GB2286146A

GB2286146A - Fibre reinforced composite artefact

Info

Publication number: GB2286146A
Application number: GB9501247A
Authority: GB
Inventors: Roger Davidson; Steven Brabon; Anthony John Hammond
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1994-02-03
Filing date: 1995-01-23
Publication date: 1995-08-09
Anticipated expiration: 2015-01-23
Also published as: GB2286146B; GB9501247D0

Abstract

An artefact, such as a hip implant, with controlled mechanical properties is manufactured by filament winding a matrix precursor impregnated fibre material onto a mandrel (11; Fig. 3 not shown). At least part of the periphery in cross section of the mandrel corresponds with a boundary in longitudinal section of a product, preferably more than one product positioned end to end. The shape of the wound product is modified if necessary by applying thereto a preformed shaped piece of compatible matrix precursor impregnated fibre material (13; Fig. 5 not shown). At least one and preferably a multiplicity of product artefacts are generated by cutting from the wound product after curing. <IMAGE>

Description

Fibre Reinforced Composite Artefact The invention relates to a fibre reinforced composite artefact, more particularly in a form suitable for use as a femoral implant and to a method and apparatus for use in manufacturing such an artefact.

An increasing number of products are now manufactured from fibre reinforced composite materials.

In these, advantage can be taken of the flexibility provided by control of the density and alignment of the fibres to tailor the mechanical properties of the product most appropriately for its application. The use of filament winding machines provides for good control of filament density and alignment, but the available shapes and the relationship of the filament alignment to the shapes are limited. Sheet lay up and moulding techniques tend to be labour intensive and time consuming, nor is it easy to ensure good uniformity of product.

We have now devised a technique for adapting filament winding and, if appropriate, combining this with moulding, to mass produce economically shaped artefacts having longitudinally extending fibres.

The invention provides in one of its aspects a method of manufacturing a fibre reinforced composite artefact comprising filament winding a suitable matrix precursor impregnated fibre material onto a mandrel, modifying the shape of the wound product by applying thereto a preformed shaped piece of compatible matrix precursor impregnated fibre material to at least one peripheral location, converting (as by curing) the matrix precursor to product matrix and cutting the product in a direction parallel with the winding axis to generate at least one shaped artefact therefrom.

Preferably the product is also cut in a direction transverse to the winding axis to generate a plurality of artefacts. Such transverse cuts can be repeated to generate a multiplicity of artefacts. For mass production purposes it is convenient to arrange for all the multiplicity of artefacts to be identically shaped.

For this purpose, it may be necessary to modify the shape of the wound product by applying preformed shaped pieces of compatible matrix precursor impregnated fibre material to at least two peripherally spaced locations. It will generally be necessary to carry out machining to produce the final form of artefact after cutting from the aforesaid product.

The invention includes an artefact when made by a method as described above. The invention finds particular application in the fabrication of femoral prostheses. Hip replacements, consisting of femoral and acetabular components, are extensively used to replace diseased and age damaged hip joints, thereby giving patients mobility and a much improved quality of life.

Existing prostheses are usually based on either stainless steel, or alloys of cobalt-chromium-molybdenum or titanium forged or cast to the approximate size.

Subsequently the stem is machined to a diameter of 9mm18mm - a range of sizes is needed to accommodate a range of patient sizes. There are many different designs which may or may not be cemented into position in the femur.

In modular designs the cobalt-chromium or ceramic ball head of the joint is attached to a conical trunnion by a machined, tapered, Morse fitting. The femoral component incorporates a straight, polished, distal stem and a stiffened, extended, proximal region designed to prevent the implant rotating. For total replacement, the acetabular female part of the joint is replaced with a dome-shaped cobalt-chromium cup carefully screwed into the pre-reamed acetabulum. A moulded ultra-high molecular weight polyethylene (UHMWPE) liner is pushed into the cup to form the bearing surface with the modular head. Plasma sprayed porous coatings or hydroxyapatite may also be applied to both the outer surface of the acetabular cup and the proximal half of the prostheses.

The femoral part of a hip replacement is inserted into a hole reamed into the femur and may be bonded into place with a polymethyl methacrylate (PMMA) bone cement or rely on a tight fit and subsequent bone growth.

Problems arise because metal implants have a much greater stiffness than bone (200 GPa against 15-30 GPa) causing the bonded region to be unevenly loaded. If the loading of the cortical bone is too light, the bone density decreases resulting in fracture or loosening of the implant. High loading stimulates normal bone growth.

Mechanically the implant may behave as a cantilever resulting in implant failure at the region of stress concentration, possibly exacerbated by environmental stress corrosion. Also, as a result of immersion in the body environment all the alloys used in orthopaedics release ions. These can be damaging to surrounding bone tissue and the long term consequences for the patient are unknown. In addition the PMMA cement can cause bone necrosis which is probably due to the exotherm generated when polymerisation occurs.

At least some of these problems are ameliorated by a femoral hip implant fabricated in accordance with the present invention. In particular, the method provides for design control of the mechanical properties of the implant so that these match reasonably closely those of bone, whilst retaining the required strengths.

