Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02475638 2008-03-27
Individually-Contoured Seat Cushion
and Shape Capturing and Fabricating Method for Seat Cushion
Field of the Invention
This invention relates to seat cushions used to support an individual in a
seated or otherwise reclined position. More particularly, the present
invention
relates to a new and improved technique for capturing the anatomical shape of
an
individual in a seated or reclined position, and to a new and improved use of
that
captured anatomical shape to fabricate a support contour of a cushion which
complements the anatomy and posture of the user. The present invention allows
the anatomical shape of the individual to be captured effectively,
conveniently,
accurately and inexpensively without the use of relatively expensive and
sophisticated measurement equipment, among other things. The present
invention is particularly useful in fabricating seat cushions for wheelchairs,
although certain aspects of the invention are not limited specifically to such
use.
Background of the Invention
A wheelchair seat cushion must perform a number of important
functions. The seat cushion should be comfortable and capable of providing
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proper support for optimal posture and posture control for a considerable
length of
time. The seat cushion should also assist, or at least not materially hinder,
the
user in maneuvering the wheelchair, permit a useful range of motion from the
pelvis and upper torso of the person, and create stability and security for
the
person within the wheelchair. Perhaps most importantly, the seat cushion
should
help prevent and reduce the incidence of pressure ulcers created by prolonged
sitting on the cushion without adequate pressure relief. Pressure ulcers can
become a very serious health problem for individuals who must remain
constantly
in contact with the support cushion, and it is important to avoid such
pressure
ulcers.
Wheelchair users like everyone are of substantially different sizes, weights
and shapes. Many wheelchair users have physical disabilities and associated
posture and postural control impairments such as those typically caused by
congenital disorders. Other wheelchair users, such as those who have been
disabled by acquired or traumatic injuries, may have a more typical size and
shape. In all of these cases, the support contour of the wheelchair seat
cushion
must safely support the anatomy of the user, whether the anatomy is abnormal
or
more typical. Wheelchair seat cushions must fit and perform properly to
prevent
further physical impairment and pressure ulcers. The cushion must also enhance
the functional capabilities of the user by supporting independence in
activities of
daily living. There are a number of different theories or approaches for
configuring
the support contour of a wheelchair seat cushion to avoid pressure ulcers and
to
provide adequate postural alignment.
To provide the best individualized support, the cushion must accommodate
the anatomical particularities and preferences of the user. Custom wheelchair
cushions are used for this purpose. Most custom wheelchair cushions are
created
from an impression of the anatomy of the user. After capturing a shape of the
user's anatomy, the captured shape is used to construct a mold for the
cushion.
Then the mold is used to fabricate the cushion, including the support contour
which interfaces with the user's anatomy from which the shape was originally
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captured. There are a number of different theories for configuring the support
contour to address the perceived needs and requirements of the user.
The most prevalent approach used to configure the support contour of a
custom cushion, at least at the time of filing hereof, is to distribute the
weight of the
user substantially uniformly over the entire support contour. The uniform
pressure
distribution is theorized to reduce the incidence of pressure ulcers because
the
uniform pressure distribution is thought to avoid localized high-pressure
points
which cause pressure ulcers. The substantial conformance of the support
contour
to the anatomical shape of the user is also believed to encourage the user
toward
proper postural alignment.
A new support theory is based on offloading and isolating pressure and
shear forces from the skin surrounding the bony prominences of the user's
pelvic
area skeletal structure. Applying this support theory involves configuring the
support contour with additional clearance, and therefore achieving greater
pressure relief, around the ischial tuberosities, the greater trochantors, the
coccyx
and the sacrum in the pelvic area, while transferring more support to the
broader
tissue and musculature below the proximal thigh leg bones and at the posterior
lateral buttocks. Pressure and shear forces on the skin around the bony
prominences is relieved, and pressure is transferred to the broader tissue
areas to
encourage proper postural alignment. The pressure transferred to the broader
tissue areas encourages proper postural alignment, while making offloading
possible.
To execute successfully any of the different support theories, the support
contour of the cushion must be created relative to the captured shape of the
individual. Otherwise, the user can not be supported adequately to achieve the
desired objectives of the support theory.
