Measuring In-Socket Skin Temperature

If you were to go running outside and return home with a higher body temperature, you probably would remove your heavy sweatshirt. When amputees go running and the temperature of their residual limbs increase, they are not always able to remove their prostheses.

That is how Glenn Klute, PhD, principal investigator/researcher for the Department of Veterans Affairs’ Center of Excellence for Limb Loss Prevention and Prosthetic Engineering in Seattle, describes his research.

Klute and his colleague, William Ledoux, PhD, collaborated on “In-socket Skin Temperatures and Perception of Comfort Over a Whole Day,” a study of four participants in 2006.

Choosing research topics

Much of the Center’s work is built on the foundation of patient feedback from the clinic. Klute explains that his research team relies heavily on VA clinicians to help them find appropriate and important research problems. This particular topic was emphasized during a 2-day amputee focus group in October 2007.

“What we were trying to do was let the key topics emerge that are problems for patients,” he said. “One of them was problems with the socket; in particular, issues related to sweat, moisture and discomfort.”

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Most of the team’s research projects are centered around creating better devices. Depending on the project, that can involve testing existing devices to determine their effectiveness or fashioning new things.

With a background in bioengineering, Klute enjoys building interventions such as these.

“I find that [to be] a lot of fun,” he said. “It does not feel like work.”

The Center of Excellence for Limb Loss Prevention and Prosthetic Engineering concentrates on four areas: injury prevention, mobility enhancement, patient comfort and sensory enhancement. Across this spectrum, research topics on Klute’s agenda include preventing injuries with better shock absorbers; a pylon prosthetic system that facilitates turning motion; and an intervention to improve patients’ perception of their residual limbs.

Most of the funding for this research directly comes from the Department of Veterans Affairs.

Tackling the problem

The first step in tackling an issue such as in-socket warmth is defining the problem. The team needed to determine just how warm it gets inside a socket, how much moisture accumulates and under what circumstances it becomes an issue for patients. Then team members set out to answer those questions from a few different viewpoints.

On the engineering side, they examined the thermal material properties of commercially available socket and liner systems to find which had low thermal conductivity and which had high thermal conductivity. Those systems, Klute said, would be comfortable prostheses to wear, especially in warm climates or when exercising.

“As it turns out, every [liner] we tested – something like two dozen – was a great insulator,” he said. “We were hoping maybe a couple of them were great conductors of heat, and those would be the ones we would recommend to clinicians to prescribe to their patients. It turns out that was not the case.”

Klute and his team set out exploring the problem differently. They asked 11 participants to sit in the lab for one hour, and then walk on a treadmill at a comfortable speed for one half hour. After that brief activity, the participants sat in the lab for another hour.

“What we found was when you sit there for a half an hour, just putting on your socket, your skin temperatures go up about 0.5°. Then when you walk on a treadmill, in a controlled exercise environment, your temperatures went up about 2.2°,” he said.

The researchers found that, during the time that participants sat after walking on the treadmill, their temperatures only dropped approximately 0.5°.

“So what that told us was your skin gets hot, but the socket system and the liner suspension system is such a great insulator that it traps the heat in there,” Klute said.

In addition, these socket systems prevent that moisture from evaporating, stifling the body’s natural cooling system. When someone’s body temperature rises, it causes that person to sweat. When that sweat evaporates, the person’s body is cooled down. Because socket systems prevent that cycle, amputees may find they have a more difficult time lowering their body temperatures.

Considering environmental factors

Researchers then approached the topic from a different angle to determine if the environment played a role. Participants were given temperature-measuring instrumentation to wear for 8 hours while they went about their daily routines. As part of that experiment, they also received voice recorders and a wristwatch set to chime every 30 minutes, reminding them to answer two questions: On a scale of 1 to 10, how warm or cold does your whole body feel, and how warm or cold does your leg feel?

The equipment revealed that the temperature of participants’ legs increased rather slowly throughout the day, while participants’ perceptions of warmth remained steady.

