ENDOPROSTHETIC BONE JOINT DEVICES

Abstract: Endoprosthetic bone joint devices commonly comprise one or more components having one part adapted for securement with a bone of the joint and another part adapted for mutual articulatory engagement with another such component or natural integer of the joint. This invention provides such a component which is substantially wholly porous, but in varying manner with greater porosity in the one part than the other part. This variation preferably involves pore diameters in the one and other parts within respective ranges from 10 to 250 //c and substantially zero to 150 //c. Also, the component is preferably made of fibre reinforced material with the reinforcement conforming to a predetermined pattern, a suitable material being carbon - fibre - reinforced carbon.

Description: ~ l ~ EJfDOPROSTHETIC BONE JOINT DEVICES Endoprosthetic bone joint devices have been the subject of considerable development during the past few decades and are now widely employed in routine clinical practice. The development in question has given rise to improvements in many respects, among which important ones are the biocom- patability of the materials used in the devices, the frictional and other mechanical properties of such materials whereby the articulatory and wear characteristics of the devices have been enhanced, and the techniques for securing the devices to bones. However, considerable as the improvementshave been, there is room for further improvement in most respects when viewed against the overall performance of natural healthy joints. The object of the present invention is to afford some such further improvement and to this end provides an endoprosthetic bone joint device comprising a component of which one part is adapted for securement with bone and another part is adapted for mutual articulatory engagement with another integer of the joint, said component being substantially wholly porous but in varying manner with greater porosity in said one part than said other part. Preferably the desired variation in porosity involves differences in pore size in a diametral sense, with the pores being interlinked throughout the component rather than being of a closed cell form. The intended benefit of the proposed component is that it promotes anastomosis, that is to say the growth of living tissue into the component. This tissue in-grσwth occurs in a fibrous form followed by ossification within the larger pores of the one part to provide secure anchorage of the component, while the tissue in the other part, and particularly at the outer surface, assumes a fibrocartilagenic form to provide enhanced articulatory properties. The anastomosis at the outer surface can be additionally promoted by the provision of grooving therein. O PI -IPO Suitable ranges of pore size for the one and other parts of the component are respectively 10-250 *m and from substantially zero, or 5 Men say. "to 150 _un, with the smaller regions of such ranges being preferred. The reason for this last preference is evident from the fact that the character of the component will progressively improve during a period of time following surgical implantation, but the component requires certain initial properties which, generally speaking, will be more satisfactorily achieved with higher density for the component. For the same reasons, the component may well be adapted to afford securement of a mechanical nature to provide some measure of stability during the anastomotic period after implantation. A particularly important aspect of the present invention is that of the materials to be used for its implementation because the materials clearly must satisfy various requirements such as biocompatability, strength, etc., as well as be capable of formation with the desired varying porosity. In practice it is preferred that these materials be of so-called carbon-carbon or carbon-fibre-reinforced carbon (CFRC) form. Satisfaction by this preference of some of the materials requirements is immediately evident because carbon is of proven biocompatability in a variety of forms and is of proven high strength in fibre form composites. Other benefits of the use of CFRC are less immediately evident. The structure of bone in the region of the joints and elsewhere is not random or arbitrary but has a patterned form best designed to suit the functions of the bone. More specifically bone has a trabecullar structure which is generally orientated in the region of the joints to best sustain the loads transmitted therethrough. CFRC is produced, in very general terms, by impregnating carbon fibre material with a resin or some other binder and then pyrolising the resultant composite or by pyrolytic deposition on carbon fibre OM S ~ WIP material of carbon from hydrocarbon gases. In either event, it will be appreciated that the fibrous reinforcement can be structured in a similar manner to bone to deploy the strength of the material in a correspondingly optimum manner. Moreover, such a structure can and indeed normally will, provide a complement.ary pore pattern of a similar nature whereby the anastomosis provides a structure which further enhances the simulation of bone. A further benefit of CFRC is that it is normally porous and its porosity can be controlled to a satisfactory extent for the present purpose. This control involves a suitable choice of initial fibrous material in respect of fibre size, form of weave or other collective fabrication, etc., and choice of in-fill procedure. In this last respect, production of a CFRC component according to the invention suitably involves both impregnation and deposition as mentioned above. For example, the eventual component may involve production in separate parts with respectively different porosity attained by differing extents of isothermal carbon vapour deposition, which parts are then bonded with resin thereafter pyrolised, or a single component matrix of carbon fibre material and binder can be pyrolised and then subjected to thermal-gradient carbon vapour deposition. The principal features of the invention having been described above, it is useful to illustrate the invention further, by way of example, with reference to the accompanying drawings which schematically illustrate an endoprosthetic hip joint device comprising two components according to the invention, and in which:- Figures 1 and 2 are respective side and plan views of the acetabular component of the device; and Figures 3 and 4 are respective corresponding views of the humeral component of the device. The acetabular component of Figures 1 and 2 comprises a substantially hemispherical cup body 10 with inner surface 11 and outer surface 12, the body having a key 13 formed by a projection of its outer surface. More particularly the key projection is longest at the rim of the cup where it extends radially outwardly parallel to the rim in a generally rectangular shape, as seen in Figure 2, the remainder of the key being convergently tapered partway towards the cup apex in tangential manner as seen in Figure 1. The cup is formed from CFRC as described above with greater porosity at the surface 12 than 11 where it is least. In use the cup is located in the acetabulum, with the key in the acetabular notch, a suitable seating having been prepared for the component by reaming or other preparation whereby the cup seats in a close fit with the key acting against rotation of the cup relative to the bone. The femoral component of Figures 3 and 4 comprises a cap body 20 of an overall shape formed by a hemispherical portion leading smoothly at its rim into a right circular cylindrical projection. The inner and outer surfaces are denoted at 21 and 22, the former extending inwardly along an axially directed strip portion thereof to form a key 23 of circular arcuate transverse cross-sectional profile, and the latter being complementary in its hemispherical part with the cup 10 for mutual articulatory engagement therein. The cap is formed from CFRC as described above with greater porosity at the surface 22 than 21 where it is least. In use the cap is located over the head of the femur after suitable surgical shaping of the latter to receive tl f e cap in a close fit with the key acting against rotation relative to the bone. The device formed by these components is similar in general geometry and intended application to others which are already available, but these others differ in the use of metal/plastics materials combinations which do not allow OMPI - 5 - anastomosis. The present invention is in fact more generally applicable in other known configurations for various joints and is in no way intended to be limited by the illustrated example.