There are a number of sophisticated methods and devices available for use
to determine and capture anatomical shapes. One type of device is a seating
simulator. A seating simulator uses a relatively large chair-like structure in
which
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flexible bags or containers of beads are confined. The user is seated on the
bags
and the beads distribute themselves around the user's anatomy. A vacuum is
then
applied within the bags, and the exterior pressure on the bags forces the
beads to
hold the conforming position. The user is then removed, and the user's shape
is
captured. Any adjustments are thereafter made. To translate the captured shape
into information which can be used to create the custom cushion, relatively
sophisticated electronic mapping equipment is moved over the shape held by the
bag. A multiplicity of different points across the shape are measured, and the
measurements are transferred electronically to a software computational
algorithm
or program which defines a mathematical simulation of the captured shape of
the
users anatomy. This simulation is thereafter used to create a mold from which
the
cushion is formed. Alternatively, a plaster or other material casting is made
of the
captured shaped directly from the bag while the captured shape is held. The
casting is shipped to a cushion manufacturer for interpretation and
fabrication of a
custom cushion having the desired support contour.
Another type of shape-capturing device uses a two-dimensional grid of
plungers or rod-like elements which are brought into contact with the users
anatomy. The relative displacement or movement of the plungers due to contact
with the anatomy is measured. The measurement data is then transferred
electronically and is used by a computational algorithm or program which
defines a
mathematical simulation of the captured shape. Thereafter, the simulation is
used
to create a mold from which the cushion is formed.
The described types of shape-capturing equipment are sophisticated and
relatively expensive to use, and that expense must be charged as part of the
price
of the custom cushion. The size of the equipment makes it inconvenient for
transportation to the user, which can be a problem or at least an
inconvenience if
the user cannot travel comfortably. At the very least, the practical cost of
the
cushion is exaggerated by the added travel expenses and inconvenience to the
user.
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Even though the described types of shape-capturing and shape-simulating
equipment may make hundreds or even thousands of measurements at different
locations over the anatomy of the individual, those numbers of measurements
might still be relatively coarse, particularly in locations where significant
changes
in contour of the anatomy occur. The computational shape-simulating programs
must interpolate the measurements, and that interpolation may not be entirely
accurate. As a consequence, the cushion may not be as comfortable or effective
as desired.
A more significant problem with the described types of shape-capturing and
shape-simulating equipment is that it does not measure a fully-loaded
anatomical
shape. A fully-loaded shape is one which accurately reflects the effect of the
full
weight of the individual against a resistance. The resistance will be from the
cushion once it is fabricated. In the case of the shape-capturing equipment
which
uses beads confined in bags, certain limited areas of the entire anatomy will
contact the beads with exaggerated loading, while other areas of the entire
anatomy will not naturally contact the beads at all. For example, sifting on
the bag
will transfer virtually the entire weight of the individual from the tissue
surrounding
the ischial tuberosities to the beads in the bag. Relatively less or little
weight will
be transferred from the lateral posterior buttocks and sides of the pelvic
area to the
beads. In order to simulate the shape of these low weight transfer areas, the
beads in the bag must be pushed up against the anatomy. However, pushing the
beads against the anatomy does not result in the same shape as would occur
from
a fully loaded condition in which the weight of the individual is fully and
naturally
resisted over the entire contact area. The combination of areas of exaggerated
loading and simulated loading creates distortion in the captured shape as
compared to a fully loaded shape, and this distortion may be reflected by
insufficiencies in the support from the cushion.
In the type of shape-capturing equipment which utilizes a grid of plungers,
resilient foam or spring-like devices surround each plunger. The resiliency of
the
foam or springs is intended to create resistance to the weight of the anatomy,
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thereby allowing the user to sit down on or recline against the plungers.
However,
the foam or springs do not uniformly load the anatomy. Those portions of the
anatomy which depress the plungers to a greater extent will be resisted by
greater
pressure compared to the resistance from plungers contacting other areas of
the
anatomy which are less depressed. The amount of resistance increases with an
increase in the distance traveled by the plunger. The shape captured from such
devices is therefore a non-uniformly loaded shape, in the sense that the
resistance
is not uniform over the entire area of the anatomy. A cushion formed from a
nonuniformly loaded shape may be inadequate.
Another problem with shape-capturing equipment is that the captured shape
is based on a static position and posture of the user. For the equipment to
capture
the image accurately, the user must remain still. In actual use, the user is
almost
always moving on the cushion. For example, the user is rocking his or her
upper
torso when turning the wheels of the wheelchair. The user may be leaning from
one side to the other when reaching or may be leaning forward to work at a
desk.
A support contour which is configured from a static anatomical shape is prone
to
create pressures and shear forces at bony prominences, resulting in an
increased
risk of pressure ulcers, due to the natural movement of the user.
