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April 2004


Categorizing Cushions

By Stephen Sprigle, PhD, PT
Wheelchair cushions serve multiple roles. They address tissue integrity issues such as load distribution, and they serve as a platform from which functional movements are performed.

Hundreds of wheelchair cushion models are commercially available and cover a wide range of materials and designs. This broad selection brings greater options to the end user, but having more options places greater importance on the ability to make a correct cushion selection. Currently, no system or process exists that allows for an objective comparison of cushions across manufacturers. By understanding different classifications and descriptions, clinicians will be better able to match cushion characteristics to the needs of their clients.

Standardized measurements and cushion categorization are supposed to assist clinicians in selecting appropriate equipment. While standards and guidelines should not be used as the sole driver in selecting a device, they can be a useful tool to help clinicians narrow the choices of potential products. This article will briefly describe three means to characterize or categorize cushions: material construction, surface characteristics, and wheelchair coding categories. The intent is to help define common terms that can be used to describe cushions and the new manner in which many payors categorize cushions.

MATERIALS AND SURFACE TYPES
Many different types of materials are used in wheelchair cushions, and quite often a combination of materials is used. All materials have beneficial features and weaknesses, therefore a good clinical sense of material construction can help in the selection of an appropriate cushion. Many cushions use a combination of materials to highlight their beneficial features while minimizing their respective drawbacks.

Cushion materials influence many variables that impact tissue viability and functional use of cushions. Most cushions are designed to provide some level of distributing body weight over the areas of the buttocks and thighs. In addition, materials influence the means by which a cushion can manage body heat and moisture, friction, stability, and related functional activities. The following are the major forms of cushion material:
  • Foam: a lightweight cellular material that can exhibit many different properties; it describes most elastic or polyurethane foams employed in cushions. Foam is often described by its density (weight per unit volume) and stiffness (resistance to deformation).
  • Viscoelastic foam: foam material that has both elastic (displacement-dependent) and viscous (time-dependent) properties. Since the viscous properties differentiate these materials from regular or elastic “foam,” viscoelastic foam is defined by time-dependent behaviors such as stress relaxation, creep, and hysteresis.
  • Nondeforming foam: support material that does not deflect or deform under clinical loads; often used in cushion bases and characterized by high stiffness.
  • Viscoelastic fluid: relatively incompressible substance that can flow under small stresses and exhibits both elastic (ability to store energy) and viscous (resistance to flow) properties.
  • Air cushion: a cushion with an impermeable membrane containing air.
  • Water cushion: a cushion with an impermeable membrane containing water.
  • Solid elastomer and solid gel: solid rubber-like, relatively incompressible polymer that resists and recovers from deformation.
  • Cushion with displacing solid elements: a cushion made of solid, relatively incompressible components that displace under load.


    • The following are primary characteristics of different cushion surfaces:

  • Contour: cushion surface that is shaped to fit or reflect the form of the human body, most specifically, the buttocks. Cushions may be contoured to a general or nonspecific shape or custom contoured to a particular user’s shape.
  • Cutout: cushion surface having a disruption or removal of material to alter the load-bearing characteristics of the surface or to create room for an insert of material. A cutout, by definition, does not reflect the form or shape of the human body.
  • Segmented: cushion surface divided into separate and distinct segments of gridtop design.
  • Convoluted foam: cushion surface composed of convex protrusions separated by depressions or sulci, often called “egg-crate,” but includes checkerboard and other designs.
CODING CONSIDERATIONS
The third and arguably the most important manner in which cushions are categorized is coding as durable medical equipment (DME). The Centers for Medicare and Medicaid Services (CMS) defines coverage and coding policies for all DME that is covered by Medicare. Wheelchair cushion codes have just been redefined, and new codes will take effect on July 1, 2004. Clinicians and users might be inclined to seek a certain class or category of cushion based on its name. However, understanding the basis and general requirements of cushion codes will, hopefully, minimize any inclination to prescribe based on a name.

Cushions will now be assigned to four general categories: general use wheelchair cushion, skin protection cushion, positioning cushion, and skin protection and positioning cushion. For the first time, the definitions and coding criteria have been presented for public comment and revised, and are now being disseminated.

CUSHION CONCEPTS
Definitions of cushions center around four concepts: immersion, interface pressure measurements, positioning capability, and lifespan. All cushions must meet immersion or interface pressure criteria and lifespan requirements. Positioning and skin protection classifications carry additional requirements.

Immersion. A cushion’s ability to permit the buttocks to immerse is measured using a simulation test. A mechanical model of the load-bearing portions of the pelvis is loaded onto the cushion and the depth of immersion is measured. The cushion codes use two depths of immersion, 2.5 and 4 cm, to define certain cushion codes. All cushions must permit 2.5 cm of immersion, and the skin protection and skin protection/positioning cushions must offer 4 cm of immersion.

