ArticlePDF Available

Seat cushion selection

February 1990
Journal of rehabilitation research and development. Clinical supplement / Veterans Administration 27(2):49-73

Source
PubMed

Authors:

Martin Ferguson-Pell

University of Alberta

. Weight of Selected Commercial Cushions*

…

Figures - uploaded by Martin Ferguson-Pell

Content may be subject to copyright.

Content uploaded by Martin Ferguson-Pell

Content may be subject to copyright.

Technical Considerations

Seat Cushion Selection

by Martin W. Ferguson-Pell, PhD

. Ferguson-Pell is Director, Center for Rehabilitation Technology and Chief of Rehabilitation Engineering Unit,

Orthopedic Engineering and Research Center, Helen Hayes Hospital, West Haverstraw, NY.

INTRODUCTION

t is natural to ask why the selection of wheelchair

cushions warrants any detailed discussion

. One

would assume that the two or three cushions which

do the job well for most people would survive the

rigors of the medical marketplace

. Surprisingly, this

is not the case

: engineers and others have not

developed a universal cushion that meets the needs

of the majority of users

. In addition to different

wheelchair cushions on the market, there are many

custom designs which offer hundreds of options.

Usually only one or two will "do the job well" for a

given client

. As a result, clinicians face the task of

matching an individual's needs with the properties

of a confusing array of available cushions. To make

matters worse, our basic knowledge of how the

seated subject and the cushion mechanically interact

is very limited, owing to the mechanical complexity

of the tissues involved. Therefore, a scientific

approach to designing cushions, and instruments to

help fit them has been slow to develop, but real

progress has been made in the last decade.

A wealth of information does exist through the

clinical observations and experience of expert cush-

ion prescribers

. But a central data/knowledge base

or any other mechanism allowing us to learn

through collective experience, has not yet been devel-

oped

. Thus, much of the advice given to therapists

new to this field is anecdotal and parochial

The objective of this chapter is to give practical

guidelines and necessary background for approach-

ing the problem in a systematic and rational way.

The primary role of a wheelchair cushion is to

provide an effective platform from which the user

may perform a wide range of tasks

. It is difficult to

work at a table or desk or to propel the wheelchair,

if the sling seat is the only support

. The wheelchair

cushion also improves comfort, aids posture, and

absorbs shock during propulsion over uneven sur-

faces

. For many, the cushion performs a crucial

function by reducing the concentration of pressure

in tissues, thereby helping to prevent the formation

of pressure sores.

Selecting a seat cushion should be considered an

integral part of the process of prescribing the correct

wheelchair

. Failure to recognize that the wheelchair

and the seat cushion should be prescribed at the

same time can waste human and material resources,

and result in a far-from-optimal seating system

. All

too often, the seat cushion (and other critical

"accessories") are prescribed as an afterthought;

choice is then restricted to accommodate constraints

introduced by the dimensions of the wheelchair.

Some wheelchair users with skeletal deformity

or very limited postural control, require seat inserts

that help control the position and stability of the

trunk and hips (36)

. Seat inserts frequently require

special components and shapes

. Recent develop-

ments using modular components are expanding on

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

the traditional fabrication techniques involving

plywood and foam construction

. Custom-shaped

shells can now be produced by

: 1) vacuum forming

of sheet plastics

; 2) resin-bonded vacuum consolida-

tion of glass or styrofoam beads

; 3) forming

shapable matrices of lockable plastic components;

and, 4) foam molding may be performed around a

user while he/she is held in an optimum sitting

position in the wheelchair

. In a few hours, these

methods can provide a finished custom-formed

postural support which intimately follows complex

curvatures and may also help to reduce localized

pressure (19).

Specialized seat inserts are normally prescribed

by seating specialists who have received comprehen-

sive training in clinical evaluation, the biomechanics

of sitting, and fabrication techniques

. However,

some of the basic principles used in prescribing

sophisticated seat inserts also apply to prescribing

wheelchair cushions

. A review of these principles

will certainly be useful.

The information in this chapter is organized

into three main categories:

General Principles

Evaluating the Client

Some Useful Techniques for

. Matching

Cushion with Client

with the addition of two short categories:

Wheelchair Cushion Fitting Clinics,

outlin-

ing what is needed to establish an effective

clinic

; and

E. CUSHFIT,

describing an expert system pro-

gram that guides the novice through the fitting

procedure.

. GENERAL PRINCIPLES

When we sit on a cushion, a number of

interacting factors determine whether it is comfort-

able, functional, and clinically safe

. Many of these

factors are interrelated

; some factors are attributed

to the properties of the cushion, and others due to

the characteristics and needs of the user.

Factors that affect comfort include:

poor distribution of stresses in soft tissues

moisture accumulation

heat accumulation and loss

compromised stability

Factors that determine functionality include:

stability provided

weight of the cushion

frictional properties of the cushion and

cover

cushion thickness

appearance

cost

durability and the need for user mainte-

nance.

Factors that determine clinical safety include:

poor distribution of stresses on soft tissues

stability provided

frictional properties of the cushion and

cover

moisture accumulation

heat accumulation and loss

durability and the need for user mainte-

nance

flammability.

POOR DISTRIBUTION OF STRESSES IN SOFT

TISSUES

tress is defined as force divided by the area over

which it acts

. Higher stresses occur if either the

force acting over a given area becomes larger or if a

given force acts over a smaller area

(Figures la and

ib).

Causes

Localized stresses in soft tissues are an inevita-

ble consequence of sitting

. Only when we are

suspended in water or are weightless in space, are

the stresses on our tissues evenly distributed

. When

our body weight is supported on a solid surface such

as a cushion, localized stresses are generated which

compress and deform the soft tissues

. The changes

in the shape of the soft tissues result in occlusion of

blood vessels and lymphatics, and stimulate nerve

endings which may signal discomfort to the central

nervous system.

The stresses can be divided into two groups:

those that act perpendicular to the skin, called

normal stresses (in everyday speech we use the term

pressure) and those that act parallel to the skin,

called shear stresses.

When a person is in the sitting position in a

wheelchair, the total force acting on the buttocks

FERGUSON-PELL

: Seat Cushion Selection

NORMAL

FORCE



Force F

Area



I x b

Normal stress =



Ixb

Figure la.

Normal stress

will be increased if F is made larger or if the area is made smaller

. Normal stress causes materials to compress

(compression) or expand (tension) without significant change in shape

Force F

Area



h x b

Shear



hxb

SHEAR

FORCE

Figure lb.

Shear stress

will be increased if F is made larger or if the area over which it acts is made smaller

. Shear stresses cause the shape of

materials to be changed without significant change in thickness.

and thighs is body weight minus the supporting forces

of the footrest and backrest

. The greatest propor-

tion of body weight, when sitting, is therefore sup-

ported by the area over the ischial tuberosities (ITs).

Bone resists even large forces with barely

perceptible deformation

. Muscle and fat are far

more vulnerable because they deform more readily,

causing blood vessels to be occluded

. The force due

to body weight, when sitting, is imposed by the ITs

on the gluteal muscles and fat, which become

deformed due to the compressive force

. In addition

to the force of the ITs and the amount of tissue

supporting them, tissue stiffness or tone also deter-

mines how much deformation (and therefore dam-

age potential) is developed in the fat and muscle.

Wheelchair users with little muscle tone in their

gluteal area are less able to resist tissue deformation,

and are therefore more vulnerable to discomfort and

tissue damage.

It is important to note that heavy individuals

are not necessarily more vulnerable to tissue break-

down (17)

. They normally have more fat and muscle

padding, which compensate for the additional body

weight that has to be supported

. At greater risk is an

emaciated person with flaccid gluteal tissues

. The

combination of inadequate padding and poor mus-

cle tone produces higher than average deformation

in the tissues that remain

. Those who have

lost

JRRD Clinical Supplement No

. 2

: Choosinga

Wheelchair System

Figure

Shaped cushions help share the supporting forces F, and F

more equally between the ischial tuberosities (F,) and the trochanters

)

. On cushions with no shaping F, is considerably larger than F

and the ITs have to compress the cushion nearly 2 inches before

the trochanters make contact.

wwio/



0000ff

."--"'

Convex base filler accommodates sling seat sag

Cushion

tissue overlying the ITs due to previous pressure

sores are also particularly vulnerable.

Prevention

Stresses in the soft tissues are not usually evenly

distributed, and are thought to be greatest in the

muscle close to the ITs, the area in which pressure

sores seem to start

. Necrosis migrates outward to the

skin as cell constituents are released and cause

damage to nearby compromised tissue.

If weightbearing can be shared equally by the

ITs and the nearby lateral posterior aspect of the

trochanters, the force at each IT is half of that when

only the ITs are loaded, and more soft tissues can

share the burden

. Weight distribution is, therefore,

an important principle used for designing cushions

that are shaped to encourage trochanteric and thigh

support

(Figure 2)

MOISTURE ACCUMULATION

oisture accumulation results in discomfort

and, in some cases, an increased risk of soft

tissue damage

. Many factors determine the causes

and prevention of moisture accumulation.

Causes

Generation of excessive quantities

heat.

Sweat is normally generated to assist in the

thermoregulation of the body by the evaporation of

moisture to cool the surface of the skin

. Normally,

sweating is suppressed locally by pressure

. Following

spinal cord injury, however, sweating can occur in

an uncontrolled manner independent of thermo-

regulation

. The reduction of pressure by use of

sophisticated cushion systems can, in effect, reduce

suppression of sweating due to pressure, and thus

FERGUSON-PELL

: Seat Cushion Selection

pose potential difficulties in keeping the skin dry

and free from damage due to maceration.

Poor exchange of air

If there is poor exchange

of air in the supported area and the supported area

is thermally insulated by the cushion, the interface

temperature can exceed 38 degrees Centigrade,

whereupon sweating increases rapidly with increas-

ing temperature.

Use of impermeable covers

Moisture accu-

mulation can also occur if materials in close con-

tact with the skin do not "breathe

." Vapor-

impermeable covers prevent moisture-laden air

from being diluted by drier air, thereby preventing

newly-formed sweat from evaporating or being

absorbed

. Furthermore, because sweat is not being

evaporated, natural environmental cooling cannot

occur, resulting in more heat build-up and more

sweating.

Measurement

Continuous humidity or moisture monitors

have not proved to be reliable in measuring the

microclimate between the skin and cushion

. A

simple moisture-sensing patch is available* that has

been used successfully in research projects (14,35).

We are not aware of any centers using humidity

sensors in routine clinical evaluation

: common

sense, direct client feedback, and experience seem to

be adequate.

Prevention

Methods for preventing moisture build-up in-

clude the use of cushion and cover materials that

encourage air exchange between the cushion and

skin

. Cushions with good heat dissipation character-

istics help to reduce moisture build-up, if they are

appropriately covered with absorbent materials like

sheepskin or jersey knit fabrics

. Wearing

cotton/flannel underwear also helps to reduce mois-

ture build-up.

Water vapor permeability, absorbency, heat

capacity, and thermal insulation of materials used in

cushions are readily measured in the laboratory, but

are rarely quoted by manufacturers

. As we become

more sophisticated and learn how to apply this

information, disclosure of such data should become

more routine.

