New Discoveries

Pressure Ulcer Research, Part 3

Bringing Technologies Together to Shape Tomorrow's Seat Cushions

When Amit Gefen, Ph.D., a professor of biomedical engineering at Tel Aviv University, began studying chronic wounds, he was interested in something he could not easily observe: what happens to the soft tissues and skeleton of our feet when we walk. Gefen noticed the lack of research in the field compared to “research resources that we biomedical engineers see in fields like orthopaedics, cardiovascular, cancer,” he says.

So Gefen focused on bringing those resources, such as computational modeling, into the chronic wound spectrum. Along the way, he became interested in spinal cord injury (SCI) patients because of their vulnerability to pressure ulcers and because so many were injured early in life.

“These are people with a life ahead of them potentially,” he says. “But one in 10 of them will die of pressure ulcers.”

What could it mean if this part of the healthcare industry better understood how, when and why tissue injuries occur?

The Various Tools Involved

In part 1 (Mobility Management June 2014), Gefen discussed tissue engineering — creating human tissues in lab settings that can then be subjected to the types of loads that occur when people sit. Tissue engineering, Gefen says, allows researchers to “see down to the level of individual cells, what these loads will do, how these loads will affect the cells.” The ability to create cells — healthy versions as well as ones that simulate a previous pressure ulcer and scarring — enables researchers to carry out their studies without the involvement of numerous human subjects, which is always a research challenge for the small seating & mobility field.

In part 2 (MM August 2014), Gefen talked about how computer models simulate the clinical conditions that seating & mobility clients often present with, such as spasms, postural asymmetries and muscle atrophy. While a range of additional factors unique to each patient make it impossible to predict individual client outcomes, thanks to computer models Gefen says, “At least you can get some understanding of the trends of events. Much like in cancer. When you see a tumor of a certain size, based on knowledge you can evaluate how it’s going to evolve. But that doesn’t mean you will be able to predict whether that patient will live or die, or how much time it will take.”

Computer models can drastically reduce the time it would take for researchers to sign up enough human subjects to create a study. “The computer simulations basically substitute for other people’s experiences because other people’s experiences will take huge clinical trials, which you cannot really do in this field,” Gefen points out.

Gefen used other tools as well, including an “open” style of MRI that enables subjects to be in a seated position during the scanning process. In the sitting MRI studies, participants initially sat on a rubber tire so their buttocks were suspended in the air. Gefen identified ischial tuberosities, gluteal muscle and fat. When presenting his findings to clinicians, Gefen’s slides include images taken during the MRI studies.

After doing the initial scans with participants sitting on the tire, “Then we took the rubber tire out, and people were just sitting normally,” he says. Pointing to a slide, he adds, “Later on we did it on cushions as well, but this is on a rigid support. You can see what’s going on here. You can see that the muscle is deformed to at least 50 percent of its original thickness.”

Technologies Used in Tandem

With this foundational information thanks to the MRI studies, Gefen says, “Now when you add the computational modeling, you have the structure of before the loading occurred, and you have it now when the loading is active. You transfer the structures into a computer environment, and basically all you have to tell the computer to do is to duplicate how much the bone here sags toward the sitting surface. Then you get beautiful diagrams of the distribution of internal tissue deformation in the bodies of actual human subjects as they sit.”

From there, Gefen used computer models to add a wrinkle: What if the participants being studied had SCI?

Gefen knows that tissues of able-bodied people and people with SCI are different. In February, he published “Tissue Changes in Patients Following Spinal Cord Injury and Implications for Wheelchair Cushions and Tissue Loading: a Literature Review” in Ostomy Wound Management.

The bottom line of the study: Due to immobility, patients’ bodies change after SCI, and those changes occur quickly.

In his article, Gefen lists changes such as weight and fat mass gain, and skeletal muscle atrophy and fat infiltration into muscles. He adds that these changes happen as early as a year after injury.

“Then you can do the same [computer simulations] with SCI patients,” Gefen says. “Now with SCI patients, it’s a completely different world, because they lose muscle. They lose so much sometimes you don’t see it: Their bones are changing, the fat is taking over.

“Basically you now have this very thin layer of muscle — it’s also softer muscle because it contains a lot of fat — which is now being subjected to more load, because the trunk weighs more. It’s a completely different structure, and then the internal loads will be different as well. Actually, they’ll become much higher.”

