Clinically Speaking

Know Your Media: The Impact of Materials on Cushion Performance

There are literally hundreds of wheelchair cushion products available, and that’s just the ones with a Healthcare Common Procedure Coding System (HCPCS) code. A cushion must have a HCPCS code in order to be billed for under Medicare — and many other insurance programs that follow aspects of Medicare guidelines related to durable medical equipment. There are also many other products marketed directly to consumers as “cash and carry” items with the intent of providing comfort. Together, this amounts to an overwhelming list of products.

Within the rehab scope of seat support products, the HCPCS codes classify cushions for skin protection, positioning, skin protection & positioning, and adjustable versions of these. The variety of mediums used in these products is also quite vast: foams, fluids, air, gel, thermoplastic elastomers. Many cushions are actually hybrids, cushions constructed from a combination of these materials. Hybrids frequently utilize a contoured foam base for support and stability, with another medium — such as air or viscous fluid — for pressure distribution.

Understanding Materials’ Properties

In the world of rehabilitation service provision, seating products are best compared and contrasted based on their material properties: density, stiffness, resiliency, dampening and envelopment. Different materials have stronger or weaker performance in each of these areas. It is important to understand these properties:

Density is the ratio of a cushion’s weight to its volume (measured in lbs./cubic foot). This property is important because of its parallel relationship with cost and durability. Denser foam has a greater number of smaller cells, and these tiny cells also have thicker walls. It is generally more expensive because more raw chemicals are used to create it. Denser foam typically has greater durability because there is more foam per cubic foot to provide support.

Stiffness is determined by an indentation load defl ection test, which measures the amount of force required to indent foam 25 percent of its thickness. If the foam is too stiff, the pelvis will not sink into the material. If the foam is not stiff enough, the pelvis will collapse through the foam and may “bottom out.”

Resiliency describes a material’s ability to recover its shape after a load has been removed (a cushion that regains its properties overnight after a day of use would be described as having good long-term resiliency). Short-term resiliency is rapid recovery from a small change in load, such as small adjustments to seated posture.

Dampening is the ability of the material to reduce impacts. Foam and air products tend to dampen impacts better than viscoelastic foams or viscous fluids.

Envelopment describes the immersion of the pelvis/buttocks into the cushion material. Greater submersion into the cushion material increases sitting stability and can reduce peak pressure. For all of the new cushions on the market today, the materials they are composed of fall into fairly specific categories: foam, viscoelastic foam, thermoplastic cellular matrix, air filled, viscous fluid filled, solid gel and hybrids of these.

Foam has been the staple of wheelchair seating for decades and remains an integral component of the majority of wheelchair cushions available today. Foam is available in a multitude of compositions, which vary the performance of the material. Foams are available in open-cell and closed-cell forms.

Viscoelastic foam, like so many other things in our industry, was created not to meet the needs of wheelchair users, but for an entirely different application: space travel. Because of their higher viscosity, a consistently applied mass moves slowly into viscoelastic foam. When weight is removed from viscoelastic foam, the material slowly returns to its original shape, a property of this material known as memory. Viscoelastic foam properties are altered by temperature, becoming firmer in colder weather and softer in climates with high heat.

Thermoplastic cellular matrix describes an open, six-sided cell construction design. These thermoplastic elastomers are constructed in various cell wall thicknesses for different amounts of structural support and stiffness. These materials are often touted for the ventilation properties of their structure.

Air-filled products feature an air-containment bladder, which may be single or multi chambered. Various thicknesses of containment devices are available. The principle of air products is envelopment, and the thickness of the containment bladder and the level of inflation can both alter the envelopment. Air-chambered products must be adjusted for changes in altitude and occasionally with extreme changes of temperature.

Viscous fluid is a component of a number of different cushion products available today. The fl uid materials utilized in these products are quite diverse and have different viscosity (the property of a fluid or semi-fluid that causes it to resist flowing). The higher the viscosity of a fluid, the less it will flow, as the molecular structures of the material do not easily glide across each other. Water is an excellent example of a fluid with low viscosity. Some of the viscous fluids utilized in cushions are also subject to changes with temperature, becoming more solid in colder temperatures and more viscous in higher temperature ranges. It is always important to consider the environments and conditions that products will be used in. Viscous fluids are commonly seen in hybrid cushions with foam bases, which provide contour and support for the pelvic structures.

Viscoelastic polymer is often referred to as a gel. This dense material is higher in weight than most viscous fluids and does not flow, but it offers good dampening and resiliency. This material is often seen in hybrid applications as an overlay to a foam base, which may be contoured.

Too Many Choices?

Hybrids, as I mentioned, are combinations of these materials and are very common. Many of today’s cushion products incorporate two or more media to achieve positioning, stability, suspension and weight distribution. Foam bases are frequently utilized because of the versatility to shape and contour the materials. Combining different materials can enhance the success of accommodating a postural asymmetry with enhanced weight distribution.

But do we really need all of these options? I believe the answer is yes, for two reasons. First: the creativity of bringing new technologies to old problems. Fifty years ago, seating interventions consisted only of foam, perhaps different densities glued together and with a limited variety available. Today, foams are engineered to specific characteristics with almost endless possibilities. Every new medium that is brought to our industry can be a chance for us to step outside of the box and rethink how we approach meeting the seating needs of the consumers we work with.

The other reason that we need the vast array of materials and cushion products is that there is NOT one solution for all needs. There is not one cushion product that works for all consumers. There are so many factors that influence skin integrity, muscle tone and orthopedic structures, and seating is one aspect of healthcare intervention.

That’s even more of a reason to try new things: We never know what the next great thing will be!

This article originally appeared in the May 2011 issue of Mobility Management.

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