*Humidial Model HHH-20-90, Humidian Corp. 465 Mt

. Vernon Ave

. Box 464, Colton, CA 92324

Some cushions naturally pump air that is

trapped in their structure when compressed (e

foams)

. The Jay cushion may now be ordered with

an Air-exchange cover that uses this principle

(Figure 3)

This effect can contribute to maintaining

comfortable moisture levels at the cushion/skin

interface, if the cushion is fitted with an air-

permeable cover

. Air exchange is an added incentive

to encourage "push-ups," even for those who are

not particularly at risk of developing pressure sores,

but who do experience discomfort due to moisture

accumulation.

HEAT ACCUMULATION AND LOSS

Causes

When a warm buttock makes contact with a

cold cushion, the buttock will cool down and the

cushion will warm up until the cushion-tissue inter-

face reaches an equilibrium temperature

. Some

cushions (e

., gels) feel colder than others (e

foam) because fluids and gels conduct heat more

rapidly than foams and air.

Another important thermal property of a cush-

ion is its heat capacity, or the amount of heat

required to raise its temperature one degree

. Heat

capacity varies with the mass of the cushion and the

type of material.

Measurement

In principle, heat accumulation can be mea-

sured using thermistors or thermocouples

. However,

several problems reduce practicability for routine

clinical evaluation

. As indicated in the studies of

Stewart

et al

(35) and Ferguson-Pell

et al

(14)

equilibrium of skin temperature requires sitting for

more than an hour.

It is still unclear how we might quantify heat

build-up for clinical interpretation

. We could mea-

sure the equilibrium temperature, but the transitory

coolness of a cushion that has not reached equilib-

rium may give considerable relief

. We could mea-

sure the area under the time-temperature plot for a

typical sitting period, but this requires a fairly

sophisticated data-logging system

. Similarly, we

could measure the time taken to reach a critical skin

temperature—38 degrees Centigrade—above which

sweating becomes increasingly profuse in able-

JRRD Clinical Supplement No

. 2

: Choosing

Wheelchair System

Figure 3.

"Standard" Jay cushion with cover removed.

bodied individuals

. Whichever method is used,

prolonged monitoring is required, preferably over a

range of environmental temperatures

. In practice, it

is sufficient to predict how different cushions

respond to heat build-up by knowing their thermal

properties as reported in the studies of Stewart

et al.

(35),

Ferguson-Pellet al

. (14),

and

Figure 4.

Prevention

Foams are poor conductors of heat and have a

low heat capacity

. A thin layer of foam (plus cover)

warms up to skin temperature when one sits on it,

but does not draw much heat from the body's

tissues

. In warm environments, or during physical

exercise, the body attempts to lose heat but is

prevented from doing so in the buttocks area due to

the insulating foam of the cushion

. This region may

therefore begin to heat, resulting in uncomfortable

dampness.

On the other hand, gels and fluid-filled cush-

ions are moderate conductors of heat and have a

high heat capacity

. They drain heat from the

buttocks and continue to do so for a long time

(Figure

. For energetic wheelchair users, the

thermal properties of the gel help to dissipate

metabolic heat

. Gel cushions are also beneficial in

warm environments if effective water vapor and

sweat removal can be assured

. However, in cold

environments or for weak individuals who have

difficulty in staying warm, these cushions make

additional demands on the metabolic system

. Fur-

thermore, gels and fluid-filled cushions are heavy

FERGUSON-PELL

: Seat Cushion Selection

Figure 4.

Changes in skin

temperature and moisture content of the

buttock/cushion interface for an able-bodied subject for 2

hours continuous sitting

. (Reproduced with permission, Oxford

University Press, 1986

(up to 35 pounds), which can reduce wheelchair

performance as well as present problems in stowing

the wheelchairs in the car

. Some cushions are

designed to obtain the best properties of both

materials by using a modular approach, with the gel

close to the skin, and foam for the bulk of the

cushion

. Because the mass of the gel is relatively

small, the heat capacity is smaller than a total gel

cushion

. It achieves less cooling with less metabolic

heat drain

. Studies completed by Stewart

et al

(35)

and Ferguson-Pell (14) discuss experiments on the

thermal properties of cushions in detail.

COMPROMISED STABILITY

he effect that the cushion has on stability, real

or perceived, can be critical—especially for users

with poor trunk control.

Causes

A number of mechanical properties of the

cushion influence the stability of the support sur-

face

. The ability of the cushion to deform, both in

direct compression (vertical loading) and in shear,

must be considered.

Highly compressible cushions offer very little

resistance to body weight and tend to function like

an overcompressed spring

. If one rides over a bump

in a wheelchair the "hardness" of the sling seat can

be felt through the cushion

. Overcompressed cush-

ions are said to "bottom out

At the other extreme, incompressible cushions

are ineffective in preventing pressure localization.

Because they are less compressed, the user often

feels as if he/she is sitting on a cushion that is too

thick

. In addition, the cushion may appear to be

moderately deformable in shear, and may feel

unstable during manual propulsion.

Cushions that fall between these two extremes

have advantages, such as improved pressure distri-

bution, but may also feel "springy." Compromises

between cushion stability and pressure distributing

capacity often have to be made in practice—usually

by accepting a small increase in the accepted

pressure over bony areas.

Material thickness can influence the sense of

instability, the cushion's compressibility, and its

pressure distributing properties.

Measurement

At present we are unaware of any routine meth-

od to measure trunk stability

. We rely upon direct

feedback from the user

. Persons with spinal lesions

at a high level often have acute awareness of their

trunk stability that is far more subtle and complex

than position measurement systems are able to detect.

In the laboratory, we test the effectiveness of a

material or cushion in absorbing impact energy by

measuring hysteresis, which is represented by the

area between the load-deformation curves for load-

ing and unloading conditions

(Figure 5)

Hysteresis

is normally expressed as a percentage, representing

the energy lost during the loading/unloading cycle.

Prevention

Deformability and cushion thickness alone do

not control the destabilization of the user

. Some

cushions have the capacity to absorb energy when

they are deformed

. Gels, fluid-filled cushions, some

foam cushions, and simple air pillows behave fairly

elastically when sat upon, and create a feeling of

instability during propulsion or when riding over

rough ground

. Because they are elastic, they tend to

Roho

Foam



Gel

-2-

Water

-4-



120

Time (min)

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

DEFORMATION

Figure 5.

Energy dissipation of cushion materials can be determined by

performing a simple mechanized test

. The area within the

load-deformation curve when the sample is loaded and then

unloaded provides a measure of energy dissipation

. The greater

the shaded area the more energy is dissipated by the cushion

transmit the energy of dynamic loading to the body

via the gluteal tissues for dissipation.

Viscoelastic materials (Temperfoam is an exam-

ple) have the capacity to absorb energy on impact,

like shock absorbers, which results in a real and

perceived sense of stability

. Viscoelastic materials

are time-dependent, their stiffness depending upon

the rate of loading

. Impact loads do not cause as

much cushion deformation as slowly applied loads,

and may therefore tend to produce high, localized,

instantaneous pressure

. Another consideration is

that because viscoelastic materials compress increas-

ingly over time, after an hour of sitting they may

bottom out, producing areas of localized pressure.

The deformation of these materials is also

dependent upon the temperature of the material.

When cold, they may feel very hard until softened

by body heat

. When very warm, viscoelastic foams

easily bottom out

. Caution should be exercised in

using these materials above or below the normal

room temperature range.

Some air-filled and water-filled cushions behave

viscously, too

. Roho cushions, for example, consist

of interconnected air bladders

(Figures 6a and 6b).

Energy dissipated:

Polyurethane

foam

Energy dissipated:

Temperfoam

Figure 6a.

Roho wheelchair cushions (upper "normal" and lower "low" profiles)

FERGUSON-PELL

: Seat Cushion Selection

Figure 6b.

Roho "high" profile cushion.

If a sudden localized force is applied, the air in the

loaded cells is forced into neighboring cells, thereby

absorbing the impact energy

. In practice, Roho

cushions provide far more stability than might be

expected, because rapid destabilizing movements are

damped by the cushion due to air flow resistance

between the cells.

WEIGHT OF THE CUSHIONS

eavy cushions are of particular concern to users

with limited upper limb strength, those who

transfer independently from chair to car, and

athletes who require minimal wheelchair weight for

maximum performance

. Gel or fluid cushions are

usually heaviest, weighing in some cases over 20

pounds (9 kg)

Table 1

is derived in part from Jay

(21) and lists the weights of some commercial

cushions that weigh more than 3 pounds.

FRICTIONAL PROPERTIES

rictional properties are particularly important

during transfers, as the user can easily fall if the

cushion slips away

. Yet, moderately low top surface

friction is helpful during transfers, for users whose

upper limbs are weak

. One solution to this problem

is to anchor the cushion with ties or Velcro.

Although ties are useful in maintaining the position

of the cushion during sitting, they are difficult if not

impossible for the user to install independently prior

to the transfer

. For clients who have to fold the

JRRD Clinical Supplement No

. 2

: Choosing

Wheelchair System

Table 1.

Weight of Selected Commercial Cushions*

Cushion Name

Description

Approx

. Weight

Modultex

Sculptured foam

.5 2

Vasiopara

Sculptured foam

6 .0 2 .8

+ sheepskin

10 .0 4 .5

Reston Flotation

Solid gel

.0 6

Spence Omega 5000

Solid gel

14 .0 6

Aberdeen

Thixotropic gel

14 .0 6

SML Comfort

Thixotropic gel

12 .0 5

Carter's Carterflow

Gel-foam

6 .0 3

Grosvenor Medical

Gel-foam

.0 4

Knoche Silicone Gel-foam

6 .0

Jay reg

. adult

Gel-foam

Seabird

Gel-foam

.0 3

Hydromedica

Water-foam

.0 9

Western Medical

Water-foam

Jobst Hydrofloat

Water-foam

25 .0

Ardo Water

.0 7

Lyco

Water

10 .0

Roho

Air

.0 2

Portalife

Foam-air mechanical

12 .0

Safe-Seat

Foam-air mechanical

12 .0

Healing Environment Alternating air pressure

*Derived from Jay P

Choosing the Best Wheelchair Cushion

Washington, D

: RESNA, 1984.

chair frequently, ties are not practicable and most



CUSHION THICKNESS

wheelchair users do not use them even when they are

supplied by the cushion manufacturer.

Mating strips of hook-and-loop fastener (e

Velcro) may be sewn to the underside of the cushion

cover and attached to the sling seat

. We have

experienced difficulty achieving a reliable long-term

bond between the fastener strip and the sling seat,

and when they fail, an ugly residue is left that is

difficult to remove.

Instead, we use three of the sling seat attach-

ment screws on each side of the frame for holding

the loop fastener strip to the frame

. A secondary

advantage of using hook-and-loop fasteners is that

the orientation of the cushion (top-bottom/front-

back) is automatically defined.

Jay (1984) suggests that friction can be increased

between the cushion and sling seat by using anti-skid

netting, as used with rugs

. Often, however, friction

alone is not sufficient to prevent the cushion from

being knocked out of place during the transfer

henever possible, the cushion and wheelchair

should be prescribed at the same time in or-

der that wheelchair dimensions accommodate the

thickness of the cushion

. It is important to recognize

that the cushion will be compressed in use when

determining the position of the user with respect to

the seat and back

. This compression is uneven

because of the localized distribution of pressure

owing to the weight and anatomy of the individual

(22, 31).

