Fluctuation in a patient’s residual limb is a common issue for amputees and can cause socket discomfort and pain for many prosthesis wearers. Researchers at the University of Washington (UW) are particularly interested in this issue and have been studying interface stresses between the socket and residual limb for more than a decade.

“We determined that one of the biggest factors affecting interface stress distributions was the change in volume of the residual limb,” Joan E. Sanders, PhD, professor of bioengineering at UW, told O&P Business News. “We would get different results in the morning vs. the afternoon, for example, or from one day to another, and we realized that was what we really needed to measure and look at in more detail.”

Bioelectrical impedance analysis

Sanders and her colleagues decided to further investigate residual limb fluctuation caused by fluid volume, but to do that, they needed an accurate way to measure it.

“We needed a way to measure limb volume while the socket is being worn, and in trying to figure out ways to do that, we came across the idea of bioelectrical impedance analysis,” Sanders said.

Joan E. Sanders

Bioelectrical impedance analysis, which is traditionally used to measure body composition, uses electrical voltage measurements to quantify the opposition that tissues present when a current is applied.

“Using the current injection signal and the voltage sensing signals, we can figure out the impedance of the tissues, which is the resistance of the tissues. That’s related to how much fluid is in the tissues, especially extracellular fluid,” Sanders said.

The researchers, in collaboration with two industry partners, developed their own tool to measure fluid volume, which they modeled after a modified version of a commercial device.

To track fluid volume, electrodes are placed on the side of the patient’s leg and the bottom of the residual limb before donning the socket. The electrodes are connected to a circuit board, which is worn around the patient’s waist and transfers data to a computer or tablet.

“We are able to a get a measure of extracellular fluid volume, and that’s really what is of interest in prosthetics, because it is the extracellular fluid volume changes that are causing the diurnal residual limb volume fluctuations that are clinically relevant,” Sanders said.

Fluid volume profiles

Sanders and her colleagues are currently testing the device at clinics in the United States and Canada.

“Right now, we have six copies of the instrument, and we are collecting a lot of data,” Sanders said. “The research is leading us to conclude people have different profiles, limb fluid volume change profiles. Some people always gain fluid volume when they walk, and some people when they sit lose a tremendous amount of volume.”

The device is currently intended for use in a clinical setting by practitioners.

“We initially see this would be a device that a clinician would have in the office, and when the patient comes in, the clinician would use it to monitor the person during a series of tests,” Sanders said. “So walking, sitting or standing, there would be a certain protocol they would run through.”

Prosthetists can then use the information from the device to determine a patient’s fluid volume profile and create an accommodative prosthetic socket.

“From the information from those tests, the practitioner would [learn] how the person’s limb volume changes, and the practitioner would use that insight to make decisions about prosthetic design, as well as volume accommodation approaches,” Sanders said. “So you can imagine a prosthesis being designed to match the person’s physiology.”

The device is currently still in the research phase of development, but Sanders plans to have a commercial tool available in the near future.

“We have several corporate partners, and the goal is to make a clinical diagnostic instrument,” Sanders said. “I am hoping that we have something out within the next couple of years.”

Eventually, Sanders hopes that the device will be small enough that it can be worn by the patient for a long time in order to get a better assessment of how a patient’s limb changes during activities of daily living. — by Megan Gilbride

Disclosure: Funding for this research was provided by the National Institutes of Health and the Department of Defense Congressionally Directed Medical Research Program.