Accordingly the invention further provides a method of manufacturing a femoral implant comprising filament winding a suitable matrix precursor impregnated fibre material onto a mandrel which has a peripheral shape in cross-section at least part of which corresponds with one boundary of the shape in longitudinal section of the desired implant, converting the matrix precursor to product matrix and cutting the product in a direction parallel with the winding axis to generate therefrom at least one artefact of a shape corresponding to that of the desired implant.

Again, this approach lends itself to mass production by fabrication of a winding of substantial axial extent, the product from which can then be cut in a direction transverse to the winding axis to generate a plurality of slices from which artefacts are then produced by crosscutting in a direction parallel with the winding axis.

Preferably the mandrel has a peripheral shape in crosssection at least part of which corresponds with one boundary of the shape in longitudinal section of two or more of said desired implants positioned end to end so that when the product is cut in a direction parallel with the winding axis at an appropriate plurality of locations1 there is generated two or more artefacts of a shape corresponding to that of the desired implant from the or each 3600 radial extent of the winding.

For controlling the mechanical properties of the product, the winding is applied in a plurality of layers having differing helical winding angles. The sequence of layers and helical winding angles are chosen so that the implant produced has a stiffness similar to that of human bone.

To produce the required femoral implant shape, we have found it desirable to modify the shape of the filament wound product, prior to converting the matrix precursor to product matrix, by applying thereto at least one preformed shaped piece of compatible matrix precursor impregnated fibre material to at least one peripheral location.

The invention includes apparatus for carrying out the aforesaid method comprising a helical filament winding machine having a mandrel which has a peripheral shape in cross section at least part of which corresponds with one boundary of the shape in longitudinal section of a femoral implant.

The invention also includes a femoral implant when made by the aforesaid method.

A specific example of method, apparatus and femoral implant artefact embodying the invention will now be described with reference to the drawings filed herewith, in which: Figure 1 is a perspective view of a complete conventional modular implant the stem of which is fabricated from metal alloy, Figure 2 is a perspective view of a wound composite product removed from the mandrel on which it has been wound and with a ring cut from it, Figure 3 is a plan view of a mandrel, Figure 4 is a cross-sectional view of composite material formed by winding onto the mandrel of Figure 3, the inner periphery of which corresponds with the outer periphery in section of the mandrel, Figure 5 is a detail sectional view of part of a mandrel and compression plate arrangement, Figure 6 is a perspective view of a pre-formed composite shoulder section, Figure 7 is a diagrammatic sectional view of components of apparatus for fabricating the shoulder section shown in Figure 6, Figures 8a, 8b, and Sc show the final stages in fabrication of the composite femoral implant, being respectively as cut from the composite formed on the mandrel (Figure 2), after partial machining and as fully machined.

Figure 1 shows a conventional metallic femoral implant. Figure Sc shows the final carbon fibre composite product of the method to be described, the external shape closely matching that of the stem of the conventional implant.

A composite product of the shape shown in Figure 2 is produced by a combination of winding onto a mandrel and application, before curing, of pre-formed shoulder sections. The product shown in Figure 2 is sliced into a plurality of rings, one of which is shown in the figure, of width corresponding to the desired width of final implant prior to machining. Each ring as shown, when appropriately cut, provides four such blanks (Figure 8a) from which an implant of desired final shape is readily machined.

Figures 3 and 4 show the design of mandrel, the essential characteristic of which is that its periphery in section corresponds with the internal boundary formed by positioning four components of the shape shown in Figure 8a end to end in a ring.

A mandrel 11 is mounted in a conventional filament winding machine and wound with a matrix precursor impregnated fibre material to produce an extended product 12 with a cross sectional shape shown in Figure 4.

For this purpose, a standard filament winding fibre resin system was used, specifically Toray T300 12K 50B carbon fibre and Ciba Geigy's epoxy resin MY750 HY917 DY070 mixed in the ratio by weight of 100:87:0.5, to ensure a stoichiometric cure.

The winding was controlled to provide fourteen layers of hoop windings at 3mm pitch and helical winding at i520 using four tows at once in a sequence of two layers of hoop, four layers of helical and two layers of hoop. This pattern was reversed halfway through to give a balanced structure. The tension in the fibre was 2Kg and the thickness of the winding 13mm.

This winding procedure produced a structure as illustrated at 12 in Figure 4 which is modified by applying preformed shoulder sections 13 as illustrated in Figures 5, 6 and 7.

The shoulder sections 13 were made from five layers of Fothergill's G806 0/90 woven carbon fibre cloth interspersed with four layers of 6.4mm carbon fibre felt wetted with Ciba Geigy's 1927GB cold cure epoxy resin system mixed in the stoichiometric ratio of 100:36 parts by weight of resin to hardener. Each length of shoulder section corresponding to the length of the winding was laid up in a right angle V block 14 (Figure 7) and compressed with an 80mm radius male tool 15.