Because of the necessity to use shape-capturing equipment and because
the user must stay stationary while the shape is captured in such equipment, a
wheelchair user is not able to test and evaluate the support contour of the
cushion
before it is formed. The ability of the cushion to protect the skin, to assist
the user
in manipulating the wheelchair, or at least not to inhibit the user in
maneuvering the
wheelchair are important aspects of the cushion. If the cushion positions the
user
too far forward, or too far rearward, or too low or high, the center of
gravity within
the wheelchair and the ability to maneuver the wheelchair and comfort and
safety
of the user can be impaired. However, these actual-use aspects of the cushion
are only determinable after the cushion has been fabricated.
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Summary of the Invention
This invention involves capturing an accurate and fully-loaded anatomical
shape of the user. The shape may be captured in a wheelchair or other
environment of intended use of the cushion, rather than in an artificial
environment
created by a seating-simulator or other shape-capturing equipment. The shape
may be captured in a way that reflects changes in position of the anatomy
resulting
from a range of typical movement of the user. Consequently, the cushion formed
from the captured shape will better accommodate the normal movements and
range of positions of the user during actual use of the cushion. As part of
capturing the shape, the user may actually test the shape in a limited sense
to
determine its general suitability before the shape is used to form the
cushion. The
shape-capturing equipment of the invention is relatively inexpensive, and it
can be
transported easily to the location of the user, rather than requiring the user
to
travel.
These and other aspects of the invention are realized in a method of
capturing a negative impression of an anatomical portion of a person. The
method
involves selecting impression foam having a crush characteristic of
substantially
constant crushing force over a predetermined range of collapse distances. The
anatomical portion of the person is forced into the impression foam to create
the
negative impression by collapsing the impression foam. The impression foam is
collapsed to an extent which falls within the predetermined range of constant-
force
collapse distances over the entire negative impression. By collapsing the
impression foam from the force from the anatomical portion in this manner, the
captured impression reflects the anatomical portion of the person in a fully-
loaded
or equally-loaded condition. Such a captured impression results and a more
effective support contour.
Preferably, the crush characteristic of the impression foam is such that the
predetermined range of constant force collapse distances is approximately 80%
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90% of an initial thickness of non-collapsed impression foam. The constant
crushing force is preferably within the range of 1.50 to 1.85 pounds per
square
inch. The crush characteristic preferably exhibits a relative lack of
structural shear
force resistance, causing the captured negative impression to be substantially
free
of displacement or deformation at edges relative to the shape of the
anatomical
portion. The force to create the negative impression is preferably obtained by
sitting or reclining the person on the impression foam, or otherwise allowing
the
weight to the person to create the negative impression.
The negative impression is particularly useful for creating a support contour
of a wheelchair seat cushion for supporting a pelvic and proximal thigh
anatomical
portions of a wheelchair user. The impression foam may be positioned on the
wheelchair seat support structure itself to obtain the negative impression
under the
same circumstances that the wheelchair seat cushion will be used.
Alternatively,
the impression foam may be used in place of a bag of beads on a seating
simulator. In capturing the negative impression, the wheelchair user is
preferably
moved through a range of movement that reflects the normal range of movement
in the wheelchair. Because of the ability of the impression foam to support
the
wheelchair user, the impression foam can be tested on the wheelchair in a
limited
manner to simulate the comfort and maneuverability available from the seat
cushion created from the negative impression.
The negative impression may be adjusted to provide further relief for certain
portions of the anatomy, such as at the ischial tuberosities, greater
trochanters,
coccyx and sacrum, and the perineal area of the person. Enhanced support areas
may also be obtained in the final cushion by creating protuberances in a
positive
mold taken from the negative impression at the lateral posterior buttocks or
the
proximal thighs of the person. The greater relief and enhanced support areas
are
particularly beneficial for wheelchair users, although they are also
beneficial in
other seating and support situations, such as office chairs.
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Because of the relative ease of capturing the negative impression, the
impression may be obtained at the location of the user and then transported by
mail to the manufacturer of the support contour. A container maybe positioned
around the impression foam to protect the impression foam from inadvertent
indentions which are unrelated to the captured negative impression of the
user's
anatomical portion.
Another aspect of the invention involves a method of fabricating a cushion.
The method involves capturing a negative impression of the anatomical portion,
creating a positive mold configuration of the anatomical portion using the
captured
negative impression, and fusing together a plurality of resilient plastic
beads into a
support structure which encompasses at least a portion of the positive mold
configuration. The resilient plastic beads are fused together at contact
points
which permit spaces between the beads to establish air ventilation
permeability
within the seat cushion.
