GB2284830A

GB2284830A - Sructural member

Info

Publication number: GB2284830A
Application number: GB9425295A
Authority: GB
Inventors: Anthony W Broad
Original assignee: Bond Helicopters Ltd
Current assignee: Bond Helicopters Ltd
Priority date: 1993-12-15
Filing date: 1994-12-15
Publication date: 1995-06-21
Also published as: GB9425295D0; GB9325606D0

Abstract

A structural member such as a hip prosthesis (1) is formed as a composite, e.g. of epoxy/carbon fibre, in which an inner core of longitudinal fibres (12) is surrounded by an outer layer of fibres extending around the axis of the member. The outer layer may conveniently be a by-directional cloth. It may be a woven or knitted fabric spirally wound round the inner core. <IMAGE>

Description

STRUCTURAL NENBER This invention relates to structural members and components and particularly prostheses such as hip joint implants.

At present, a hip joint comprises a femoral component and an acetabular component. The femoral component has an elongate stem which is fitted into a patient's femur and carries a modular head or ball; the acetabular component provides the socket. The femoral component is normally manufactured from Cobalt Chrome and has to meet the British Standard on orthopaedic joint prostheses BS7251 Part 5. This requires that the component be tested for defect by application of a minimum loading of an applied force of between 200N and 300N applied at a frequency of 1OHz.

However, although strong, metallic implants have shortcomings. In particular, for a satisfactory joint, the patient's bone has to adhere to the implant. But, with metal, the patient's defence system recognises the implant as a foreign body and attacks the implant corroding it in a way which gives off toxic ions. The bone reacts against the toxicity and over a number of years retracts away from the metal. As a result, the implant becomes loose. This means that a metal implant only has a life of about 15 years and whilst this may be acceptable in cases of rheumatoid arthritis which tends to be caused by old age, it is not acceptable in hip replacements due to osteo-arthritis which can affect people at any time of life.

It is therefore desirable to find a material which is more compatible to bone to provide a longer life. This material ideally must be of similar or less cost than the current metal implant and ideally should have a modulus similar to bone to allow flexure in a similar fashion - something not presently possible with metal.

In addition, it must be made the same shape whilst having the same strength characteristics.

As a replacement material, one immediately thinks of plastics. However, in converting a metal item to plastic, the plastic equivalent is normally made bigger to provide comparable strength characteristics. In the case of an implant the optimum shape is already determined an increase in overall dimension would not be acceptable.

In June 1989, it was reported in Advanced Composites Engineering that ICI Composites was working on a composite hip joint made of thermoplastics material with reinforcing carbon fibres. This was said to offer greater corrosion and chemical resistance and greater bio-compatibility than current materials. However, we are not aware of an acceptable hip joint being made in this way.

An object of the present invention is therefore to provide an improved structural member, particularly a joint prosthesis.

According to the present invention, a structural member comprises an elongate body of fibre reinforced resin including a first plurality and a second plurality of fibre lengths, the second plurality of fibre lengths being orientated generally longitudinally of the body to resist tensile stresses and the first plurality of fibre lengths being disposed about the second fibre lengths and extending about the longitudinal axis of the body for resisting torsional and compressive stresses.

Preferably the resin is epoxy resin.

Preferably the fibre lengths are a plurality of fibres, for example carbon fibres.

The second plurality of fibres are preferably sufficiently long to extend substantially the whole length of the body and are unidirectional. The first plurality of fibres may be a spiral winding of fibres, woven or knitted fibres and encapsulating the second plurality of fibres. Preferably, however, the first plurality of fibres is a cloth of bi-directional fibres which may encapsulate substantially the whole portion of the body or only a selected portion thereof which may be particularly susceptible to torsional stresses.

Preferably, the structural member is a prosthesis and therefore according to a preferred aspect of the invention, a structural prosthesis comprises an elongate body of fibre reinforced epoxy resin, fibre lengths being orientated, longitudinally of the body to resist tensile and compressive stresses, and about the longitudinal axis of the body to resist torsional stresses.

Suitably the prosthesis is a femoral component for a hip joint which includes an elongate stem and short trunnion section slightly offset from the axis of the stem. Suitably, some of the longitudinal fibres extend from the stem and curve slightly into the trunnion portion. Further fibre lengths may also extend along the trunnion section. The fibre lengths orientated about the longitudinal axis of the body are preferably bi-directional and, although they extend substantially over the whole length of the body the critical area to be covered is between 25mm and 90mm below the centre of the head of the component which is the area most prone to failure.

Long fibre lengths are preferred, since sites for structural failure are avoided or minimised. Chopped fibres are susceptible to fibre wash and are carried away with the applied resin in the production process and single strand fibre randomly arranged is liable to be moved away from the inlet port for resin resulting in a localised structural defect site. Long fibre lengths generally orientated in the direction of injection of epoxy resin are preferred as the best orientation for structural performance and resisting movement by the applied resin.

The invention will now be described by way of example with reference to accompanying drawings in which: Fig 1 is an illustration of a preferred hip joint prosthesis; and, Figs 2 and 3 illustrate fibre orientations.

