Comparing lab-based and community wheelchair caster failures

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Comparing lab-based and community wheelchair caster failures

Anand Mhatre1 , Jon Pearlman1

1University of Pittsburgh



Wheelchair casters are reported to experience frequent failures in the community with different failure modes. [1– 9] Nearly one-third of the wheelchair part failures are caster failures as per studies documenting wheelchair incidents and repairs in the United States and Scotland. [2,8] In the less-resourced parts of the world, casters are known to fail within few weeks to two years of wheelchair use [4–7,9,10]. Failures such as caster stem bolt fractures can be catastrophic as they cause the wheelchair to tip and the user to fall out of the wheelchair [2,11]. Further, failure-induced breakdowns can affect the users socially and economically [7,9]. 

To reduce the incidence of caster failures and its effects, international humanitarian organizations are advocating the provision of high-quality, appropriate products including wheelchairs. [12–15] The World Health Organization’s Guidelines on the provision of manual wheelchairs in less-resourced settings has recommended the development of suitable product testing standards [16]. Based on this recommendation, the Standards Working Group of the International Society of Wheelchair Professionals (ISWP) has developed a suite of appropriate wheelchair quality testing standards including a caster durability testing standard. [1,9] Accordingly, the ISWP’s Standards Working Group has developed a standard protocol for caster testing which subjects casters to shocks, corrosion and abrasion exposures in the laboratory based on the exposures seen in the community. A recent caster testing study found that corrosion and abrasion affect the caster durability, bearing durability and failure mode. As a part of the standard development, the authors seek to validate the testing outcomes to community evidence.

In this study, the efficiency of the caster testing standard to reproduce community-representative failures in the laboratory is being evaluated. This study goal was accomplished in two steps:

  1. Collecting caster failures from the community through various data sources.
  2. Evaluating the correlation between the community and laboratory-based failures.



The caster failure data from the community was collected from three sources as shown in Table 1. The period of use for the casters ranged from 3 months to 6 years.


Table 1. Data Collection Sources

Community failure modes that were found with more than 20% of the samples of each model were used for comparison. The laboratory-based failures were failures found with caster testing which included corrosion, abrasion and shock conditions [20]. Due to limitations with the number of samples for testing, only eight samples of each model were tested through the three testing conditions.



Figure 1 shows the models evaluated in this study and Table 2 shows the details of their failure modes collected from the community.

Figure 1. Caster models A-F from left to right. Table 2. Community Failure Modes



As per the study results, around 50-100% of the caster failure modes observed in the community are replicated with laboratory-based testing. This demonstrates the strong external validity of the ISWP caster durability testing protocol. This correlation shows a significant improvement in the community failure prediction accuracy of the testing protocol which was about 40% in the preliminary testing study [1]. All of the matched failure modes are due to corrosion and abrasion which overall support the inclusion of these environmental factors in the caster testing protocol as well as wheelchair durability testing (ISO 7176), which historically has not been performed. 

Few of the common testing and community failures did not correlate for some models. For instance, fork fractures with model A and stem bearing fractures with model F were frequently observed with testing but not in the community. This may be attributed to the variation in the quality of parts that are used on these models. 

One of the goals of caster testing is that the time to failure matches between community and laboratory. This warrants continuous testing, failure data collection, modifying test exposures based on failure comparisons and additional investigation into the inclusion of additional testing factors such as impacts and dust. This iterative development can certainly improve the prediction accuracy of the testing protocol. For dissemination purposes, the ISWP Standards Working Group members are in the process of developing and publishing a caster durability testing standard with the International Organization for Standardization (ISO).



Caster failures found in the community were compared with laboratory-testing failures in this study. There was a high degree of correlation between caster failures seen in the two settings which demonstrates the strong external validity of the testing protocol. To improve the validity, further data collection, laboratory testing and the addition of relevant testing factors are necessary.



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