Selection guidelines for providers and consumers
By Allen Siekman
Wheelchair cushions are available in a staggering number of designs and are made from a variety of materials. Understanding some basic principles relative to how cushions work, what materials are employed in their production and how they are used will help you to select an appropriate cushion.
The type of cushion that best meets the needs of the user will depend on the individual’s physical health, risk factors for pressure sores, seated stability, how much time he or she spends in a wheelchair and a number of other factors.
Wheelchair cushions can provide comfort for increased sitting time, stability to help with performing everyday tasks such as reaching and mobility in the chair, and protection against pressure sore development.
Human anatomy in the seated position
The human body was not designed for sitting. Our skeletal and muscular systems are built for walking on two legs. That being the case, people spend a lot of time sitting, and a significant number of individuals are not able to stand or walk because of accidents, disease or age-related limitations.
People who sit for extended periods of time during the day may require specialized seating equipment for increased comfort, controlled posture or protection against the development of skin ulcers (also known as bed sores or pressure sores).
To help understand this, let us take a look at the human pelvis. Several primary areas are important to support the pelvis and upper torso when in a seated position. The areas that are in contact with the seat cushion are the most important for this discussion. They are formed by a combination of skeletal components and are surrounded
by layers of soft tissue, giving us the familiar shapes of the buttocks and thighs.
The parts of the pelvis that have a major role in supporting the body in a seated posture include the ischial tuberosities (ITs), the greater and lesser trochanter (at the hip joint) and the long bone of the femur (see Figure 3). The trochanteric shelf is formed by the long bone of the femur and trochanter area; it is a great place to support weight and shift load away from the ITs; it also improves lateral stability in seating.
The first areas to discuss are the two prominent bony structures called the ITs. Most people may not know the name, but when you say, “Those two sharp bones that hurt when you sit too long on the bleacher seats,” they immediately understand.
The IT area of the pelvis is the lowest point of the pelvis when you are in a seated position. Viewed from the side, the ITs are lower than the hip joint. In the average adult, the distance between the lowest point of the ITs and the lowest part of the hip joint (the trochanter) is approximately 40 mm (1.57 in) (see Figure 4).
In addition to being lowest point of the pelvis, the ITs have sharp, pointed contours. Approximately 80 per cent of all pressure sores among wheelchair users occur at the ITs.
When in a seated posture with feet on the floor or on wheelchair foot rests and arms resting on armrests, the buttocks and thighs support approximately 65 per cent of a person’s body weight. For example, a 200-pound person will have 130 pounds of weight distributed on the buttocks and thighs. In addition, peak pressures are centred on the IT area.
The sacrum and coccyx (tailbone) are other potential areas of high pressure in the seated position. The coccyx is another sharp, bony prominence that is not ideally suited for significant weight bearing and is at increased risk for pressure sores. The coccyx is higher than the ITs, so the risk is not as great unless the person sits in a “slouched” posture, but the risk is still present.
Most wheelchair cushions support the body in two ways: immersion and envelopment. Immersion is how far you sink into the cushion when you sit on it, while envelopment is how well the cushion wraps around you and copies your exact shape. As a general rule, foam cushions are good at immersion and not as good at envelopment. Fluid cushions are generally better at envelopment, but may not be as stable as the foam cushions. It seems that there is always a trade-off between the two.
People who lack sensation or who have difficulty changing position or getting out of their wheelchair are at increased risk of developing a skin ulcer or “bed sore.”
Approximately 80 per cent of all wheelchair-related skin ulcers occur at the sharp “pelvic bones” (ITs).
When should a skin-protection cushion be considered?
A cushion designed for skin protection should be used if any of the following apply:
• The user sits in a wheelchair for more than 30 minutes at a time.
• The user has diminished or lack of skin sensation.
• The user is unable to change positions when seated.
• The user is unable to get out of the wheelchair without help.
• The user has a history of one or more pressure sores.
Heat and moisture
Heat and moisture build-up is a problem when sitting on any cushion. When a person sits on a cushion, his or her skin temperature in the contact area will increase. After sitting for 30 minutes or longer on most cushions, the temperature of the skin increases to the point that the body tries to cool the skin by sweating.
A waterproof cover on a cushion can trap heat and cause elevated skin temperatures and sweating. Besides being uncomfortable, high skin temperatures and moisture caused by perspiration can cause the skin to soften, increase the risk of skin rash, promote the spread of bacteria and increases the risk of pressure sores.
It is not uncommon for skin temperatures to reach up to 39°C (102°F) and for relative humidity levels at the skin to reach 70 per cent or greater after 30–120 minutes of sitting. If heat and perspiration are problems for the wheelchair user, some cushions that can reduce temperature rises and maintain skin humidity at 5 per cent or less. Careful consideration of the heat and moisture properties of a cushion can prevent many skin-related problems from occurring.
