 which extend in spaced apart parallel chordal directions. The outer surface of the backing is plasma sprayed with hydroxyapatite (HA) coating which is osteo-conductive and stimulates bone growth. 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 the strakes on the cup. Similarly two chordal grooves are cut to accept the fins 19. The cup is now placed in position and lightly tapped, the strakes and fins holding it accurately as required. The joint is now reassembled and it has been found that the pressure of the ball of the femoral prosthesis, if such a prosthesis is used, or the natural ball shape of the bone, acts to pressurise the cup and hold it in position whilst bone growth takes place. In the construction described the invention is applied to an acetabular cup but it can be applied to other types of prostheses in which its advantageous construction allows it to be used. In the constructions described above the bearing liner and backing are moulded together so that the liner is bonded to the backing by the moulding process. If desired however, the bearing liner and backing can be retained together by a mechanical interlock between the two components. Thus the liner or the backing could have elements which engage in the other component or they could be fastened together by separate means such as studs or screws or any other suitable mechanical interlock arrangement.

Claims: A cementless prosthetic bearing element comprising a backing (1) moulded from a rigid polymeric material which is moulded to and supports a bearing liner (2) made from an elastomeric polyurethane material and which has a bearing surface (3), characterised in that said backing (1) comprises a part-spherical main portion (4) and two independent arms (5) (6) projecting therefrom and which are separated from each other, wherein said backing (1) is moulded with an outer surface having a rim (13) and moulded raised engagement features which include at least two spaced apart upwardly projecting engagement fins (19) normal to the surface and which extend inwardly away from the rim (13) of the outer surface of the part-spherical main portion (4) in parallel chordal directions towards the independent arms (5)(6). A cementless prosthetic bearing element as claimed in claim 1 characterised in that said engagement features also include projecting strakes (18). A cementless prosthetic bearing element as claimed in claim 1 or claim 2 characterised in that said engagement features are of substantially triangular cross-section. A cementless prosthetic bearing element as claimed in any one of claims 2 or 3 characterised in that said outer surface is substantially part-spherical and the projecting strakes (18) extend in radial directions. having a rim (13) and A cementless prosthetic bearing element as claimed in any one of preceding claims characterised in that said rigid polymeric material has a minimum hardness value of 65 N/mm 2 and said bearing liner is made from a "soft" elastomeric polyurethane material having a hardness value of 3.0 to 9.0 N/mm 2 . A cementless prosthetic bearing element as claimed in any one of the preceding claims characterised in that said backing (1) is made from polyurethane. A cementless prosthetic bearing element as claimed in claim 6 characterised in that said polyurethane is Corothane 75D. A cementless prosthetic bearing element as claimed in any one of prceding claims 1 to 5 characterised in that said backing is made from carbon fibre reinforced polybutyleneterphthlate (CFR-PBT) . A cementless prosthetic bearing element as claimed in any one of preceding claims 1 to 5 characterised in that said backing (1) is made from polyetheretherketone (CFR-PEEK) . A cementless prosthetic bearing element as claimed in any one of preceding claims 1 to 5 characterised in that said backing (1) is made from polymethylmethacrylate. A cementless prosthetic bearing element as claimed in any one of the proceeding claims 1 to 10 characterised in that the bearing liner (2) is made from Corothane 80A. A cementless prosthetic bearing element as claimed in any one of the preceding claims 1 to 11 characterised in that it is in the form of an acetabular cup. A cementless prosthetic bearing element as claimed in any one of the preceding claims characterised in that the inner bearing surface (3) is substantially part-spherical over a portion thereof spaced substantially opposite a separation or opening (12) between the arms (5) (6) of the backing (1) . A cementless prosthetic bearing element as claimed in any one of the preceding claims characterised in that the bearing liner (2) has independent arms (5) (6) similar to the backing with a separation or opening (12) between them. A cementless prosthetic bearing element as claimed in claim 14 characterised in that said bearing liner (2) is of substantially the same configuration as the backing (1). A cementless prosthetic bearing element as claimed in any one of the preceding claims characterised in that the bearing liner (2) is substantially hemispherical and extends across the separation or opening between the arms (5) (6) of the backing (1).

Inventors: FIELD RICHARD EDDY, RUSHTON NEIL, STORER ANDY, FIELD, RICHARD EDDY, RUSHTON, NEIL, STORER, ANDY
Applicants: BENOIST GIRARD SAS
Application year: 2001
Main classification: A61F2
IPC codes: A61F2, A61L27