Clinical study of
the Custom-made Seat Cushion
Computer-aided
design/computer-integrated manufacturing of a
customized seat to prevent sores : biomechanical
and clinical validation.
R. Ragavan, P. Benoit, F.
Ohanna, Centre Propara, 34195 Montpellier cedex 5
Summary: When some patients are sitting,
ischiums, trochanters and coccyx endure excessive
pressures that cause sores.
This study aims at proving
the efficiency of a new technology Expert-Seat
that uses pressure cells, model lings and
mathematical transforms in order to design with a
digital milling machine a customized seat adapted
to the patient's morphology. In order to assess
this customized seat, the study compared it with
other currently used seats regarding average
pressure at ischiatic areas (two areas of 14 cm2).
The results of this study on 60 sore-risk
patients show that the average pressure at the
ischium level is significantly lower with Expert-Seat
than with the other evaluated seats. The average
measured ischiatic pressures amount to 30 mmHg
with this new seat and to 87 mmHg with the other
kinds of seats.
Introduction: In spite of the vast range of
available seats, ischiatic sores occur frequently
and involve well known medical, economic, social
and psychological consequences 1,2,3,4. The
interface pressure measurement is important in
evaluating and comparing suPforts, but not enough
to meet the biomechanical requirements of the
sitting position' 6, 7. The biomechanical part of
the sitting position can be divided in four
levels (Fig. 1) : ,
At level 1 : the weight and size (of the trunk)
determine the forces born by the
ischiums.
At level 2 : the ischium form, the distance
ischium-skin and the tissue quality determine the
compressing and shearing forces acting on the
stressed tissues.
At level 3 : the body flattening on the support
determines the interface constraints.
At level 4 : the physical properties of the
support determine the transformation of vertical
forces in horizontal ones.
The systematic study of the supports have led to
important works, particularly those of Cochran
and al.8. .Moreover vascular explorations (echo-doppler,
laser-doppler) and imagery explorations (radiology,
echography, magnetic resonance) have helped to
understand better the relations between pressures
(interfaces/inside) and hypoxia of stressed
tissues 5.6.10, 11, 12.13.14.15.
The aim of this pilot study was to design a
support meeting the following requirements :
(a) moulding individually as much of buttock mass
as possible
(b) using different density materials to combine
support rigidity and interface softr1ess (c)
reducing the deformation of stressed tissues
(d) creating two levels in the seat, in order to
increase femoral pressures and to decrease
ischiatic pressures.
The seats used as reference are those more
currently used in the common medical practice and
have been chosen according to thousand
evaluations of the seat with patients with sore
risk 16.
Population: These 60 cases include 24 women (40
%) and 36 men (60 %). Average age: 40,7 years .:t.
2,9 (from 12 to 87 years). Average weight: 57,2
kg .:t. 1, 7 (from 35 to 100 kg).
Average size: 164,5 cm .:t. 1 ,6 (from 140 to 188
cm).
These 60 cases have the following pathologies :
40 medullary injuries, 3 spinal bifida, 3
myopathies, 3 IMOC, 6 geriatric neurologies (Parkinson,
Alzheimer, ...), 1 concussion, 4 hemiplegies.
The personal seats are made of :
- air for 34 of them
- foam for 15 of them
- gel-foam for 6 of them
- gel for 4 of them
- water for 1 of them.
35 patients (58 %) had skin problem backgrounds,
including 30 of them (50 %) with sore and 5 of
them (8 %) with flush.
Material: The material used includes :
a) the pressure mapping system Expert-Seat
b) printer
(c) the seat components
(d) the CNC milling machine.
During measurement the patient sat in his
personal wheelchair. Pressure is measured with a
soft matrix of capacitive sensors of 45 cm x 45
cm. Expert-Seat software analyses datas.
Expert-Seat software processes the data obtained
during printing and sends them through Internet
for computer-integrated manufacturing.
The customized seat is produced in two steps with
a high precision digital milling machine: first
with the support material and then with the
interface material.
The milled seat is integrated in a very elastic
Iycra cover, commonly used with the current seats.
Functional valuation includes 13 parameters that
reflect the patient's life at the time when he
used his customized seat in his wheelchair. The
appreciation of each parameter has an objective
and a subjective part : good or bad valuation of
each parameter depends on the previous patient's
life with his personal seat.
