DEVELOPMENT, EVALUATION AND IMPLEMENTATION OF WHEELCHAIR SEAT CUSHION TESTING STANDARDS
The International Organization of Standardization (ISO) has developed test protocols that quantify the physical and mechanical characteristics of wheelchair seat cushions that are considered relevant to their influence on tissue integrity (ISO16840-2). The draft standard contains a total of nine test methods, of which we have focused on four tests, namely recovery, load-deflection and hysteresis, lateral and forward stiffness, loaded contour depth and overload deflection.
The first goal of this study was development of the recovery test protocol. Recovery characterizes the short-term (25sec) and long-term (20min) resilient tendencies of a cushion. The test was repeated three times on three sets of eight cushions. The same sets of cushions were tested by two other laboratories to verify protocol reproducibility. The test had high intra-lab repeatability, but low inter-lab reproducibility. The reproducibility is affected mainly due to the high sensitivity of the test. The results also suggested that the long-term recovery test be converted to a pass or fail binary test.
The goal of the second part of the study was to implement these tests and to evaluate their reliability by analyzing their repeatability and ability to differentiate between the cushions. Each test was run on a set of 21 commercially available cushions. Reliability and repeatability was evaluated using the ICC (intra class correlation coefficient) and RC (repeatability coefficient). The load-deflection and hysteresis characterizes the cushions hysteresis at 8N, 250N and 500N. The 250N value was the most reliable measure. The lateral and forward stiffness test measures the peak and 60sec force required to displace the cushion indenter by 10mm. The test showed high reliability. The loaded contour depth (LCD) and overload deflection test measures the cushions depth of immersion in loaded (135N) and overloaded (180N) states. The LCD test was also highly reliable. However, lack of variability for the overload test suggests that it be converted into a binary test, to check if the cushion has bottomed out or not. The recovery test displayed high intra-lab repeatability, but its reliability is questionable due to its poor ability to distinguish between cushions. Overall, the test results suggest that time interval between replications should be increased to atleast 30mins to reduce the potential of a systematic trend between readings. The RC can be used as a precision statement for each test, thus giving us a baseline for acceptable variations between its replications. In order to validate these tests completely, we need to establish inter-lab reproducibility, conduct research to investigate the clinical resolution for each test and clinically validate these test parameters.
Advisor:David M. Brienza; Patricia Karg; Rory A. Cooper
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:06/20/2005