A further aspect of the invention involves cushions, seat cushions and
support contours for cushions which are formed by the methods of the present
invention.
According to one aspect then, there is provided a method of capturing a
negative impression of an anatomical portion of a person, comprising:
selecting a
piece of impression foam having a crush resistance characteristic of
substantially
constant crushing force per unit area over a predetermined range of collapse
distances; forcing the anatomical portion into the piece of impression foam to
create the negative impression by collapsing the impression foam; and
collapsing
the impression foam only within the predetermined range of collapse distances
within which the crushing force per unit area is substantially constant while
creating
the negative impression.
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In accordance with another aspect, there is provided a method for creating a
support contour for a seat cushion by which to support pelvic and proximal
thigh
anatomical portions of a person while sitting, comprising: selecting
impression
foam having a crush resistance characteristic of substantially constant
crushing
force per unit area over a predetermined range of collapse distances; sitting
the
person on the impression foam to force the anatomical portions which will be
supported on the support contour into the impression foam to create a negative
impression by collapsing the impression foam to an extent which falls within
the
predetermined range of collapse distances at every location of the negative
impression contacted by the anatomical portions which will be supported on the
support contour; removing the person from sitting on the impression foam after
collapsing the impression foam; further collapsing the impression foam at
selected
relief areas of the negative impression to create an adjusted negative
impression;
locating the selected relief areas to obtain further clearance from the
support
contour at the location of at least one of the ischial tuberosities, greater
trochanters, coccyx and sacrum, and the perineal area of the person; using the
negative impression as a mold to create a positive mold configuration of the
anatomical portions; and removing material from the positive mold
configuration at
selected support areas to create an adjusted positive mold configuration.
In accordance with a further aspect, there is provided a method for creating
a support contour for a seat cushion by which to support pelvic and proximal
thigh
anatomical portions of a person while sitting, comprising: selecting
impression
foam having a crush resistance characteristic of substantially constant
crushing
force per unit area over a predetermined range of collapse distances; sitting
the
person on the impression foam to force the anatomical portions which will be
supported on the support contour into the impression foam to create a negative
impression by collapsing the impression foam to an extent which falls within
the
predetermined range of collapse distances at every location of the negative
impression contacted by the anatomical portions which will be supported on the
support contour; removing the person from sitting on the impression foam after
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collapsing the impression foam; using the negative impression as a mold to
create
a positive mold configuration of the anatomical portions; and removing
material
from the positive mold configuration at selected support areas which will be
part of
the support contour to create an adjusted positive mold configuration.
In accordance with yet another aspect, there is provided a method of
fabricating a seat cushion having a support contour for supporting a person at
pelvic and proximal thigh anatomical portions of the person, comprising:
capturing
a negative impression of the pelvic and proximal thigh anatomical portions;
creating a positive mold configuration of the anatomical portions using the
captured
negative impression; fusing together a plurality of resilient plastic beads
into a
support structure which encompasses at least a portion of the positive mold
configuration to define the support contour for the seat cushion: and fusing
the
resilient plastic beads together at contact points which permit spaces between
the
beads to establish air ventilation permeability within the support structure.
In accordance with another aspect, there is provided a method of fabricating
a cushion having a support structure for supporting a person in a wheelchair,
comprising: utilizing a matrix of resilient fused-together plastic beads as
the
support structure, the plastic beads in the matrix being fused together at
contact
points which permit spaces between the beads to establish air ventilation
permeability within the matrix; shaping a human-interface side into the matrix
of
resilient fused-together plastic beads, the human-interface side defining a
support
contour which contacts the person; and configuring another side of the matrix
of
resilient fused-together plastic beads to contact the wheelchair.
A more complete appreciation of the scope of the present invention and the
manner in which it achieves the above-noted and other improvements can be
obtained by reference to the following detailed description of presently
preferred
embodiments taken in connection with the accompanying drawings, which are
briefly summarized below, and by reference to the appended claims.
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Brief Description of the Drawings
Fig. 1 is a perspective view of a block impression foam which is substantially
enclosed within a container structure, exemplifying one type of use of
impression
foam in accordance with the present invention.
Fig. 2 is a graph illustrating a relationship of crush distance and crush
resistance characteristics of the impression foam, shown in Fig. 1, which is
used in
accordance with the present invention.
Fig. 3 is a flowchart of steps illustrating a method using the impression foam
shown in Figs. 1 and 2 to capture an anatomical shape used to create a support
contour of a cushion, in accordance with the present invention. Figs. 4-18 are
graphical representations of steps in the method shown in Fig. 3.
