Metal backing for inclusion in the manufacture of a prosthetic component

Abstract: A metal backing for inclusion in the manufacture of a prosthetic component which has a synthetic plastics material bearing element and comprising a metallic element (1) having a front portion with means for attachment to a bone and a rear portion (6) at least part of which is formed by a grill (8) having exposed front and rear faces and which is intended to be embedded within said bearing member (20) during construction.

Description: This invention relates to a metal backing for inclusion in the manufacture of a prosthetic component and to a metal component incorporating such a backing. There are often difficulties in securing a metal backing to the synthetic plastics material of a bearing member, sometimes because of the limited space available. Metal-backed patella components are well known within the industry. They have been subject to limitations on performance related to the small available space for their construction. Generally the factor affecting performance is the reliable and long term attachment of the plastic bearing elements to the metal backing which is sued to provide the fixation surface of the device, either for cemented use or for biological ingrowth type applications. Traditional products have used riveting or moulding techniques to provide for the interconnection of parts, these generally being geometric interference caused by this form of gross metal features on the metal backing, posts, holes, rims, etc. EP-A-0 522 999 (corresponding to US 5 181 924) shows the use of a metal grid on one side of a patella component to provide attachment of a slide member, the material thereof being intended to embed into the metal grid. US-A-4 997 445 shows the use of a metallic wire screen which is bonded to the upper surface of a base and a polyethylene bearing portion part of which penetrates the porous openings in the screen to enhance the securement of this polyethylene portion to the base. In both the above earlier constructions however the synthetic plastics material can only penetrate as far a the depth of the grid or screen because these members are located against flat support surfaces and the present invention is intended to provide a superior form of attachment utilising a metal grill. The present invention also has applications with regard to other metal backed components, for example tibial components, elbow and shoulder components and hip cups. According to the present invention a metal backing for inclusion in the manufacture of a prosthetic component which has a synthetic plastics bearing element comprises a metallic element having a front portion with means for attachment to a bone and a rear portion, at least part of said rear portion being formed by a grill characterised in that said grill (8) is spaced away from said metallic element (1) thus having exposed front and rear faces and which grill is adapted to be embedded within said bearing element during construction. Thus, due to the construction of the metallic element the synthetic plastics bearing material of the bearing element can be moulded so that both exposed faces of the grill are covered and the grill is embedded within the synthetic plastics material bearing element. Preferably the grill is located on a metal support and is provided with spacers on its front face to space said front face away from said support. The grill can be cast integrally into said metal support during manufacture. In one preferred embodiment the front portion of the metal backing is provided with means for attachment to a bone with cement. In an alternative construction the metal backing can be provided with means for mechanical attachment to a bone. Thus, the front portion can be provided with a biological ingrowth surface to facilitate bone ingrowth. With this arrangement the front portion of the metal backing can be provided with a reticular metal wire mesh and this mesh can be cast integrally into one side of a metal support on the other side of which said grill is located. The invention also includes a prosthetic metal backed component including a metal backing set forth above and a synthetic plastics material bearing element having a rear bearing surface and a front portion within which the grill is embedded. With this arrangement the grill is preferably moulded integrally into the bearing element during manufacture. As mentioned above the invention is particularly although not exclusively applicable for use with a patella component. The invention can be performed in various ways but one embodiment showing the application of the invention to a patella component will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is an isometric view of a metal backing for inclusion in the manufacture of a prosthetic patella component; Figure 2 is a cross-sectional side elevation through the component shown in Figure 1; Figure 3 is a plan view from beneath of the component shown in Figures 1 and 2; Figure 4 is a plan view from above of the same component; Figure 5 is an enlarged cross-sectional view of part of Figure 2; Figure 6 is an enlarged part-plan view showing the grill; Figure 7 is an enlarged part-cross-sectional elevation on the line VII-VII of Figure 2; Figure 8 is a cross-sectional side elevation of a complete prosthetic metal backed bearing component embodying the metal backing shown in Figures 1 to 7; and, Figure 9 is a plan view of the component shown in Figure 8. figures 1 to 7 show a metal backing for inclusion in the manufacture of a prosthetic patella component and comprises a shaped plate like metal support 1 on the front face of which is provided three location pins 3 and a central boss 4. On this face there is also a surrounding rim 5. The rear face 6 is substantially flat apart from a projecting boss 7. Mounted on the boss 7 is a grill 8 which has exposed front and rear faces and an outer rim 9. The grill is formed from substantially oval section members, best shown in Figure 5, and is spaced away from the face 6 by integral supports 10. The space between the rim 5 on the front face and the boss is filled with a reticular metal mesh 11 as is most clearly shown in Figure 7. The metal mesh 11 provides an ingrowth surface on the front face which is intended to be next to the prepared bone and the grill is intended to be embedded in a synthetic plastics material bearing element to be described hereunder. The grill and wire mesh are integrally cast with the metal support 1 using an investment casting technique where wax patterns for the castings are either assembled prior to casting, or ceramic inserts are used. The metal backing is used with a synthetic plastics material bearing element 20 as shown in Figure 8. In plan view the bearing element 20 is substantially the same shape as the support 1. Subsequent to the casting and finishing of the metal component, this component is then used as an insert within a plastic compression mould and polymer powder, for example ultra-high molecular weight polyethylene (UHMWP) bearing material is loaded on top of the backing plate and compression moulded in place to yield the construction shown in Figure 8. During this process the plastics material under the pressure and heat of the moulding operation forms a continuous structure underneath the grill 8 thereby providing an excellent mechanical lock between the two parts. Due to this construction it is possible to reduce the thickness of the metal structure so that a larger thickness of bearing plastic can be formed above it. This is in part because of the efficiency of the mechanical interlocking system but also because the grill construction reinforces the metal substrate of the component. A biological fixation surface may be formed by the same general process, or by other techniques well known in the industry. In the arrangement described the metallic mesh 11 is intended to provide ingrowth but if desired any suitable other surface could be used or a cement fixation surface can be applied in any manner well known in the art. As described above, the invention is applied to a metal backed patella component but it can be used in other metal backed components for example tibial components, elbow and shoulder components and hip cups where the ability to provide a rigid fixation in a restricted area will allow increased thickness of bearing material.

