Sensor System Under Development Will Help Accommodate Prosthesis Fit Changes

Prosthetic lower limb fit and comfort can be a difficult issue for many prosthetists and amputees, but a sensor under development could offer a solution.

Sandia National Laboratories researcher Jason Wheeler, PhD, and colleagues in the Intelligent Systems, Robotics and Cybernetics group at the lab are developing a sensor to place within an amputee’s prosthetic socket that detects both normal pressure and shear forces and accommodates these changes.

Wheeler said the sensor offers performance benefits and convenience.

“The ability to make adjustments to accommodate for volume change and shape change of the limb without having to doff the socket and add socks and then put the socket back on — that is both a cumbersome and an inconvenient process for them to have to go through — this can do that for them automatically,” Wheeler told O&P Business News. Additionally, “because we have not done anything to the socket itself, [the amputee] can use the socket they have already been fit with. It is just a change in liners.” Thus, the prosthetist does not need to refit a socket – the amputee can simply remove their conventional liner to try the sensor system.

The greatest challenge the researchers have faced in developing the sensor has been the “engineering tradeoffs,” according to Wheeler. “It was just those tradeoffs between size and performance,” he said. “We could improve the performance by making [the sensor] bigger, but then we were worried about the comfort and integration of the sensor.”

Jason Wheeler, PhD, of Sandia National Laboratories, demonstrates the lab’s new sensor and line system for lower limb prostheses.

Jason Wheeler, PhD, of Sandia National Laboratories, demonstrates the lab’s new sensor and line system for lower limb prostheses.

Images: Randy Montoya, Sandia National Laboratories

Measurement of shear forces

The ability to measure shear forces is the most important attribute of the device, Wheeler said. Shear forces – which put pressure along the surface of the skin rather than into the skin – are important for prosthesis wearers because they can cause rubbing, blisters and abrasions.

“We talked to a lot of clinicians and researchers in prosthetics, and they had indicated that measuring shear pressures was important,” Wheeler said. “We were not able to find any kind of sensor of the appropriate size and performance that we thought would fit this application. When that occurred, we started [asking ourselves], Well, if we were going to design such a sensor, how might we do it?”

After experimenting with a number of different sensors, the group settled on an optical sensor because this type of sensor is inexpensive, the components are relatively easy to obtain and assemble.

“It also met a lot of the performance requirements that we were shooting for in terms of ability to measure both normal and shear pressure and be of an appropriate size and not drift over time,” Wheeler said. “It was a 2-year research and development effort between trying to find an appropriate commercially available sensor and, when that did not work out, trying to iterate on what we felt was the best new type of sensor we could develop to meet that application.”

“I would estimate at least 95% of the research in socket pressure is all normal pressure just because there is not technology available that can measure shear [pressure] that is appropriately sized. So that is the most novel component, is just to be able to measure things that have not been measured before,” Wheeler said.

Liners increase comfort

To increase the comfort of the amputee while wearing the sensor, the researchers also created liners that automatically adjust the socket shape. The liners contain bladders to which fluid can be added or removed in order to improve fit. The liners are made of an elastomeric material and the bladders can be filled using valves and pressurized liquid outside the liner.

Wheeler and colleagues created a sensor which can be placed in an amputee’s prosthetic socket and detects pressure in order to accomodate changes in the limb.

Wheeler and colleagues created a sensor which can be placed in an amputee’s prosthetic socket and detects pressure in order to accomodate changes in the limb.

“The initial intent was to design a complete system with sensors that monitor the fit, and then if there are deviations from what the prosthetist intended, then you could make adjustments using liquid in the bladders. However, we do believe [the bladders] can be used independently in some configurations as well,” Wheeler said.

While adding socks increases volume everywhere and distributes pressure unevenly, using the bladder system allows the wearer to add volume locally. The researchers would like to further research the best times to add and remove fluid in order to develop bladders that make these adjustments automatically.

More testing needed

The next step in developing the system is to try it with more amputees to maximize its utility.

“We have tested so far on a relatively low number of amputees; the sensor liners on only one or two amputees and the bladder liners on about 10 [amputees]. But we want to increase the number so we can get a little more information about [the system’s] utility,” Wheeler said.

The researchers also hope to test the sensor and bladder systems as a closed loop system.

“We will be pursuing that over the next year or two and in parallel with that we will be continuing to talk to potential clinical distributors and manufacturers to look at getting this out and available to the clinical community.”

Wheeler said the Sandia team expects the system to become commercially available within 3 [years] years to 5 years.

“As a research tool it could be available sooner in much smaller quantities. But in terms of a clinical device that can be easily gotten by a prosthetist, for instance, I would say 3 to 5 years is a reasonable time frame,” Wheeler said. — by Amanda Alexander

Disclosure: Wheeler has no relevant financial disclosures.

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