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Fig. 4 is a perspective view of the impression foam enclosed within the
container shown in Fig. 1, positioned on a seat support structure of a
wheelchair,
illustrating the practice of one of the steps of the method shown in Fig. 3.
Fig. 5 is a perspective view of a block of impression foam similar to that
shown in Fig. 1, positioned in a shell seat of a wheelchair, illustrating the
practice
of one of the steps of the method shown in Fig. 3.
Fig. 6 is a perspective view of the impression foam enclosed within the
container shown in Fig. 1, positioned on a conventional shape-capturing
simulator
machine, illustrating the practice of one of the steps of the method shown in
Fig. 3.
Fig. 7 is a perspective view of an individual seated on the impression foam
in the wheelchair shown in Fig. 4, showing an assistant applying additional
pressure to the seated individual when creating an impression of the
anatomical
shape, illustrating the practice of a step of the method shown in Fig. 3.
Fig. 8 is a perspective view similar to Fig. 7, illustrating an additional
aspect
of the step shown in Fig. 7, and also illustrating the creation of a dynamic
impression in accordance with the practice of steps of the method shown in
Fig. 3.
Fig. 9 is a perspective view of the impression foam enclosed within the
container structure shown in Fig. 1, in which an impression of the anatomical
shape of an individual has been captured by executing the steps shown in Figs.
4,
7 and 8, illustrating the practice of another step of the method shown in Fig.
3.
Fig. 10 is a perspective view showing adjustments to the shape captured
within the impression foam shown in Fig. 9 to provide areas of greater relief
in the
support contour of the cushion, illustrating the practice of another step of
the
method shown in Fig. 3.
Fig. 11 is a perspective view illustrating the relief-adjusted shape shown in
Fig. 10, around which a mold form has been placed and into which liquid
molding
material is placed, illustrating the practice of another step of the method
shown in
Fig. 3.
CA 02475638 2004-07-23
Fig. 12 is a perspective view of a positive mold of the relief-adjusted
impression shown in Fig. 10, after hardening the molding material shown in
Fig.
11, illustrating the practice of another step of the method shown in Fig. 3.
Fig. 13 is a perspective view showing adjustments to the positive mold
shown in Fig. 9, to provide areas of enhanced protrusion in the support
contour of
the cushion, illustrating the practice of another step of the method shown in
Fig. 3.
Fig. 14 is a perspective view of the protrusion-enhanced positive mold
shown in Fig. 13, inserted within a cushion-forming form, illustrating the
practice of
another step of the method shown in Fig. 3.
Fig. 15 is a perspective view of the protrusion-enhanced positive mold
within the cushion-forming form shown in Fig. 13, into which plastic beads are
added, illustrating the practice of another step of the method shown in Fig.
3.
Fig. 16 is a perspective view of the cushion-forming form shown in Fig. 15,
with an upper enclosure placed thereon to confine the plastic beads within the
cushion-forming form as a part of creating a support structure from the beads,
illustrating the practice of another step of the method shown in Fig. 3.
Fig. 17 is a perspective view of the positive mold shown in Fig. 13 and the
support structure formed around the positive mold as shown in Fig. 15,
illustrating
the practice of another step of the method shown in Fig. 3.
Fig. 18 is a perspective view of the support structure shown in Fig. 16 with
the cushion-forming form removed, illustrating the practice of another step of
the
method_shown in Fig. 3.
Fig. 19 is a perspective view of the finished cushion made by trimming and
contouring the support structure shown in Fig. 18.
Detailed Description
This invention makes use of a piece or block 30 of impression foam, shown
in Fig. 1, to capture the shape of an anatomical portion of the individual, to
fabricate a cushion 31 (Fig. 19) having a support contour 33 (Fig. 19) to
support
that captured part of the anatomy of the individual. The anatomical shape is
captured by contacting the individual with the block 30 of impression foam,
such as
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by having an individual sit or recline on block 30 (shown in Figs. 7 and 8).
The
weight or force of the individual crushes or indents the impression foam block
30
into an impression 35 (Fig. 9) which is a highly accurate negative impression
of the
individual's anatomical shape which contacted the impression foam.