For users with critical requirements, the cushion

should be seen as an extension of his/her gluteal

padding and the wheelchair dimensions selected

in accordance with the prescribed cushion

. For

those with less critical needs, compromises can be

made in both cushion and chair specifications,

when they are evaluated together

. Correct sitting

height is crucial for optimal performance in the

wheelchair.

FERGUSON-PELL

: Seat Cushion Selection

APPEARANCE

or many people, appearance is quite important

and should not be overlooked

. Strategic use of

cover materials, colors, and designs can be em-

ployed to disguise the utilitarian appearance of the

"core" of the cushion.

COST

t should be remembered by prescriber, user, and

insurer that the cheapest cushion is not necessarily

the most economical

. Some cushions are less durable

than others, and for most wheelchair users, long-

term rather than crisis economics should be em-

ployed

. Many clinicians have expressed great con-

cern over the cost of some cushions, especially those

designed specifically for users vulnerable to pressure

sores

. In most cases, these costs are fully justified

when the complexities of manufacture, development

and distribution costs, and the relatively small size

of the market are considered

. According to a study

by Noble (29), the average number of inpatient days

required to heal an ischial pressure sore is 74 days.

With per diem rates in rehabilitation hospitals

exceeding $500, the expense of a correctly prescribed

cushion can be easily justified in economic terms

alone, since it is likely to significantly reduce the risk

of tissue breakdown.

DURABILITY AND NEED FOR USER

MAINTENANCE

aterials used in the construction of wheelchair

cushions vary greatly in their durability and

longevity, and some cushion types require user

maintenance.

All foam cushions deteriorate with time, even

when they are not used

. This is caused by increased

brittleness of the polymer matrix, leading to frac-

tures in the foam and softening of the cushion.

Nobel

et al

. (30),

found that storage over a even few

months caused changes in the properties of foam

cushions

Fatigue

Fatigue is a deterioration in the supportive

properties of the cushion due to prolonged use, and

is a function of the time in use and the magnitude

and number of stresses

. Most foam cushions do not

tolerate tension ("pulling") and shear ("tearing")

well

. Certain cushion designs, particularly those

incorporating cut-outs, can produce very high ten-

sion and shear in the foam matrix, requiring them to

be replaced more often than simple slabs of foam.

Exposure to moisture also shortens the life of

foam cushions

. Viscoelastic (Temperfoam) cushions

are particularly susceptible to moisture, fatigue, and

tearing

. Gels also age by developing hard or consol-

idated local regions

. Sometimes these regions can be

"kneaded" to break up the lumps and prolong the

life of the cushion

. Both prescriber and user should

be alerted to deterioration in gels, as these hard

regions can pose a risk to tissue integrity.

Maintenance

Some cushions require the user to actively

monitor and service them

. Roho and other air-filled

cushions require that the amount of air in the

cushion be at a specific level for that individual.

Alternating pressure (ripple) cushions incorpo-

rate a battery-powered air pump and valve manifold

to control the inflation of different panels or regions

of the cushion

. Maintenance for these cushions

centers on regularly charging the battery.

Cushions requiring maintenance to ensure prop-

er function should be prescribed only when the user

or caregiver is physically able and willing to do so.

THE COVER

he cover is an important component in the

cushion system

. Some of the contributions to

the performance made by the cover have already

been referred to

: moisture build-up, frictional

properties, and appearance

. Mechanically, the

cover can modify the stiffness of the cushion

core due to the "hammock effect" (7,15,34)

. Un-

less the cover is stretchable, it will not follow

the surface of the compressed cushion, causing

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

the cushion to be harder than without the cover and,

in turn, producing higher localized pressures over

bony areas.

Users who experience frequent incontinence

require some waterproofing to protect the core of

the cushion if it is an absorbent type (e

., foam).

Otherwise, frequent washing should be all that is

required

. (It should of course be noted that good

medical programs to manage incontinence are far

more preferable than waterproof covers

.) Most users

with only infrequent accidents may prefer to risk

wetting their cushion than to using a vinyl type

cover

. Simple foam cushions can be rinsed and spun

dry, although it should be remembered that this

shortens their useful life

.* Viscoelastic foams should

not get wet because they are irreparably damaged by

fluids

. Jay (21) points out that one of the difficulties

with waterproof, nonabsorbent covers is that should

an accident occur, puddling of urine on the cover

can be very embarrassing

. One approach is to use an

inner waterproof liner (e

., plastic waste-bin line)

wrapped around the core of the cushion which can

then be covered with a stretchable water absorbent

material.

Sheepskin covers are widely used and offer

many advantages

. They absorb moisture (there-

by reducing wetness due to sweating), contain

lanolin, are washable, and have intrinsic mechan-

ical properties that offer support and help to re-

duce pressure localization and shear. Lanolin,

when in direct contact with skin, provides addi-

tional protection from moisture and friction.

The appearance of sheepskin is highly acceptable to

most users and it can be dyed

. Denne (9,10)

undertook studies that describe in detail both

the pressure-distributing properties of sheepskin and

the changes that can occur following repeated

washings.

Brushing or combing the fleece helps to pre-

vent felting of the wool fibers. Jay (21) and Menec

et al

(27) discuss in depth the benefits of sheep-

skin, compare natural and synthetic fleeces, and

recommend methods for washing and caring for

them.

*Editor's Note

Washing a foam cushion will severely damage the

foam's integrity

. A soiled foam cushion can be `dry' wiped clean and

then dusted with cornstarch or crushed mothballs, or a mixture of the

two to remove dampness and odor

FLAMMABILITY

a noninstitutional setting, flammability stan-

dards for cushions and covers need be no more

stringent than for ordinary furniture and clothing,

unless the user is confused, or has severe disabilities

and is unable to get out of the chair unassisted

. The

greatest dangers arise in institutions where large

numbers of cushions and mattresses can present a

very serious fire hazard

. When burned, plastic

foams produce extremely poisonous gases that may

incapacitate even more rapidly than the fire they

support.

Smoking presents the greatest fire hazard to

foam cushions in the home

. Some covers can form

an effective barrier between a dropped cigarette and

the cushion, but may lose their fire retardant

properties with frequent washing.

Roho, air-filled, and fluid-filled cushions are

more resistant to burning

. Plastic foams that have

been treated with a fire retardant can be obtained.

"Scott Foams" produce a foam called "Pyrell" that

is particularly resistant to burning.

. EVALUATING THE CLIENT

Before attempting to fit a cushion, it is impor-

tant to identify the needs of the prospective user

. A

simple evaluation form may be helpful, ensuring

that key points are covered and the goals for the

user (and cushion) are understood and documented.

Techniques for achieving these goals come with

experience, but we hope with the help of expert

system computer programs such as CUSHFIT

(13,20), that we can make the process less special-

ized and more systematic in the future.

Many of the most difficult problems faced by

wheelchair prescribers are associated with attempts

to prevent pressure sores

. When evaluating users

vulnerable to pressure sores, it is important to

determine whether:

sensation is total, partial, or absent,

any clinical conditions exist, placing them at

higher than normal risk,

they regularly relieve tissue stresses by move-

ment,

they spend prolonged periods of time sitting

in their chair,

FERGUSON-PELL

: Seat Cushion Selection

they produce unacceptably high localized

stresses over bony prominences,

they are subject to heat and moisture accu-

mulation due to exertion, environmental

temperatures, and/or reflex sweating,

they have a good sitting posture,

they have good trunk stability,

an adverse skin condition exists.

Our approach to cushion fitting divides these

factors into two groups

. Type I are those that can be

modified by a cushion/seating system, and Type II,

those that are intrinsic to the user and place him/her

at greater risk for breakdown.

Type I factors include:

highly localized stresses

heat accumulation

moisture accumulation

poor sitting posture and positioning

poor trunk stability.

Type II factors include:

degree of sensation

regular relief of stresses by movement.

(Note

: some cushions accomplish weight

shifting by alternating air pressure in local

regions of the cushion

Establishing Maximum Allowable Interface

Pressures

Reswick and Rogers (32) developed guidelines

for maximum acceptable pressures at the

skin/cushion interface, associated with a range of

time intervals for which the pressure is applied

(Figure

. A number of important assumptions

underlie these guidelines and should be seriously

considered when implementing them clinically:

Their guidelines are intended for use with

spinal cord injured outpatients.

They are intended to be used by associating

the total sitting time with a maximum

pressure over ischial tuberositates

. It is as-

sumed that the client performs push-ups,

etc

., with a frequency typical of the average

spinal cord injured wheelchair user

. This

average frequency is not indicated nor is a

correction factor given for other frequencies.

Some users may be at a higher risk for

breakdown than others due to clinical fac-

tors (such as diabetes, presence of scar tissue

Hours continuous pressure

Figure

Acceptable pressure/time interval guideline

: Reswick and

Rogers (1976).

or persistent skin redness, recent spinal cord

injury) and reducing the maximum recom-

mended pressure may be indicated.

Differences in the tolerance of tissues overly-

ing different bony prominences (e

., coccyx

is much less tolerant than ischial

tuberositates) are not reflected in the guide-

lines.

Presence of other stresses—particularly shear

(coccygeal sitters) or repetitive loading (very

active user)—should be accommodated by

reducing the maximum recommended pres-

sure.

Users with a tendency for self-neglect, taking

physical risks, or whose family, social, or

career commitments are unusually demand-

ing, may tend to ignore or delay responding

to precursors or actual breakdown

. They

require a recommended pressure-time guide-

line which pulls them back further from the

threshold conditions for breakdown

. This

can be accomplished by taking rests from

sitting during the day, by really performing

push-ups, by reducing interface pressure—or

preferably all three.

For many years, there has been a debate in the

literature concerning maximum allowable pressures

[see for example: Kosiak (24)

; Cochran and Slater

(8)

; Edberg

et al

. (11)

;

Reswick and Rogers (32);

Shaded area represents

margin of uncertainty

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

Agris and Spira (1)

; Graebe (18); Noble (29)].

Discussion has centered on linking maximum al-

lowable interface pressures with physiological

parameters such as capillary perfusion pressure.

These arguments are fraught with difficulty

. For

example, there is a large pressure drop across

a capillary ranging from 30 + mmHg on the

arteriole side to nearly zero on the venule side.

Presumably, if sufficient pressure is applied to close

off the outlet (venule) side of the capillary, no blood

will flow.

I believe that interface pressure is only a rela-

tive indicator of the stresses and deformations

occurring deep in the tissue, and that we should not

attempt to link absolute values of interface pressure

to physiological processes

. Reswick and Rogers (32)

contribute positively to the clinical use of pressure

measurement, by at least emphasizing that in all

probability, there is no one maximum allowable

pressure to suit all individuals. They also expand the

clinical guidelines to include a second variable—

time

At Helen Hayes Hospital we have expanded this

concept to include other factors, and have estab-

lished a simple scoring system that provides a

maximum allowable pressure for each bony area at

risk (Table 2)

The basis for our scoring system is

purely pragmatic, based on extensive experience

with large numbers of spinal cord injured individu-

als

. Part of our current research is designed to form,

in collaboration with five other centers, a large

database that will determine how effective this

scoring system is, and whether additional factors

need to be included

Sitting Posture and Positioning

Scoliosis and pelvic obliquity cause uneven

pressure distribution over the ischial tuberositates

and greater trochanters, while rotation of the pelvis

will result in excessive sacrococcygeal pressure.

