At face value, immersion, envelopment and off-loading seem to be straightforward terms, even in the complex realm of seating people with significant mobility impairments.

But combine those terms with the variety of materials used to create today’s wheelchair seat cushions, then add clients with unique clinical needs. Suddenly, those common terms become much more complicated.

Starting with Definitions

Susan Wilson, VP of Supracor, said of the three terms, “According to the 2015 draft of RESNA national standards, ‘Immersion is the depth to which a body penetrates into a seat cushion from an uppermost plane; envelopment is the ability of a cushion to conform, so to fit or mold around the irregular shape of the body; and off-loading is the clinical practice of reducing or removing pressure from one area of the body to another in an effort to reduce risk to injury — e.g., reducing the pressure under the ischial tuberosities and increasing pressure on the thighs or other parts of the seated body.”

From those basic definitions, cushion strategies can become very diverse, in part because of the wide array of media that can be used to create wheelchair seat cushions.

For example, Wilson noted for Supracor products, “Stimulite cushions achieve envelopment and immersion as a result of their cellular matrix. Unlike most cushioning materials — including foams, gels and air bladders, which are isotropic and have the same resistance from different angles — Stimulite is anisotropic with different degrees of resistance in its length, width and thickness. This varying resistance enables the cushion to naturally contour to the body. When sitting on the cushion, the cells flex, allowing the body to immerse into the cushion. Since Stimulite cushions provide uniform pressure distribution throughout the cellular matrix, off-loading is generally not necessary.”

Compare Supracor’s situation with that of Sunrise Medical, which uses a multitude of materials, including different types of foam, air cells and gels in its cushion line.

Jeff Rogers, senior product manager of pediatrics and seating, said of Sunrise’s strategy, “For us, the big [term] is immersion — when a person sits into a product, trying to evenly distribute pressure is our goal. When no two butts are the same, it’s always a challenge to conform around people. You can’t create a cookie-cutter approach, so you’ve got to have products that can fluctuate. We have a full product line that tries to handle this in multiple different directions. We have cushions that do off-loading and try to load where possible, like with the J3 [cushion]. We have cushions that fully plan to immerse, like the J2 Deep. Then we have ones that do a little bit of everything with a good, balanced approach. That would be the Fusion. We have different ways of reaching that goal, because we know everyone presents a little bit differently.”

Measuring Immersion

As can happen when defining terms, what seems simple on paper can become much more complex when applied to actual clients.

Tom Whelan, Ki Mobility’s VP of product development, explained the nuances of immersion, for example.

“Immersion has been characterized as the linear distance that you sink into the cushion,” he said. “The problem with immersion, like everything else in science, is where do you start measuring from on a highly contoured surface? You start to run into problems using immersion as a comparative measure.

“Conceptually, it’s easy. If I sink into the cushion more, I’m going to create at least the opportunity to have more surface area to distribute my load over. But now if you want to start talking about immersion in four different cushions, how do you characterize immersion? Because if it’s a linear dimension from a starting point to a finishing point, the starting point becomes the subjective part of the measure; the finishing point is straightforward.”

How do you measure immersion, for example, in a contoured cushion?

“If you have a cushion that has a slight upward contour on the outside of the cushion that isn’t really material to loading,” Whelan said, “do you measure from that point? If it’s highly precontoured, and it matches your butt, do you measure from where you first contact the cushion to where you end? Are those comparisons meaningful?

“To talk about immersion and just say that immersing is important and it’s one way to look at a cushion is easy. To drive it into a scientific measure that gives you values that a clinician could use is incredibly difficult.”

Medicare’s cushion testing, Whelan added, measures immersion — by sinking an “indenter,” a mechanical model representing a human rear end, into the cushion.

“[Medicare testing] is purely an immersion test,” Whelan said. “The problem is that when you use a cross-sectional analog of a pelvis, which is what [Medicare’s] indenter is, you have to have a material that responds to a cross-sectional analog the same as it would respond to a complete butt. And that’s not rational.

