Technical note
ISO 16840-2:2007 load deflection and hysteresis measurements for a sample of wheelchair seating cushions

https://doi.org/10.1016/j.medengphy.2013.10.010Get rights and content

Abstract

Load deflection and hysteresis measurements were made on 37 wheelchair seating cushions according to ISO 16840-2:2007. Load deflection plots for all 37 cushions are reported and fundamental aspects of graph interpretation discussed. ISO hysteresis data are also reported and interpretation discussed.

Introduction

Wheelchair seating cushions must fulfil a variety of requirements to meet an individual's specific rehabilitation aims, including managing comfort, tissue integrity, postural control, postural alignment and functional enablement. Clinical selection of the best seating support surface however continues to be based principally on custom and practice, the individual clinician's experience, seating theory, user trial and, if available, interface pressure mapping. The reason for this must be, in part at least, the lack of evidence available to guide prescription [1].

The evidence required to facilitate more objective prescription of cushions includes detailed information about the intended user's diagnosis, associated physical and cognitive complications, other aspects of their health, postural presentation, ability, lifestyle, environment, and rehabilitation goals. There is also however a need for objective information about the performance of the available cushions.

Several measures have already been defined and some are in use. Kuncir et al. [2] for example describe compliance factor and compressibility factor. Other measures are from the furniture industry, such as indentation force deflection [3]. Some manufacturers of wheelchair seating cushions also provide information on specific products. Qbitus Products (Halifax, UK) for example, publish Linear Load Limit. Invacare (Elyria, OH), by contrast, publish Loaded Contour Depth, Overload Deflection and Impact Damping data [4], which are defined in the ISO 16840-2:2007 standard [5], for cushions in the Flo-tech range. Other manufacturers however do not provide any objective measures at all. The paucity and inconsistency of data therefore makes rigorous comparison of cushions difficult or impossible.

ISO 16840-2:2007: Wheelchair seating—Part 2: Determination of physical and mechanical characteristics of devices intended to manage tissue integrity—Seat cushions, was published in 2007. Standards are important as they can facilitate the production of transparent data that can be globally understood, allowing objective comparisons of products. This can increase the safety, quality and reliability of design and hence provision, and means manufacturers cannot make unsubstantiated claims. ISO 16840-2:2007 is the first version of this standard and was current at the time of testing. A revision however is in preparation at the time of writing.

ISO 16840-2:2007 details a set of measures which describe static and dynamic/elastic characteristics of wheelchair seating cushions which are relevant to tissue integrity. Tests are accompanied by rationale linking each test to clinically relevant features of cushions such as pressure redistribution and shock absorption. In its introduction the standard also states, “The link to clinical efficacy, although implied, has not been validated,” and goes on to express the intention that, “this part of ISO 16840 will evolve when the evidence of clinical relevance is confirmed.” The emergence of this evidence however is unlikely unless the standard and its resulting data are familiar to and better understood by clinicians. Although the standard was not developed for clinicians to apply directly in clinical decision making, it is hoped that this Technical note will begin the process of developing better understanding amongst clinicians, and hence may lead to theories of clinical effectiveness which draw upon their valuable experiential knowledge.

The aim of this study therefore is to begin this process by examining the results from one of the tests for a selection of wheelchair seating cushions with a view to identifying aspects of the data most salient to differentiating the cushions according to clinical potential. The test examined in this study is the load deflection and hysteresis test described in section 9 of the standard, and which considers the compression characteristics of the cushion as it is loaded and unloaded.

Section snippets

Method

Load deflection and hysteresis measurements were made on 37 wheelchair seating cushions according to section 9 of ISO 16840-2:2007 [5]. For full details of test procedures please refer to section 9.2 of this standard.

The cushions tested are listed in Table 1. The cushions represented a variety of manufacturers’ designs in current clinical use and also a range of foam types used in custom-made wheelchair seating systems. Cushions were 410 mm wide by 460 mm long where available, which was the size

Results

For full details of test report requirements please refer to section 9.3 and 9.4 of ISO 16840-2:2007 [5]. Sample load deflection plots from selected cushions are shown in Fig. 2, Fig. 4. Plots for all 37 cushions are given in Figs. 6 and 7 online. Hysteresis for the full set of 37 cushions at 250 N ranged between 0.041 and 0.371 with a mean value of 0.142 and a mode of 0.099. At 500 N they ranged from 0.023 to 0.223 with a mean of 0.090 and a mode of 0.069. The full set of hysteresis values is

Discussion

ISO 16840-2:2007 states that all the tests it describes are “intended to differentiate performance characteristics between cushions and are not appropriate for ranking or scoring cushions or for directly matching these characteristics with the requirements of individual users”. However it is desirable that, if possible, this information be interpretable to inform clinical decision making. The standard does not provide explicit guidance on clinical interpretation, but it does provide a rationale

Conclusion

This study has acquired data which confirms that the ISO 16840-2:2007 load deflection and hysteresis test achieves its objective of differentiating wheelchair cushion performance. The ISO's rationale for this test however also links it with stability, pressure management, and shock absorption. This study has therefore also examined the potential for the load deflection and hysteresis measures to inform on these clinically relevant features, and has highlighted the following factors pertinent to

Funding

None.

Ethical approval

Not required.

Conflict of interest

None declared.

References (5)

There are more references available in the full text version of this article.

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