Ensuring Assistive Technology is Fit for Purpose: Meeting the Standards or Pushing them Higher?

Authors:  Laura Finney, Noel McQuaid, Chris Anderson

The ISO Standard for Wheelchair Seating, ISO 16840, is currently in development. Due to the lack of quantitative loading data, some parts of the standard, e.g. Impact strength, are broadly based on the ISO standard for Wheelchairs, ISO 7176.  Although ISO 7176 has been in circulation for many years, it was originally intended for application to basic NHS service wheelchairs and its application to seating is not straightforward.

When James Leckey Design were developing the Mygo seating system 5 years ago, ISO standards for Seating were unavailable. To ensure the Mygo seat was Fit for Purpose, it was subjected to a wide variety of in house tests, including; Finite Element Analysis, Clinical Trials and Part 81 of the Wheelchair standard, for Static, Impact and Fatigue Strength.  No issues were encountered.

Now after 3.5 years in production and over 3,500 Mygos prescribed, there has recently been one instance of the backrest failing with a young boy, aged 7 years, who exhibits a strong extensor pattern. Interested to understand the loads involved, Leckey used an experimental approach to investigate this case in more detail.

Using a strain gauge load cell and actuator assembly, the backrest was loaded repetitively until failure. It was necessary to perform a series of experiments to replicate the type of failure encountered. These included: increasing the magnitude of the load to 40% above maximum user weight, varying the materials used and changing the point of load application to 110mm offset from midline.

Depending on the material composition of the backrest, and the magnitude and position of the load, the number of cycles to failure was found to vary between 4,500 and 27,000 cycles. However, the maximum number of repetitive loading cycles for a postural backrest as proposed by draft version of ISO 13840-32 is 1000 cycles.

This study highlights the difference between ISO standards which are based on theory and actual clinical findings, and underlines the need for standards to be based on quantitative data, representing loads which such assistive technology is subjected to in everyday use.  Leckey are working alongside WestMARC Wheelchair Services to support such a project at the University of Strathclyde in Glasgow.

A presentation of this study will be supported by a real-time demonstration of the Mygo seating system undergoing a cyclical load test in the exhibitor section of the conference.

References

1. ISO 7176-8 Wheelchairs – Requirements and test methods for static, impact and fatigue strengths

2. ISO 16840-3 (draft) Wheelchair Seating – Determination of static, impact and repetitive load strengths for postural support devices