Claims: CLAIMS 1. An endoprosthetic bone joint device comprising a component of which one part is adapted for securement with bone and another part is adapted for mutual articulatory engagement with another integer of the joint, said component being substantially wholly porous but in varying manner with greater porosity in said one part than said other part. 2. A device according to claim 1 wherein said porosity involves variation in pore diameter in said one and other parts within respective ranges from 10 to 2_>0 j m and from substantially zero to I5O -I11. 3. A device according to claim 1 or 2 wherein said component is made of a fibre reinforced material with such reinforcement being arranged to conform to a predetermined pattern therewithin. 4. A device according to claim 1, 2 or 3 wherein said component is made of carbon - fibre - reinforced carbon. 5. A device according to claim 4 wherein said component is made from initially separate portions respectively to form said one and other parts, with said portions being subjected to respectively different extents of isothermal carbon vapour deposition to afford correspondingly different porosity, and said portions thereafter being bonded together. 6. A device according to claim 4 wherein said component is formed to have said varying porosity by thermal-gradient carbon vapour deposition. 7- A device according to any preceding claim comprising a further component as aforesaid to act as said other integer. 8. An endoprosthetic hip joint device substantially as herein described with reference to the accompanying drawings.

Inventors: KNIGHT M
Applicants: KNIGHT M
Application year: 1980
Main classification: A61F1
IPC codes: A61F2, A61L27