THE SCIENCE OF SEATING
High quality polyurethane foam can be used to make a highly effective cushion. The efficacy does not just come from the quality of the materials, but also the specific shapes, densities and coverings. All aspects of design are critical to the cushion's performance. Let's cover some specific design concepts used throughout the Axiom line.
Surface Tension Reduction
Improved pressure distribution and stability are directly related to the ability of the body to immerse itself in the cushion. Less immersion means less surface area to spread the load over and a smaller base of support for stability. Foam compresses under load to allow the body to immerse. Surface tension in the foam and its coverings can limit that immersion. This is the result of tension forming as the foam and cover materials attempt to stretch to accept the body. This tension can also change the shape the tissues supporting the bony prominences take under load, thus concentrating forces at the point of greatest immersion, usually the area around the Ischial Tuberosities.
Polyurethane foam is made by the formation of a gas at the same time as the urethane polymerization is occurring; resulting in cells with elastic walls. As a result, the foam acts like a spring; it compresses under load and rebounds when the load is removed. There is a reaction force "pushing back" in any compressed area. The greater the compression the greater the reaction force. In a flat foam cushion the area of greatest immersion is the area of greatest compression and therefore it has the greatest reaction force. Unfortunately, this is most likely to occur around the tissue supporting the Ischial Tuberosities; the area where we are trying to reduce pressure. One way to reduce this reaction force is to Pre-contour the cushion to match the shape of the buttocks by removing foam.
While Pre-Contouring is a very effective design technique, it can be difficult to execute because humans vary in size and shape, as well as the quality and amount of tissue surrounding the bones we sit on. Cushion widths relate directly to the overall width of the individual and their wheelchair. A paraplegic individual may have very little tissue outside of their trochanters. So, their overall width may closely relate to their bi-trochanteric width. Conversely, another person may have considerable redundant tissue lateral to their trochanters and therefore have a much smaller bi-trochanteric width relative to their overall width. Since loading of the trochanters improves stability by widening the base of support and reduces the load the tissue that supports the Ischial Tuberosities must bear, designers must understand the anthropometry of the pelvis to design an effective cushion.