Are Wheelchair Cushions Like Razors?

Has anyone noticed that the number of blades on razors keep multiplying?  Is there a top end where the increasing number of blades no longer provides a better shave, or had we already reached that number few blades ago?  Similar to razor blade manufacturers, wheelchair seat cushion manufacturers seem to tout their products’ thicknesses.  For example, here is a sample excerpt: 

… the high profile (4" cells) line of products are going to offer the most immersion (sinking in) into the cushion. This greater immersion will increase the force (weight) redistribution of the client thus decreasing peak pressures and … preventing pressure ulcers.

If 4” offers the most immersion, what about 6” or 12”?  Is there a top end where there is no more benefit, and is 4” that top end?  Would all 4” seat cushions (all of which are designed to permit same immersion levels), whether made out of metal ball bearings, goose down feathers, or water balloons, be indistinguishable from one another?  What about a 4” seat cushion whose top surface is made of “cheese cutting” wires that immerse to similar levels?  I know which one I am NOT choosing for my seat!

Seat cushion thickness and immersion are important considerations, but the underlying support materials may be more important.  Compressive materials, like foams and gels, cannot easily disperse peak pressures.  Displacement materials, like water and air, can better disperse and lower peak pressures.  However, even displacement materials, if surrounded by stiff forms, will not be able to disperse and lower pressures easily.  The product’s skin interface has to easily conform to a person’s body and provide broad support for best dispersion and lowest pressures.

When choosing a wheelchair seat cushion, please consider that while a razor’s performance may improve as the number of blades increases, a cushion’s thickness may not be as important as the type of underlying support material and product design.


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The most important performance parameter is shape compliance to minimize deformation. More air cells improve shape resolution improving shape compliance.
Higher air cell height increases contact area but has diminishing returns with respect to providing support forces.
5 AUGUST 2014

Robert H Graebe

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