Many cushion manufacturers now offer cushion covers with a breathable fabric or mesh top layer to help prevent excessive heat and moisture build-up. These covers are an improvement over a waterproof non-breathable cover, but they may not provide enough protection if the cushion itself is not also designed with heat and moisture in mind.
A few companies have focused on maximizing airflow by using innovative designs and materials. Some of these new-generation cushions are completely waterproof and can even be used in the shower or bathtub. By providing air passages through the cushion from top to bottom and using fully breathable covers, they offer comfort, moisture control and skin protection, together with temperature control.
Wheelchair seat cushions come in a staggering variety of choices, from a simple piece of foam to complex cushions featuring multiple-density foams, foams with flexible gel layers or cushions with fluid bladders (air and/or liquid).
Most cushions provide support by allowing the body to sink or immerse into the cushion. When a person sits, the first point of contact with the cushion is the IT area. Cushions that successfully provide comfort and reduce the risk of pressure sore development share a common design feature: they redistribute pressure away from the sharp boney prominences at the centre of the pelvis and shift those pressures to the rest of the seated support surface at
the hips and thighs. Some cushions do this well, others not so well.
Most cushions change shape when you sit on them.
The cushion may be flat or contoured, but it will begin to adjust
to the shape of the person who sits on it. Most cushions work in this way. For example, cushions made from softer foams will compress, allowing the body to immerse into the cushion, which enables the cushion to change shape and adapt to the user. Some cushions have a fluid interface with the user; in this configuration, the fluid will move out of the way of high pressure and flow to areas of low pressure as the cushion changes shape and attempts to equalize support.
The key to the function of these cushions is that the material used to fabricate them has the ability to change shape under load. The foam compresses or the fluid moves.
When sitting on a thin foam or fluid cushion, it is possible for the pelvis to sink all the way through the cushion until the pelvis is sitting on the top of the wheelchair seat. This is known as “bottoming out.” If there is no foam or fluid separating the pelvis from the wheelchair seat, the pressure on the ITs (pelvic bones) increases, causing discomfort, increases in skin temperature and moisture, and a greater risk of pressure sores.
Wheelchair cushions are most commonly made from foam. Many foam cushions offer excellent support, stability and comfort, while others prioritize protection from skin ulcers. However, many inexpensive cushions are either too thin or too soft and deliver poor comfort and durability, and they may bottom out. These may be used for short periods of time, such as when moving from room to room or to or from a car or van, but should not be used for long-term seating, or if the user is already at high risk for pressure sores.
Most foam wheelchair cushions are made from flexible polyurethane foam. Some cushions use viscoelastic (“memory”) foam or a combination of both types. A newly introduced series of cushions use waterproof EVA foam (a softer formulation of the foam used in Crocs sandals). Using this firmer foam provides increased stability and allows air passages to be molded into the cushion, leading to improved airflow during use. In testing, this type of cushion provided excellent heat and moisture control.
Many foam cushions offer excellent comfort, support and protection. Just because they are made of foam does not mean that they cannot perform equal to or in some cases better than the most expensive fluid or combination cushions. It is important to evaluate individual cushions, rather than the type of cushion, when choosing which will work best for you.
How foam cushions work
When foam is compressed, it offers resistance as the foam changes from a flat sheet to a contoured surface. Although the analogy is not perfect, you can think of most polyurethane foams as a series of springs. Picture a foam cushion as hundreds of little coil springs standing on their ends, much like the springs in a mattress. As load is applied to a wheelchair cushion, the first “springs” that are compressed are the ones under the ITs; these springs compress the furthest as load is applied over the entire cushion surface. Coil springs increase in resistance the further they are compressed.
The spring-like quality of polyurethane foam responds in a similar way. The pressure pushing back against the load increases as the foam is compressed. Since the foam is compressed most under the IT area, this is the area of greatest pressure.
Another way to achieve comfort and good pressure distribution is to design cushions with bladders that contain fluid (air or liquid). Both materials are fluid in make-up, but differ in their physical properties. It is the nature of a fluid to move away from areas of high pressure to areas of low pressure. This means that a fluid-type cushion will allow immersion and will also provide greater levels of envelopment as the cushion adjusts to the shape of the object pushing against it.
The most well-known fluid cushion has multiple air bladders. The original design had a connection between each of the air bladders. When you sit on such a cushion, the air (gaseous fluid) moves from areas of high pressure toward areas of low pressure. This type of cushion equalizes the pressure over the complete seating surface and reduces peak pressure at areas of high risk, as long as you can adjust the air volume and prevent “bottoming out.”
Fluid cushions that use a liquid instead of a gas follow the same laws of physics, with fluid moving away from areas of high pressure and filling areas of low pressure. Because of the higher viscosity of most liquids as compared with gases, liquid fluid cushions tend to adapt to the shape of the user more slowly than air-filled cushions. This can improve stability, but the pressure-relief principles are the same in both air- and liquid-filled cushions.
