**study is to be used as a reference only and was not performed on the Aqua-Aire
cushion**
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, PhDDelft University of Technology
Delft, The Netherlands
Summary of Conclusions
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- Cushions using Air and Water Reduces surface stress by 28% to 39% more than Foam
- Cushions using Air and Water Reduces surface stress by 25% more than Gel.
- Occlusion of blood flow is seen as one of the most important factors behind pressure sores and discomfort.
- All authors agree that the most important cause of pressure sores is the mechanical load (pressure and shear) on the skin.
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 by 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 of 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 in 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.
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