INTRODUCTION

Given the role of support surfaces in the daily life of wheelchair users and its importance in the prevention of tissue injuries, the selection process for cushions should be careful and informed. Knowledge of a cushion’s mechanical characteristics is essential to the decision-making process of users and clinicians as they choose the most suitable type of seat to meet the user’s requirements. The International Organization for Standardization (ISO) has published a wheelchair seating standard (ISO 16840-2) to determine the physical and mechanical characteristics of seat cushions that are relevant to maintain the integrity of the tissues. The document includes seven tests to characterize cushion performance, including the impact damping under normal loading conditions test (IDT), which is designed to measure the ability of a cushion to reduce impact loading through acceleration responses.

The document states that the IDT “identifies the characteristics of a wheelchair cushion, which reduce impact loading of tissues and help to maintain postural stability. The cushion’s ability to absorb vibration and impact decreases peak pressures associated with impact loading such as rolling off a curb or other obstacle. Impact damping is related to hysteresis”. Therefore, an ideal cushion would be able to absorb the impact and not transfer this energy to the tissue.

Because manufacturers’, clinicians’, and researchers’ understanding of the ISO standards could vary, improvements in the documents have been made over the years so that the information in a more understandable and valuable to its users involved groups. This paper presents a literature review of how different stakeholder groups have applied the ISO standards, summarizing their findings and the recommendations they make to improve the comprehension of the methods and to bring relevant information in the characterization of wheelchair cushions. The purpose of this literature review is to give an overview and act as a basis for further investigations of wheelchair seat cushion performance.

METHODS

For the current review, published papers were analyzed and discussed. Information such as the author’s purpose for using the ISO 16840-2 standard as well as the parameters reported, the main findings, the approach of the studies, and the main recommendations to improve the standard were examined. Although ISO 16840-2 defines a series of tests, the most popular among the researchers is the Impact Damping Test (IDT). The IDT describes the dynamic ability to absorb vibration and peak pressures associated with sudden high loading. This characteristic is believed to provide a measure of the cushion’s ability to maintain postural stability under dynamic loading. 

Table 1 shows a summary of the studies and publications identified as applying the IDT of ISO 16840-2. All the authors followed the method defined in the standard for preconditioning and IDT and used a rigid cushion loading indenter (RCLI), a rigid hinged plate, a support block, and an accelerometer. Briefly, the IDT consists in placing the cushion on the rigid hinged plate that is loaded with the RCLI with an accelerometer on the top surface. The support block is removed allowing the cushion and RCLI to drop, and acceleration recorded; the complete description of all test procedures and apparatus is referred in the ISO 16840-2:2007 [1].

Chung was the first to investigate the ID characteristics of wheelchair cushions by performing a short test on three wheelchair cushions made from different materials [2]. He calculated the four parameters specified in the standard for reporting outcomes, which are: 1) the mean number of rebounds higher than 10% of the peak acceleration, 2) the mean of the peak rebound acceleration relative to the baseline, 3) the mean of the second highest rebound acceleration relative to the baseline, and 4) the mean of the ratio of the second and the peak rebound acceleration. Additionally, the authors obtained the first, second, and third impact, and the ratio of the third to the second impact acceleration. Curve fitting indicated that the response is not a simple harmonic oscillation and suggested a more rigorous analysis to characterize the damping properties of the test cushions. Since then, other authors have analyzed more deeply the impact response either for assessing or improving the methodology. In 2010, Sprigle et al. [3] reported on a study assessing the IDT with five cushions of different compositions with the aim of improving the methodology and distinguishing the ID performance of the cushions. In addition to the four criteria established in the standard, they analyzed the magnitude of the first and second impact acceleration and the ratio of the second impact to the first impact. Their results confirmed the results of Chung [2] and showed that the ID response is not reflected by a simple damped harmonic and that the impact acceleration exhibited was both reliable and sensitive. Therefore, they suggested that the impact acceleration should be reported as a result of this test. They also indicated that the ISO IDT method must include an explicit positioning of the accelerometer on the RCLI, and a mechanism to ensure the distance between the accelerometer and the axis of rotation is consistent.

 

Table 1. Studies applying ISO 16840-2

Author	Journal, Year	Number of Cushions	Parameters measured	Purpose of the study
B.M. Chung	IFMBE Proceedings, 2009	3	# rebounds, 1st, 2nd high peak (rebounds), ratio 2nd – 1st 1st, 2nd, 3rd impact, ratio 3rd – 2nd	Investigate characteristics of cushions
Sprigle et al.	Assist Technol, 2010	5	# rebounds, 1st, 2nd high peak (rebounds), ratio 2nd – 1st Magnitude of 1st and 2nd, ratio 2nd – 1st impact acceleration	To assess and improve the test
Ferguson-Pell et al.	JRRD, 2015	36	# rebounds, 1st, 2nd high peak (rebounds), ratio 2nd – 1st The first 3 impact acceleration, ratio 1st to the 2nd, ∆t 1st impact LCD, OD (to predict membership) Frequency components analysis	To assist in the selection of cushion and distinguish performance of cushions
Hillman et al.	Assist Technol, 2017	37	# Rebounds, the 1st and 2nd highest rebound acceleration, ratio 2nd – 1st *Magnitude of 1st and 2nd impact accelerations, ratio 2nd – 1st (to correlate) Load Deflection and Hysteresis h250, h500	Examine the relationship between IDT and hysteresis
Tyler Freeto et al.	JTV, 2017	5	Hysteresis, IDT (Max/Min acceleration) LCD, OD	To assess 2 prototype cushions by comparing with commercial cushions

