Seating & Positioning
Anatomy of a Seat Cushion
There Are High Expectations for the Immediate Interface Between Client & Seating System, But Are Those Expectations Always Fair?
- By Laurie Watanabe
- Feb 01, 2013
Consider the wheelchair seat cushion, a relatively modest portion of the seating & mobility system for a client with significant and lasting injury, illness or genetic condition.
The cushion is just one part of that system. But a lot is expected of it by both clients and clinicians.
“The cushion is the direct interface with sitting, and therefore balance, and therefore comfort,” says Tricia Garven, PT, ATP, clinical applications manager for The ROHO Group.
As this “front-line” interface, cushions are readily blamed by consumers — and sometimes the healthcare professionals working with them — when something about the seating system isn’t quite right.
Of course, it’s not always the cushion’s fault. So knowing what a cushion should be expected to accomplish — as well as what it should not be expected to accomplish — is an important step toward creating and maintaining a seating system that works on all necessary levels.
Defining Cushion Expectations
Susan Cwiertnia, PT, MS, clinical education specialist for VARILITE, says of expectations for a seat cushion, “Some of the main functions are pressure management and skin protection, positioning, stability,
skin temperature and moisture control, friction and shear management, vibration damping and of course, comfort.”
Sharon Pratt, PT, Seating Solutions LLC, says a seat cushion should also “facilitate function from a mobility and physiological perspective.”
But she adds, “I consider my job as ‘taking the work out of sitting for my clients.’ In doing so, each time I am involved with a wheelchair/seating prescription, I like to envision the ‘sitting footprint’ of the client. The seat support is a critical component of the inferior seating support surface (inferior sitting footprint). I personally like to think of optimizing the client’s sitting ‘footprint’ every time I prescribe a seating system.
“The seat cushion is of course just one critical component of this; another critical inferior support surface is the foot support. It should be noted that the seat support can’t provide a successful outcome in isolation. Other components, such as a back support for example, are also necessary to consider in the big picture.”
Cwiertnia adds that cushion covers are also designed to perform a specific role as part of the overall seating system.
“Covers are often designed to complement the design and function of a cushion and play a role for moisture and temperature management of the skin,” she explains. “Covers made out a mesh-type material oft en are more breathable and build less heat near the skin than a waterproof cover designed to protect the cushion from incontinence.
“Some manufacturers utilize the cover to help meet flammability requirements. Covers can even contribute to skin protection with special designs to prevent the hammock effect or materials incorporated to enhance pressure distribution.”
“The seat cushion cover is basically an opportunity to create tension,” Garven says. “Surface tension on top of the cushion is going to limit how much you can immerse into the product.”
When it comes to cushion covers, Pratt says, one size — or style or design — does not fit all.
“The cover can be instrumental to the success of the cushion performance in terms of posture, skin and function management,” she notes. “It is extremely important that the cover that was designed for use with the cushion is in fact the one that is always used.”
Garven adds, “With the JAY J3 cushion and the ROHO Hybrid Elite cushion, there’s actually a piece of foam sewn into the cover — so the cushion is not going to work as intended without the cover.”
How Media Make a Cushion
All seat cushions have the same basic goals, but they accomplish those goals through different means.
For example, some cushions use different media in various parts of the cushion.
“Frequently, the cushion base is designed to provide support and shape to the cushion,” says Cwiertnia. “The base may function to provide contours for positioning to softer materials used on top, or be used to make the cushion more rigid and supportive when used with a sling-seat style of wheelchair.
“The materials used mostly depend on the design and function of the base. Oft en, closed-cell foam is used because it is firm for the support and shape while still being a lightweight material. Inserts used as a rigidizing base are often made from wood or plastic.”
For the middle and top layers, meanwhile, many different media can be used.
Says Pratt, “The purposes of the seat cushion’s middle/top layers are many. In consideration of product design and materials used, we have many factors to consider, mostly depending upon the clinical goals.”