Basically, these requirements suggest that to have therapeutic value, a cushion must allow the buttocks to immerse a minimum of 2.5 cm. In addition, to provide skin protection, a cushion must allow 4 cm of immersion. This value was chosen because it reflects the anatomical relationship between the inferior aspect of the ischial tuberosities and the trochanters. In short, a cushion is deemed to have skin protection characteristics if it permits enough immersion to involve the lateral aspect of the buttocks in load redistribution.

Interface pressure measurement. Interface pressure has long been used clinically and in research studies to characterize the pressure redistribution characteristics of cushions. The new wheelchair codes allow for interface pressure tests to be used in defining cushion categories.

Interface pressures are measured while 10 wheelchair users sit on the test cushion and on a reference cushion—a 3-inch-thick flat foam cushion with 45-pound ILD. A peak pressure index (PPI) is calculated from the interface pressure measurements, and the test cushion value is compared to that of the reference cushion.

To meet reimbursement criteria, the average peak pressure index must be less than 125% of that of the reference cushion within the area of the ischial tuberosities and sacrum/coccyx. A skin protection cushion has a much lower pressure threshold. Its PPI must be 85% of that of the reference cushion. Therefore, general use and positioning cushions may have a PPI that is 25% greater than a flat foam cushion, whereas skin protection and skin protection/positioning cushions must have a PPI that is 15% less than that on foam.

Positioning capability. Positioning is defined by the presence of certain features that are designed to align or support the body. A cushion is deemed to have positioning capabilities if it has two or more of the following structural features: a pre-ischial bar or ridge, two lateral pelvic supports, a medial thigh support, or two lateral thigh supports. These features must be at least 25 mm in height in an unloaded condition, but a provision is made for cushions with a planar surface that have positioning features embedded in the design such as firmer material in relation to the supporting material. Some types of air cushions may also be defined as positioning cushions if they address postural asymmetries via the adjustment of air in two or more compartments.

Lifespan. Cushions will now be required to last a minimum length of time under normal use. Manufacturers will have to document that certain tests are passed after simulated or actual use. General use cushions must be tested after 12 months of use, and skin protection, positioning, and skin protection/positioning cushions must be tested after 18 months of use. The Statistical Analysis Durable Medical Equipment Regional Carrier does not specify what type of test simulates use, so manufacturers will have to describe the methods used. These life-span durations are also reflective of the minimum warranty period that must be supported by cushion manufacturers.

CORRECT CUSHION USE
Given the wide range of cushion options available, one can easily assume that several cushions can meet the unique needs of most wheelchair users. Clinicians should assume that more than one cushion could be adequate for a given person in a given situation. Therefore, one of the most important issues that must be addressed is not the specific brand or model of cushion, but its condition and use. Two very simple questions should be considered: one, is the cushion in good repair and, two, is it oriented properly?

This might sound elementary, but because a person must be out of a wheelchair and the cushion cover removed to check its condition, this inspection does not always occur. Simply looking for signs of fatigue, improper orientation, or other misuse can reduce the risks associated with a worn cushion or a poor fit. The mechanism and signs of failure should be discussed with the user or staff upon setup and delivery. In fact, in the best case, signs of failure should be described in the product literature.

Fatigue of foams is indicated by cracks and flaking on the surface and by inability to recover when unloaded. If you remove the cover of a flat foam cushion and the foam appears compressed or contoured, then it is fatigued and should be replaced. Solid gel cushions require a covering material to protect the gel. If this material is cracked or torn, or if the solid gel material is exposed, the cushion should be replaced. A fluid cushion, whether air, water, or viscous fluid, is encased by an impermeable cover and fatigue is indicated by a leak of fluid. Cushions that use viscous fluid can also indicate fatigue if the fluid becomes nonhomogeneous, meaning some areas become softer or firmer than others.

Orientation is an important issue for contoured or cutout cushions and any cushion that has site-specific loading areas. Checking orientation might be considered elementary, but it also requires the person and cover to be removed, making time a factor. Many wheelchair cushions are contoured or designed so that the buttocks rest in a particular location, therefore, orientation can make the difference between an effective cushion and a harmful one.

CONCLUSIONS
In summary, many types of wheelchair cushions are commercially available. These products exhibit a wide range of materials and designs, but most products are inherently similar to others. Clinicians can benefit from a basic understanding of how cushions can be characterized or described.

This article described several terms to be used when discussing cushions with clients, vendors, and manufacturers. Common terminology often helps communication. In addition, the government categorizes cushions for the purpose of coverage and reimbursement. Because new categories will be recognized, clinicians should understand what each category means and does not mean.

Finally, clinicians need to acknowledge that several cushions might meet the needs of their clients. However, how well a cushion is used and the proper time to replace or repair a cushion are factors that can have a large impact on positive clinical outcomes.

Stephen Sprigle, PhD, PT, is director, Center of Assistive Technology & Environmental Access, Georgia Institute of Technology, Atlanta.

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