Engineering the next generation of prosthetic limbs

Jonny Williamson discusses the design and manufacture of next generation lower limb prosthetics with the recently announced Royal Academy of Engineering research fellow, Dr Alex Dickinson.

Pride of Britain Award - 11 members from the UK Team take the stage
The Invictus Games British Armed Forces Team takes the stage to receive a Pride of Britain Award.

Events such as the Paralympics and the recent Invictus Games have offered a fantastic opportunity to raise society’s awareness of people living with disabilities, however for amputees, there remain a number of fundamental challenges that need to be solved in regards to the design and manufacture of the prosthetic limbs – human interface.

That’s the viewpoint of University of Southampton’s Dr Alex Dickinson, who is working in Southampton’s Bioengineering Science Research Group to apply his past research and analysis of orthopaedic implants to external prosthetics.

A prosthetic limb is produced from standard components and attached to the body using a personalised socket. According to Dickinson, the predominant issue with today’s prosthetic limbs stems from the accepted way measurements of the residual limb – the cut bones and the reconstructed soft tissue structures created during the amputation surgery – are captured, and how that translates to the resulting prosthetic socket.

An amputee will return to their prosthetist an average of nine times within the first year, he notes, in order to arrive at a socket fit that’s both comfortable and allows them to begin performing some of the activities they enjoyed previously.

Dr Alex Dickinson performs a scan on a residual limb model (Image courtesy of Institute for Life Sciences, University of Southampton (WEB)
Dickinson performs a scan on a residual limb model (Image courtesy of Institute for Life Sciences, University of Southampton).

“Each patient requires a bespoke socket and if it is uncomfortable, they may abandon even the most advanced prosthetic limb, and they won’t be able to achieve their rehabilitation goals and return to a level of function that really should be possible in 2015.”

Prosthetists are highly trained and understand how the soft tissues are likely to respond to differing load factors, continues Dickinson, but their job is extremely challenging because the shape of the residual limb varies throughout a single day, as a result of temperature and hydration, and often shrinks in the months following amputation, because of muscle atrophy.

Current measuring methods (plaster molds, calipers and standard tension tape measure) contact the residual limb and may distort its shape, further complicating measurement.

What Dickinson proposes is to utilise modern non-contact shape measurement technologies and biomechanical analysis to create accurate, dynamic models of residual limb-socket interaction in lower limb amputees to better predict how the residual tissues deform and respond to loads arising from various activities.

This will enable surgical prosthetic treatments to be better planned and specific to the individual, and speed up and reduce the discomfort of rehabilitation.

The developed software performs precise, high resolution and non-contact measurements of residual limb shape
The developed software performs precise, high resolution and non-contact measurements of residual limb shape.

With inter-patient variability so prevalent, another challenge facing Dickinson and his team is to arrive at a method that’s likely to work for the largest group of individuals.

“We’ve been working with the Disablement Services Centre in Portsmouth [a prosthetics fitting and manufacturing service] and we have ethical approval with them to evaluate the shape of the residual limbs in its amputee cohort.

What we’ve produced over the past year is a software package that takes our more accurate residual limb scans and performs a series of automated tests to validate them for reliability and repeatability.

“The next step is to firstly, offer this measurement technology to prosthetists so that they can monitor patients over a longer period of time and assess how their residual limb changes; and secondly, compare the residual limb shapes across a whole range of patients and their sockets, and work with prosthetists and physiotherapists to see how well those patients progress through rehabilitation.”

In this way, Dickinson wants to see whether trends can be identified in successful rehabilitation and trends where amputation doesn’t go so well, and assess if there’s a way to inform prosthetists’ practice for the better.

“All of our preliminary work and results so far are based on residual limb casts from the Portsmouth patient group, which we hope to present next year at the world congress of the International Society for Prosthetics and Orthotics [the global governing organisation for prosthetists and orthotists] and then to start working with patients themselves.

“At the end of the five year Royal Academy of Engineering research fellowship, we want to be in a position where we can provide advanced engineering tools to support the excellent work the prosthetics companies and the prosthetists do to help lower limb amputees, and ultimately make a difference to the amputees, clinics and UK industry.”