Wheelchair Cushion Differences

by Stephen Sprigle, Ph.D., PT

Understanding the materials used to make wheelchair cushions can help you choose your best seat and back.

Materials used for wheelchair cushions have come a long way. You now have quite a choice based on your individual need. Cushions protect skin from damage that can lead to pressure ulcers, and they affect many functional activities such as reaching, transfers and wheelchair propulsion.

Many researchers and clinicians who have studied cushions have developed a common conclusion: ‘No one cushion is best for all people’. Because so many cushions are available commercially, wheelchair users should be able to find one that adequately meets their needs.

How do you do that? First you need to understand the materials used to make cushions. Cushions are designed to distribute body weight over its surface or to re-distribute pressure away from areas considered 'at risk' for pressure ulcer formation. Many different materials and combination of materials are used in cushions, all of which have both positive and negative characteristics. In addition, several terms are used to describe cushions.

Becoming familiar with the words used to describe materials and construction will help you select the right product. These definitions are used by the Paralyzed Veterans of America and by the Rehabilitation Engineering and Assistive Technology Society of North America. A few of these terms are:

Foam or flexible matrix: a lightweight, flexible cellular material used in support surfaces.
Viscoelastic foam or matrix: a compressible cellular material that has both elastic (spring-like) and viscous (time-dependent) properties. Viscoelastic foam is different from regular foam by having time-dependent behaviors such as creep, stress relaxation, and hysteresis. This type of foam is sometimes called ‘memory foam’ because it maintains the shape of an indentor (like your hand) before springing back to its original shape. NASA invented the first visco-elastic foam, T-foam, about 40 years ago.
Non-deforming foam or matrix: a support material that does not compress, deflect, or deform under sitting forces; often-used in cushion bases (Sunrise Medical Quickie Jay; Otto Bock Cloud [shown here]cushions use non-deforming foam bases).
Bonded foam: adhesion of foam material including different foams being glued together (The Invacare Ultimate cushion is made with bonded foam)
Viscoelastic fluid: a relatively incompressible substance with viscous properties; viscous fluids do not flow as readily as water and include maple syrup and grease. (Otto Bock Floam, Sunrise Medical Quickie JayFlo are viscoelastic fluids.)
Air cushion: a cushion with an impermeable (air-tight) membrane containing air. (Crown Enhancer [shown here])
Water cushion: a cushion with an impermeable membrane containing water.
Gel or solid elastomer: solid rubber-like, relatively incompressible material.
Segmented cushion: a cushion whose surface is divided into separate and distinct segments. (Span America’s PRT)
Convoluted cushion: cushion surface composed of convex protrusions separated by depressions or sulci; often called ‘egg-crate’. (Span America)
Contoured: shaped to fit or reflect the form or shape of the body, specifically, the buttocks. (Invacare Ultimate)
Cut-out cushion: surface having a disruption or removal of material to alter the load bearing characteristics of the surface.

Certain characteristics can be used when judging the appropriateness of support surfaces. Recently, a panel of health care professionals, researchers and industry representatives identified nine characteristics that impact efficacy and should be considered when selecting support surfaces (Krouskop & van Rijswijk, 1995) (Table 1). The management of forces (pressure, friction), heat and moisture directly affect the health of skin.

Important Support Surfaces Characteristics

Redistribution of Pressure -- A cushion should support your body weight without causing harm to your skin.
Skin Temperature Control -- You should not sweat when seated on a cushion. Some materials such as foam are naturally hotter than other materials such as gel.
Skin Moisture Control -- A cushion should help keep your skin dry and should not allow moisture or wetness to build-up next to the skin because wet skin is more easily damaged.
Patient/Product Friction -- The friction between the cushion cover and your clothes should not be too great (prevents you from transferring) or too little (allows you to keep sliding off the cushion).
Life Expectancy -- You should know how long a cushion is expected to last so it can be replaced before problems arise.
Flammability -- A cushion should not ignite into flames if a cigarette is dropped on it.
Fail Safety -- You should know what happens to your cushion if it breaks or becomes unusable.
Infection Control -- A cushion should not encourage or promote the formation of bacteria or other germs.
Product Service Requirements -- An owner’s manual should tell you how to clean and maintain your cushion and who to contact if a problem arises.

Knowing the durability of a cushion is important so you know how often it should be replaced. Typically, cushions should be replaced about every two to five years with all-foam cushions having a shorter life compared to other materials. Set-up and maintenance requirements should also be considered when selecting cushions. If you are unsure about how to use a cushion, chances are you will use it incorrectly.

Support surfaces can be thought of as having one or more of these three characteristics to distribute load (the weight of your body): compression, displacement and tension. Quite often, cushions are created using different materials and therefore, use a combination of these characteristics to distribute load.

Materials that compress include foam and air. Solid gel, water and viscous fluid displace when loaded (bear weight) as these are incompressible materials. Covering materials such as vinyl, cotton, and Lycra support the body in tension. Regardless of the materials used, the effectiveness of a support surface depends on its ability to distribute pressure without hindering function or increasing the potential for skin damage.

Foams are typically light and vary widely in stiffness and density. Stiffness refers to the firmness or hardness of a material, whereas density relates to a material’s weight per volume. Foams used in wheelchair cushions have medium stiffness in the range of 40 to 60 IFD (Indentation Force Deflection is a standard measure of foam stiffness with a higher number indicating a stiffer foam) while bed overlays can be softer (30 IFD) because loads are less when lying down since your weight is distributed over a larger area. Foams that are appropriate for a mattress will be too soft for use in sitting. Egg-crate mattresses and other foam mattresses should never be cut into pieces for use as wheelchair cushions. Foams used as support surfaces should have a higher density (thicker and more difficult to compress) than non-supportive foams and be classified as high resilience (HR) foams.

Proper stiffness, density and resilience help ensure adequate support and an effective lifespan. Foam envelops the buttocks well, but can be hot and is damaged by moisture and light, so a protective cover should be used. Covers made from fabric that does not stretch and is fitted tight around a cushion tend to negate the compressive benefits of foam.

Foam absorbs impact loads well, so it can help absorb the shock of going over bumps or obstacles.

Gels and viscous fluids are heavier than foams and, therefore, are often used in combination with foams. These materials are good at minimizing heat and are typically encased by a protective cover so are easily cleaned. Because of their incompressible nature, viscous fluid and gel cushions often use a contoured non-deforming foam base or a flat, compressible foam base to better enable the cushion to conform around the body. Gels and fluids absorb vibration but cannot absorb impact well. Therefore, these materials can absorb the vibrations that one might experience in a car, but not the impact one gets when bumping over a curbcut.

Air is highly compressible and would make an excellent support surface, except for its requirement of an impermeable membrane. Therefore, the efficacy of air cushions is directly related to the design of its container and proper inflation. Some encompass the body well while others do not. Some dissipate heat well, all can handle moisture without harm and most will absorb impact loads. Because air cushions are adjustable, they can be inflated to meet the needs of the user, but this adjustability also means that they must be maintained properly. An improperly inflated air cushion can be either too hard or two soft, and both are potentially damaging situations. If an air cushion is right for you, be sure to read the instructions so you inflate the cushion properly.

In summary, many characteristics must be considered before you select a cushion. These include cushion materials as well as other design characteristics. This article cannot tell you which material is best for you, but it can act as a source of information. All the cushion materials described in the article have positive and negative features. By better understanding these features, you’ll be better able to make choices about your wheelchair cushion. Anytime you decide to change wheelchair cushions, a clinician experienced in wheelchair seating should evaluate you and allow you to try out a few sample cushions, as well.