Page 21 - Mobility Management, September 2017

have a highly precontoured foam, where that indenter relates to the contours can make all the difference in the world.”
So Medicare’s current test, Whelan said, is limited in its ability to capture the entire picture.
“I could literally build a piece of concrete that’s exactly shaped like the indenter and just put a little tiny bit of foam at the bottom of it, and I could pass the Medicare test with a cushion that would provide absolutely no clinical benefit whatsoever.”
Adding Envelopment
Whelan said envelopment is even trickier.
“If immersion is considered the depth you sink into,” he
explained, “envelopment can be considered the quality of that depth. When you sank into the cushion and you formed a loaded contour shape — your body’s ‘squishy material’ matched up with the squishy material you’re sitting in, and there was a resulting loaded contour shape — how that loaded contour shape envel- oped you is what we’re trying to measure.
“I can have two cushions that by definition have the exact same immersion, but one of them could be concentrating more pressure on the ischial tuberosities (ITs) than the other cushion. Immersion is the same, and you could argue that the surface area might be the same of that loaded contour shape. You have
identical loads and identical surface areas. But not an identical distribution of force over that identical surface area. If I have a cushion that has less equalization between the trochanters and the ITs, the envelopment is not going to be as good as one that has greater equalization between those two.”
The good news for the future: Whelan said, “Now there is a draft standard for a laboratory test of envelopment using an instrumented indenter.”
Measuring Magnitude
While the third term of the seating trinity is often referred to as off-loading, Whelan said, “If you follow the science of seating presentations that were put together, it’s actually immersion, envelopment and magnitude. Offloading is a technique; magnitude is the measure of the technique.”
So with that context, Whelan said, “Magnitude is the absolute value of the loads at specific points, like the ischial tuberosities and the trochanters. You can have two cushions, same immer- sion, exact same envelopment as far as parity goes between the trochanters and the ischials, but you could still have higher forces at work because it didn’t load the other areas as well.”
Combine the three measurements — immersion, envelopment and magnitude of off-loading — and Whelan said, “At the end of
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