To facilitate transportation and use of the block 30 of impression foam,
shown in Fig. 1, the block 30 is typically enclosed in a relatively rigid
container 32,
formed by sidewalls 34 and 36, a back wall 38 and a bottom wall 40. The
relatively rigid container 32 protects the block 30 of impression foam so that
inadvertent or unintended crushing of the block 30 will not occur except when
placed in contact with the individual's anatomy. The container 32 does not
include
a wall which extends across the front vertical surface of the foam block 30
between
the sidewalls 34 and 36 and the bottom wall 40, so that the user may sit on
the
foam block 30 and have his or her legs bend over the front surface of the foam
block (Figs. 7 and 8). Although not shown in Fig. 1, a lid structure may fit
over the
foam block 30 and the rigid container 32 to protect the upper and front
surfaces of
the foam block 30 prior to its use in capturing the anatomical shape.
A highly accurate shape 35 (Fig. 9) or negative impression is obtained from
the crush characteristics of the impression foam block 30. The crush
characteristics are illustrated by the curve 42 shown in Fig. 2. The amount of
force
required to crush the foam over a considerable distance is referenced at point
44.
The crush force 44 remains essentially constant from point 46 to approximately
point 48. When the foam has been crushed to the extent represented by point
48,
the crushing force increases substantially and almost instantaneously within a
relatively slight further crushing distance. Crushing the impression foam past
point
48 requires substantially increased force, as shown by the almost vertical
extension of the curve 42 past point 48.
It is within the range of crushing distances between points and 46 and 48
that the impression foam of the present invention should be used. Point 46
represents the uncrushed surface of the foam block 30 (Fig. 1) and point 48
represents the maximum depth to which the impression foam can be collapsed
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from its original surface while experiencing approximately constant crush
force 44.
The distance represented between the points 46 and 48 is related to the
initial
thickness of the foam block 30 (Fig. 1). The distance between points 46 and 50
represents a typical initial thickness of a foam block which will achieve a
constant
force or resistance crushing depth between points 46 and 48.
The considerable degree or distance of collapse at the relatively constant
resistance or force 44 allows a fully-loaded impression of the anatomical
shape to
be captured. A fully-loaded impression or shape 35 (Fig. 9) is that created by
equal resistance on and over substantially all portions of the shape 35 as it
is fully
formed, without more resistance being applied in some areas of the anatomical
shape compared to the amount of resistance applied in other areas. As a result
of
forming the negative impression of the anatomical shape by crushing the foam
with
approximately equal resistance independent of the extent or amount of crushing
or
indentation, the characteristics of the negative impression 35 (Fig. 9) more
accurately reflect the true, equally-loaded or fully-loaded anatomical shape.
By
obtaining the negative impression of a fully-loaded anatomical shape, the
shape of
the cushion support contour will better interact with the anatomy to support
the
user as desired.
In addition to the desired crush-distance characteristics of the impression
foam shown in Fig. 2, the impression foam has an extremely low modulus of
elasticity, making it very brittle and inelastic. The crushed portions of the
impression foam are permanently collapsed, and will rebound only
insignificantly
when the crushing force is removed. This characteristic allows the impression
form to retain the shape that was pressed into it. Coupled with its
brittleness, the
foam has very slight compressive strength. The slight compression strength
allows for accurate and precise deformation of the foam without deflection of
the
forcing source. The impression foam also has a relative lack of structural
shear
force resistance. Shear force resistance refers to the ability of the
structural lattice
to resist relative sliding motion through a plane in the lattice. The
structural lattice
strength of the impression foam allows shear without deformation of any
adjoining
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areas. Consequently, the resulting impression is almost exact complement of
the
shape which created the impression, with virtually no displacement or
deformation
around the edges of the impression.
The impression foam collapses or crushes by first crushing the layers of the
foam lattice adjacent to the forcing shape which causes the impression. As the
crushing progresses, thin layers of crushed or failed material build up
adjacent to
the forcing shape because those failed layers have been fully compressed, in
the
same manner as full compression of the entire thickness of the impression foam
is
represented by the distance between points 48 and 50 shown in Fig. 2. Thin
layers of the impression foam material beneath the fully crushed layers
surrounding the forcing shape continue to crush at the uniform resistance or
force
represented by point 44 in Fig. 2 until the extent of the crushing reaches
point 48.
Impression foam which has proved satisfactory has a preferred crushing
force resistance of approximately 1.56 pounds per square inch. In general,
however, an acceptable range of crushing force resistance will be within the
range
of 1.50-1.85 pounds per square inch. Higher crushing forces will not allow the
weight of the user to indent into the impression foam an adequate amount.
Lower
crushing force resistance will cause the impression foam to crumble and crack,
making the impression useless.