Claims: A metal backing for inclusion in the manufacture of a prosthetic component which has a synthetic plastics material bearing element (20) and comprising a metallic element (1) having a front portion with means (3)(11) for attachment to a bone and a rear portion, at least part of said rear portion being formed by a grill (8) characterized in that said grill (8) is spaced away from said metallic element (1) thus having exposed front and rear faces and which grill is adapted to be embedded within said bearing element (20) during construction. A metal backing as claimed in claim 1 characterised in that said grill (8) is located on a metal support (1) and is provided with spacers (10) on a first face of said grill (8) to space said front face away from said support (1). A metal backing as claimed in claim 1 or claim 2 characterised in that said grill (8) is cast integrally into said metal support (1) during manufacture. A metal backing as claimed in claims 1 to 3 characterised in that a first portion of the metal backing is provided with means for attachment to a bone with cement. A metal backing as claimed in any one of claims 1 to 4 characterised in that said front portion is provided with means (3) for mechanical attachment to a bone. A metal backing as claimed in claim 5 characterised in that said front portion is provided with a biological ingrowth surface mesh (11) to facilitate bone ingrowth. A metal backing as claimed in claim 4, claim 5 or claim 6 characterised in that said first portion of the metal backing is provided with a reticular metal wire mesh (11). A metal backing as claimed in claim 7 characterised in that said metal wire mesh (11) is cast integrally into one side of a metal support (1) on the other side of which said grill (8) is located. A prosthetic bearing component including a metal backing as claimed in any one of preceding claims 1 to 8 and characterised by a synthetic plastics material bearing element (20) having a rear bearing surface and a front portion within which the grill (8) is embedded. A prosthetic bearing element as claimed in claim 9 characterised in that said grill (8) is moulded into said bearing element during manufacture. A prosthetic bearing element as claimed in claim 9 or claim 10 characterised in that said component is a patella component.

Inventors: ASHBY ALAN, SCHWARTZ MEL JR, SLEMON DECLAN PATRICK, ASHBY, ALAN, SCHWARTZ, MEL, JR., SLEMON, DECLAN PATRICK
Applicants: HOWMEDICA, HOWMEDICA INTERNATIONAL INC.
Application year: 1995
Main classification: A61F2
IPC codes: A61F2