Understanding the Cushion: How the Cushion Works
Last week we talked about the importance of increasing surface area contact to redistribute pressure for skin protection. Today, we will talk about two ways in which we can achieve this pressure redistribution: Immersion and offloading.
Immersion is how much the client sinks into the surface. Immersion works by increasing the surface area contact and therefore redistributing the pressure. Remember from physics in our university that Pressure = Force/Area. The easiest way to describe immersion is to think about how much we sink into a surface, in this case a cushion. When our client sinks into their cushion the amount of surface area contact is increased as opposed to if they are sitting on top of a cushion. A great example of a cushion that works through immersion is a ROHO cushion. ROHO has specific instructions on how to properly adjust the cushion in order to have the full amount of immersion. This is designed to provide the greatest surface area contact and therefore achieve the optimal amount of pressure redistribution. Imagine now if we had a fully inflated ROHO cushion and the client was sitting on top of the cushion… would we still get that immersion into the cushion? No, we would lose the benefits of immersion in this case and decrease the optimal amount of pressure redistribution.
While immersion is a great design not every cushion can work through immersion. Sometimes we don’t want to “sink” down into the cushion, or maybe we can sink down a small amount, but not enough to provide that full amount of pressure redistribution. Let’s take for example a flat foam cushion. Should our client be able to sink down far into a single layer flat piece of foam? If our clients are sinking down far into a single layer flat piece of foam, what is protecting them from bottoming out onto the hard surface under this foam? Foam can often be layered to allow for some immersion on the top while still giving a base for support, but this is why we typically see foam working through the principle of offloading.
Offloading is just what the name suggests! With offloading the cushion is designed to off-load the pressure under a particular area. In this case, typically we see offloading designed to decrease the pressure under the ITs, ischial tuberosities. The ITs, or your sit bones, are one of the points of highest risk of skin breakdown due to it being the lowest point on the pelvis and therefore having the most amount of pressure placed on this area if sitting on for example a flat piece of foam. The easiest way to think about offloading is to think about a tyre. It you were to sit on a car tyre you would have the open ring of the tyre under your ITs and therefore no pressure would be applied to that area. This is true offloading. What we have to remember though is that although we have offloaded in one area, we have decreased the over all surface area contact. Therefore, we have increased the pressure to the areas that have contact, Pressure= Force/Area. These areas might be able to withstand higher pressures, but it is important to keep in mind. What is more common is to provide partial offloading. This is what we see with pelvic/ischial wells, through contouring and material layering. Partial offloading works by trying to load through the femurs first and thereby allowing for partial offloading of the ITs, sacrum, and coccyx.
It is important that you can look at a cushion and understand how a cushion will work for your client. This is how you are able to narrow down the options for your client and determine which cushions may be appropriate.
If you are interested in furthering your knowledge on the materials and principles behind cushions reach out to Rachel Fabiniak at Rachel.Fabiniak@permobil.com to schedule a clinical education course in your area.