Changes in Wheelchair Cushions as a Result of Simulated Use Protocols

Maureen Linden, MSBME; Stephen Sprigle, PH.D., MSPT

Center for Assistive and Environmental Access, Georgia Institute of Technology

Abstract

The expected lifespan of a wheelchair cushion and its durability properties can adversely influence the performance of the cushion through degradation of mechanical properties or failure of key components. This study investigated how the performance as use of the cushion is simulated. Use of 90 cushions was simulated and failure mechanisms were noted. Seven of the cushions exhibited signs of failure resulting from simulated use. An additional nine cushions showed signs of wear. Loaded Contour Depth (LCD) and Overload Depth (OLD) were calculated before and after simulated use according to SADMERC protocols. Eight cushions that met the OLD criteria prior to simulated use did not meet the criteria following simulated use.

Keywords:

cushion, fatigue, failure mechanisms

Background

The expected lifespan of a wheelchair cushion and the cushion's durability must be taken into consideration when choosing a cushion. The mechanical properties of the cushion can degrade over time, and repeated loading and stress may cause components of the cushion to fail. As a result, cushions may not continue to provide performance throughout their lifespan. The Statistical Analysis Durable Medical Equipment Regional Carrier (SADMERC) sets lifespan criteria of 12 months for general use cushions and 18 months for cushions that provide pressure relieving and/or positioning benefits.

More information is needed regarding how the therapeutic effects of wheelchair cushions change as the cushion ages. This study simulates the use of 83 cushion and looks at the failure modes as defined by SADMERC's criteria for pressure relief and positioning, as well as looking at material failure of the cushions.

Methods

Wheelchair cushions were accepted from the manufacturer to be tested according to SADMERC test protocol. A total of 96 standard adult and bariatric wheelchair cushions were accepted for testing. These cushions were dimensioned and the initial Loaded Contour Depth (LCD) and Overload Depths (OLD) were measured.

Figure1: Loaded contour jig - 40 mm schematic (Click image for larger view)

The LCD is a measure of how well the cushion envelops the buttocks to distribute load. A cushion loading indentor (Figure 1) simulates the spacing of the bony prominences of the pelvis. The loaded contour depth is determined to be the difference in the height of the unloaded cushion, and the height of the cushion after a standard load (based on anticipated user weight) is applied and 5 minutes has elapsed. The OLD is a measure of whether the load imparted during the LCD measurement has caused the cushion to deflect to its limit (i.e. bottom out). An additional overload weight is added to the LCD load, and difference between the height at standard load and height at overload is determined.

When submitting cushions the manufacturer specifies simulated use protocol based on codes for general use, positioning, pressure-relieving, or positioning and pressure-relieving cushions. Each of these categories has specific criterion for LCD, OLD, and height of positioning features. If these criterion were not met, the simulated use protocols were not performed to spare costs to the manufacturer. Six of the submitted cushions did not meet the initial requirements, and were removed from the study.

Use was simulated according to the protocol defined below. The cushion was inspected for signs of failure following each step of the protocol. If signs of failure were found, further testing on the cushion was not completed. Seven cushions failed during simulated use. An additional eight cushions showed signs of wear that were not considered failure. This was typically wear to the cushion cover material during laundering. Simulated use tests were completed on these cushions.

Laundering:

If the cushion was supplied with a removable cover, the cover was removed and exposed to five repeated washing and drying cycles.

Accelerated Heat Aging was designed to simulate deterioration of cushion materials due to the cushion being stored prior to use (i.e., shelf-life). The protocol for heat aging assumes that the cushions are in storage for 8 weeks prior to being used by a client. Assuming cushions are stored at room temperature, the specific heat equation determined a time-temperature protocol. The cushions were placed in a convection oven and at 70 ° C for 48 hours.