Results: Analysis of the interface pressures :
Average value for the computer-aided design/computer-integrated
manufacturing seats: 30 mmHg +/- 2,0 (from 10 to
76 mmHg).
Average value for personal seats: 87 mmHg +/- 7,0
(from 25 to 246 mmHg).
Maximal ischiatic pressures: 47 mmHg +/- 3,0 with
computer-aided design/computer- integrated
manufacturing seats and 128 mmHg +/- 9,0 with
personal seats.
Average support area for computer-aided design/computer-integrated
manufacturing seats: 1528 cm2 +/- 37,1 (from 1014
to 2303 cm2) and 1421 cm2 +/- 30;7 (from 1070 to
2304 cm2) for personal seats.
The pressure differences between the two seat
groups are statistically significant with 95% of
significance (p< 0,01) for ischiatic pressures
and p< 0,05 for support area.
The functional valuation shows the improvement
percentages of ten parameters :
a) 70% for comfort
b) 60% for position and re-positioning
c) 52 % for release of bearing points
d) 33% for pain
e ) 25 % for transfers
(f) 25 % for propelling and handling the
wheelchair
g) 18 % for using upper limbs
h) 13 % for handling and maintenance of the seat
i) 10 % for spasticity
j) 5 % for vesical probing.
Parameters
valuated by YES / NO:
Parameter 11 : During this study no patient had
cutaneous problem at the ischium area.
Parameter 12 : 87 % of patients wish to use the
customized seat in the future, against 13 % who
do not wish to use it with its present form. Some
of these patients are interested in trying the
seat again after specific changes.
Discussion: This work includes two different
complementary components: technical matter and
functional matter.
Measurement requires technical knowledge and
medical checking in order to get the best
printing of the seat for each patient.
In some cases it may be necessary to correct an
imbalance of the trunk or pelvis, or to release
perineum or other areas with abnormal bearing
points.
There is a manual part in printing: to mark the
prominent bones such as trochanters, coccyx and
osteoms and the corresponding areas on the seat
printing.
This manual checking is necessarily repeated when
placing the customized seat under the patient's
buttocks. These data are the basic parameters of
the customized seat concept.
Mathematical part of the technique is completely
automated with a computer and with calculation
algorithms that have been validated by many
previous research works.
This is a particularly interesting innovation for
the technique because it allows to repeat and
modify printing at will in order to rectify
measurements.
Other innovation of this new technique: The
equipment is entirely portable and can be used
wherever the patient is (hospitals, nursing home,
at home, ...).
This mobility is a revolutionary improvement
compared with other works on this matter.
This advantage is underlined by the easy use in
various centres.
The possibility to act upon printing by
positioning areas of deep milling and areas of
level differences enables a specialist to have
specific results according to the characteristics
of a seat with problems.
For example a depression is recommended for area
with recurring cutaneous weakness in order to
help tissue cicatrisation even when sitting, and
for perineum areas to make the vesical probing
easier in sitting position.
The functional valuation shows corroborative
relevant matters and improvement percentages from
52 % to 70 % for parameters of comfort, position,
re-positioning and release of bearing points.
This confirms that these matters are intimately
related with each other and that their global
value in subjective appreciation terms is the
most important part of the comparative valuation
with other seats.
A new innovation has just improved the nearly
definitive version of the seat: a system to
locate positioning.
This system provides confirmation and safety to
the patient and the people taking care of him and
avoids any possible gap between the buttocks and
the milling areas. A prototype has been
successfully used and the system is now tested
for validation . This system is particularly
helpful for patients with no position references
or who are installed in their wheelchair by
someone.
Conclusion: Computer-aided design/computer-integrated
manufacturing of a customized seat enables sore-risk
patients to sit with interface pressures that are
much lower than those with mass-produced seats.
The customized seat can also partly correct some
imbalances in sitting position and improve the
comfort of patients in wheelchair.
However a skilled staff is necessary to produce
this seat. Using modern technique to treat and
transmit data enables to use it for any patient.
The final version of this kind of seat integrates
a visual system for positioning references that
is anew matter for active prevention and active
sore treatment.
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