Tech Briefs

Even custom-fit sockets are not the complete answer.

Diabetes is the leading cause of limb loss, accounting for more than 65,000 amputations a year nationwide. In addition, there were more than 1,500 major limb amputations from US battle injuries in Iraq and Afghanistan from 2003 to 2013. That’s a lot of people needing comfortably fitting prostheses. When an amputee walks on a prosthetic leg during the day, the natural fluid in the leg shifts and the muscles tend to shrink a bit. That can pose a problem for the fit of the prosthesis.

Fig. 1 – Sandia National Laboratories researcher Jason Wheeler demonstrates a liner aimed at helping prosthetic limbs fit better. The project has worked with several different types of prosthetic limbs. (Credit: Randy Montoya)
Researcher Jason Wheeler at Sandia National Laboratories has been studying prosthetics for a decade and is part of an Intelligent Systems, Robotics, and Cyber-netics group working to develop a sensor to tell how a limb changes, along with a system that automatically accommodates those changes. After additional testing and refinements, he hopes to find a company that’s willing to market the sensor system.

How It Works

The interface, or socket, between a prosthesis and a limb is custom-made, starting with a cast of the area. The socket follows that contour, and a clinician adjusts it for the best fit.

In the case of a leg, the prosthesis bears the weight of the wearer when standing or moving. But Wheeler said tissues in your leg, unlike tissues on the bottom of your foot, aren’t well-suited for that pressure. In addition, a limb doesn’t stay the same shape during the day because of fluid fluctuations, and, of course, people gain or lose weight. Thus, a custom-fit socket doesn’t always fit.

Robotics researchers developed a small sensor, about the size of a quarter, that is placed inside the socket to monitor fit and detect any changes. What’s unique about this sensor is that it detects pressure in three different directions: normal pressure and shear forces in two directions on the skin. Shear forces cause problems like rubbing, blisters, and abrasions, and no current appropriately sized commercial sensing system can monitor them, Wheeler said.

The engineers at Sandia invented the three-axis pressure sensor, incorporated into a liner that slips into the socket of a prosthesis. Sensors can be distributed to measure three directions at several sites. Other designers have placed pressure sensors in sockets, but those measured only normal pressure, Wheeler said.

“The thing that prevents people from wearing a prosthetic or being satisfied with their prosthesis is comfort,” he said. “Even if you’ve got a high-technology limb, if it’s not comfortable, people won’t wear it.”

The sensor is part of the effort to develop systems that can adjust socket shape to changes in limb shape. Sandia’s system automatically adjusts socket shape by moving fluid into bladders inside the liners that amputees normally wear to improve a socket’s fit and comfort. Standard liners are like a stretchy, cushiony sock, just a few millimeters thick, made of a gel-like material that’s a bit sticky to help hold everything in place. (See Figure 1)

Since modifying a custom socket would be expensive and cumbersome and could require several fittings, Sandia adapted its technology to fit inside a liner made of elastomeric material similar in thickness to a gel liner.

The Sandia system adjusts to limb changes by placing bladders inside the liners, and filling the bladders using valves and pressurized liquid on the outside of the liner. Prototypes have been developed to fill and empty the bladders automatically, but Wheeler said more research is needed to determine when it’s best to add and remove fluid. He added that the technology “is getting mature enough to where before too long, if we want it to be successful, we’re going to have to hand it off to a commercial entity to market it.”