Cyclic loading protocols:

The dynamic loading protocol simulates clinical pressure relief maneuvers (PRM) over the life of the cushion. This maneuver completely removes the user weight from the cushion surface. Load patterns were estimated using an average user weight and an estimate of 2 PRM/hour over a 12 hour day for the life span of the cushion. Cushions were loaded using the ISO16840-2 buttock model (1) sized for the cushion being tested. Cushions with a lifespan of 12 months were subjected to 8700 cycles of 0-750N loading, while cushions with an 18 month lifespan were subjected to 13,000 cycles at the same load.

The static dynamic protocol models static load with activity-based weight shifts during the life-span of the cushion. The protocol estimates 8 weight shifts/hour for a 12 hour day. Weight shifts do not represent complete unloading of the cushion, but merely transfer of loading to other parts of the wheelchair, cyclic loading was applied with an ISO16840-2 buttock model. General use cushions were subjected to 35,000 cycles of 400-600N loading, while specialized cushions were subjected to 52,500 cycles.

LCD and OLD were re-measured for each cushion that successfully completed simulated use. Pre and Post Simulated Use data was compared for a total of 83 cushions.

Results

The cushions that were submitted and tested consisted of those made with a variety of foams, viscoelastic foams, multi-density foam layers, foams with viscous fluid inserts or overlays, and gel only and viscous fluid only cushions. No air cushions were included in this study, as none were submitted for testing to this facility.

Seven cushions failed during the simulated use tests. Three of these failures occurred during accelerated heat aging when a viscous fluid expanded as a result heat transfer, causing the containment compartment to fail. The remaining four failures occurred during the cyclic loading protocols as a result of failure of gel or viscous fluid containment compartments.

LCD and OLD were recalculated for cushions that survived the simulated use tests. The table below shows descriptive statistics for the metrics.

No cushions failed the LCD criteria on post test, while eight cushions failed the OLD criteria on post test. Paired comparisons between the pre-simulated use and post-simulated use measures show statistical significance at the p < 0.01 level for the LCD metric, but no statistical significance existed for the OLD metric.

Discussion & Conclusions

The difference between pre and post simulation for the loaded contour depth metric was statistically significant. None of the cushions that completed simulated use failed the LCD criteria on post test. Contrarily, while the OLD metric did not show statistical significance, 8 cushions failed the OLD metric following simulated use. This would suggest that the difference in the loaded contour depth created by cushion wear causes the cushions to bottom out, which while increasing envelopment, reduces pressure-relieving characteristics. This conclusion is also supported by an increase in the averages, minimums and maximums of the LCD metric, but not a similar change in the OLD metrics.

Six of the cushions that failed post OLD criteria were solely foam in composition. An additional cushion had a gel overlay, while the remaining had a gel insert between foam layers. The majority of failures were foam only, and none of the gel only cushions failed; suggesting that the gel material resists mechanical wear better than foam.

All of the cushions material failures were from cushions that contained viscous fluid or gel. These failures were divided evenly between heat expansion of the gel during accelerated aging and failures of the gel containment compartment during loading. This would suggest that gel products do not survive storage as well as foam, and gel compartments are subject to mechanical fatigue.

The protocols described above are only one of several simulated use protocols that are in use today. In addition to mechanical wear and accelerated aging, other protocols examine ultra-violet light decomposition and contamination by biologic microbes. A comparison of the failure rates on these protocols would determine whether these additional simulated use techniques cause failures of the cushions at higher rates or different mechanisms.

The study was limited only to cushions submitted by manufacturers. The cohort not include air cushions. A more balanced study sample would include these cushions as well as cushion of other material categories.

References

1. ANSI/RESNA Subcommittee on Wheelchair Seating Standards, Wheelchair Seating-Part 2: Test methods for devices that manage tissue integrity - Seat Cushions, ISO 16840-2.

ACKNOWLEDGEMENTS

Funding for this study was provided in part by NIDRR project number H133E030035.

Maureen Linden
CATEA, Georgia Institute of Technology
490 10 th Street
Atlanta GA 30332
Phone: 404-894-9087
Maureen.linden@coa.gatech.edu