Measuring Pressure interface and shear on cushions fitted correctly versus upside down


At a recent seating seminar therapists were asked if they had experienced a client sitting on their wheelchair cushion placed upside down? The answer was “Yes” 80% had experienced this more than once. Asked if, in their opinion, would this place their client at further risk the majority answered “Yes”!

The Problem

If the cushion is placed incorrectly in chair, how can we minimise risk for pressure & shear?
The following 3 cushion types had their data measured:

1. Vicair Aircell cushions
2. Flotation cushion
3. Gel cushion

The tools used were a pressure-map and an iShear. iShear is the world’s first tool to measure total shear force in the seat of a wheelchair. It is placed under the cushion and communicates the results to an app through a bluetooth connection.
The total shear force (TSF) in the seat plane measured by the iShear is the anterior-posterior force parallel to the seat plane. It’s measured between the cushion and the seat base interface.


The Vicair Aircell technology cushions performed significantly better than flotation and gel cushions for pressure and shear when used upside down. Further evaluation and clinical research is suggested.

The Research

We rely on the following statistics presented on the right of the pressure mapping image and interpreted in combination to help objectively contrast pressure care products and personal positioning. Comfort, fit, temperature, moisture, set up and posture are also important considerations.

Shear force

This indicates the total shear force in the seated plane. It is accepted that high shear force overtime is associated with deep tissue injury and tissue torsion. A reduction in this force reduces risk. Shear force or sliding force is often inversely proportional to peak pressure. It tends to increase with time and, when it is high, it can be considered there is some sliding on the seated surface. The rotational component of the Shear mapper can quantify some postural instability by measuring pelvic rotation.

Dispersion index (Regional distribution)

Under 50% is linked in literature to lower risk of pressure injury development. This is a measure of tissue load specifically over the vulnerable IT area.

Coefficient of variance or CoV

The measure of rate of change in a tissue’s weight bearing. If this is over 50% there is some risk for a tissue torsion/stress/strain/shear injury. COV can be misleadingly high if weight bearing is not immersive.

Peak pressure Index

Indicates the area of highest point pressures.

Sensing area

This is the size of the weight bearing area. The larger the area the better for pressure distribution.
Pressure mat is orientated with the front right of the cushion at the bottom right of the screen. Shear map is oriented as the icon indicates. Rotation depicted by arrow and figures indicate magnitude.

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