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 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.    




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