After curing for twenty four hours at room temperature, four shoulder sections 13 are symmetrically positioned around the winding 12 as shown (for one shoulder section) in Figure 5. Clamping pressure is applied via metal plates 16, 17 and rubber blocks 18 serve to locate the shoulder sections 13 and transfer pressure from the metal plates 16, 17. Curing of the resin matrix is then completed, this requiring, for the wound component four hours at 1000C followed by four hours at 1600C, the final post cure being carried out in an oven after removal from compression jig and metal plates 16, 17.

The product is then sliced transversely to the winding axis as illustrated in Figure 2 and the slicing repeated to produce a plurality of rings from the wound product. Each ring is then cut at four places in a direction parallel to the winding axis thereby to produce blanks of the form illustrated in Figure 8a. Figure 8b shows the partly machined blank and Figure Sc a fully machined blank the surface of which is then sealed with a suitable coating and the artefact sterilised and coupled with a conventional modular metallic head through a tapered Morse fitting.

It should be noted that when the blanks (Figure 8a) are cut from the ring slices, there is a small amount of "spring-in", a feature commonly observed in wound ring and cylindrical sections. This has to be allowed for in the design of the shape of the mandrel.

Measurements indicated an apparent flexural modulus of approximately 40GPa and the ability to resist application of a 3.3KN load in straight or off-axis compression without any visible damage. The tensile/compressive modulus is of the order 60GPa. By reducing the relative number of hoop wound layers in the winding, it is possible to match more closely the longitudinal and torsional characteristics of the host bone whilst retaining adequate strength. For example, we have found a good match using a winding sequence in which the ratio of hoop to helical at i 520 is one to four, the helical windings being distributed evenly across the thickness of the winding.

The invention is not restricted to the details of the foregoing example.

Claims

1. A method of manufacturing a fibre reinforced composite artefact comprising filament winding a suitable matrix precursor impregnated fibre material onto a mandrel, modifying the shape of the wound product by applying thereto a pre-formed shaped piece of compatible matrix precursor impregnated fibre material to at least one peripheral location, converting (as by curing) the matrix precursor to product matrix and cutting the product in a direction parallel with the winding axis to generate at least one shaped artefact therefrom.

2. A method as claimed in Claim 1, wherein the product is also cut in a direction transverse to the winding axis to generate a plurality of artefacts.

3. A method as claimed in Claim 2, wherein the product is repeatedly cut in a direction transverse to the winding axis to generate a multiplicity of artefacts.

4. A method as claimed in Claim 3, wherein the multiplicity of artefacts are all identically shaped.

5. A method as claimed in any of the preceding Claims, wherein the shape of the wound product is modified by applying preformed shaped pieces of compatible matrix precursor impregnated fibre material to at least two peripherally spaced locations.

6. A method as claimed in any of the preceding Claims, wherein the artefacts are subjected to machining after cutting from the product.

7. An artefact when made by the method of any of the preceding Claims.

8. A method of manufacturing a femoral implant comprising filament winding a suitable matrix precursor impregnated fibre material onto a mandrel which has a peripheral shape in cross-section at least part of which corresponds with one boundary of the shape in longitudinal section of the desired implant, converting the matrix precursor to product matrix and cutting the product in a direction parallel with the winding axis to generate therefrom at least one artefact of a shape corresponding to that of the desired implant.

9. A method as claimed in Claim 8, wherein the product is also cut in a direction transverse to the winding axis to generate a plurality of artefacts.

10. A method as claimed in Claim 9 wherein the product is repeatedly cut in a direction transverse to the winding axis to generate a multiplicity of artefacts.

11. A method as claimed in any of Claims 8 to 10, wherein the mandrel has a peripheral shape in crosssection at least part of which corresponds with one boundary of the shape in longitudinal section of two or more of said desired implants positioned end to end and wherein the said product is cut in a direction parallel with the winding axis at an appropriate plurality of locations to generate two or more artefacts of a shape corresponding to that of the desired implant from the or each 3600 radial extent of the winding.

12. A method as claimed in any of Claims 8 to 11, wherein the winding is applied in a plurality of layers applied at differing helical winding angles.

13. A method as claimed in Claim 12, wherein the sequence of layers and helical winding angles are chosen so that the implant produced has a stiffness similar to that of human bone.

14. A method as claimed in any one of Claims 8 to 13, wherein the shape of the filament wound product is modified, prior to converting the matrix precursor to product matrix, by applying thereto at least one preformed shaped piece of compatible matrix precursor impregnated fibre material to at least one peripheral location.

15. Apparatus for carrying out the method as claimed in any of Claims 7 to 14 comprising a helical filament winding machine having a mandrel which has a peripheral shape in cross-section at least part of which corresponds with one boundary of the shape in longitudinal section of the desired implant.

16. A femoral implant when made by the method of any of Claims 8 to 14.

17. A femoral implant comprising fibre reinforced composite material in which the fibres extend longitudinally and are in layers with an angle between the direction of fibres in one layer relative to those in another layer.

18. A method substantially as herein described with reference to the accompanying drawings.

19. A femoral implant substantially as herein described with reference to and illustrated in Figures 8a, 8b or Sc of the drawings filed herewith.