In Fig 1 a joint prosthesis 1 has an elongate body 2 with a trunnion 3 and a distal stem 4.

The testing procedures for testing a prosthesis comprise two defined loads. The first is an almost vertical compressive force across the prosthesis between the trunnion and distal stem. The second is a torsional load resulting from the first load case in combination with the test attitude.

In this embodiment of the invention, the prosthesis 1 is provided with sufficient compressive/torsional strength by the use of a combination of both unidirectional and bi-directional carbon fibre fabrics reinforcing an epoxy resin. Carbon fibre was selected for its bio-compatibility and its high strength and an epoxy resin system was selected for fibre reinforcement on its bio-compatible properties.

The body 2 of the prosthesis 1 comprises a laminate construction built up of an outer skin 10 of the bidirectional carbon fibre material constituting the aforesaid first fibre lengths and an inner core 12 of the uni-directional carbon fibre material constituting the aforesaid second fibre lengths. The outer skin (Fig 2) has an average thickness of 0.71mm being made up of two ply of the bi-directional material. The orientation of the outer skin is such that the warp direction of the cloth is aligned substantially parallel to the centre line L of the distal stem (4).

This arrangement increases the torsional strength of the body by adding a 0/90 degree surface ply and, secondly, the bi-directional material prevents a bursting effect attributed to the compressive loading cycle.

The uni-directional inner core (Fig 3) takes up the remaining volume of the body. Orientated in such a way as to maximise the compressive strength of the core, the fibres are separated into thin strips and then laid end on into the mould. The direction of the fibres is split 50/50.

Firstly, fibres 12a running along the inside of the curved portion 5 of the prosthesis 1 from the domed end 6 of the distal stem 4 to the termination of the trunnion 3, this being a continuous unbroken fibre run.

Secondly, fibres 12b running along the outside surface of the prosthesis from the same start and finish points as previously described but traversing the 90 degree corner 7 of the proximal region 8 and the somewhat gentler radius leading from the proximal region 8 to the neck portion 9, this again being a continuous unbroken fibre run.

Finally, in order not to leave an unreinforced portion in the central area of the proximal region, this is subsequently packed using flat ply 13 of unidirectional carbon fibres. Both the outer skin and inner core are diagrammatically shown on Figs 2 and 3 respectively, which illustrate warp directions and ply thickness.

The volumetric composition of the preferred prosthesis in accordance with the invention is as follows: Total volume of prosthesis 37.5 cm3 Volume of outer skin 10.2 cm3 (assuming 0.71mm thickness all over) Volume of uni-directional 27.3 cm3 inner core The approximate resin to fibre ratio is 37.7%:62.3% respectively for the uni-directional fibres and 42%:58% for the outer skin.

In production, the carbon fibres of the outer skin are placed in a split mould and the uni-directional carbon fibres orientated as explained. The mould is then closed and epoxy resin applied. The fibre reinforced epoxy resin is then cured.

In an alternative arrangement the outer skin is formed as a "sock" of desired shape and uni-directional fibres to form the core are fed into the sock. The fibres are then placed in a mould and epoxy resin applied to form the final shape.

Claims

1 A structural member comprising an elongate body of fibre reinforced resin, including a first plurality and a second plurality of fibre lengths, the second plurality of fibre lengths being orientated generally longitudinally of the body to resist tensile stresses and the first plurality of fibre lengths being disposed about the second fibre lengths and extending about the longitudinal axis of the body for resisting torsional and compressive stresses.

2 A structural member according to Claim 1, in which the resin is epoxy resin.

3 A structural member according to Claim 1 or Claim 2, in which the fibre lengths are a plurality of fibres.

4 A structural member according to any preceding Claim, in which the fibre lengths are of carbon fibre.

5 A structural member according to any preceding claim, in which the second plurality of fibres are sufficiently long to extend substantially the whole length of the body and are uni-directional.

6 A structural member according to any preceding claim, in which the first plurality of fibres is a spiral winding of fibres, woven or knitted fibres and encapsulating the second plurality of fibres.

7 A structural member according to Claim 6, in which the first plurality is a cloth of bi-directional fibres.

8 A structural member according to Claim 7, in which the cloth encapsulates substantially the whole body.

9 A structural member according to Claim 7, in which the cloth encapsulates a selected portion of the body.

10 A structural member according to any preceding claim which is a prosthesis.

11 A structural prosthesis comprising an elongate body of fibre reinforced epoxy resin, fibre lengths being orientated, longitudinally of the body to resist tensile and compressive stresses, and about the longitudinal axis of the body to resist torsional stresses.

12 A prosthesis according to Claim 11 which is a femoral component for a hip joint and which includes an elongate stem and a short trunnion section slightly offset from the axis of the stem.

13 A prosthesis according to Claim 12, in which some of the longitudinal fibres extend from the stem and curve slightly into the trunnion portion.

14 A prosthesis according to Claim 13, in which further fibre lengths also extend along the trunnion.

15 A prosthesis according to any of Claims 11 to 14, in which the fibre lengths orientated about the longitudinal axis of the body are bi-directional.