Participants did notice the temperature of their legs, however, after an exercise bout because it increased rapidly.

“Then they kind of forget about it after awhile,” Klute said. “In some sense that is good news, because the patients only notice they are uncomfortable right around periods of activity, but in between, it is this general feeling of ‘Could be better.’”

Most of the participants, he notes, said they were too warm all the time.

Following this experiment, researchers and three participants completed a snowshoeing exercise in the mountains. These participants were appropriately dressed for walking through the snow at approximately 30°F, and both their body temperatures and leg temperatures increased with the activity.

“Even though they were out in the chilly weather, when they stopped walking around, neither their core body temperature nor their residual limb skin temperature dropped,” Klute said.

A study investigator places temperature sensors on a participant Eight sensors are arranged on the anterior portion of the participant’s residual limb
A study investigator places temperature sensors on a participant (left). The sensors, each about the size of a grain of rice, were taped to the participant’s skin prior to donning the liner and socket in order to map differences in skin temperature. Right, eight sensors are arranged on the anterior portion of the participant’s residual limb.
Images reprinted with permission of the Dept. of Veterans Affairs

Determining results

“The take-home messages so far are that environment does not play much of a role, activity plays a big role, and that the materials we are using for sockets and liners now are great insulators,” Klute said.

The engineering members of Klute’s research team realized that the way to resolve each of those issues was by building a better socket. So far, the team has come up with a few diverse options.

One approach is what Klute refers to as “active cooling.” Essentially, the socket would consist of lightweight components providing a type of air conditioning system for the residual limb.

“I could probably get away with adding maybe a half a pound of weight, maybe a pound at the most, if it makes somebody feel really comfortable. But you can’t add 5 pounds of weight – nobody wants to carry that around,” Klute said. His team is in the process of testing that idea.

Another of the team’s burgeoning strategies would provide electrical impulses to stimulate preferentially cold receptors, making the amputee feel cooler without decreasing body temperature.

“This is what I think is exciting about it: If we can preferentially stimulate your cold receptors in the skin of your leg, would you have a physiological response and vasoconstrict so that you get less blood flow to the surface of your skin, which then would actually make you cooler?”

The experiment uses a laser Doppler system to ensure that blood flow remains healthy and physiological responses are appropriate. Klute adds that a system like this might not appear in clinic for 20 years.

“Or if we could build [an active cooling system] that only weighs 5 pounds, that could keep your leg at 31°-32°C, that would be comfortable no matter what you are doing,” he said. “And if that could last all day on a quarter-pound battery pack, maybe everybody would want one.”

Taking next steps

A study participant walks on a treadmill while tethered to a data recording instrument
A study participant walks on a treadmill while tethered to a data recording instrument.
Image reprinted with permission of the Dept. of Veterans Affairs.

Advanced technology ensures that more and better prostheses improve gait and performance for amputees. In the quest for improved quality of life, however, one cannot neglect comfort.

“Hand in hand with all that is quality of life issues,” Klute said. “Yes, mobility is important but so is your level of comfort. There are feet out there that are high performance, but people don’t wear them all day long.”

Klute’s team is not the first to address this issue. Other researchers have attempted solutions as well. A research team at Shriners Hospital in New England built a prototype in the early 1990s, Klute said. For many reasons, however, some devices never make it to the market.

“Maybe people have found a solution, but it so severely compromised the suspension that it was not a good solution,” he said. “You have to balance all these parameters, like performance with comfort. Ideally you would like a limb system that could adapt appropriately so when you need cooling, it could provide it, and when you need performance, like you’re running to try and catch the bus, maybe right at that time what you need is the best suspension you can get.”

For now, Klute’s team plans to focus on an active cooling system and cold receptor stimulation, though he continues exploring other avenues.

“If we could measure just how much perspiration accumulates, could we build a system that could provide enough evaporative cooling and solve the problem in a different way?”

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Stephanie Z. Pavlou is a staff writer for O&P Business News.

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