There was the power wheelchair user with post-polio syndrome who preferred a thin and flat, visco-elastic polymer gel-type cushion mounted to a solid seat pan in order to slide himself forward in the seat to use his urinal independently.

Then there was the woman with multiple sclerosis and a limited ability to move and shift her weight who needed a plastic honeycomb-type cushion because she lived in a humid climate where perspiration was a problem. And finally there was the person with high-level tetraplegia and recurrent issues with pressure sores who wanted an air flotation cushion for optimal pressure relief.
While these wheelchair users were quite different, they all had one thing in common. Each of their wheelchair cushions fell in the category of “low pressure and equalization pad” for use in a wheelchair and shared a single Healthcare Common Procedure Coding System (HCPCS) code (E0192), which covers the great majority of wheelchair cushions on the market.

With such a range of cushion types sharing a common code and the same Medicare allowable, the process of selecting and providing the right cushion becomes similar to a physician choosing the right medication to treat an ailment—medications have benefits, side effects, and performance variations and so do cushions. To select the most appropriate cushion intervention, a supplier must be familiar with the common materials used in wheelchair cushions and their advantages, disadvantages, and applications.

Cushion Elements
There are several requirements of wheelchair cushions. They must meet the functional needs of the user (for example, allowing the user to transfer on and off the cushion independently); be comfortable; be low weight, if the user propels a manual wheelchair or needs to lift the cushion in and out of the wheelchair; be relatively easy to use and maintain; provide postural stabilization as well as pressure relief; and handle issues of heat and moisture. But because there is no one cushion that can address all of these needs, a supplier must prioritize these goals based on the user’s needs and preferences.
Pressure sore management tends to get the most attention when selecting a wheelchair cushion, and pressure mapping devices are often used to assist in the selection of a cushion. However, pressure alone is not always the single cause of pressure sores. Extrinsically, pressure sores can be the result of downward and shear forces as well as heat and moisture build-up. Intrinsically, pressure sores can be the result of immobility, loss of sensation, poor skin integrity, and poor nutrition. Pressure mapping therefore may not always be the best determinant of the most appropriate cushion, and optimal pressure relief may not be the priority of the user.

The materials used in wheelchair cushions have various properties. Density affects the weight of the cushion. Material stiffness or softness contributes to the cushion’s ability to provide immersion, stability, and pressure distribution. The specific heat of the material contributes to temperature and moisture management. The coefficient of friction between the material and the user affects postural stabilization as well as shear forces. Finally, the cushion covers can also affect the performance and properties of these materials.

So how does one begin to determine that, for example, a person with severe spastic cerebral palsy, who wants to be able to shift and move freely on the wheelchair seat to reach and access objects, may prefer a flat foam cushion, or that an older female nursing home resident may prefer a compartmentalized viscous fluid cushion to accommodate her pelvic obliquity associated with a hip resection, provide comfort, and reduce maintenance?

A useful place to start is to individually review the most common materials used in wheelchair cushions. These include solid bases (such as wood, metals, or plastic), foam, fluids, viscous fluids, visco-elastic polymers or gels, and air.

Firm Surfaces
Sitting on a firm surface, such as a solid board, can provide a sense of stability but no immersion or pressure distribution. It is uncommon for people to sit directly on hard surfaces; however, they are often used as an underlying base of support. Users who choose a firm surface for stability and the ability to move around on the surface for postural changes and transfers are willing to forgo the greater comfort of softer materials. Solid materials also tend to have a greater longevity and conduct heat away from the body. However, a firm surface is not right for everybody. Many people cannot tolerate a solid base of support, especially if they have used flexible sling upholstery as a base of support for a long time.

Foam
Another common material is foam—a good absorber of shock and vibration due to its resilient qualities. There are many types of foam available, and the performance of foam cushions varies depending on the quality, density, and elasticity of the foam. Open-cell foam tends to be low cost, but it loses resilience over time as it breaks down due to repeated and prolonged loading and exposure to elements in the environment. Closed-cell foams have greater elasticity and are slower to lose resilience. However, over time all foams will eventually lose their ability to distribute pressure, stabilize, and absorb shock when they break down and “bottom out.”

Foam can be contoured in a generic shape to provide further pressure distribution; however, the fit of generically contoured cushions is only as sensitive as the shape of the user. Custom-contoured foam cushions provide a specific fit and accommodate severe postural deformities, but customization requires sophisticated equipment, processes, and skill level. Finally, foam can be contained in an airtight bladder with an air valve and compressed to the shape of the user, giving good pressure distribution.

Fluid-filled
Bladders are also used with materials other than foam. Often they are filled with fluids, such as water, to provide some immersion and pressure distribution. However, water-filled bladders can be heavy and unstable as the fluid moves around, and they can bottom out if the fluid is displaced.

One solution to this problem is the use of bladders filled with viscous fluids rather than water. These cushions allow for immersion, like water, but viscous fluids move more slowly and thereby provide more stability. However, like water, they also tend to bottom out.

To avoid fluid-filled cushions bottoming out, manufacturers have developed cushions with viscous fluids that are compartmentalized in modular components. This also allows for adjustment to accommodate deformities or to provide greater pressure relief in specific areas.

One fact to remember about fluids is that they have a very different specific heat compared to foam and other materials and therefore may pull heat away from the body, which can help resolve issues of heat and moisture build-up. However, while this may be helpful in some cases, it can also be too much of a good thing. Some heat is needed to prevent vasoconstriction and inhibited blood flow. To compensate for this, manufacturers are introducing phase-change materials to assist in heat management.

Visco-elastic Polymers and Gels
Visco-elastic polymers (also referred to as gels) have different characteristics. Although the material tends to be much heavier than the others (except water), it is very durable, stable, and relatively easy to maintain. It has some immersion and resilience to provide pressure distribution and absorb shock. Furthermore, it tends not to insulate body heat but also does not absorb or dissipate moisture. Some manufacturers have configured this material with open spaces to assist with moisture and fluid dissipation.

Air
Air-filled cushions or air-flotation systems can provide good immersion and pressure distribution. In addition, they tend to be lighter than cushions composed of other materials. However, they are sensitive to volume. Overfilled systems will increase pressure and underfilled systems may bottom out. And, like fluid-filled systems, air-filled cushions also may provide an unstable sitting surface depending on how the air is compartmentalized. In addition, these systems require more maintenance as air volume may leak out over time and the volume is sensitive to climate and atmospheric pressure. Finally, because the user immerses in the cushion, air-filled cushions can make sliding across the surface to change position or transfer in and out of the wheelchair more difficult.

Honeycomb
Another configuration of material includes a plastic honeycomb matrix. These types of cushions are not intended to provide immersion and pressure distribution. Rather they provide the greatest heat, moisture, and fluid dissipation. They are durable and easy to maintain, and the firmness allows the user to move across the surface for postural shifts and transfers.

As you can probably tell from these short examples, keeping on top of the multitude of wheelchair cushions available to serve the many different user needs and preferences can be a challenge. To meet it, rehabilitation technology suppliers should work closely with the user and clinicians to identify the priorities among the user’s needs and match those needs to the characteristics of the available options.

Mark R. Schmeler, MS, OTR/L, ATP, is the director of the Center for Assistive Technology at the University of Pittsburgh Medical Center Health System and instructor in the Department of Rehabilitation Science & Technology at the University of Pittsburgh.