S C I E N C E O F S E AT I N G

Transcription

1 S C I E N C E O F S E AT I N G GLOSSARY OF TERMS Like the modern running shoe, today s wheelchair seat cushion is a complex mix of design terminology: firmness layering, pre-contouring, segmentation, tension reduction, microclimatic material, hydrostatic force, etc. To best understand the meaning, relevance, and value of this terminology, we ve developed the JAY Science of Seating Glossary. Through the use of this guide, it is our hope clinicians, users, and dealers can all make a better informed wheelchair seating choice. improving people s lives

2 Foam Density vs Foam Firmness Foam Density vs Foam Firmness Foam density is a measure of PCF (Pounds per Cubic Foot). Foam firmness, or spring, is its IFD (Indentation Force Deflection). PCF and IFD define foam s durability and pressure-relieving properties. Dense foam can be very soft (low spring or IFD) and non-dense lightweight foam can be very firm (high spring or IFD). To say foam is firm says nothing about its density. This is important because density, not firmness, influences durability. Regardless of foam s ILD (or spring), it is important its density remain above 2.5 PCF (pounds per cubic foot). Manufacturers who use low density foams for a specified ILD (or spring) may reduce cost, but ultimately sacrifice durability. -- See Polyurethane Foam Association, In Touch article, Volume 1, Number 2, May 1991 Firmness Layering Firmness Layering Firmness Layering is using multiple layers of materials with varying firmness to change a cushion s ability to distribute load. In a seated position, the maximum load of the human body rests on the ischial tuberosity bones, placing the tissue beneath these bones at risk for breakdown. A common form of firmness layering is to use a firmer material under the thighs and a softer material under the ischial tuberosities to redistribute the load to the thigh region, helping to reduce the chance of tissue breakdown under the ischial tuberosities. Redistributing load is not the only reason to use firmness layering. It can also aid in positioning and overall stability. Frequently a firm layer of material is used as a base to provide stability followed by a softer upper layer(s) for comfort. It is always important to determine why a manufacturer is using firmness layering.

3 Hydrostatic Force Hydrostatic Force Hydrostatic force is a force that is in balance or equally distributed. Hydrostatic forces occur in objects that immerse in fluids (liquid or gas). This is why people say they feel weightless when floating in water. Hydrostatic loading evenly distributes pressure to reduce buildup under sensitive areas of the pelvis, which helps prevent the formation of decubitus ulcers (pressure sores or bed sores). Considerations Because it is impractical to sit in a bowl of water, fluids (liquid or gas) must be held in a closed container. It is imperative that these containers be flexible and deep enough to easily reshape (allowing displacement) to accommodate the contour of the pelvis. If a fluid can't displace, it won't hydrostatically distribute the load. Microclimatic Material Microclimatic Material Microclimatic material is a material that encourages the passage of air, heat, and water vapor from the user. Although more conclusive studies are ongoing, tissue breakdown has been attributed to excess heat and moisture. Does the material have enough depth to allow the full passage of heat and moisture? Frequently thin air-exchange covers sit on top of impervious materials that trap both heat and moisture. Without a thick air conducting cover layer, or a base material that transfers heat and moisture, the thin air-exchange cover is less than effective.

4 Positioning Capability Positioning Capability Positioning capability is the creation of a shape by using one or a combination of the following techniques; pre-contouring, modifying the cushion material, adding or subtracting components, or controlling the volume and location of fluid. Flexible postural abnormalities frequently become fixed when not corrected by using a positioning technique. Fixed postural abnormalities need proper support to ensure a comfortable and safe sitting experience. Without the ability to properly position, users who have either flexible or fixed postural abnormalities are at risk for increased skeletal problems. A few key considerations to ask when evaluating a cushion s Positioning Capability: Can the cushion support or correct the postural abnormality without modification? Can the cushion material be modified? Can positioning components be added or subtracted? Does the cushion help reduce posterior pelvic tilt? Can the volume of fluid (air or liquid), when used, be controlled bilaterally? Pre-contouring Pre-contouring Pre-contouring is the removal or reshaping of material to create a new shape that changes the cushion s distribution of seated pressure. Wooden or plastic school chairs are seldom flat. Without pre-contouring, they would be uncomfortable. In the same way, without pre-contouring, cushion materials with poor pressure-reducing, positioning, and/or stabilizing capabilities often cause discomfort and sometimes the formation of decubitus ulcers (pressure sores or bed sores). Assessing the need to pre-contour a cushion is based on many factors. Four important ones include: The need to further redistribute pressure The need to minimize reaction forces caused by a compressed material The need to position the sitter The need to stabilize the sitter Any time pressure-reduction, positioning, and/or stability are important; pre-contouring should be a design consideration. Cushions made from a single homogenous material without pre-contouring often fail to address the need to reduce pressure, improve position, or properly stabilize.

5 Reaction Force Reduction Reaction Force Reduction Reaction force is the resulting force from compressing or deforming a solid elastic material. The maximum reaction force occurs at the area of highest compression. For instance, when you push a finger down into a block of foam, the highest reaction force is at your fingertip. When a pelvis is seated, the ischial tuberosity bones are positioned at the maximum point of compression, resulting in high reaction force that often contribute to tissue breakdown and the formation of decubitus ulcers (pressure sores or bed sores). This is why it is always important to reduce the reaction force as much as possible. While foams, elastomers, and gels help spread load over a larger area to reduce peak pressure area, the reaction force they produce frequently offsets this benefit. The problem can be reduced by using techniques such as pre-contouring, stiffness layering, and segmentation to lessen the reaction forces. Without these techniques, individuals at risk for skin breakdown may develop decubitus ulcers (pressure sores or bed sores). Segmentation Segmentation Segmentation is the division of a material into segments to reduce tension within the material. Without segmentation, material tension prevents the pelvis from sinking fully into a material such as foam, gel or an air bladder. Segmentation allows these materials to better conform to the shape of the pelvis, which effectively reduces peak pressures. Segmentation reduces tension, but in the case of foam, it may potentially weaken t he material s inherent durability.

6 Stability Stability Stability is a cushion s ability to provide both lateral and forward stability. Poor postural stability can easily lead to: Skeletal deformities such as kyphosis, scoliosis, and lordosis. A poor hip position that compromises the pressure-relieving properties of the cushion. The increased likelihood of falling out of a seat or being unable to return to upright. It is important to properly assess an individual s movement stability the ability to move a chair without becoming unstable. Individuals with weak movement stability need a cushion that firmly supports the trochanters and prevents posterior pelvic tilt. Cushions that fail to provide adequate stability place these individuals at risk. Tension Reduction Tension Reduction Tension is the state of being stretched or strained. Tension is a common concern in cover materials, but it can also be a problem with the actual cushion material. Foam is a prime example of a tension producing cushion material. Sinking the pelvis into a cushion fluid (liquid or air) or a pre-molded shape significantly increases contact area and helps reduce peak pressures areas. When taut cover materials are used, this benefit is minimized. Even external materials, such as clothing or a lift-sling, can ruin the pressure-reducing capability of a cushion. In addition, the cushion material itself should not create tension. Tight fluid containers, non segmented foam, or foam without pre-contouring all run the risk of producing too much tension.

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