Interactions with Seat Cushions

Regarding those higher loads, Gefen says, “This is where some of the cushions don’t adapt. This is where a cushion needs to respond. Only certain technologies, for example air cell-based cushions, are able to respond to these changes, which is a feature we term as ‘adjustability.’”

But he adds that when the wheelchair cushion is selected soon after injury — “when a patient has a nearly healthy type of anatomy” — the client may end up with a cushion that doesn’t fit well due to changes that happen post injury. For example, a contoured foam cushion that is fitted close to the time of the injury and that the client keeps on using will very soon be a misfit (as the body changes), which can, as a result, place the individual at a risk for serious pressure ulcers.

Using other kinds of technology could help clinicians and cushion manufacturers better understand what typically happens to many SCI patients, and create products with those changes in mind.

“You can ask many, many questions with this MRI technology,” Gefen says. “You can ask what would be the effect on posture. You can use different cushions in the MRI. You can directly measure the internal tissue deformations. But you can’t look at the distributions or deformations in the localized deformations, say, just under the bone with just the MRI. There you need the computer model. Then you can add these cushions into the model to see how they behave.”

Using computer models, Gefen says, “You can see in real time what’s happening to the loads in the tissues. Also what’s happening to the cushion. You can see that you get most of your loads here, at the interaction between the bone and the muscle, which is what we expect. And then you can ask: If this is the level of deformation, how would that correlate with what we know about the tolerance of cells and tissues that we get from the more biological-oriented research that we do with tissues?”

Standard Cushions vs. More Complex Cushions

So by creating tissues and loading them, studying how they react down to the cell level, and then by using computer models to extrapolate the information to include, for instance, scars that remain from a previous pressure ulcer — Gefen says it’s possible to differentiate between a standard foam seat cushion and a seat cushion more complex in design and media.

“Now you can say, ‘Suppose I look at an air cell-based cushion, a ROHO type. I also look at the standard foam cushion. So the first thing you see is the level of immersion: The level of envelopment is substantially different. One of the things that came out of our research is that the levels of tissue deformation depend very strongly on the levels of immersion and envelopment. So this is a body immersed in foam, and this is a body immersed in a ROHO cushion. If you look at the level of deformation or stresses, you see they are on different orders of magnitude. They are scales apart: 10,000 times different! Because the levels of deformation in that interface are so sensitive to the level of envelopment that if you increase the level of envelopment, you can substantially decrease the level of deformation. And this is shown in the simulation.”

How can that be applied to the recommendation of seating products?

“The application of that is going back again to the injury thresholds — soft tissues in general can sustain high levels of deformations for just short times, and with low levels of deformations, [patients] can live with them for longer times, for a couple of hours. So if the levels of deformations are very high, you meet the injury threshold curve in a short time. If you have this high envelopment and immersion and your levels of deformation are very low, [sitting tolerance is higher].

“This is what these models are showing, the trends of effect. You can also add ‘what-if’ questions. What if that patient already had a pressure ulcer? What if that pressure ulcer affected only his skin, or what if it was a deep-tissue injury which left a scar that goes all the way through? Scar tissue is much more stiff than fat, and it’s also stiff er than muscle. If you’ve ever cleaned a chicken breast, you remember that when you cleaned the tendon out, it’s thicker than the muscle. A tendon is like a scar. So you have this stick-like inclusion there that’s stiff , so that’s why we say it creates a lot of stress, in it and around it. Because you need more force to deform it and more force in that area of tissue — there’s more stress.”

Obviously, being able to quantify how and why different media do a better job of pressure distribution is very significant to all stakeholders, from manufacturers who design cushions to clinicians and ATPs who recommend the products, funding sources who pay for them, and of course, the consumers who use them.

“The educational value of this is huge,” Gefen confirms. “The purpose would be more than just visualizing and understanding why. The purpose would be again in the context of risk assessment: To look at deep scars, to put a patient at more risk again or not with respect to the support surface and to the cushion that the patient is being prescribed. This is more than what you would be able to conclude by just looking at patients.”

Editor’s Note: This three-part series will soon be available in a single-pdf format that’s easy to download and share. Watch Mobility Management and the eMobility newsletter for an alert when the pdf — which will include special bonus content — is ready

This article originally appeared in the September 2014 issue of Mobility Management.

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