Methods for controlling these problems are dis-

cussed in depth by Zacharkow (37).

Wheelchair adjustments are critical during eval-

uation for the cushion, if the optimal posture and

positioning of the user are to be preserved

. If special

supports are required, they should be in place during

the evaluation

. The use of an adjustable evaluation

wheelchair, such as that described by Shapcott

et al.

(33),

may prove to be helpful in optimizing concur-

rent cushion and wheelchair prescription.

Armrests and footrests can affect cushion per-

formance dramatically

. Support for the arms can

reduce ischial pressures 25-35 percent (4)

. Footrests

should be set, if possible, so that the thighs are

horizontal and not bearing their own weight

. If the

footrests are too high, they transfer much of the

weight of the thighs to the ischial tuberositates

. If

they are too low, they can promote sacrococcygeal

sitting.

Bush (6) and Brattgard and Severinson (4) have

also demonstrated marked reductions in ischial

pressure (up to 25 percent) by reclining the backrest

angle

. Zacharkow (37) suggests that a seat angle of

10 degrees and backrest angle of 15 degrees are good

average values to assure good posture, stability,

comfort, and function

. Increasing the seat angle

excessively can transfer thigh weight to the ITs, and

increasing backrest angle can produce greater shear

stresses.

Table 2.

Recommended Maximum Acceptable Pressure

Ischial



Trochanters



Sacrum

Tuberosities

Coccyx

40 60

<20 <20

60 80 40 40

80 80 40 40

Note—These maximum recommended pressures assume a normal clinical status, push-ups performed every 15 minutes and no other factors that

would increase the risk of breakdown

. Prolonged sitting( > 10 hrs) and infrequent pressure relief are weighted by reducing each of the maximum

allowable pressures by 10 mmHg per factor

. If risk is already high, then sitting time and/or pressure relief frequency

must

be brought to normal

levels.

Level of risk

High—no sensation

; history of sores at site

of measurement

Moderate—no sensation

; no history of

tissue breakdown

Low—partial or full sensation

; no history

of tissue breakdown

FERGUSON-PELL

: Seat Cushion Selection

Sensation

In routine clinical evaluations, we normally rely

upon the client's description of residual sensation.

Pressure Relief by Movement and Total Sitting

Time

Pressure relief by movement and total sitting

time are factors at the technological threshold of

being directly monitored for routine evaluation.

Fisher and Patterson (16), Merbitz

et al

. (28),

and

Burn

et al

(5) are developing portable devices that

will provide information about pressure relief and

total sitting behavior

. Merbitz

et al

(28) and Fisher

and Patterson (16) have collected sequential data in

the hospital environment for up to 1 month

. Burn

(5) now have a device capable of measuring

pressure relief behavior and habitual sitting asym-

metry, for more than a month in the

home/workplace, without the need for battery re-

charging or data downloading

. Until these systems

complete preliminary development and clinical eval-

uation, we must rely upon the reports of the user.

Adverse Skin Condition

Adverse skin condition is monitored by direct

observation

. Intense activity continues in an attempt

to identify methods for detecting early signs of

tissue breakdown

. These on-going studies involve

measurements of

: skin pO

, laser Doppler assess-

ment of skin blood flow

; ultrasonic properties of

skin

; and local temperature using thermography or

liquid crystal film

. In the meantime, the following

scale is used for classifying skin condition associated

with the development of pressure sores (3):

Class 1

: skin discoloration

Class 2

: superficial pressure sore

Class 3

: destruction of skin—no cavity

Class 4

: destruction of skin—cavity.

We also recommend that the presence of scar-

ring be recorded

. An instant photograph of prob-

lematic skin conditions kept in the medical record, is

valuable for the review of progress and planning.

. SOME USEFUL TECHNIQUES FOR

MATCHING CUSHION WITH CLIENT

Devices for Measuring Interface Pressure

Devices for measuring the pressure (normal

stress) between the skin and the cushion can be used

to locate regions of localized pressure, and indicate

how they are distributed by a cushion

. It is

important to remember, however, that pressure

measurements made at the skin's surface are

thought to be much lower than those deep in the

tissues, where pressure sores initially develop

. For

users with sensation, subjective feedback of comfort

is far more desirable than rigid adherence to a

method involving maximum interface pressure.

Pressure sensors fall into the following catego-

ries

discrete sensor, single measurement

multiple sensor, single measurement

discrete sensor, continuous measurement

4) multiple sensor, continuous measurement.

Until recently, sensor types (1) and (2) have

been most widely used in cushion-fitting clinics.

Two discrete sensor systems providing single or

snapshot type measurements are currently available

commercially in the USA:

Type (1) Skin Pressure Evaluator (SPE)

(Figure 8)

Camp International Inc.

PO Box 89

Jackson, MI 49204

1-800-492-1088

[A fuller technical description of this device is given

by Reswick and Rogers (32)

One multiple sensor snapshot or single reading

device is commercially available:

Type (2) TIRR Pressure Evaluator Pad (PEP)

(Figure 9)

Tee Kay-Applied Technologies

11915 Meadowtrail Lane

Stafford, TX 77477

(713) 495-6838

[A fuller technical description of this device is given

by Krouskop

et al

. (25)

One suitable continuous reading device is com-

mercially available:

Type (3 and 4) Oxford Pressure Monitor

(OPM)

Camp International Inc.

PO Box 89

Jackson, MI 49204

1-800-492-1088

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

Figure 8.

Skin Pressure Evaluator.

Appropriate configuring of the OPM can pro-

vide single sensor continuous measurement (type 3)

or multiple sensor (type 4) capability.

The SPE consists of a 4-inch diameter polyeth-

ylene sack lined on its inner surface with an

electrically conductive grid

. It is inflated manually

using a sphygmomanometer which also indicates the

pressure inside the sack

. The grid functions as a

switch which controls a small light on the sphygmo-

manometer

. When the pressure inside the sack

exceeds the pressure applied by the body, the two

grids are separated and the light is extinguished

. As

air pressure is reduced using the bleed screw on the

sphygmomanometer, a critical point is reached

where the light is suddenly illuminated

. Bleeding of

air from the sack is stopped by the operator and the

air pressure measured indicating the maximum

(peak) pressure over the bony area

. In fact, the air

sack does provide some support for the tissues

surrounding the center of the pressure distribution.

This results in readings that are somewhat lower

than the true peak pressure

. The size of this error

depends upon how localized the peak pressure is

(14)

. In effect, this means that unacceptably high

pressures are even higher than measured, whereas

"acceptable" pressures may be marginally greater

than indicated.

The PEP pressure monitor consists of an array

of 144 discrete pressure sensors approximately 1

.25

inches between centers

. A similar principle of

operation is used in this device in that the sensor

matrix is formed from a flexible sheet of plastic,

each divided from the next using a welded seam

. All

the air cells are, however, interconnected and are

inflated from a single air pressure source

. Each cell

FERGUSON-PELL

: Seat Cushion Selection

Figure 9.

TIRR Pressure Evaluation Pad.

is coated with a ribbon of electrically conductive

paint that functions in a similar way to the metal

grid of the SPE

. The commercially produced system

is designed to provide a readout using an array of

lights

. Several research centers have connected the

matrix of sensors to a personal computer and have

developed software that can plot pressure contours

directly using a printer

. The matrix is inflated to

80-90 mm Hg at which point it is assumed that the

lights on the display array will be extinguished

. If

the local pressures are higher than the maximum

inflation pressure, cells that are "on" are recorded

as ">90 mm Hg

." The pressure in the matrix is

then reduced using a bleed screw in increments of

5-10 mm Hg and the pattern of lights are recorded

at each increment

. This system, therefore, provides

a direct contour map of the interface pressures

. Its

disadvantages include relatively high cost, a maxi-

mum pressure range limited to about 90 mm Hg,

and only moderate durability of the sensor matrix

(which is subject to replacements due to fatigue in

the conductive paint).

The OPM consists of a 3 x 4 matrix of sensors,

again incorporating air cells sealed into a flexible

plastic sheet

. This design does not, however, utilize

electrical contacts and the sensors are therefore

more robust than the SPE or PEP

. The control unit

that provides the inflation source for the cells and

for a readout display automatically scans the sen-

sors, rapidly reading the pressure at each sensor

every 2 to 3 seconds

. The device employs the

following principle

. When a cell is collapsed due to

the pressure applied by the body, the flow rate of air

to the cell is very small

. Suddenly, when the air

pressure in the cell is equal to the applied pressure,

the cell inflates and the flow rate of air increases

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

rapidly

. By monitoring both the inflation pressure

and the flow rate of air to the cell during the

inflation cycle, a "knee" in the pressure versus the

flow rate curve can be detected and the pressure

displayed

. The output can be displayed using a serial

interface to a personal computer or by using the

small printer provided to produce a hard copy

record.

Measuring Interface Pressure

The number and location of the sites for

pressure measurement should be guided by clinical

judgment

. Most often, pressures are measured un-

der the Ischia, trochanters, and coccyx, and fre-

quently, one or two sites are of particular concern.

To save time, usually only the pressure at problem

areas is measured until a cushion has been identified

that provides adequate readings at these sites

. We

then check the remaining areas of interest, which

will invariably be satisfactory

. Two exceptions exist,

scoliosis is present

. Relieving the high-pres-

sure side may well overload the opposite side

. Both

should be measured throughout the evaluation.

the user is a coccygeal sitter

. Relieving

sacrococcygeal pressure by reducing posterior pelvic

tilt can also result in a substantial increase in ischial

pressure

. The sacrococcyeal region and at least one

IT should be monitored throughout the evaluation.

All sites clinically determined to be vulnerable

to breakdown must be measured, and the results

documented, prior to issuing the cushion.

The air cell may be inserted from either side of

the wheelchair, or from behind. The transducers are

located by palpation under bony prominences

. It is

often helpful to remove screws from the lower-back

upholstery and bend it up

. This is particularly

helpful in checking the fit and pressures on cut-out

cushions.

To measure pressure on each cushion:

ask or assist the user to lift his/her buttocks

off the cushion;

palpate the bony prominence and place the

pressure sensor pad under the center of this area;

hold it in place as the user resumes a relaxed sitting

position;

remove your hand without disrupting the

placement of the pressure sensor pad (sometimes a

little talcum powder for your hands will make this a

little easier);

4)check the user's posture;

close the valve and inflate the sensor pad

until the indicator lamp goes off

; slowly release the

air and record the pressure reading at the moment

the lights go on;

repeat the process of palpation, placement

and measurement.

Usually, two readings suffice for each site

. If

successive readings are significantly different (>5

mm Hg), check placement and the user's posture,

and take readings until consistent values are ob-

tained

. Occasionally, the indicator lamp may fail to

go out, even if inflated to high pressures, because of

a fold in the sensor pad or possibly because it has a

damaged cell

. If this happens, reposition the sensor

pad and repeat the measurement

. If this does not

resolve the problem after two or three attempts,

inspect the metal grid of the cell for damage

(delamination)

. When using viscoelastic foams

(Temperfoams) wait 5-10 minutes after placement of

the air cell before taking your set of readings to

ensure that the material has conformed to the shape

of the client.

