This month, let's focus on materials commonly used in wheelchair cushions and how choice of material can affect skin protection.
There are many different types of materials that are used in the manufacturing of wheelchair cushions. Materials include foam, elastomers, gel, fluid, and air. The interaction of a body with the material will affect the cushion's ability to offer skin protection. Let's look at the different materials.
There are different types of foam, such as open-cell or closed-cell, as well as different degrees of springiness and density. The Indentation Load Deflection (ILD), or Indentation Force Deflection (IFD), measures the degree of springiness of the foam. Foam that has a lower ILD or IFD measurement will feel softer than foam with a higher ILD or IFD measurement. Foam that is softer and more pliable will contour around a client's buttocks when the client is sitting on the cushion. This contour helps to create a greater surface contact area, for a more uniform distribution of pressure1. Foam that is too soft, however, may result in the client completely compressing the foam and "bottoming out" when he/she sits on the cushion1, which then results in increased pressure points at the bony prominences.
There is a difference between foam density and foam springiness. Both are measurements applied to foam. Density, which is measured in pounds per cubic foot (PCF), relates to the amount of air that is in the material2. Two samples of foam may have the same density (amount of air in the material), but different springiness. Another two samples of foam may have the same springiness, but different density. Greater density in foam provides for greater durability, helping to maintain the height of the material and the springiness of the material for longer3.
New advancements in the design of foam cushions have been made. For example, there are now foam cushions on the market for which the density of the foam increases as the size of the cushion becomes larger. This is exciting news! This means that the density of the foam in the cushion should better match the client's needs; a heavier client, who requires a wider cushion, will have an appropriate degree of density to the foam to support the client and maintain the integrity of the cushion for longer. There is less risk of completely compressing the foam and bottoming out.
This material is a polymer (type of plastic) that has been shaped into a "honeycomb" by the manufacturer. The honeycomb responds like foam when a client sits on it. Similar to foam, the honeycomb can be contoured by the cushion manufacturer for standard shapes2.
Fluid and Gel
Often the terms "gel" and "fluid" are used interchangeably when describing cushion materials; however, this is incorrect. There is a difference in the way that gel, which is a semi-solid material, and the way that fluid, a material with some viscosity or flow, respond when loaded (i.e. when a person sits on the material). A gel does not allow sufficient displacement of the material to permit a client's buttocks to fully immerse into the material, while a fluid has the ability to conform to the unique shape of the buttocks of the client sitting on the fluid. This conformity to one's shape facilitates a more even distribution of pressure, thereby offering skin protection to the user.
Fluid works by moving within its container to come to a constant, equal pressure over the entire seated surface2. Manufacturers will use segmentation in the design of the fluid packs to control the flow of the fluid. Segmentation also reduces the surface tension of the fluid pack to allow for immersion.
Fluid also provides better skin temperature control than other materials by conducting heat away from the skin4. (Heat and moisture are factors associated with skin breakdown.) That being said, some types of fluids may freeze if left outdoors in a frigid environment – it may be best to keep the cushion indoors to avoid this.
Fluid also offers skin protection through its ability to adjust to body movements4. Adjustment to body movements reduces the shearing forces that would otherwise be present through horizontal shearing forces on the cushion1.
Maintenance of the fluid (i.e. kneading the fluid) is required to ensure that the fluid is properly dispersed within the pack and that bottoming out does not occur.
Air in wheelchair cushions has long been associated with skin protection. Air allows for the immersion of the seated body into the air chambers, with the surface of the chambers conforming to the shape of the client's body. Providing that the cushion has been properly inflated to allow for immersion into the cushion, the air baffles or chambers work to provide an area on which the pelvis and lower extremities can "float", thereby reducing potential peak pressures1. Air and fluid work in much the same way – the air shifts conform to the load and to equalize the pressure over the seated surface2. Like fluid, air also reduces shear forces1.
Maintenance is required to ensure adequate inflation and to avoid bottoming out.
Combination of Materials
Of course, there are cushions that are made with a combination of materials. The combination of materials, such as a foam base with an air insert, allow for more than one clinical goal to be met through the use of the cushion, such as skin protection with stability.
Clinical consideration must go into the choice of material for a wheelchair cushion – not only for how the material will help to achieve seating goals for a particular client, but also for how well the client and/or caregiver can consistently perform the maintenance required for a certain cushion. Other factors, such as ease of transfers or weight of the cushion, should be considered. Lastly, remember that there is no one cushion that is best for all clients.
- Consortium for Spinal Cord Medicine Clinical Practice Guidelines. (2000). Pressure Ulcer Prevention and Treatment Following Spinal Cord Injury: A Clinical Practice Guideline for Health-Care Professionals. Paralyzed Veterans of America.
- Cooper, R. (1998). Wheelchair Selection and Configuration. New York: Demos Medical Publishing.
- Pratt, S. (2006). Seating for Function & Mobility. A Clinical Perspective, Linking Clinical Thinking with Technology. Sunrise Medical Inc.
- Batavia, M. (2010). The Wheelchair Evaluation. A Clinician's Guide. (2nd ed.). Sudbury, MA: Jones and Bartlett Publishers.