For example, she says, if the clinician is seeking lateral stability for the client, “a cushion may be selected for trial that is designed such that the ischials can sink into the materials at least 2" without bottoming out and the precontouring, if it exists, needs to respect the anatomical dimensions of the trochanters and femoral loading area. The cover and the materials beneath the cover need to work in harmony, creating minimal tension so as to allow this optimal immersion of the ischials. If using a fluid material beneath the pelvis, segmentation becomes an important factor. This may reduce the flow of the fluid from one side to another, enhancing stability.”
If skin protection is one of the clinician’s goals, Pratt says, “We need to think about selecting materials that allow immersion without bottoming out as well as optimal envelopment with the goal of pressure redistribution. This means that we want the ischials to sink in while providing an optimal shape relationship with the client’s unique buttocks and femoral shape so as we have an overall reduction in the magnitude of the pressure.
“A desirable shape relationship can be achieved with fluids for example that have in the design of the container low surface tension and a volume that permits the appropriate depth of immersion. Others use the intimacy of custom molding/shaping to achieve this intimate shape relationship.”
Other cushions, meanwhile, use only air cells to accomplish what other cushions accomplish with several different media.
Garven says it’s all a matter of prioritizing a client’s multiple needs. An air-cell cushion, she explains, “would be higher on the skinprotection side, lower on the stability and positioning side than something that’s more foam based, more contoured or formed.”
Air-cell cushions can, however, still provide significant support.
“You have to be able to control where the air is going,” Garven says. “If that air does not have some mechanism to control the location of the air, it’s not going to provide stability and positioning.
“Of course, air is going to be good for skin protection; it’s going to allow immersion and envelopment and match the shape of the person. But unless you control where the air is going and you control the flow of the air, you miss out on those other goals. And depending on your client, that may or may not be what they need.”
Garven points out that some cushions combine air cells with other media to accomplish different goals.
“That’s why many different companies that use air singularly or in combination have a version of that product that has multiple valves,” she says. Controlling air direction can be used to create proper positioning for such conditions as obliquities, Garven adds.
Foam is also a popular choice for middle or top layers of cushions, particularly because so many different densities are currently available.
“Foam is often layered so that a softer or less stiff layer is in contact with the user to provide comfort and envelopment,” Cwiertnia says. “A middle layer is used to provide load distribution and stability because oft en the softer foam is not stiff enough to do so.
“The foam thickness that is used depends on the stiffness and density of the foam. Stiffness allows foam to support and distribute load by how much it compresses, while density is a measure of the weight per volume of the foam. Density in foam helps to provide support by thickening the cell walls. Those two features combined can determine the weight capacity of a foam cushion.”
As for other media frequently used in today’s cushions, Cwiertnia says, “Fluid cushions such as air and visco-fluid are better known for pressure distribution and skin protection. These media displace and allow for immersion, which creates more surface area for pressure distribution as well as good envelopment.
“A benefit of a solid gel insert would be tissue cooling, at least until the temperature in the gel equalizes with the skin temperature.”
Contours, Profiles & Other Cushion Shapes
Cushions also accomplish their goals by via special physical designs, such as different heights of air cells, or built-in contouring.
“Some cushions are pre-contoured or cut with indentations that will try to fit the body shape for the majority of users,” Cwiertnia says. “These contours provide additional stability, positioning and surface contact area for pressure distribution. Since one shape can never fit all, a pre-contoured cushion may or may not be appropriate and should be evaluated on a case-by-case basis.”
Meanwhile, in air cushions, the height of the cells “is going to correlate with the amount of immersion,” Garven says. “Immersion is how deep you go (into the cushion), how much sinking in you do. The higher the cell, the more you can immerse. That’s going to be (the same) for foam, that’s going to be for anything that you sink into. The taller it is, the more immersion you get.”
Clinicians seek immersion, Garven adds, because of pressure.
“Pressure is a problem, but it’s not pressure in general that’s the problem, because we never eliminate pressure,” she says. “We’re redistributing that pressure because we’re trying to avoid peak pressures.