The average ratio of the original height or thickness of the impression foam
(the distance between points 46 and 50, Fig. 2) and the distance that the
impression foam will crush under relatively uniform force or resistance (the
distance between points 46 and 48, Fig. 2) is preferably equal to or greater
than 5
to 1 (5:1), meaning that the impression foam will crush or collapse to at
least 80%
of its initial thickness under the uniform resistance or constant force 44.
The
average ratio of the original starting thickness of the impression foam to the
maximum compressed height at the asymptotic limit of the vertically extending
portion of the curve 42 (Fig. 2), is approximately 10 to 1 (10:1), meaning
that the
impression foam will crush or collapse to about 90% of its initial thickness
at the
point 50.
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Impression foam having these preferred characteristics is similar to the type
of foam used by florists in creating flower arrangements. However, much of the
floral foam does not crush sufficiently within the preferred range of crushing
force
resistance. Some of the other preferred characteristics of the impression foam
may also be met by typical floral foam. However, manufacturers of floral foam
are
able to adjust their fabrication processes to achieve impression foam having
the
characteristics preferred for use in this invention.
The impression foam is used to fabricate a seat cushion by practicing the
method 56 shown in Fig. 3. The steps shown in Fig. 3 are referenced by
individual
reference numbers and are also illustrated graphically in Figs. 4-18. The
details of
the method 56 shown in Fig. 3 are explained in conjunction with Figs. 4-18.
The first step 58 of the method 56 shown in Fig. 3 is to select a particular
configuration of a block 30 of impression foam (Fig. 1) and orient it to
capture the
negative impression or shape of the anatomical portion of the user's body.
Selecting the particular configuration of the foam block 30 is related to
orienting it
for capturing the shape. For example, a foam block 30 in the rigid container
32
may be positioned on a seat support structure of a conventional wheelchair 60,
as
shown in Fig. 4. In this case, the user will sit directly on the foam block 30
while
positioned in the wheelchair 60, in the same manner that the user would sit on
the
cushion formed by the shape captured by the foam block 30.
Another example of practicing the step 58 (Fig. 3) is shown in Fig. 5. There,
the foam block 30 is removed from the rigid container 32 (Fig. 1) and the foam
block 30 is shaped to be received within a pan seat portion 62 of a shell seat
64
which is attached to a wheelchair 66. In this case, the user will sit directly
on the
foam block 30 while positioned in the shell seat 64, in the same manner that
the
user would sit on a cushion fitted into the shell seat 64.
Fig. 6 shows another example of positioning and orienting the foam block
to capture the negative impression of an anatomical shape. The foam block 30
and the rigid container 32 are attached to a horizontal seat portion 68 of a
30 conventional seating simulator 70. The seating simulator 70 is of the type
which
CA 02475638 2004-07-23
normally includes a bag 72 containing beads which will be conformed around
part
of the anatomy of the user and then held in place by vacuum applied to the
bag.
In the circumstance shown in Fig. 6, the foam block 30 and the rigid container
32
are used in place of a bag, which would normally be positioned on the
horizontal
seat portion 68 of the simulator 70. The conventional bag 72 is still used on
the
vertical back part of the seating simulation.
Once the foam block has been oriented, as shown in Figs. 4-6, the user sits
down on the foam block 30, or otherwise the portion of his or her anatomy to
be
supported by the cushion is forced into contact with the foam block 30 (74,
Fig. 3).
Fig. 7 illustrates the user sitting on the foam block 30 within the wheelchair
60
(also see Fig. 4). The weight of the user crushes the impression foam 30 and
forms a negative impression 35 (Fig. 9) of the shape of the portion of the
users
anatomy in contact with the foam block 30.
In addition to the normal weight of the user, it may be advantageous for an
assistant to press the user more firmly down into the foam block, as shown at
76 in
Fig. 3 and in Figs. 7 and 8. Pressing the user down in the manner shown in
Figs.
7 and 8 assures that a complete fully-loaded impression of the anatomical
portion
of the user is created. Despite the relatively small crush resistance force of
the
impression foam, the relatively large surface area of the buttocks and legs
may
partially resist the creation of a complete impression 35. Pressing the user
down
as shown in Figs. 7 and 8 does not diminish or otherwise change the fully-
loaded
characteristics of the impression created, because the added force is
distributed
throughout the entire anatomical area which is being captured by the negative
impression in the impression foam 30. None of the added pressure is unequally
distributed to impact adversely the fully-loaded or equally-loaded impression
35
created by crushing the impression foam. Adding the extra pressure or force to
the anatomical portion from which the negative impression is created is
optional.