When evaluating cushions of different thick-

nesses, it is important to remember that the

footplates, armrests, and/or other wheelchair com-

ponents must be readjusted for each cushion.

Location of Bony Prominences

Location of bony prominences should be deter-

mined when fitting cushions which have been, or

will be, custom-formed to provide local relief of

bony areas (cut-out cushions

; ideally commercially

preformed contoured cushions such as the

Vasiopara and the Jay,

Figure 10)

It is wise to

accurately measure the location relative to the

wheelchair, and the separation of the two ITs, and

possibly the coccyx.

A number of techniques can be employed:

Palpation

With the user side-lying, hips and

knees flexed to 90 degrees, with a 90-degree thigh to

trunk angle, the ischial tuberositates are palpated.

The distance is measured from the most prominent

part of one tuberosity to the corresponding point on

the other. This technique is extremely simple and

requires no equipment, but unlike the methods

FERGUSON-PELL

: Seat Cushion Selection

Figure 10.

Jay "Combi Posture Seat

described below it does not provide an indication of

the location of the ITs relative to the seat and back

of the wheelchair, and hence their location on the

cushion to be prescribed.

Carbonized paper

Carbonized paper can be

used to produce an imprint of bony areas

. One of

the difficulties with this technique can be the

smearing of the imprint in the process of achieving

the normal sitting position.

A simple way to reduce this problem involves

constructing an air pillow from vinyl or other heat

sealable plastic and fitting it with a suitable inflation

tube and valve. A sheet of the carbonized paper (we

use Shutrak gait analysis paper*) is placed on a firm

surface in the chair (seatboard plus 1 inch foam

*Notecare Inc

., 2435 Professional Dr

., Suite C, Santa Rosa, CA 95401

works well), and the air pillow is then placed on top

and inflated sufficiently to support the user's weight

without bottoming out

. Thus, no localized pressure

is transmitted to the carbonized paper

. It is desirable

to make the impression with the user sitting at the

normal sitting height in the chair, with armrests and

footrests adjusted appropriately.

With the user correctly positioned, the air is

released from the pillow and an imprint is made.

The pillow is then reinflated using a hand-pump and

the person carefully transferred. The measurements

of inter-IT distance and location of bony areas

relative to the chair can be made, and if required, a

hard copy kept for the record.

Bead-bag

This technique is used to make

temporary "casts" of users for molding custom

postural seating systems

. It can also be used to

obtain an impression of the gluteal region. The areas

of deepest impression correspond to the bony

prominences

. This technique works best with indi-

viduals who are not "well-padded

A thin, flexible sack preferably made from

latex, is filled with enough glass or Styrofoam beads

to produce a layer 1-inch thick when the bag is

flattened

. The user is asked to sit on the sack, again

on a firm surface, and the air is then evacuated

using a vacuum pump

. This leaves a hard shell-like

impression of the gluteal region, and the deep

indentations of the ITs can be identified.

Barograph

This device is described in full by

Mayo-Smith and Cochran (26)

. A Plexiglas plate,

which is illuminated from the side with fluorescent

lamps, supports a rubber sheet which has been

molded to form conical projections on one surface.

When viewed from below, cones which have been

compressed reflect more brightly than the

uncompressed cones

. When sat upon, areas of

highest pressure cause greatest compression of the

cones, and the result is an image with a bright region

corresponding to the bony areas

. It produces a

dynamic image indicating the effects caused by

changes in posture, reduced pelvic obliquity, rota-

tion, and other factors.

TIRR PEP Pressure evaluator

One of the

attributes of this sensor is its capacity to produce a

direct pressure map of the gluteal region

. Bony areas

can therefore be identified easily as areas of peak

pressure, and their location measured

JRRD Clinical Supplement No

. 2

Choosing aWheelchair System

Removing Sling-Seat Sag

We frequently encounter wheelchairs with ex-

cessive sling seat sag, and often, it is severe enough

to justify replacing the seat canvas

. The following

techniques are helpful in eliminating the conse-

quences of sag

: knee adduction, high trochanteric

pressures, reduced stability, and discomfort.

One technique that has been used for many

years is to place a plywood board across the seat

frame or use drop-hooks to suspend it from the

frame

. Thin boards can also be placed in the base of

the cushion, but they have to be manageable and

they tend to flex

. These methods are effective, but

they can make independent transfers more difficult

when the chair has to be folded

. Also, bases that are

screwed to the frame of the chair tend to affect the

ride

. Wheelchairs need to flex when traveling over

uneven terrain because, unlike automobiles, they are

not routinely fitted with a suspension system.

Our solution to this problem has been to form a

base for the cushion from Plastazote or Ethafoam.

The base is cut to a convex shape on its lower

surface, so that it fills the concave space generated

by the sling-seat

. The base can be attached to the

cushion and placed within the cover

. As an integral

part of the cushion, it is lightweight and doesn't

interfere with transferring

The Cut-Out Cushion

The principle of load distribution has already

been discussed

(Figure

. Cut-out cushions help by

redistributing the body weight normally borne by

the ITs, more equally with the posterior aspect of

the greater trochanters

. The concept was first

introduced by Reswick and Rogers (32) and has been

discussed in depth by Ferguson-Pell et al

. (12,13),

Key and Manley (23), and Zacharkow (38).

A rectangular region from a slab of firm

polyurethane foam (density 3

.25-3

.75 pounds/cubic

feet

; indentation load deflection (ILO) (25 percent)

60-80 pounds) is removed with an electric carving

knife

. The size of the cut-out is determined precisely

for each individual, based on the measured location

and separation of the ITs

. A margin of 1

.25 inches

lateral and 1

.5 inches anterior to the center of each

IT is allowed, making the cut-out slightly larger in

width and depth than the inter-ischial and seatback-

*Alimed, 297 Hyde St

., Dedham, MA 02026 (800-966-4262), supplies

these bases, preformed for easy use

to-ischia distances

. Persons with a natural tendency

to sit obliquely, or those who sit slightly off-center,

should be accommodated with a correspondingly

shaped cut-out, if correction is not indicated

. The

depth of the cut-out should not exceed half the

thickness of the cushion

. Cushions should not be

less than 3 inches thick and at least 4 inches is

preferred

. To reduce shear at the margin of the

cut-out, a chamfer 1 inch deep at 30 degrees can be

cut along the periphery.

Variations of this design include using low

density foam fillers and cut-outs on the under-

surface of the foam instead of the top

. Undercut-

outs can also be filled with low density foam

. We

recommend that an Ethafoam base be used with all

cut-out cushions so that the cushion does not

"fold" when placed in a sagging sling seat

. The

Ethafoam base itself could be cut, in addition to, or

instead of the cushion, allowing for more foam

between the buttocks and base while benefiting from

the modified shape, without gaining additional total

thickness.

This approach requires experience to use effec-

tively, but provides fine control of the relative

loading of the trochanters and ITs

. Cut-outs can

also be used to provide sacrococcygeal sitters with-

out independently controlling anterior pelvic rota-

tion

. Zacharkow (37) suggests that this can often be

achieved using a lumbar pad and a 95-degree

thigh-to-trunk angle.

When using cut-out cushions it should be noted

that

: 1) a flexible cover should always be used with

cut-out cushions

; 2) a firm base is always required to

prevent inwards folding of cut-out dimensions due

to sling seat sag

; 3) the overall dimensions of the

cushion should be cut precisely, to ensure that the

cushion is located securely in the same place each

time

. Velcro attachment as described above is

recommended

; 4) dimensions of the cut-out must be

measured for each individual

; 5) the durability of

cut-out cushions is not as good as plain foam

cushions

. They should be replaced on the average of

every 6 months

. Thus, this type of cushion should

only be prescribed when the means for reliable

follow-up are present, and the user can be relied

upon to meet scheduled appointments

; and, 6) both

trochanteric and ischial pressures must always be

measured when evaluating cut-out cushions, because

it is crucial that goal pressures for both sites are

met

. Where possible, an accepted pressure a little

FERGUSON-PELL

: Seat Cushion Selection

lower than the goal pressure should be attempted, in

order to allow for changes in the foam in the first

few weeks of use.

Despite these constraints, we find that cut-out

cushions are extremely useful for users with pro-

nounced tissue wasting in the gluteal region, when

more straightforward cushion systems fail to reduce

interface pressures sufficiently, or are not applicable

for other reasons.

The Modular Cushion

The modular cushion is designed to assist those

at low-risk for pressure sores whose needs cannot be

met by the simple foam cushion

. The modular

cushion concept (15) consists of up to four layers of

component materials

. Each layer is selected to

contribute to the overall properties of the cushion

A wide range of commercially produced materials is

available

. The system makes use of combinations of

three classes of materials ordinarily used alone:

foams, temperfoams, and gels

. The construction of

a typical cushion is described below and illustrated

Figure 11.

COVER:

Purpose

: to provide a cover that is adequate for

carrying, as well as contacting body surface.

Support Segment (bottom, sides, back)

: a

smooth, strong, light, flexible, easily cleaned mate-

rial that is adequate to support and transport the

cushion.

Body Contact Segment

: a durable, porous,

two-way stretch, absorbent, easily cleaned, cotton/

polyester jersey-type fabric.

Figure 11.

Schematic of a modular cushion showing

: A) body contact segment of cover

; B) support segment of cover

; C) top layer of support

system

; D) middle and bottom layers

; and, E) convex base

. (Reprinted from Ferguson-Pell

et al

., J Rehabil Res Dev 23(3):65,

1986

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

TOP LAYER:

Purpose

: to encourage air circulation and re-

duce sitting pressure

. (This layer may be omitted

when heat dissipaters such as gels are used for the

middle layer

.) A soft open-cell or reticulated foam;

thickness 0

.5-1

.0 inches is recommended

. If pressure

relief is a critical factor, a denser, easily washable or

disposable material may be selected instead

. (Note:

if necessary for the protection from fluids, the

underlying middle and bottom layers may be en-

closed in a thin, loose, waterproof plastic bag

MIDDLE LAYER:

Purpose

: to reduce pressure and shear forces,

and control heat dissipation

. The qualities of this

layer may be selected to suit the most urgent needs

of the user

. Various medium or high density foams

and gel components

; 1

.0-2

.0 inches thick can be

used

BOTTOM LAYER:

Purpose

: similar to middle layer, but this layer

is selected primarily to augment the favorable

qualities of the middle layer, and offset any unfa-

vorable qualities

. Viscoelastic foams may be used to

improve conformance and reduce pressures

; firm

foams may be used to counter a tendency to bottom

out

BASE:

An Ethafoam base, as described above, is

required to reduce the effects of sling seat sag.

. WHEELCHAIR CUSHION FITTING CLINICS

Reswick and Rogers (32) established the first

wheelchair cushion fitting clinic at Rancho Los

Amigos Hospital

. Their clinic focused primarily on

the needs of spinal cord injured patients at risk for

developing pressure sores

. Through this clinic

evolved the design and application of the SPE

pressure evaluator and the cut-out cushion.

