Ahhh…Comfort! UK Research Takes Next Generation “Smart Limb” to New Level

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Unless the prosthesis provides a comfortable, pain-free experience for the amputee, the most technologically advanced componentry available may simply end up in the closet. Several surveys have shown that poor socket fit and residual limb pain are primary reasons for dissatisfaction with lower-limb prostheses.

The socket is the vital interface between the residual limb and the prosthesis. Alignment, friction, temperature, and moisture accumulation inside the socket all play a part in prosthetic fit, comfort, and function, as do such factors as the amputee's comorbidities and soft-tissue properties. Limb shape changes, volume, and pressure distribution in the socket—factors which can fluctuate throughout the day as well as over weeks and months—also play major roles.

How to better overcome these challenges to achieve optimal socket fit, function, and comfort has been a perennial challenge for researchers, manufacturers, and prosthetists.

Combining research, clinical, and manu­facturing expertise, a team from the United Kingdom (UK) is using cutting-edge technologies to find better solutions.

Chas A Blatchford & Sons Ltd, Basingstoke, Hampshire, and Bour­nemouth University, Poole, Dorset, are collaborating on developing the next generation of "smart" prosthetic limbs. The new prosthesis will use low-weight, high-strength materials, innovative sensor-integration technology, and what they describe as "a powerful application of artificial intelligence used for assessment and provision of a graphical user interface design, initially combined with a passive feedback control mechanism."

The innovative, non-invasive technique for measuring and monitoring socket pressure will not only assist in modifying the socket during fabrication, but also will be used for future automatic adjustment of fit as socket pressures change during daily activities.

The new socket may help up to 75 percent of amputee soldiers return to active duty and may also be used by the National Health Service (NHS), according to the university. The new socket may cut costs by reducing the repeated need to manufacture new sockets to meet an amputee's changing requirements, plus it may reduce the number of visits for adjustments in order to obtain comfortable fit.

According to Bournemouth University, the new interface technology is the first of its kind that will provide quantitative feedback during the fitting process, as well as providing real-time data necessary for active fit corrections. "It will compensate for changes in limb volume due to fluid build-up and muscle wastage and provide a high level of fit during walking, sitting, standing, and maneuvering over rough terrains," according to a November 2009 Bournemouth University press release.

Regarding the actual mechanism that would be controlled by the innovative computer technology to adjust pressure distribution, Saeed Zahedi, technical director for Blatchford, says that systems already exist to accomplish this and could possibly be used.

Zahedi

"Comfort and fit now depend much on the experience and skill of the prosthetist," points out Siamak Noroozi, PhD, professor, School of Computing, Engineering & Design, chair of Advanced Technology, and director of Bournemouth University's Design Simulation Research Centre. "But, by being able to measure and quantify pressures and by developing software for adjustment in situ, the socket can respond much more effectively to volume changes throughout the day. The measurement system is non-invasive, so it doesn't change the characteristics of the socket."

It has been thought that most socket fitting problems were caused by changes in the residual limb, Zahedi says. "However, we have found recently that pressure distribution is greatly influenced by the foot and ankle interaction and natural compliance with the ground, so what happens at the socket interface mirrors what actually happens with the foot-and-ankle complex and the ground. For instance, we discovered that when an amputee stands on an inclined surface, he will make compensation to achieve a comfortable position. If we can minimize the amount of pressure forces being applied within the socket, we can minimize the amount of compensation needed for a comfortable position—hence, achieving the goal of comfort in all conditions."

Solving Other Problems

Blatchford has developed advanced prosthetic ankles that even out pressure distribution and is working on microprocessor-controlled knees and ankles that would "talk" to each other to overcome some of the challenges faced by bilateral amputees, according to a news story, "Amputee Mobility Fix Is Socket Science," by Siobhan Wagner in The Engineer February 22. Blatchford also has received funding from the Ministry of Defence (MoD) to develop a fluid management system to work out sweat and keep the interface dry, preventing socket slip, according to Wagner.

