Know Your Media: The Impact of Materials on Cushion Performance
- By Stephanie Tanguay
- May 01, 2011
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
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
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
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
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.