The clinic was established, not only to prescribe

wheelchair cushions but also to provide a focus for

developing and reinforcing self-help methods of

pressure sore prevention, to monitor skin condition

on a regular basis, and to provide ongoing educa-

tion

Ferguson-Pell

et al

. (12),

Key and Manley (23),

Noble (29), Krouskop

et al

. (25),

and others devel-

oped similar clinics around the world, with many

local variations in techniques and materials

. Each

clinic claims marked reductions in pressure sore

incidence

. Ferguson-Pell

et al

(1985a), monitoring a

spinal cord injured population, noted a reduction in

admission for treatment of ischial pressure sores

from 12 percent to 5 percent following adoption of

the cushion fitting clinic concept

. Similar successes

were reported at the other clinics.

The following components are required to

establish an effective cushion fitting clinic:

Human resources

Most clinics operate within a

team approach framework

. Although details of

fitting the cushion may have to be resolved by one

or two cushion-fitting specialists, it is important that

the rehabilitation team be involved in setting goals.

In general, physical and occupational therapists

have the primary skills and training required to be

responsible for the clinic, and to work closely with

the physician.

It is important to assign one or two therapists

to be responsible for the clinic, and to develop an

experience-base which will be vital in solving more

difficult cases

. This leaves the clinic vulnerable,

because should key personnel leave, the skill-base is

lost and the clinic will take several steps backwards

while the new staff develops experience

. These are

the dangers of specialization. But the alternative—

rotating responsibility for the clinic—can lead to

communication problems, lack of continuity for the

client, and a reduced skill-base

. CUSHFIT (see

following section in this chapter) has been designed

to reduce the problems of specialization

. It is

recommended that the clinic be open at regular

times each week (half-day blocks will be needed) and

that new evaluations, which take longer, should be

separated from follow-ups.

Material resources

Instruments for measuring

interface pressure and the location of the ITs will be

required

. In addition, an ample supply of various

densities of foam and frequently prescribed com-

mercial cushions should be available

. If the cushion

prescribed is commercially produced, an order

should be placed in the usual way, keeping the

evaluation stock intact

. Custom-made cushions can

often be made on the spot and fitted with one of a

number of standard covers

. Temporary foam cush-

ions may also be supplied while waiting for the

FERGUSON-PEEL

: Seat Cushion Selection

commercial cushion to be delivered or a special

fabrication to be completed.

Documentation

It is crucial that detailed

records be kept for each evaluation

. Results from

previous evaluations can save many hours of fol-

low-up by avoiding duplication of effort

. Documen-

tation should include

: 1) summarizing the clinical

evaluation of the user

; 2) setting goals that are

acceptable to the team and the user

; 3) recording the

observations and measurements made during the

evaluation

; and, 4) reporting use protocols, fol-

low-up frequency, precautions, etc.

Scheduling

The clinic should have the means to

reliably follow clients on a long-term basis. This

may have to be a rather paternalistic process, but

necessary since clients may be using cushions which

deteriorate rapidly

. Their awareness of the need to

follow-up must be reinforced continuously, backed

up by telephone calls and reminder cards

solution for a minimum number of different cush-

ions evaluated (critical path approach).

CUSHFIT offers a number of benefits:

it automatically documents, stores on disk,

and prints in approved medical format all

observations made during the evaluation;

it ensures that consistent goals and protocols

are maintained in the clinic, thereby provid-

ing quality assurance;

automatically develops a database for the

clinic which may be used for administrative

and research purposes;

reduces the need for specialist therapists to

operate the clinic and reduces the impact of

staff turnover;

incorporates a scheduling program to auto-

matically keep track of clients by generating

form letters and updating central ambula-

tory care records.

. CUSHFIT

CUSHFIT is an expert system computer pro-

gram that has been developed by the Orthopaedic

Engineering and Research Center of the Helen

Hayes Hospital (13,20)

. Recognizing the problems

of therapist specialization discussed above,

CUSHFIT has been designed to guide the less

specialized therapist through the fitting procedure in

an effective and efficient manner.

The program operates on the IBM PC and XT

personal computers equipped with 256K resident

memory, two disk drives, color graphics display,

and a printer.

Initially, basic clinical and administrative infor-

mation is collected

. Observations of skin status at

each bony site, and other factors, are used to weigh

the prospective user's level of risk for developing

pressure sores

. Maximum acceptable pressures (goal

pressures) are then established, as described in

Table

. At all times, the therapist has full control of the

program and is able to interactively modify both the

goals recommended by the program, and cushions

that it specifies.

Pressure readings are obtained on two reference

cushions (medium and firm foam) and are used by

the program to establish a sequence of cushions,

commercial and custom, to be evaluated

. The

sequence has been designed to reach a satisfactory

CONCLUSION

hen selecting a seat cushion, basic physical

and clinical principles should be applied to

ensure that the best choice is made

. Unless there is

no alternative, the seat cushion should always be

prescribed simultaneously with the wheelchair

. This

ensures that we do not have to compromise our

goals unilaterally (to either the cushion or chair,

whichever happens to be prescribed last) and that

dimensional adjustments can be made to accommo-

date ergonomic and functional needs.

It should be recognized that seat cushion

selection is not a simple process—no single cushion,

at present, meets the needs of all users

. The number

of parameters which have to be controlled is

considerable, and unless we adopt a highly struc-

tured approach to cushion selection, one of two

frequently encountered problems can occur

. We

could limit our choice of candidate cushions to a

small, manageable number with consequent compro-

mises in the final result

. Alternatively, we could try

to juggle all the factors at once, and become so

confused by options that the "best fit" would be

made by chance rather than design.

This article has discussed ways of keeping our

options open while maintaining a professionally

based strategy for selection

. Pivotal to the process is

an understanding of the properties of cushions and

JRRD Clinical Supplement No

. 2

: Choosing a Wheelchair System

the principles of correct seating and positioning.

Success has been catalyzed by the availability and

correct use of measurement tools that provide both

quantitative and qualitative feedback of how the

client and the cushion interact.

ACKNOWLEDGMENT

Drawings for

Figures 1,

2, 4,

7, were done by

Samuel McFarland

. Photographs for

Figures

8, 9,

and

are printed with permission of the

manufacturers.

The author acknowledges that many aspects of

the information in this article were developed from

programs begun in 1970 by Dr

. G

. Cochran at

the Helen Hayes Hospital and supported by the

Veterans Administration and the Paralyzed Veterans

of America.

REFERENCES

Agris J, Spira M

Pressure ulcers

: Prevention and

treatment

Clin Symp

31(5)

:2-8, 1979.

Bader DL, Gwillim J, Newson TP, Harris D

Pressure

measurement at the patient support interface

. In:

Biomechanical Measurement in Orthopaedic Practice,

Whittle M, Harris D (Eds

.), 145-150

. Oxford

: Oxford

University Press, 1985.

Barbenel JC, Jordan MM, Nicol SM, Clark MO

Inci-

dence of pressure sores in Greater Glasgow Health Board

Area

Lancet

:548-550, 1983.

4. Brattgard S-0, Severinsson K

Investigations of pressure,

temperature and humidity in the sitting area in a

wheelchair

. In

Biomechanics

VI-B

International Series

on Biomechanics,

Asmussen E, Jorgensen K (Eds

.),

270-273

. Baltimore

: University Park Press, 1978.

Burn TG, Ferguson-Pell MW, Hurwitz DE

Pressure

relief monitoring device

. In

Proceedings of the 8th

Annual Conference on Rehabilitation Technology 5

:42-

. Washington, DC

: Association for the Advancement of

Rehabilitation Technology, 1985.

Bush CA

Study of pressure on skin under ischial

tuberositates and thighs during sitting

Arch Phys Med

Rehabil

:202-213, 1969.

Chow WW, Juvinall RC, Cockrell JL

Effects and

characteristics of cushion covering membranes

. In

Bed-

sore Biomechanics,

Kenedi RM, Cowden JM, Scales JT

(Eds

.), 95-102

. Baltimore

: University Park Press, 1976.

Cochran GVB, Slater G

Experimental evaluation of

wheelchair cushions

: Report of a pilot study

Bull

Prosthet Res

BPR10-20

:29-61, 1973

Denne WA

An objective assessment of the sheepskin

used for decubitus prophylaxes

Rheum Rehabil

:23,

1979.

10.

Denne WA

Fleece mechanics

Care, Science and Practice

2(4)

:14-15, 1983.

11.

Edberg LE, Cerny K, Stauffer ES

Prevention and treat-

ment of pressure sores

Phys Ther

53(3)

:246-252, 1973.

12.

Ferguson-Pell MW, Wilkie IC, Reswick JB, Barbenel JC:

Pressure sore prevention for the wheelchair-bound spinal

cord injured patient

Paraplegia

:42-51, 1983.

13.

Ferguson-Pell MW

Establishing a pressure sore preven-

tion service

. In

Chronic Ulcers of the Skin,

Lee B (Ed

.),

203-212

. New York

: McGraw-Hill, 1985a.

14.

Ferguson-Pell MW, Reddy NP, Stewart SFC, Palmieri V,

Cochran GVB

Measurement of physical parameters at

the patient support interface

. In

Biomechanical Measure-

ment in Orthopaedic Practice,

Whittle M, Harris D

(Eds

.), 133-144

. Oxford

: Oxford University Press, 1985b.

15.

Ferguson-Pell MW, Cochran GVB, Palmieri V, Brunski

Development of a modular wheelchair cushion for

spinal cord injured persons

J Rehabil Res Dev

23(3)

:63-

76, 1986.

16.

Fisher SV, Patterson R

Long-term pressure recordings

under the ischial tuberositates of tetraplegics

Paraplegia

:99-106, 1983.

17.

Garber SL, Krouskop TA

Body build and its relationship

to pressure distribution in the seated wheelchair patient.

Arch Phys Med Rehabil

:17-20, 1982.

18.

Graebe RH

Static forces-cushion

Care, Science and

Practice

1(1)

:50-53, 1981.

19.

Hobson DA, Nwaobi OM

The relationship between

posture and ischial pressure for the high risk population.

Proceedings of the 8th Annual Conference of Rehabil-

itation Technology

:338-340

. Washington DC: Associa-

tion for the Advancement of Rehabilitation Technology,

1985.

20.

Hurwitz DE, Ferguson-Pell MW, Burn TG, Cardi M

integrated approach to pressure sore prevention for the

spinal cord injured

. In

Proceedings of the 8th Annual

Conference on Rehabilitation Technology

:142-144.

Washington DC

: Association for the Advancement of

Rehabilitation Technology, 1985.

21.

Jay P

Choosing the best wheelchair cushion

. Distr.

RESNA Publications Service, Suite 700, 1101 Connecticut

Ave NW, Washington DC 20036, 1984.

22.

Kadaba MP, Ferguson-Pell MW, Palmieri VR, Cochran

GVB

Ultrasonic mapping of the buttock-cushion inter-

face contour

Arch Phys Med Rehabil

:467-469, 1984.

23.

Key AG, Manley MT

Pressure redistribution in wheel-

chair cushion for paraplegics

: Its application and evalua-

tion

Paraplegia

:403-412, 1978-79.

24.

