Cushion Properties

JAY Fusion cushion with property icons

Why is this cushion made like this and is it right for my client? Wheelchair cushions are created using scientific properties to address the needs of wheelchair users. Understanding these properties will help the clinician select a cushion based on the desired effect that they are trying to achieve. This resource explains the technology used in cushions and the clinical application that should be considered when choosing a cushion. Examples of cushions which demonstrate these properties are also provided, however all cushions use these properties and are key to making your cushion selection.

Firmness

Firmness

Foam firmness relates to the amount of stiffness or softness in a foam.

Indentation Force Deflection (IFD), also referred to as Indentation Load Deflection (ILD), is a measurement of foam firmness.

  • High IFD is stiff
  • Low IFD is soft

Clinical Application

Firmness does not describe density or durability, it describes the spring (ability to compress and rebound) of a foam.

The softness/firmness of the foam will impact positioning and pressure.

The clinician must select foams that are durable, yet provide the level of stiffness or softness to achieve the positioning goals.

The clinician must also consider the foam's reaction to heat (including body heat) because foam (i.e. memory foam) may soften when sitting on it, thus reducing the IFD.


Density

Density

Foam density relates to the durability of a foam.

Foam can be either high density or low density and measured in Pounds per Cubic Foot (PCF).

Firm foam can be either high density or low density.

  • High PCF is more durable
  • Low PCF is less durable

Clinical Application

Density is important to consider when selecting a cushion so it lasts the intended amount of years. Regardless of foam's IFD, it is important that high density foam is used. This is important because density, not firmness, influences durability.

Manufacturers who use low density foams may reduce cost, but ultimately sacrifice durability.

Cushions that use low density foams may compress over time (bottom out). They harden, lose shape, and no longer provide appropriate support and pressure distribution.


Reaction Force Reduction

Reaction force reduction

Reaction force reduction is the ability of the material to reduce the resulting force from compressing or deforming a solid elastic material.

The maximum reaction force occurs at the area of highest compression (i.e. when you push a finger down into a block of foam, the highest reaction force is at your fingertip).

Clinical Application

It is important to have a cushion which reduces reaction force as much as possible.

When a pelvis is seated, the ischial tuberosity bones are positioned at the maximum point of compression, resulting in high reaction force that often contributes to tissue breakdown and the formation of pressure injuries.


Hydrostatic Force

Hydrostatic Force

Hydrostatic force is the amount of force of a fluid at rest. It is the force that occurs in objects that immerse in fluids (liquid or gas). Hydrostatic forces that are in balance or equally distributed will have less pressure against the tissue.

Clinical Application

Hydrostatic loading evenly distributes pressure to reduce buildup under sensitive areas of the pelvis, which helps prevent the formation of pressure injuries.

Hydrostatic forces that are in balance give the feeling of weightlessness or floating in a pool.

It's important to provide the correct amount of fluid to enable proper reduction of hydrostatic forces.


Tension Reduction

Tension reduction

Tension reduction is the use of materials to reduce the effects of creating tension on the tissue.

Tension is the state of being stretched or strained when the materials attempt to accept the body.

The more stretch within a material, the more tension is reduced. An example is comparing stretch denim with original denim.

Clinical Application

Tension can cause increased forces at the point of greatest immersion, usually the area around the ischial tuberosities.

Foam is a prime example of a tension-producing cushion material, whereas fluids are tension-reducing.

Using materials such as 4-way stretch in covers reduces tension on the tissues. Although tension is a common concern in cover materials, it can also be a problem with the actual cushion material.

Sinking the pelvis into a cushion fluid (JAY fluid or air) or a pre-molded shape significantly increases contact area and helps reduce peak pressure areas.


Microclimate

Microclimate

Microclimate is the temperature and moisture created in a particular area. Too much heat may cause sweating, leading to moisture. Too much cooling may interfere with healing.

Clinical Application

The microclimate of an area while sitting on a cushion must be monitored to prevent temperature build-up which leads to moisture, maceration, and pressure injury.

Clinicians should consider microclimatic materials which cool the area and delay temperature build-up. Microclimatic materials will also reduce perspiration (moisture).

Although tissue breakdown has been attributed to excess heat and moisture, skin that is too dry is also more prone to pressure injury.

Microclimatic material encourages the passage of air, heat, and water vapor from the user; therefore materials that promote a stable microclimate in the cushion should be considered.


Vibration Dampening

Vibration Dampening

Vibration dampening is the use of material to disperse the energy caused by vibrations by reducing the amplitude and frequency of the vibration.

Whole-body vibration (WBV) is the vibration transmitted by supporting surfaces to the entire human body.

Some materials can reduce or dampen this effect of WBV.

Clinical Application

Cushions which provide vibration dampening to reduce the WBV are important for those who experience pain, fatigue, and nerve damage.

A vibration-reducing material should be able to perform over a wide range of temperatures while still absorbing shock and vibration energy.

Visco foam is an example of a material that performs well in absorbing shock and vibration.

Download the Cushion Properties Table

References

  1. Brienza, D., Kelsey, S., Karg, P., Allegretti, A., Olson, M., Schmeler, M., Holm, M. (2010). A Randomized Clinical Trial on Preventing Pressure Ulcers with Wheelchair Seat Cushions. Journal of the American Geriatrics Society, 58(12), 2308-2314. doi: 10.1111/j.1532-5415.2010.03168.x
  2. Call, E., Hetzel, T., Mclean, C., Burton, J. N., & Oberg, C. (2017). Off loading wheelchair cushion provides best case reduction in tissue deformation as indicated by MRI. Journal of Tissue Viability, 26(3), 172-179. doi: 10.1016/j.jtv.2017.05.002
  3. Garcia-Mendez, Y., Pearlman, J. L., Boninger, M. L., & Cooper, R. A. (2013). Health risks of vibration exposure to wheelchair users in the community. The Journal of Spinal Cord Medicine, 36(4), 365-375. doi: 10.1179/2045772313y.0000000124
  4. Shabshin, N., Zoizner, G., Herman, A., Ougortsin, V., & Gefen, A. (2010). Use of weight-bearing MRI for evaluating wheelchair cushions based on internal soft-tissue deformations under ischial tuberosities. The Journal of Rehabilitation Research and Development, 47(1), 31. doi: 10.1682/jrrd.2009.07.0105
  5. Watanabe, L. (2017, September). Immersion, envelopment and off-loading. Mobility Management. Retrieved from https://mobilitymgmt.com/Articles/2017/09/01/Seating-Strategies.aspx

Published: 4/8/2020

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