When a person experiences a leg amputation, the residual limb will shrink after the surgery. Putting weight onto a prosthetic leg may cause discomfort in the socket, the connection point of the limb to the prosthesis. Different sockets of various sizes and fabric "socks" may be used to cushion the impact and adjust the fit.

UW research scientist Dave Gardner places electrodes on a patient’s residual limb. (Credit: Mary Levin, University of Washington)

Many people who use prostheses experience pain on a daily basis where their skin meets the socket, particularly those who have diabetes or other diseases that affect their physiology. To increase their comfort and their use of the prosthetic, engineers at the University of Washington, Seattle, are reaching ways to build better sockets. They have developed a device that tracks how much a person’s limb swells and shrinks when inside the prosthetic socket. This data, they say could help doctors and patients predict how and when their limbs will swell, which could be used to build smarter sockets.

Soft tissues in a prosthesis socket swell and shrink often during the day. This happens when users increase physical activity, sit or stand, or eat salty foods. In a fixed socket, these fluid volume changes can be particularly painful, forcing people to seek relief by removing their artificial limb or adjusting the snugness of the fit by adding or removing fabric socks.

If physicians can track when an individual typically experiences volume changes in his or her prosthesis socket, they can better fit patients with artificial limbs and reduce the amount of pain, they say.

The new checkbook-sized portable device measures increase or decrease of fluid volume in a patient’s limb by receiving data from small electrodes placed in different spots on the leg. Instrument electronics can be worn in a belt pack and include a circuit board that calculates the fluid volume change in the leg tissues, transmitting the data wirelessly to a computer or storing it on the device.

Researchers are testing the device by asking patients to go through a routine that includes sitting, standing and walking as the device records fluid changes. Data is transmitted wirelessly to a tablet that displays the changes in limb size about 15 times a second.

Longer term, the researchers want to build a device that patients could wear for a couple of weeks or longer to monitor changes in their limb size as they go about their daily routines.

The hope is that prosthetic limb sockets will become more robust and flexible, accommodating natural changes in swelling without causing discomfort or inconvenience.

This summer the team will work with patients across the US and Canada who use vacuum-suction technology to help keep their residual limbs snug in the sockets and adjust the fit when tissues swell and shrink. Patients say this vacuum can help improve socket comfort and reduce pain, but the technology requires careful maintenance and it can be disruptive when the noisy vacuum turns on.