Kosiak M

Etiology and pathology of ischemic ulcers.

Arch Phys Med Rehabil 40

:62-69,

1959.

25.

Krouskop TA, Noble PC, Garber SL, Spencer WA

The

effectiveness of preventive management in reducing the

occurrence of pressure sores

J Rehabil Res Dev

20(1)

:74-

83, 1983

FERGUSON-PELT

: Seat Cushion Selection

26.

Mayo-Smith W, Cochran GVB

Wheelchair cushion mod-

ifications

: Device for locating high pressure regions

Arch

Phys Med Rehabil

:135-136, 1981.

27.

Menec JM, King

MW,

Leach SJ, Shannon EE

Designing

polyester fleece decubitus pads

. In

Proceedings, 2nd

International Conference on Rehabilitation Engineering,

Ottawa, 465-466, 1984.

28.

Merbitz CT, King RB, Bleiberg J

Continuous direct

recording of wheelchair pressure relief behavior

Arch

Phys Med Rehabil

64(abstr), 1983.

29.

Noble

: The prevention of pressure sores in persons

with spinal cord injuries

. In

International Exchange of

Information in Rehabilitation

New York

: World Rehabil-

itation Fund Inc

., 1981.

30.

Noble PC, Goode B, Krouskop TA, Crisp B

Influence of

environmental aging upon the loadbearing properties

of polyurethane foam

J Rehabil Res Dev

22(3) :31-38,

1984.

31.

Reger SI, Chung KC, Martin G, Mclaurin CA

Shape

and pressure distribution on wheelchair cushions

. In

Proceedings of the 8th Annual Conference on Rehabil-

itation Technology,

:341-343

. Washington DC

: Associa-

tion for the Advancement of Rehabilitation Technology,

1985.

32.

Reswick JB, Rogers JE

Experience at Rancho Los

Amigos Hospital with devices and techniques to prevent

pressure sores

. In

Bedsore Biomechanics,

Kenedi RM,

Cowden JM, Scales JT (Eds

.), 301-310

. Baltimore

: Uni-

versity Park Press, 1976.

33.

Shapcott N, Heinrich C, Brubaker C, Ferguson-Pell, M:

A methodology to assist in the prescription of wheel-

chairs

. In

Proceedings of the 9th Annual Conference on

Rehabilitation Technology

:378-380

. Washington DC:

Association for the Advancement of Rehabilitation Tech-

nology, 1986.

34.

Small CF

Beds

. PhD diss

., University of Strathclyde,

Glasgow, Scotland, 1977.

35.

Stewart SFC, Palmieri V, Cochran GVB

Wheelchair

cushion effect on skin temperature, heat flux and relative

humidity

Arch Phys Med Rehabil

:229-232, 1980.

36.

Trefler E

Seating for children with cerebral palsy

: A

resource manual

. Washington DC, 1984.

37.

Zacharkow D

Wheelchair Posture and Pressure Sores.

Springfield, IL

: CC Thomas, 1984

Novel portable device to analyze the moisture permeability of car seat

Article

Jul 2018

The comfort performance of car seat is important factor while producing car seats, each layer of the car seat is tested separately on classical testing machines, which lacks the real car seat performance when all layers are sandwiched. The complication of car seat design and the testing method bring a great demand of portable device which can measure the comfort performance of the real car seat. In this research a novel portable device is designed which work with special heat flux sensor and the device is connected to computer by USB port and values of heat flux temperature of the water and temperature of the surface is provided by the software. Heat flux sensors measure the heat transfer through a surface, and are expressed in kw/m2. The software controls the heating plate adjustment using PID controller. The device is tested with real car seat and shows repeatable and reproducible results.

Modeling and Experimental of the Mechanical Behavior of Seat Cushion for Wheelchair Users

Article

Full-text available

Jun 2019

The cushions assistance to the pressure ulcers (PUs) prevention has like objective to improve wheelchair users by decreasing the effects of pressure between the both surfaces. Innovating honeycomb cushion (HC) containing thermoplastic polyurethane (TPU) which has many advantages was studied using a biomechanical and digital model for simulation. We used the finite element method (FEM) for the cushion simulation and modelization. This method led to the structural decomposition of the cushion in the aim to evaluate his mechanical behavior. This comportment was simulated numerically by FEM on cell of the cushion, then on the whole cushion. The tests of compression carried out with the electromechanical press (INSTRON 33R4204 machine) on HC permitted to determine curves related to his mechanical behavior. The result helps the comparison with the numerical model developed in this study. This result permits to study the distribution of pressure, shearing, friction and the microclimate which are sources of PUs through HC. The idea following this work is to optimize the HC in the aim to present a new product satisfying the needs for wheelchair users. The design of HC for prevention assistance of the scabs in only one layer is possible and present many advantages: optimization of the manufacturing process, improved ventilation, facilitated cleaning and decreasing cost.

Off-loading pressure relief with foam cut out cushions: experiences from Rancho los Amigos National Rehabilitation Center

Article

Full-text available

Nov 2023

This paper explores the efficacy of the cushion fitting technique using foam cut out cushions for off-loading bony prominences in the sitting position, with a particular focus on reducing the high risk of developing pressure injuries among aging wheelchair users. This technique, historically employed at Rancho Los Amigos National Rehabilitation Center, has shown promising results in reducing pressure injuries for patients with spinal cord injuries. However, its widespread adoption remains limited. This manuscript aims to raise awareness about foam cut out cushions, its historical context, and its contemporary relevance by presenting customized solutions for individual patients with specific deformities. Key clinical points are highlighted, emphasizing the importance of skilled clinicians in the fitting process and the need to consider foam cut out cushions alongside other preventive measures. Case examples illustrate successful outcomes, demonstrating improved pelvic stability, posture, and off-loading of bony prominences. By promoting foam cut out cushions as a valuable cushioning option, this manuscript equips clinicians with knowledge to utilize this technique effectively. LAY ABSTRACT This paper investigates the effectiveness of using foam cut out cushions to relieve pressure on bony areas while sitting, especially for elderly wheelchair users who are prone to developing pressure injuries. This study delves into the historical background of foam cut out cushions and emphasizes their relevance in addressing the needs of modern patients. By customizing the cushions according to individual requirements, this research highlights a successful alternative for providing personalized cushion solutions to wheelchair users. The findings underscore the importance of adopting the cushion fitting technique to reduce pressure-related health risks, making it a valuable approach for enhancing the comfort and well-being of aging wheelchair users.

Effet de la variation des paramètres ergonomiques du siège d'avion sur les phénomènes biomécaniques et les actions musculaires associés au transfert assis-debout

Thesis

Dec 2018

Nadège Tebbache

Le transfert assis-debout est une des activités journalières les plus courantes, prérequis à la marche et crucial pour l'autonomie. Cette tâche nécessite une coordination importante entre les mouvements des segments corporels, et donc des activités musculaires. S'il a beaucoup été investigué par différents moyens (analyses dynamique et cinématique, électromyographie), il existe peu de standardisation sur ses déterminants. Le transfert assis debout peut être séparé en 2 phases, une posturale durant laquelle le tronc est fléchi et une focale durant laquelle les membres inférieurs sont étendus. Cette tâche est influencée par les caractéristiques du siège dans lequel la posture assise est adoptée. Certains de ses déterminants relevant du siège (e.g. la hauteur et la présence d'accoudoirs) sont connus, mais de nombreux paramètres n'ont à notre connaissance pas été investigués. En particulier, ceux qui favorisent son ergonomie en posture assise semblent présenter une dualité avec la tâche de STS. Cette thèse s'est intéressée à l'influence de l'inclinaison du dossier, de la mousse recouvrant le siège et de la présence d'une rangée de sièges à l'avant. Dans les différentes conditions, des variations des paramètres biomécaniques analysés par une plateforme de forces et des actions musculaires quantifiées par l'électromyographie ont été observées dans la phase posturale, avec ou sans conséquence sur la phase focale. Une posture initiale avec le dossier incliné vers l'arrière a nécessité une variation des niveaux d'activité des muscles actifs dans la phase posturale, principalement les muscles du tronc, sans effet sur la phase focale. Un coussin constitué d'une mousse plus dense a réduit la durée de la phase posturale et mené à des variations des actions des muscles du haut du tronc (sternocléidomastoïdien et trapèze supérieur) et de la cuisse (vaste médial), ainsi qu'à une vitesse verticale maximale plus importante lors de l'extension des membres inférieurs. Lorsque le champ avant était occupé par une rangée de sièges, la stratégie de STS s'est révélée modifiée, avec les muscles jambiers et les extenseurs du tronc plus actifs en phase posturale, action associée au contrôle plus strict du mouvement, et les muscles droit fémoral et tibial antérieur (plan ventral) plus actifs en phase focale pour l'extension. Cette stratégie plus coûteuse a permis d'obtenir une vitesse verticale à la décharge du siège plus importante, compensant la vitesse horizontale plus faible. Ces résultats mettent en avant les adaptations à des contraintes variées des paramètres de la phase posturale pour la réussite de la tâche de transfert assis-debout. Les caractéristiques étudiées sont également liées au confort statique ressenti par le sujet. Ainsi, les caractéristiques optimales tenant compte également de la performance dynamique et permettant un compromis avec celle-ci peuvent être déduites. Ces travaux amènent donc un nouvel éclairage sur le contrôle du STS, avec des implications directes sur l'ergonomie des différentes catégories de sièges, et en particulier ceux utilisés pour le transport aérien.

Manufacturing custom-contoured wheelchair seating: A state-of-the-art review

Article

Mar 2019

Background: Custom-contoured wheelchair seating lowers risk of pressure injury and postural deterioration while custom-contoured wheelchair seating lowers risk of pressure injury and postural deterioration while increasing the stability and functional activity of the wheelchair occupant. Producing custom-contoured seating systems has historically been a labour-intensive process custom-contoured seating systems is historically labour-intensive. Objectives: Evaluate the strengths and limitations of current manufacturing processes for custom-contoured wheelchair seating to suggest potential future manufacturing processes. Study design: Literature review of the state of the art. Methods: Research conducted through a literature review focused on the performance of different types of custom-contoured wheelchair seating products and processes over the last 40 years. Recent literature in orthotics and prosthetics was also consulted to assess future trends in seating. Results: There are five main manufacturing processes currently used to produce custom-contoured wheelchair seating systems. No single process is yet suitable for all wheelchair users, but many manufacturers are transitioning to computer numerical controlled (CNC) milling to reduce manual labour. Adjustable micro-modular seating and moulded seat insert manufacturing are also prevalent and offer alternative seating to soft foam carving. Conclusion: There is a need in the custom wheelchair seating sector for processes that are fast, cost-effective, produce little to no material waste, and that can effectively maintain a comfortable seating micro-climate. Additive manufacturing may meet these criteria, but further evaluation is required. Clinical relevance: This review suggests that the custom-contoured wheelchair seating manufacturers are moving away from labour-intensive processes towards digital techniques, like CNC foam milling. Additive manufacturing is a potential new process that may reduce overall costs, the lead time in preparing seats and has the potential to better manage the seating micro-climate.