Breakthrough Technology with AI

Bournemouth University researchers have achieved some cutting-edge discoveries and developments that apply to the practical ability of computerized technology to measure and adjust socket pressure distribution in real time.

The research involves artificial neural networks (ANNs) and inverse problem-solving. An artificial neural network is an information-processing system inspired by how the brain processes information, explains an overview of artificial neural networks available on the London Imperial College website (www.doc.ic.ac.uk). The key element is the structure of the system, which is composed of a large number of highly interconnected processing elements called neurones. Like people, ANNs learn by example.

An inverse problem—to put it simply—is a problem in which the answer, results, or consequences are known, but the question or the cause is not. "The inverse problem consists of using the actual result of some measurements to infer the values of the parameters that characterize the system," according to noted physics expert and author, Albert Tarantola, PhD. (Author's note: For more information on inverse problem theory and application, visit www.ipgp.fr/~tarantola/)

This task is well-suited to ANNs, which can extract meaning, patterns, and trends from complicated or imprecise data. This contrasts with typical computer programs, which figure out problems through an algorithm—a set of well-defined steps. However, ANNs and conventional algorithmic computers are not in competition; rather, they complement each other, and many tasks require systems that use both, the Imperial College website points out.

ANNs can play a vital role in advanced prosthetic technology. The Bournemouth researchers report on the successful solution to a problem in creating a hybrid analysis tool to monitor in-service loading ("Improvements in the Accuracy of an Inverse Problem Engine's Output for the Prediction of Below-Knee Prosthetic Socket Interfacial Loads," Philip Sewell, Siamak Noroozi, et al., Journal of Computational Science, 2010, in press at the time of this writing).

They point out that combining traditional analysis methods with an ANN can create a hybrid analysis tool to monitor loading in complex structures. Since an ANN predicts poorly in the high and low ends of the envelope it is trying to predict, the research team developed the ANN Difference Method to improve the accuracy of the inverse-problem engine's output. Applying the ANN Difference Method to the backpropagation [a method of training a neural net in which the initial system output is compared to the desired output, and the system is adjusted until the difference between the two is minimized] of the neural network system reduced the inherent errors at high and low ends of the envelope.

"Utilizing an experimental technique combined with an ANN can provide in-service loads on complex components in real time as part of a sophisticated embedded system," the study sums up. Solving this problem in the practical application of an ANN as part of a "smart limb" system would thus bring the limb one step closer to becoming reality.

An earlier paper ("An Artificial Intelligence Approach for Measurement and Monitoring of Pressure at the Residual Limb/Socket Interface—A Clinical Study," R. Amali, S. Noroozi, et al., Insight – Non-Destructive Testing and Condition Monitoring, July 2008) notes that although transducers and finite element analysis have been used to measure and monitor socket pressures, both techniques have limitations, making them impractical for prosthetists to use in the everyday fitting process. The paper "details the design of a Hybrid Inverse Problem Engine (HIPE) which combines Artificial Intelligence (AI) and experimental/numerical data to create a less invasive and passive approach to develop a practical clinical tool for predicting the pressure distribution at the limb/socket interface."

The authors reported that testing and validation of the HIPE under laboratory conditions showed that the technique was able to predict the location and magnitude of pressures applied manually to the socket. "A comparison of the predicted pressure distribution found using the HIPE, at the limb/socket interface of a patient in a clinical environment with photoelastic data of the actual pressure distribution, further indicated the technique's potential benefits." The authors comment that they hope the HIPE will become a general tool for monitoring prosthetic fit in a clinical environment.

Regarding the technology for measuring, modeling, monitoring and adjusting socket fit, Noroozi says, "I think we have solved a major problem." And the other pieces seem to be falling into place for taking the second-generation "smart" prosthesis to a whole new level of fit, function, and comfort.

Miki Fairley is a freelance writer based in southwest Colorado. She can be reached at

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