Past articles have looked at various materials commonly used in wheelchair cushions and different types of cushion covers and their relative ability to effect skin protection. This month, let’s think about how some materials may be formed into a cushion and how the shape and design of a cushion can effect skin protection.
First, let’s think about the profile of a wheelchair cushion. Manufacturers offer cushions in various profiles, even offering some cushion models with a choice of height, such as standard or deep height. The height of a cushion will affect the depth of immersion available for a person when sitting on a given cushion. Depth of immersion is an important factor in helping to ensure that a person’s ischial tuberosities, which are the lowest points on the seated pelvis, do not “bottom out” on the cushion. In order to immerse the ischial tuberosities, which have a high risk of skin breakdown due to potential pressure at their small surface area, the depth of the material of the cushion will need to be of a sufficient height in order to prevent increased pressure at this bony prominence. It is noteworthy that the difference in height between the ischial tuberosities and the greater trochanters when seated is about 2 to 2-1/2 inches for an adult.1 (Please refer to the below picture. The small arrow points to the ischial tuberosity, while the larger arrow points to the undersurface of the femur to illustrate the relative difference in height when seated with a neutral pelvis.)
Note: Graphic is copyright Sunrise Medical Inc.
Sometimes we try to achieve a low seat to floor height by requesting a low profile cushion, but this may not always be the best solution for a person if pressure relief and skin integrity are seating priorities. Perhaps we should try to see if there are other ways through wheelchair configuration to achieve a lower seat to floor height, rather than starting with the profile of the cushion.
Now let’s think about the shape of a cushion and how that can impact skin protection. A cushion that is contoured, with shape for a person’s buttocks and legs, will envelop the seated surface and provide more surface area contact than a cushion with no contours. A greater surface area helps to reduce pressure. In addition, it is thought that a contoured cushion may help to preserve the shape of a person’s body when seated, and that by preserving shape, body tissue will be minimally distorted, maintaining blood flow to the area.2 One of the key points specified in Spinal Cord Injury Rehabilitation Evidence Wheeled Mobility and Seating Equipment states “Contoured foam cushions compared to flat foam cushions seem to provide a seat interface that reduces the damaging effects of external loading and tissue damage.” 3
When we think about cushion shape when a combination of materials are used, we can consider how the pelvis is supported – what I mean by that is, what is the design of the cushion and how does it affect skin protection? For example, in a cushion that has a combination of materials, such as a contoured foam base with a pressure relieving material at the area for the pelvis, the well for the pelvis in the foam base can be shaped in different ways. If the well is sufficiently wide, the pelvis “floats” in the pressure relieving material, such as a fluid pack or air cells, as neither the ischial tuberosities nor the greater trochanters are supported by the edge of the well. The depth of the pressure relieving material is sufficient to ensure that the pelvis does not bottom out, provided adequate maintenance of the material has occurred. This design redirects load away from the ischial tuberosities and the greater trochanters to the femurs.
A different way to support the pelvis is by supporting the greater trochanters on the edge of the pelvic well formed by the foam base, while immersing the ischial tuberosities in pressure relieving material. A manufacturer will use anthropometric data to design the well to ensure that the shape and slope of the edges of the pelvic well will provide adequate support to the trochanters. This design redirects load away from the ischial tuberosities to the greater trochanters and the femurs. There are, however, some people who should not use a cushion with this type of design. If a person has skin breakdown at the trochanters, has a dislocated hip, or has had hip surgery, that person should not use a cushion that re-directs load from the ischial tuberosities to the trochanters. For those that can use a cushion with this type of design, the benefits include not only skin protection for the ishial tuberosities, but also lateral stability of the pelvis as the greater trochanters are supported. Stability of the pelvis can promote improved upper extremity function, as proximal stability promotes distal function.
There are other cushion designs, each with an intended effect on distributing load to achieve a seating goal. As I have said many times, it is only by taking the cover off a cushion that we can examine its materials and shape to begin to understand how the cushion may work to achieve seating goals.
So, we have looked at how height or profile of a cushion can effect skin protection through the depth of immersion that is available to ensure that bottoming out does not occur to minimize peak pressures. We have also seen how shape of a cushion can effect skin protection through contour and through design to redistribute load. Of course, we have to remember that there is no one cushion that is best for all clients. Clinical reasoning is always needed to match generic seating product parameters to client seating needs.
- Pratt, S. (2006). Seating for Function & Mobility. A Clinical Perspective, Linking Clinical Thinking with Technology. Sunrise Medical Inc.
- Cooper, R. (1998). Wheelchair Selection and Configuration. New York: Demos Medical Publishing.
- Connolly, S., Miller, W., Trenholm, K., Titus, L., Moir, S., Coulson, S., Foulon, B., Mehta, S., Aubut, J. (2010). Wheeled Mobility and Seating Equipment. Spinal Cord Injury Rehabilitation Evidence. Version 3.0