EVALUATION OF VARILITE WHEELCHAIR CUSHIONS
An independent study carried out by the Centre for Disability Research and Innovation, Institute of Orthopaedics and Musculo-Skeletal Science, UCL, Stanmore, Middlesex HA7 4LP
Cushions from the Evolution, Solo, and Stratus ranges of Varilite cushions were tested using a simulated backside (a Skeletal Embedded Loading Indentor – Figure 1) to characterize interface pressure distributions following the protocols set out in the original Committee Draft of ISO-16840-2 (Wheelchair Seating – Part 2: Test methods for devices intended to manage tissue integrity – Seat Cushions) in 2001. The cushions were measured when new, and then following a repetitive load test of 200,000 cycles, which has been designed to simulate fatigue from a lifetime of use.
The data from the Varilite cushions were compared with the results obtained from 3” thick blocks of HR45 and HR70 high resilience foam. Foam is used widely in many low cost cushions, and is usually effective for pressure care in the short term, but tends to develop a ‘set’ and break down within a short number of months. Any cushion that ‘performs’ even better than new foam obviously has advantages.
Cushion Characteristics
For a cushion to be effective, it should spread the person’s weight over as large an area as possible (the larger the area the better) – this spread of contact area equates with envelopment buttocks and thighs.
A second important characteristic is the cushion’s ability to take pressures away from bony prominences, such as ischial tuberosities, sacrum, and greater trochanters – referred to as dispersion (the lower the value the better the dispersion).
The next consideration is how effective the cushion is at reducing the maximum pressure (the lower the better), and how evenly the pressure is distributed: the latter is called Peak Pressure Index (PPI) the the lower this value, the more evenly the pressures are distributed.
The resilience of the cushions and their recovery characteristics have also been measured, but Nicholson et al (2004) question the value of these measures, indicating that they can give misleading results that may not be relevant to the different types of cushion assemblies now generally marketed: these measures also do not take into account conformability of the cushion nor the custom fit to an individual.
Results
Envelopment
The data presented in Figure 2 show that all three Varilite cushions provide up to twice as much envelopment as ordinary foam, and this characteristic changed little with fatiguing. The ‘higher’ grade the cushion, the better the envelopment. i.e. the Evolution performed better than the Solo than the Stratus.
Fig 2. Envelopment
Dispersion
The data presented in Figure 3 show that all three Varilite cushions are better at dispersing the pressure away from the bony prominences than ordinary foam and, again, the ‘higher’ grade the cushions, the better the dispersion.
Fig 3. Dispersion
Maximum Pressure
As shown in Figure 4, the maximum pressure readings underneath the test ‘backside’ were up to twice as high on ordinary foam than they were on the three Varilite cushions. The higher the ‘grade’ of the cushion, the lower the maximum pressure.
Fig 4. Max Pressure (mm Hg)
Peak Pressure Index (PPI)
This is a measure of how evenly pressures are distributed across the whole cushion. Figure 5 shows that all three Varilite cushions performed up to twice as well as ordinary foam. Before fatiguing, the ‘higher’ the grade of cushion, the better the outcome. After fatiguing, all three cushions performed similarly to each other, but all still performed signifi cantly better than unfatigued ordinary foam.
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