In a more extensive evaluation, Ferguson-Pell et al. conducted a study on 36 different cushions to assist in the decision of wheelchair cushion selection by simplifying the results of the impact damping response [4]. They followed the criteria established in the ISO IDT and incorporated the parameters and recommendations from Chung [2] and Sprigle [3]. They used a differential accelerometer and extended the measurements to include the mean of the first three impact accelerations, the ratio of the first to the second impact acceleration, and the duration of the first impact. In addition, they analyzed the loaded contour depth (LCD) and the overload deflection (OD) for correlating a classification of cushion and investigated the frequency composition of the impact response. In the study, they applied a two-step cluster analysis for classifying the cushions into two groups defining their degree of damping. They associated the first impact peak with 70% of the dissipated energy and considered that each rebound is an additional impact to the tissue, as well as emphasized that a smaller value of second impact/rebound ratio is desirable to minimize the risk of potential deep tissue injury. In the same way, the second impact and the second rebound were the most influential contributors to predict cushion membership. The fundamental frequency for all the cushions was 5.6±9.5 Hz [4]. The authors emphasized that the wheelchair cushion must be capable of damping impact pulses rapidly to avoid the impact and possible damage to the tissue.

Acknowledging that the ISO IDT standard evaluates the relationship between impact damping and hysteresis, Hillman et al. [5] investigated the correlation of these tests [6] on 37 wheelchair cushions. The results from their study showed a weak-moderate relationship of the results from these two tests, but when they analyzed the response separating the planar foam, they found out that a simple design showed a stronger correlation which may suggest that the relationship between impact damping and hysteresis is affected by the cushion materials. They found that the second rebound acceleration and the ratio of the first and the second rebound acceleration had the strongest correlation with results from the hysteresis test (h250 and h500), which were the same findings of Ferguson-Pell et al. [4] for cushion membership, thus these parameters may offer the best sensitivity when classifying cushions.

On the other hand, Tyler Freeto et al. [7] used a portion of the ISO 16840-2 tests to assess the performance of their cushion prototypes and compared the responses with commercially available cushions using the hysteresis, impact damping, and loaded contour depth and overload deflection tests. They presented a full characterization of their prototype and three more wheelchair cushions by comparing the performance among them. They did not find a strong correlation among test type results, the cushion which had the best performance in the impact damping test was not the same as the one with the best performance in the hysteresis test, which may be consistent with the results from Hillman et al. [6].

Finally, to provide information on the typical values of the IDT, we computed the mean and standard deviation of the results reported from these various studies for the parameters defined in the ISO IDT, including the first and second rebound and the ratio of the second to the first rebound. Table 2 shows the overall comparison of these results. It is noteworthy that the high standard deviation is due to of the range of values reported in the studies, which may confirm the assertion of the authors that the material of the cushion affected the performance. It could be of interest to assess the influence of the design materials in the characterization of the wheelchair cushions using the ISO 16840-2 standard.

Table 2. Comparison the primary results presented in the studies

Author	Number of Cushions	1st Rebound (m/s ^2)		2nd Rebound (m/s ^2)		Rebound Ratio
		Mean	SD	Mean	SD	Mean	SD
B.M. Chung	3	8.67	4.62	2.70	1.12	0.340	0.123
Springle et al.	5	6.47	2.38	3.81	1.95	0.573	0.202
Ferguson-Pell et al.	22	9.04	1.21	3.7	1.13	0.410	0.110
	14	10.50	0.90	6.96	1.79	0.640	0.130
Hillman et al.	37	7.72	1.71	3.93	1.72	0.497	0.158
Note: Ferguson-Pell et. al. divided their cushions into two groups based upon LCD test results.

DISCUSSION

There is a consensus among those groups using and evaluating this standard that the setup of the cushion as stated in the document should be specified and standardized. The document indicates resetting the cushion when the material remains displaced after loading or when the manufacturer specifies adjusting to the user [1]. Setting up the cushion, in the same way, could be a challenge, especially when the cushion composition includes different materials or it has multiple air sections; therefore, it might affect the results. However, further research is required to investigate the influences of cushion design materials and the ISO standard criteria. Additionally, the authors emphasized that the placement of the accelerometer with respect to the axis of rotation and the distance to the hinge is not consistent in countered cushions, so the measurements might be affected [3, 5].  

Although the impact damping test is the most studied test in ISO 16840-2 by researchers, more research is needed to assess the other tests defined for the characterization of wheelchair cushions and to correlate the relationship of the performance from each one of them. For example, ISO 16840-2 states there is a relationship between IDT and hysteresis, but the studies presented in this document do not entirely agree with this statement.

On the other hand, ISO 16840-2 states “cushions with larger hysteresis values will tend to absorb energy when used on rough surfaces or when dropping down steps, rather than transfer the impact energy to the user’s tissues” [1], but thus far, no studies have been reported to investigate the effects of that impact energy on the tissues and the possible damage.

It is noteworthy that the ISO standard, as well as the harmonized national standards published by RESNA, are always improving the methodology because the committee acknowledges the importance of clear understanding and application of the standards in achieving better results. 

CONCLUSION

In this review, we identified studies that have applied the ISO 16840-2 IDT method and summarized the level of agreement between them, the authors recommendations for improving the methodology and the range of the outcome measures for the cushions tested. These studies have produced valuable information and have shown there is perceived value in the application of the IDT. No studies were found that evaluated clinical relevance of the IDT measures such as the relationship between test results and the ability to maintain postural stability. More of these types of studies are needed to continue to provide information to improve the test methods and generate data characterizing and distinguishing cushion performance.