Bug-free prostheses: Reducing infection risk and improving reliability

In this feasibility study, we propose to investigate key controls on the coupled mechanical and biological performance of prostheses with the aim of building a unique cross-disciplinary research team to design novel antimicrobial lining materials and prosthetic interface systems. In the UK there are approximately 62,000 amputees and conservative estimates suggest that there are more than 10 million people in the world who live with limb loss. Thus, developing functional, comfortable, cheap and safe prosthetic limbs has social and economic impacts that spread through the developed world and beyond. A major cause of prostheses failure is bacterial fouling of the socket, which occurs in up to 50% of patients in the developed world. A prosthetic socket envelops the residual limb snugly to provide two main functions: (1) a solid interface to transmit forces created through the interaction with the environment onto the residual limb and (2) an attachment point for the components replacing the missing body parts. Through this tight connection between the prosthesis and the limb, a closed environment is generated that provides ideal conditions for bacterial growth. Bacteria not only cause unpleasant odour but also lead to infection and ultimately a breakdown of the whole prosthetic system. Even if a user keeps a prosthetic socket meticulously clean, it is inevitable that over the lifetime of a prosthetic socket (max 3 years) bacteria will settle. Skin problems are frequent and studies on lower limb prosthetics users have shown they occur in 34% to 50% of patients. Infections can have a major negative impact on the use of the prosthesis and hence are highly detrimental to the users' quality of life.The feasibility study will begin to collect data on the microbial populations present on the skin and within the liner. Molecular techniques will be used to sequence microbial DNA to determine the species present and catalogue the genes which may play key roles in community function. Electron microscopy will be used to image the structure of the microbial populations living upon the liner. Once these data are available, we will begin to develop multidisciplinary models of the prosthetic interface. This model development is made possible by the wide range of expertise provided by the cross-disciplinary team: we will develop both conceptual and mathematical models that describe existing interfaces and proposals for new lining materials and prosthetic interface systems.

The number of amputees and new referrals for prosthetic limbs in the UK has remained steadily high at approximately 5000/year over the last 10 years [1]. This is despite the decline in industrial accidents that was a major source of amputation 50 years ago. With 50% of referrals being older than 65, a steadily ageing population and a dramatic increase in type II diabetes, which accounts for more than 33% of referrals there is a high probability that the requirement for safe prosthetics will grow into the future. In the US where a staggering 7% of the population have diabetes and 1.9 million are estimated to live with limb loss, the number of diabetes-related amputees discharged from hospital has increased from an average of 33,000 to 84,000 [2]. Thus there is every indication that the requirement for prosthetics in the developing world will increase into the future. Therefore, the problems of infection in the prosthetic sockets that causes failure in 50% of cases and blights the life's of so many amputees has enormous implications. A solution that draws upon new materials and an understanding of the combined biological and mechanical behaviour of prosthetic sockets could dramatically improve their quality of life. There are, in addition, economic impacts both for the companies who make prosthetics and the amputees themselves. The UK has a growing bioengineering industry, which known for its high-tech solutions, that exports prosthetics around the world. Take for example Touch Bionics LtD in Livingston, Scotland who launched the award winning 8,500 i-Hand, recognised by Time magazine as one of the top 50 inventions of 2008; they are already exporting throughout North America and the Indian sub-continent. This innovation in biomechanics has not been matched by the same invention in material design for prosthetic liners. Since every prosthetic limb requires a liner there is huge market in novel materials and designs that reduce the risk of infection. Truly novel materials have a track record of tapping into markets well beyond those originally envisaged for them; companies like W. L. Gore and Associates have grown from a handful of scientists inventing a new material for the electronics industry to one of the largest textile companies in the US, marketing high-tech material like Gore-Tex to everything from sports waterproofs to artificial heart valves. Thus, investing in novel cross-disciplinary materials research can have unforeseen additional benefits. This prosaic economics of wealth creation extends down to individual patients. This is especially true in the developing world where loss of a limb often plunges an amputee into extreme poverty. Take, for example, Sierra Leone, where during the recent civil war the rebels developed the horrific tactic of chopping off the hands or legs of civilians as a way of sowing terror in the population. The majority of the population relies on subsistence agriculture, which is nigh on impossible if disabled by limb loss. Hence, providing high quality prosthetics can lift individuals and local communities out of a cycle of poverty and ill-health: Currently, even when prosthetics are supplied to countries like Sierra Leone or Liberia many can often only be used intermittently as a result of infections.