Factors affecting the supportive properties of foam wheelchair cushions

Abstract

A protocol was developed to test foam wheelchair cushions in the laboratory and clinically and to correlate the results. The laboratory procedure consisted of loading several types of foams at different temperatures, moisture contents and weights. The support properties of the foams were determined on an Instron Universal Testing Machine using a modified version of ASTM D-1564, Standard Methods of Testing Slab Flexible Urethane Foam. In the clinical procedure, the same types of foams were given to eight students at the Texas Jaycee Campus of Victoria College in Gonzales, Texas to use with their wheelchairs. The support properties of these cushions were tested periodically using the procedure developed for the laboratory cushions. Ischial tuberosity pressures were also measured using a Scimedics Pressure Evaluator. A model of the cushion and an indentor was developed and used to determine the changes in peak pressure. These changes in peak pressure were found to depend not only on the cushion support properties but also on the size and shape of the indentor. There was also found to be a three-stage response of polyurethane foam to compression. In the first stage, a relatively large load is necessary to compress the foam a small amount. The second stage is the readily compressible region where there is a gradual increase in the load necessary to compress the foam an additional amount. The third stage is reached when the foam has been deformed beyond the readily compressible region. In this stage, a large load is necessary to cause each additional deflection in the foam. When the foam is compressed beyond the readily compressible region, the peak pressure increases sharply. The Cal West foams underwent a large compression set when subjected to a combination of elevated temperature and high moisture content. The Scimedics foam suffered physical deterioration when subjected to a high moisture content at room temperature. The testing procedure developed in this study was not adequate for use with Temper-Foam. For the Cal West and Scimedics foams, the cold, wet loading configuration appears to provide the best choice for correlating the laboratory results with the clinical results.