The Cambridge-MIT Institute is pleased to announce the achievement of one of its researchers.

Dr Athina Markaki, from Cambridge University’s Department of Materials Science & Metallurgy, has been awarded one of the 2004 SET (Science, Engineering and Technology) for Britain awards for her work arising from the CMI-funded project on ‘Developing an ultra-light stainless steel sheet material’, which has developed and patented a lightweight metal 'sandwich' of microscopic stainless steel fibres bonded between two very thin stainless steel faceplates.

As a spin-off from the project, Dr Markaki conducted research into another possible application for the metal fibres, after a magnetic field has been applied to them, to connect prostheses to bone more effectively.

Replacement of hip, knee and other joints, usually as a treatment for degenerative arthritis, has a worldwide market of $5 billion and a growth rate of about 10%. These operations bring relief from pain to millions of people every year, but there is a serious problem. Prosthetic implants are attached to bone either with cement or later via bone growth into a rough surface. Loosening between bone and implant can cause problems, reducing the average prosthesis lifetime to less than 15 years. An effective method to improve the durability of implant-bone joints is urgently required.

Loosening has two main causes - poor bonding and “stress shielding”. The latter arises because conventional (metal) prostheses are stiffer than surrounding bone, which inhibits bone from being strained, and straining is essential for healthy bone growth.

Dr Markaki was one of several hundred young researchers who submitted a poster about her work to this year's SET for Britain scheme. On March 15, at the House of Commons, she was honoured with the De Montfort Award for her work, which has been published in the April edition of Biomaterials.

The team behind the CMI-funded project on 'Developing an ultra-light stainless steel sheet material' is working with an industrial consortium to develop and test the metal further, with a view to encouraging its use by vehicle manufacturers, as it could help to help cut down the body weight, and therefore the fuel consumption, of cars and trucks.

MAGNETO-MECHANICAL STIMULATION OF BONE GROWTH

A.E. Markaki and T.W. Clyne, Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, Cambridge CB2 3QZ, UK

This work introduces a completely new approach to the crucial problem of interfacial loosening, which commonly occurs with prosthetic implants. This arises partly because bone adjacent to a conventional, fully dense metallic prosthesis often fails to bond well to it. Additionally, since the metal is much stiffer than the bone, stress shielding occurs, so that straining of bone adjacent to an implant is strongly inhibited. Such straining is essential for healthy bone growth it is responsible for the beneficial effects of exercise on bone physiology. Strain levels of at least about 1millistrain (0.1%) are needed in order to stimulate significant beneficial effects.

The proposal relates to the introduction of a relatively thick layer, strongly attached to the prosthesis. This layer is highly porous, being composed of an array of metallic fibres bonded together. Bone growth into such material is known to occur readily.

The fibres are made of a magnetic material, such as ferritic stainless steel (which has excellent biocompatibility). When a magnetic field is applied, a fibrous array of this type deforms elastically, as a result of the tendency for such fibres to align with the field. In-growing bone tissue filling the inter-fibre space would be mechanically strained during such deformation. Subjecting a newly implanted prosthesis to an externally imposed magnetic field for suitable periods would thus be expected to stimulate bone growth and to promote healthy bone physiology.

A simple analytical model has been developed to predict the expected levels of deformation. This demonstrates that, using magnetic field strengths already employed for diagnostic purposes, it should be possible to generate strain levels sufficient to stimulate bone growth, provided the architecture of the fibre array conforms to certain (achievable) requirements.

Experimental measurements confirm the broad validity of the model, although no work has been done so far involving bone tissue. It may also be noted that, by suitable choice of the layer thickness, stress shielding could be reduced or eliminated, so that the long-term health of adjacent bone would be improved. Application of the magnetic field would only be required during the short period immediately after implantation, when bone-in-growth occurs.

The proposed concept should not be confused with that of an externally-applied magnetic field itself directly stimulating bone growth, which has often been suggested, but which lacks both a clear mechanistic explanation and convincing experimental evidence for its efficacy. It also differs from the idea of magnetic fields being used to generate forces within cells and tissues implanted with magnetic micro-particles, which suffers from similarly uncertainties and also presents practical difficulties.

In contrast to these two suggestions, the mechanistic basis for the proposed approach is simple, clear and well founded. A paper outlining the work done so far has recently been published in Biomaterials. No IPR protection has been sought, since it was felt that this might inhibit systematic study and development of the concept.