Testing of crutches material

Introduction

This case study conducted by One Eighty Materials Engineering focuses on the evaluation of three sets of injection-molded crutches manufactured by an orthopaedic supplier. The crutches tested were the “Permanent User Non-adjustable – Heavy Duty,” the “Permanent User Non-adjustable – Standard,” and the “Craggy Adjustable” crutches. The purpose of the study was to assess the crutches’ compliance with ISO 11334-1, a standard that specifies requirements and test methods for assistive products used for walking with one arm.

Approach and Methodology

Nine crutch samples (labelled A to I) were tested, with three samples from each set. The tests conducted included measurements, forearm withdrawal tests, static loading tests, and low temperature falling tests. For the static loading tests, the crutches were subjected to a load equivalent to 200 kg, which is greater than the crutches maximum user mass rating of 120 kg.

Results

1. Measurements: All nine crutches met the measurement requirements outlined in the standard.
2. Forearm Withdrawal Test: The crutches were subjected to a forearm withdrawal test, which involved pulling a pipe with a diameter larger than the standard requirement through the cuff opening. All crutches successfully passed this test, with the maximum opening force below the specified limit, and minimal permanent deformation.
3. Static Loading Test: The crutches were loaded in compression, and a force of 2000 N was applied for 15 seconds. All crutches passed this test without any visible cracks or breakages.
4. Low Temperature Falling Test: After subjecting the crutches to -22°C for 24 hours, Samples A, G, and H exhibited breakages, specifically in the handle end screws. The fractures occurred in the region where the thread begins, which is a known stress concentrator.

Conclusion and recommendations

Based on the test results, it can be concluded that the crutches met the requirements of ISO 11334-1 for a maximum user mass of 200 kg. They demonstrated strength and durability during the measurements, forearm withdrawal, and static loading tests. However, the handle end screws proved to be a weak point, as evidenced by the breakages observed during the low-temperature falling test.

To address this issue, it was recommended that the handle end screws be redesigned to include a shank and a longer engaged thread into the crutch. This modification would provide enhanced strength and stability to the handle end screw, which is crucial for securing the handgrip in place.

Based on the results of these tests, the orthopaedic supplier has since improved the handle end screw design. One Eighty has also since retested the low temperature falling test using crutches with the new handle end screw design. The information gathered from these tests have aided the orthopaedic supplier’s overall product design process and research.