May the force be with you – and not against you!

Support surfaces vary significantly, as do the materials used. Foam, gel and air cushions have significant variability in their function – even between two very similar looking products. This is because the manufacturing process and specific materials used to construct products does vary so much. It also helps to explain why two foam cushions, for example, may have a very different lifespan as though they may have a similar indentation force deflection, their density may not be so similar.

In addition to how they may be manufactured, the mechanics and design of the cushion influence the forces produced when the user sits on it. This impacts the way pressure is distributed, and therefore the user. Let’s explore these concepts below.

Reactive support surfaces

To know the type of support surface you are looking at, consider if it compresses; does the material ‘squish’ when sat on? Foams, and some gels and air cushions work through compression.

Here we have two images, one before any weight is placed on the surface, and the other when the surface is holding the weight of the object.

We can see that the material has compressed around the object, and this has led to a reactive force. The reactive force reflects Newton’s 3^rd law of motion ‘for every action there is an equal and opposite reaction’. Therefore, for any cushion that compresses the force will peak in the location with the most downward force, which will often be the ischial tuberosities.

The image on the right provides this within the context of a pressure cushion that compresses. The arrows indicate where the greatest opposing forces are; the ischial tuberosities.

Hydrostatic support surfaces

The other type of support surface is the one which works on displacement rather than compression. I like to think back to school where you have a large container with water and see what happens when you stick your arm in – the water level rises! This is displacement, occurring with fluids, and is a hydrostatic force at work rather than reactive.

Surfaces which displace produce an even force around the contact area of the object, rather than peak forces. Therefore translating this into a cushion, the force under the user would be quite even through the contact surface. Materials which move like a fluid, being water, gel and air, may displace and therefore create a hydrostatic force when sat on.

The material, such as being made out of gel or air, does not in itself determine whether it will behave with a reactive or hydrostatic force. The container, or what the substance is in, and its volume, are essential to also consider. A helpful question (or test to try out) to determine this is what movement is there in the cushion? By that, I am asking whether the gel or air has room to move in the container it is in, or whether it is almost or completely full, therefore not allowing movement. The unique properties of the material, height and shape also influence the ability to immerse and reduce shear, but that is for another blog!

Take the ROHO Adaptor pad for example [image right]. Here we have air trapped in the neoprene cells, but unlike other ROHO products, the air cannot move from one cell to another, it remains semi-inflated in its own cell. This means that it doesn’t need to be pumped up, but does mean that as there is nowhere for the air to displace [being into another cell]: it will provide a reactive force.

Let’s see how these forces relate to an ISO 16840-12:2021 Pressure – envelopment test completed by the University of Pittsburgh and deidentified in our Scientific report: Guidance to individualized cushion selection based on performance metrics.

This test uses sensors across four markers on each side of the buttock indenter, and they are colour coded to assist with comparisons.

The results on the left are for the ROHO Quattro High Profile, an air cushion, inflated to 0.5” clearance under test load; locked with the test load applied. When looking at the different areas (colours), they are all very similar – demonstrating hydrostatic forces in effect!

The results on the right are for the Comfort Embrace, a foam cushion. We can see that the highest pressures are measured in the yellow and red areas simulating the location of the ischial tuberosities. Here we can see the reactive force being created.

As a clinician, if your only consideration was pressure, then of the two, it would be likely you’d select the cushion on the left. However, as we know, there are so many factors when it comes to cushion prescription, that, depending on the client, the cushion on the right may actually be more clinically appropriate for some users.

Moving forward, we would encourage you to ensure a wholistic assessment is being completed when determining the most appropriate support surface for a user. This includes considering the forces impacting on the user, whether they are reactive or hydrostatic, alongside their other goals and priorities.

There are so many factors when selecting a pressure cushion that a flow chart or decision tree cannot capture the multitude of factors, with the user’s personal preference (a key factor) being one of these. Consider instead what information is needed to guide your clinical reasoning. We will shortly release a poster which has some prompts which may be beneficial. Please feel free to add this on your office wall for inspiration during that assessment process. 

References

University of Pittsburgh. (n.d.). Introducing our New Interactive Data Exploration Tool. Wheelchairstandards.com. Retrieved October 04, 2024, from https://wheelchairstandards.com/filter-envelopment-immersion-data/

Author

Roseanna Tegel BHthSc/MOT

Clinical Services Specialist