“If you use something that has fairly consistent materials, like a foam cushion, a cross section analog of a pelvis isn’t a bad indicator of how it would perform with an actual pelvis. But if you have a highly precontoured foam, where that indenter relates to the contours can make all the difference in the world.”

So Medicare’s current test, Whelan said, is limited in its ability to capture the entire picture.

“I could literally build a piece of concrete that’s exactly shaped like the indenter and just put a little tiny bit of foam at the bottom of it, and I could pass the Medicare test with a cushion that would provide absolutely no clinical benefit whatsoever.”

Adding Envelopment

Whelan said envelopment is even trickier.

“If immersion is considered the depth you sink into,” he explained, “envelopment can be considered the quality of that depth. When you sank into the cushion and you formed a loaded contour shape — your body’s ‘squishy material’ matched up with the squishy material you’re sitting in, and there was a resulting loaded contour shape — how that loaded contour shape enveloped you is what we’re trying to measure.

“I can have two cushions that by definition have the exact same immersion, but one of them could be concentrating more pressure on the ischial tuberosities (ITs) than the other cushion. Immersion is the same, and you could argue that the surface area might be the same of that loaded contour shape. You have identical loads and identical surface areas. But not an identical distribution of force over that identical surface area. If I have a cushion that has less equalization between the trochanters and the ITs, the envelopment is not going to be as good as one that has greater equalization between those two.”

The good news for the future: Whelan said, “Now there is a draft standard for a laboratory test of envelopment using an instrumented indenter.”

Measuring Magnitude

While the third term of the seating trinity is often referred to as off-loading, Whelan said, “If you follow the science of seating presentations that were put together, it’s actually immersion, envelopment and magnitude. Offloading is a technique; magnitude is the measure of the technique.”

So with that context, Whelan said, “Magnitude is the absolute value of the loads at specific points, like the ischial tuberosities and the trochanters. You can have two cushions, same immersion, exact same envelopment as far as parity goes between the trochanters and the ischials, but you could still have higher forces at work because it didn’t load the other areas as well.”

Combine the three measurements — immersion, envelopment and magnitude of off-loading — and Whelan said, “At the end of the day, what we really care about in a cushion are two things. One: How much did it take load, force, pressure if you will, off of the ischials and put it somewhere else? The cushion that can take the load from here and move it somewhere else the best is the best cushion, assuming everything else is the same. Two: That’s where off-loading comes in. In an ideal cushion, you could say off-loading and you can say directed loading. Off-loading looks at it from the ischial’s perspective. Directed loading looks at it from the cushion’s perspective. Was the cushion designed to direct the load where we want the load to go, which by definition would be offloading the ischials?

“Offloading the ischials is the universal truth. There is nobody who sits in a wheelchair more than four hours a day who doesn’t benefit from ischial offloading.”

Different Cushion Media

Given this context, the media used to create cushions can be compared to multiple roads that can lead to the same place.

“The reason we have multiple media is we know there are side effects, if you will, to every one of them,” Rogers said. “People will use air cells because they’re light, but maybe there’s a little more maintenance because you may put a hole in it. With fluid, it’s great for immersion, it’s great for no maintenance, but on the other side of things, it’s a little heavy. Same thing with foams: They’re comfortable, they feel good underneath you, but they can get hot. So putting the clinician in charge to be able to pick the right product from a wide portfolio is what we try to do. We try to balance it out.”

Whelan said, “Why do we use different media? Because every medium has a different characteristic on how it interacts with tissue. Because they have different characteristics under load that can be taken advantage of in proper design.”

He referred to the “science of seating” and the evolving ways that researchers are examining seat cushions to better determine their efficacy — and how that efficacy is best measured. These are questions as complex as the needs of the clients who depend on accurate answers.

“Recidivism of pressure injuries is probably the biggest single cause of death in wheelchair users, other than the natural course of the disability and disease processes,” Whelan said. “Most clinicians will tell you that most pressure injuries can be prevented. Yet, we still have an increasing prevalence and incidence of pressure injuries in seated postures. So that’s a bad outcome. When something is increasing, and there is an overwhelming medical opinion that it can be prevented, that generally means there’s still some ignorance at work.”