AXIOM 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 cushions performance. Lets cover some specific design concepts used throughout the Axiom line.


 

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.