A comparative analysis of the compression characteristics of a thermoplastic polyurethane 3D printed in four infill patterns for comfort applications

Article

Dec 2021
RAPID PROTOTYPING J

Purpose Most support surfaces in comfort applications and sporting equipment are made from pressure-relieving foam such as viscoelastic polyurethane. However, for some users, foam is not the best material as it acts as a thermal insulator and it may not offer adequate postural support. The additive manufacturing of such surfaces and equipment may alleviate these issues, but material and design investigation is needed to optimize the printing parameters for use in pressure relief applications. This study aims to assess the ability of an additive manufactured flexible polymer to perform similarly to a viscoelastic foam for use in comfort applications. Design/methodology/approach Three-dimensional (3D) printed samples of thermoplastic polyurethane (TPU) are tested in uniaxial compression with four different infill patterns and varying infill percentage. The behaviours of the samples are compared to a viscoelastic polyurethane foam used in various comfort applications. Findings Results indicate that TPU experiences an increase in strength with an increasing infill percentage. Findings from the study suggest that infill pattern impacts the compressive response of 3D printed material, with two-dimensional patterns inducing an elasto-plastic buckling of the cell walls in TPU depending on infill percentage. Such buckling may not be a beneficial property for comfort applications. Based on the results, the authors suggest printing from TPU with a low-density 3D infill, such as 5% gyroid. Originality/value Several common infill patterns are characterised in compression in this work, suggesting the importance of infill choices when 3D printing end-use products and design for manufacturing.

Evaluation of temperature on different wheelchair cushions using infrared thermography method

Article

Feb 2023

Newly developed wheelchair cushions, nowadays, promise to offer more comfort and protection against injury for wheelchair users. Therefore, developing novel wheelchair cushions has become a real challenge for manufacturers in recent years. In particular, thermal properties of the wheelchair cushion and their effect on the user’s comfort are very important factors in the design and optimization. This paper reports the findings of an investigation into the thermal properties of wheelchair cushions using an infrared thermography system. The experimental results showed a variation in the temperature behavior of different trademarks of cushions depending on the heating zones and the C1 cushion made in TPU material has more advantages than the other cushions made in PU foam, pharmform, and visco-foam. The results and analysis from this research will help to find a better type of cushion and contribute to collecting useful medical information to prevent pressure ulcers for bedridden patients.

Synthesis and characterization of soft polyurethane for pressure ulcer prevention

Article

May 2022
POLYM TEST

A series of super-soft polyurethanes was prepared by synthesizing lightly crosslinked polyurethane with low isocyanate:hydroxyl ([NCO]:[OH]) molar ratios. The starting materials were polymeric isocyanate (pMDI), polypropylene glycol (PPG), and 1,1,1-tris(hydroxymethyl)propane (TMP). The effects of the [NCO]:[OH] molar ratio and concentrations of the crosslinking agent TMP on the crosslink density and the thermal, dynamic-mechanical, and mechanical properties of the polyurethanes were determined. In addition, TMP generates chemical and physical crosslinks within polyurethane; therefore, data calculated using the Flory-Rehner equation and the rubber elasticity were compared. The success of the reaction between pMDI and PPG was confirmed using Fourier-transform infrared spectroscopy. The synthesized polyurethanes exhibited a mixing phase between soft and hard segments. The addition of TMP increased the crosslinking density, resulting in increased glass transition temperature, tensile properties, and hardness; however, the pressure distribution was suppressed. The synthesized polyurethane showed effective properties for pressure ulcer relief applications.

Study on Seating Comfort of Polyurethane Multilayer Seat Cushions

Article

Oct 2020

Instead of a monolayer, multi-layered cushion can be promising to develop more comfortable automotive seat if the layer materials are properly selected. In this study, we have examined the seating comfort of different types of multilayer seat cushions. Polyurethane foams (PUFs) of three different hardness, memory foam, and technogel were used in different combination to prepare four types of multilayer seat cushions. The short-term seating comfort of the seat cushions has been investigated in terms of hardness, initial hardness factor (IHF), modulus irregularity factor (MIF), sag factor (SF), and hysteresis loss as derived from the indentation force deflection (IFD) curves. Long-term comfort has been compared to the respective stress relaxation behavior of the seat cushions. It has been observed that both the short and long-term conditions of comfort are not only influenced by the characteristics of the upper layer, but also by the bottom layer (i.e., supporting material). The present work will help the seat designer to consider the material characteristics for fabricating a good automotive seat on the basis of the desired features needed to provide traveling passengers with proper short- and long-term comfort.

Automotive Textiles and Composites

Chapter

Mar 2019

Bijoy K. Behera

Automotive textiles happen to be one rewarding sector, which extensively uses technical textiles, namely for interior trims, safety devices like seatbelts and airbags, carpets, filters, battery separators, hood liners, hoses, and belt reinforcement. The potential for the growth of the automotive textile business is considered one of the most progressive sectors in over the world, as car production is exponentially increasing in almost all developing countries. Most of the developments of components, parts, pieces, and materials used in a car are primarily governed by comfort, functionality, safety, economy, and ecology. Comfort is the first criterion that values the customer. In the case of a car's upholstery, both psychological comfort and thermo‐physiological comfort are considered. The incorporation of 3D woven composites is changing automotive design and the development process; instead of one level, the textile industry will be engaged at all stages of the automotive development process.

Development of a modular wheelchair cushion for spinal cord injured persons

Article

Full-text available

Aug 1986

This paper discusses a modular wheelchair cushion system intended for patients at moderate or low risk for developing pressure sores. With the use of components with different mechanical and physical properties the modular cushion produces improved performance compared with cushions comprising the individual components alone. In addition to achieving clinically acceptable interface pressures, this approach to wheelchair cushion prescription helps to accommodate individual preferences associated with stability, temperature dissipation, and resiliency. Mechanical tests were performed to demonstrate the underlying principles of the modular cushion. Tests with able-bodied subjects identified specific cushion configurations for different weight groups, offering alternative material configurations that may be selected at the discretion of the therapist or patient.

Effects and Characteristics of Cushion Covering Membranes

Chapter

Jan 1976

The present paper reports a portion of a rather comprehensive study of the transmission of forces to the human body through pads of various types and specifications (Chow, 1974). The basic objective of such pads is, of course, to impart the necessary forces without causing damage to the human tissue. Problems commonly arise in the case of handicapped individuals who are unable to shift their bodies so as to alter the force distribution with sufficient frequency to prevent tissue damage in the form of decubitus ulcers. Although the force transmission problem was considered in a general and fundamental way, particular attention was given to the special case of transmitting body weight to a seat cushion. An important phase of this general study was the investigation of the effects and characteristics of membranes which are usually interposed between the body of a resilient pad and the human skin (as pad covers, clothing, etc.) The characteristics of these membranes profoundly influence the stresses and strains within the loaded portion of the anatomy.

Measurement of physical parameters at the patient support interface

Article

Jan 1985

Experience at Rancho Los Amigos Hospital With Devices and Techniques to Prevent Pressure Sores

Chapter

Jan 1976

Since this paper is a contribution to a symposium dealing with many aspects of tissue trauma, we propose to limit our discussion to experience only at Rancho Los Amigos Hospital in Downey, California. We do not wish to imply that our methods are superior to all others but we hope that some different and useful ideas may obtain from our limited experience. The Tissue Trauma Group of the Rehabilitation Engineering Center has been actively involved in research, development and patient service for both the sitting and the recumbent (or prostrate) patient. The approaches to these two types of problem are fundamentally different for the following reasons: (i) The projected area of support of a prostrate human being, when divided into his body weight, yields an average pressure of around so mm Hg. This is below the 30-35 mm Hg usually accepted as necessary to cause ischaemia. (ii) The projected area of support, including the feet (but not the back) of a sitting person when divided into his body weight, yields an average pressure of about 50 mm Hg. This is above the 30-35 mm Hg ‘ischaemic pressure’. The logic of reason (i) has led us to develop devices to attempt to distribute the support pressure of the recumbent patient as uniformly as possible, so that such patients may lie in any position in definitely (usually through the night) without being turned. This approach has resulted in the Rancho Flotation Bed (mud bed) and the Poly-Flotation Mattress to be described later. The problems of the seated patient suggested in reason (ii) have led to an approach that involves the development of optimum seat cushions, coupled with clinical service which relies on special pressure measurement techniques and procedures for custom modification of seat cushions and sitting positions. These will be discussed in detail. Research and development have focused on new instrumentation to indicate the potential for tissue damage, studies of various support material characteristics and the development of support systems, both passive (viscoelastic seat cushions with or without cutouts, and flotation systems) and active (devices requiring external power) systems.

Pressure measurement at the patient support interface

Article

Jan 1985

Incidence of pressure sores in the Greater Glasgow Health Board Area

Article

Oct 1977

A survey was made by questionnaire of the prevalence and severity of pressure-sores on a given date among all hospital inpatients and all patients visited by a district nurse within the Greater Glasgow Health Board area. 8.8% of patients had a pressure-sore. Those aged 70 and over accounted for 70% of the patients with sores. Chairfast patients consistently had a higher pressure-sore frequency than bedfast patients of a similar degree of helplessness.

Pressure ulcers: prevention and treatment

Article

Feb 1979
Clin Symp

An objective assessment of the sheepskins used for decubitus sore prophylaxis

Article

Mar 1979
Rheumatol Rehabil

W A Denne

SUMMARY The mechanical properties of four commonly used sheepskin-like interface materials have been investigated to assess their relative effectiveness in preventing decubitus sores. The samples were resilient and silky varieties of natural sheepskin, resilient wool woven into a fabric backing and a pure polyester simulation of sheepskin. Consideration of the factors which cause skin breakdown suggested that the relevant mechanical properties to be measured were compressibility, frictional behaviour, ability to absorb water vapour and porosity under load. The four samples exhibited similar ability to distribute load; the polyester material produced greatest friction; the resilient wool fabrics showed both greatest porosity under load and ability to absorb water vapour. It was found that the latter was the more important property for keeping the patient's skin dry. The general conclusion arising from this work is that resilient natural sheepskins are the best interface for the prevention and management of decubitus sores.

Pressure redistribution in wheelchair cushion for paraplegics: Its application and evaluation

Article

Mar 1979
Paraplegia

A survey of the first 200 cases supplied with a "Paracare" pressure redistribution wheelchair cushion. The rationale of the design and material used is discussed. The method of application and follow-up studies are given and a total evaluation of the results achieved to date are presented.

Prevention and Treatment of Pressure Sores

Article

Apr 1973

Even though easily prevented, pressure sores occur frequently. The consequences of allowing pressure sores to develop are great. Pressure sores are costly and disabling, both physically and psychologically. his article presents 1) criteria for recognizing the stages of potential skin breakdown: hyperemia, ischemia, necrosis, and ulceration; 2) areas most vulnerable to skin lesions when the patient is in different positions; 3) responsibility for pressure sore prevention; 4) prevention of pressure sores by routine relief of pressure from susceptible areas; and 5) treatment of pressure sores.

Seat cushion selection

Figures

Recommended publications

Effect of postural stress on catecholamine metabolism

Baroreceptor Reflex and Integrative Stress Responses in Chronic Fatigue Syndrome

Hemodynamic adaptation to orthostatic stress after orthotopic heart transplantation

A Bio-Psycho-Socio-Molecular Approach to Pain and Stress Management