“Immersion is important because it’s creating more contact area, more surface area, and that’s really how we’re redistributing the pressures. We’re basically allowing your thighs and more surface area to accept the load. And then also with the higher cells, that is going to allow that material to conform as closely to your shape as possible — that’s envelopment. So you get more immersion and envelopment with deeper products.”
One more important note with immersion: For it to optimally work, consumers need to do their part.
“It’s very important that they allow whatever product it is to work, and having as few layers in between the skin and product is best,” Garven says. “In an ideal world, there would be no clothes and no covers, but that’s not going to happen. The clothing that the client is wearing and any additional layers they put (between themselves and the cushion) — diapers, towels, pillow cases — are just layers that are creating tension that’s not going to let them sink into and have as much immersion and envelopment as that cushion is supposed to have.”
When to Consider Replacing a Cushion
Even high-quality cushions that are well cared for (see Cushion Care 101 sidebar) do eventually need replacing.
“The client, or provider or clinician, should check regularly for signs such as foam compression sets that have a negative impact on the client — for example leaving the client without the necessary ‘cushioning effect,’ Pratt says. She also recommends checking for “holes or punctures in cells or compartments that contain fluid/air; migration or loss of fluid/air beneath the areas of the pelvis needing greatest protection — for example, beneath the ischial tuberosities — and tears in the cover that was designed for that particular cushion.”
Garven says a change in the foam’s color can also indicate that it’s aging and needs to be checked out.
Wheelchair users, particularly those with compromised sensation, should regularly conduct skin checks, of course. But skin checks become even more important, Garven says, if consumers suspect their cushions may be wearing down.
“A big clue is to double check that skin is still intact,” she says. “Because with that foam cushion, if you don’t replace it on time, you could be sitting on a bottomed-out cushion and could be getting pressure ulcers and not even realizing it.”
In fact, consumers, clinicians and providers should check for bottoming out because it’s possible with cushions regardless of construction.
“No matter what product (they’re) sitting on, make sure it’s not bottoming out because that’s for sure going to be a problem,” Garven says. “Everything can bottom out, not just air. That’s when you know that foam needs replacing or when that gel needs massaging, or maybe there’s a leak and you didn’t notice because you didn’t open the cushion up.”
Cwiertnia points out that cushion covers also need to be checked regularly for wear: “Obvious signs of a worn cover would be torn seams, broken zippers and excessive soiling that will not clean when washed according to manufacturer instructions.”
It’s Not Always the Cushion’s Fault!
Of course, there are other reasons a cushion doesn’t work as well as it used to or as well as the clinician, provider and consumer hoped.
A client’s sizable weight gain or loss, for example, could impact how weight is distributed and could make the cushion “feel different” to the user.
“The same person can sort of have an evolving experience with the cushion,” Garven says, “especially kids who are going through puberty and their bodies are changing. They’re changing shape. Their load is changing and redistributing, and maybe they’re becoming heavier in their middle.”
For example, kids with spina bifida could notice they’re sitting differently on and in their cushions as they grow older: “ Their legs aren’t getting heavier, but their middle is.”
As the immediate interface between man and machine, seat cushions do take a lot of blame. Consumers can be quick to complain about a seat cushion that may not be at fault.
When consumers start blaming their cushions, Garven suggests encouraging them to give examples of what differences they’re noticing in their seated positioning.
“The biggest thing I would say on the consumer side is just to be aware of changes,” she says. “Maybe you don’t know exactly what’s changed, but you can identify something’s changed. You don’t feel as comfortable as you used to. You can’t reach your joystick, you can’t untilt as easily as you used to, or you’re not as deep in the wheels — you used to be deeper. These should all be cues (to check) if all the seating components are still working properly.”
Whether the seat cushion is the issue or not, having the consumer share such a list of differences can be the start of ultimately determining what modifications need to be made.
“Maybe your transfers are harder than they used to be, or you feel like you’re always leaning to the left now, and you are always losing your balance in that direction,” Garven says. “Well, maybe your pelvis has changed, and we need to change your cushion.
“Those would be clues as to what we could change to make you as functional as possible.”
This article originally appeared in the February 2013 issue of Mobility Management.