Some cushions are produced with a gel component. Technically, a gel is a material that can flow like a liquid or is semi-rigid and does not flow. For example, the material used in many cushions is a semi-solid fluid made up of a solid material in a thick liquid. The flow from areas of higher pressure to areas of lower pressure occurs more slowly than with a thinner liquid or gas.
Other cushions use a polymer to form a flat or contoured slab of material. This type of gel cushion does not have the same flow patterns as a fluid gel. In fact, the gel does not flow at all. If the plastic film containing a fluid gel is ruptured then the gel will run out. If the film membrane surrounding a polymer gel slab is cut, the gel will not run out.
A fluid gel behaves like a fluid and can equalize pressure over a broad area. A gel slab will not flow and does not have the ability to equalize pressure in the same way as a fluid, but it does provide some pressure reduction and shock absorption.
Many cushions combine resilient materials (foam or fluids) with a cushion shape that is pre-contoured to match a generic anatomical shape of a seated person. As an example, when a person sits on a soft moldable surface such as sand or snow, and then carefully gets up, an imprint will remain that represents a normal anatomical shape. The contours will be lower underneath the IT area and contour upward around the buttocks; there will be two elongated areas where the surface was compressed by the thighs.
To reduce peak pressure build-up under the IT area and to improve comfort overall, a cushion is pre-contoured so that it does not have a flat-top surface. This allows the cushion to start out with a shape that closely matches the average human anatomy. You can tell that a cushion is pre-contoured if it is fabricated with a top shape that mimics the shape of the buttocks and thighs in the seated position.
When a cushion has this generic pre-contoured configuration, the support medium does not have to compress as much to match the shape of the user, and pressures can be redistributed to the trochanteric shelf and away from the ITs more efficiently.
Cushions can be created with materials that provide a firmer surface underneath the hips and thighs and a softer surface underneath the centre of the pelvis. These cushions transfer load away from areas of peak pressure and improve pressure distribution and comfort. Using this multi-density foam technique is common. This can be done with a flat or pre-contoured cushion but still relies on the same principles of cushion support outlined above.
Custom-molded cushions are made in the exact shape of the person who will use them. They are molded either directly to the person during a fitting session, or from a digital file generated in a fitting chair and then transferred to the finished product. Custom-molded cushions may be the very best option for people with significant orthopedic and postural asymmetry.
Because of the intimate fit, this type of cushion is usually excellent at pressure distribution and protection from pressure sores. They also tend to be the most stable of all cushion styles and are often used for high-end sports applications.
The fitting process can be challenging, depending on the method used to take the shape of the user, but an accomplished technician and therapist can provide an excellent molded product.
This type of cushion is among the most expensive and has a few other drawbacks. One is the tendency for heat and moisture build-up. Many cushions have little accommodation for air circulation and can be very hot in use. The other primary drawback is the problem of maintaining the proper cushion fit after growth, weight gain or loss, or changes in orthopedic postural alignment. Molded systems are difficult to modify and often need to be replaced as a child grows.
Many cushions use a combination of foams, fluids and gels. Some simple cushion designs sandwich a liquid fluid bag between two layers of relatively soft foam. The theory is that the foam provides comfort while the fluid provides pressure distribution. These cushions are often very unstable and many people comment that it is like sitting on a ball half-filled with water.
Other combination cushions are made with multi-density foams to provide stability and pressure redistribution, while using a well-designed and -positioned air or fluid element for pressure distribution. Some of these designs are excellent and combine the beneficial properties of both foam and fluid cushions. Some use an adjustable air bladder, while others use a gel or liquid fluid component in strategic locations (usually held in with a hook and loop fastener such as Velcro).
Most manufacturers offer cushions in a range of bariatric sizes. It is common to have cushions up to 24-inch wide as a standard item. Wider cushions are also available and custom cushions can be made in very wide widths, but are not available in all models.
The cushion cover is a very important part of the cushion design and should be thoroughly evaluated. Most wheelchair cushions that have a foam component are not waterproof. This is a problem if the cushion is exposed to moisture from accidental spills or incontinence. To solve this problem, foam cushions are usually treated with a waterproof coating at the factory or protected inside a waterproof cover. Both methods work well and are used throughout the industry, but can increase heat and moisture build-up at the skin.
Note: Only medical professionals such as doctors, therapists and certified equipment suppliers are trained in the evaluation, delivery and adjustment of wheelchair cushions. They can identify all of the factors that will ensure the cushion you get is correct for you. As an individual user, you should not select cushions without consulting a wheelchair seating professional.
Allen Siekman has over 35 years clinical experience as a seating specialist, designer and educator, specializing in the design and provision of seating equipment for children and adults with moderate to severe physical challenges.
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