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Recent study finds Water & Air Cushion works better than

gel or

foam to increase

comfort.

Shear stress measured on three different cushioning materials.

R.H.M. Goossens, PhD

Delft University of Technology

Delft, The Netherlands

Shear force is an important component of the mechanical load on a

person that

is supported by a

surface. Too high shear force leads to

occlusion of blood

flow, which is seen as one of the most

important

factors behind pressure sores

and discomfort. In the present study the

influence of three

different cushioning

materials (Water and Air

Cushion, gel and foam) on shear stress is evaluated

with the shear

sensor from the Erasmus University of Rotterdam. It is

concluded that

the Water and

Air Cushion produces significant lower shear

stress than

the foam cushion in situations when a shear

force acts forward

(P=0.

001), backward (P=0.038) and in the horizontal position of the seat (P=0.

005).

When using Water and Air combined instead of foam there is a

reduction of

shear stress varying

from 28% to 39%. It is concluded that

the Water and Air

Cushion produces significant lower shear

stress than

the gel cushion in

situations when a shear force acts backward (P=0.038)

and at the

P=0.10-level

in the horizontal position of the seat (P=0.07) and

when the shear force acts

forward

(P=0.07). When using Water and Air

Cushion instead of gel there is a

reduction of shear stress

varying from

24% to 25%. No significant differences

were found between the gel

cushion and the

foam cushion. Introduction Shear

force is defined as a

force that acts parallel to a surface (whereas

pressure

acts

perpendicular to a surface).

When the shear force acts over a certain area it is called shear stress (in

accordance with the

definition of pressure as a force on a certain area).

Different authors showed that shear stress has a

significant influence on

occlusion of the blood flow within the tissue. Goossens (Goossens,

Zegers et

al. 1994) showed that shear stress had a significant influence

on the reduction

of blood flow on the

sacrum of healthy subjects. Bennet

(Bennett, Kavner- et

al. 1979; Bennett, Kavner et al. 1981;

Bennett,

Kavner et al. 1984) showed

that the combination of pressure and shear

was particularly

effective in

promoting blood flow occlusion in the palm

of the hand. Zhang (Zhang and

Roberts 1993)

came up with a

biomechanical model to estimate the influence

of pressure and shear

components on

blood flow occlusion. Occlusion of blood

flow is seen as

one of the most important factors behind

pressure sores and

discomfort.

Intermezzo pressure sores and discomfort pressure sores are

caused

factors that are classified generally as intrinsic and extrinsic.

The intrinsic factors are related to the patient's clinical condition and

both the

nature of the illness

and its severity are relevant. The extrinsic

factors, that

can be influenced directly, are concerned

with pressure,

shear, temperature

and humidity. All authors agree that the most

important cause of

pressure

sores is the mechanical load (pressure and

shear) on the skin

Although most authors agree that pressure sores are due to prolonged

tissue

ischaemia caused by

the mechanical load through which the

capillaries are

closed and diffusion of oxygen and metabolites

to the

cells is hindered, other

extra mechanisms are reported in literature.

Reddy et al. (1981)

studied the

effects of external pressure on

interstitial fluid dynamics using a simple

mathematical

model concluding

that squeezing of interstitial fluid may also

play a role in ulcer formation.

Meijer

(1991) states that it is most likely that

local blood circulation

under influence of the mechanical load

is controlled also

by regulatory

mechanisms. In a review of literature Lueder (Lueder 1983)

gave a

general overview of approaches to the assessment of comfort relevant

to the design of office

furniture. The author concluded that although

substantial research exists, little insight is available

into the meaning of

comfort. More recently Zhang et al. (Zhang 1996) concluded that

comfort and

discomfort are two different and complementary entities in

ergonomic

investigations. In an attempt

to identify the factors of comfort

and discomfort

in sitting the authors conclude that amongstother

factors,

poor biomechanics

(meaning too high a mechanical load) was one of the

factors of the cause

discomfort. In some studies this relation between

pressure and discomfort was

demonstrated

(Diebschlag and Hormann

1987; Grindley and Acres1996;

Ballard 1997; Buckle and Fernandes

1998).

In a recent study Goossens (Goossens 2000) showed that different

combinations of pressure and

shear (for example high shear and low

pressure,

and high pressure and low shear) when applied to

the outside

of the skin still

have the same effect inside the skin. In this way it was

demonstrated that

not

only pressure relates to discomfort but also shear

stress. For both aspects of

the mechanical

load (pressure and shear) it

can be concluded that a reduction

leads to less discomfort. Tissue load

lying and sitting and thus occlusion of

blood flow can be influenced in two

ways. Firstly, by

changing the mutual

positions of the body supporting

surfaces. Secondly by changing the material

and profile of the seat or

backrest. In literature mostly the influence of the

material on pressure is

evaluated. And although different kinds of cushioning

are developed to

reduce the shear stress as much as possible, no studies can be found on

their

effectiveness. The

reason for this is that pressure measurement

systems are

commercially available, and a sensor that

measures shear

stress is not.

However, in the Erasmus University of Rotterdam there is

a sensor

that can

measure shear stress acting on subjects in a sitting

and lying position

(Goossens, Snijders

et al. 1997). Aqua Aire Cushion

uses a fluid-filled bladder

(outer skin) that is designed to

dramatically

reduce friction (shear forces),

reduce pressure and absorb shock. In the

present study

the influence of three

different cushioning materials The

Water and Air Cushion, gel and foam) on

shear stress is evaluated by

means of the shear sensor from the Erasmus

University of Rotterdam.

The shear sensor that was used is 27x15x3.5 mm, in

size and thus the

contact area is 4.05 cm2. Six

of these sensors were fixed on

the cushion

with double-sided tape. They were positioned at the

location where

the

right ischial tuberosity of the subjects rests on the cushion. The subjects

sat for 2

minutes on the sensors before the measurements took place.

After

that period 100 measurements

were done in 20 seconds. In order

to vary the

shear force that acted on the seat, three different seat

angles

(5° forward, 5°

backward and horizontal 0°) were randomly installed for

each subject. The

backrest was not used during these tests. In this way

the shear force on the

seat covered the wide

range of shear forces that

can be expected in all kind of

body supporting products (saddles, office

chairs, forward tilted seats, standing

aids etc.). Three different cushions

were used, one with the

Water and Air

Cushion, a gel cushion and a

foam cushion. These cushions were positioned

upon a

layer of foam. The

entire seat on its turn was installed on a special

chair on which the

adjustments of

the angles could be made, and on which the

total shear

force on the seat could be measured. Twenty

healthy subjects were

used

in this test (mass 66 (s.d. 12) kg, length 175 (s.d. 10) cm).

In order to exclude the influence of the different kinds of trousers, the

subjects

all wore a pair of

trousers that is worn in the operation room. In

total 9

combinations were measured for each subject

(3 angles, 3

cushions), and

between every combination the

subject stood up to allow angle adjustments to be made. In every

situation (for

example angle 5°

backward) the shear stress on the right

buttock was

measured 100 times on 6 sensors, and then

averaged. The

maximum value of

the six sensors was then used for statistics. The unit

for shear

stress is kPa,

kilo pascal. (With 13.3 kPa= 100 mmHg).

Statgraphics 8.0 was used for data

analyses.

The non-parametric

Wilcoxon Signed Rank Test was used to test

the following hypotheses

with a

level of significance = 0.05:H0: There is no

difference in

maximum shear stress between the

cushions: At least one of the

cushions differs from the others, Water and Air Cushion.

Study is to be used as a reference only and was not performed on the Aqua-Aire cushion

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