The necessity to add the extra pressure or force will be determined by the
surface
area and other characteristics of the impression which must be created, the
weight
naturally applied by the user, and the crush resistance force of the foam
block.
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After establishing the desired initial negative impression, as illustrated in
Fig. 7, the user is moved through a normal range of motion as exemplified by
the
two positions of the user shown in Fig. 7 and 8 and as shown at step 78 (Fig.
3).
In the example shown in Figs. 7 and 8, the upper torso of the user is moved
from
the upright position shown in Fig. 7 to the forward pivoted position shown in
Fig. 8.
This range of motion is repeated to assure that the dynamic aspects of this
type of
user movement will be reflected in the negative impression created in the foam
block. The upright and forward-tilted positions shown in Figs. 7 and 8
generally
represent the range of motion that a wheelchair user may experience when
pushing the drive wheels of the wheelchair. Any other type of movement that
the
wheelchair user might normally make, such as side to side movement, reaching,
or
the like, may also be undertaken while the user is seated on the foam block
30. In
this manner, the effect of dynamic movement of the user will be reflected in
the
negative impression 35 in the foam block 30. The reflections of the dynamic
range
of motion adjust the configuration of the negative impression 35 (Fig. 9) so
that the
cushion formed from the negative impression will provide more accurate and
comfortable support to the user throughout his or her range of normal use
activities.
After the negative impression in the foam block has been made, additionally
loaded, and moved through a dynamic range of movement, as shown at 74, 76 and
78 in Fig. 3, respectively, the user is removed from contact with the foam
block, as
shown at step 80 (Fig. 3) and in Fig. 9. Fig. 9 shows the negative shape or
impression 35 which has been created in the foam block 30 as a result of
crushing
the impression foam.
Thereafter, the foam block with the negative impression or shape 35 is
removed and physically transferred to the cushion manufacturer. The transfer
may
occur by mail, after the foam block 30 with the negative impression 35 has
been
sufficiently protected in the rigid container 32 and a top (not shown) has
been
attached to the container 32 to allow it to be transferred without deforming
the
negative impression 35.
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Not shown in Figs. 4-9 is the possibility of positioning the foam block 30 and
rigid container 32 on a horizontal surface so that the user can sit on it,
apart from a
wheelchair or other simulating device to obtain the negative impression 35.
The
relative simplicity of using the foam block 30 and container 32 avoids the
need for
expensive seating simulators and other types of complex shape-capturing
devices.
The foam block 30 and container 32 may be taken directly to the user at the
user's
home, and an impression taken at that location, without requiring the user to
travel
to the location of the shape-capturing equipment.
Once the foam block 30 containing the negative impression 35 (Fig. 9) is
received by the cushion manufacturer, the negative impression 35 may be
adjusted in accordance with a desired support theory to provide greater relief
or
clearance in certain areas and/or enhanced support in other areas, or simply
to
simulate the captured shape without modifications. Adjusting the captured
impression 35 is shown at 84 in Fig. 3 and in Fig. 10. The adjustments to the
negative impression to provide greater relief in certain areas are made with
the
use of a shaping tool 86 to crush certain areas of the negative impression 35
to a
greater extent than they have already been crushed by the anatomical shape of
the individual. Further crushing of the specific areas will result in a
relieved
negative impression 35a which provides greater clearance or separation between
the anatomical shape and the cushion in those areas. By further crushing the
negative impression 35 in selected areas, a greater amount of separation or
clearance from the anatomy is established in those areas. The greater
clearance
may be beneficial in certain areas where there are higher risks of pressure
ulcers,
such as on the skin which surrounds the ischial tuberosities, the greater
trochanters, the coccyx and sacrum, and in the perineal or genital area where
the
skin may be prone to breakdown due to heat and moisture. This application of
one
type of support theory is described more completely in the above-identified
U.S.
patent application Serial No. 10/628,860.
If the negative impression 35 is taken by a person qualified to modify the
negative impression to provide greater relief, the adjustment to the negative
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impression 35 can be evaluated by the user prior to completing the cushion.
The
foam block with the adjusted negative impression 35a is placed back into the
wheelchair of the user, and the user can use that foam block as an example of
the
pressure-relieved aspects of the completed cushion, by maneuvering the
wheelchair and moving his or her body within the wheelchair. Of course the
extent
of movement is limited so as not to break down the foam, but nevertheless
enough
movement may be accomplished for the user to determine the acceptability of
the
pressure-relief and clearance aspects of the support contour of the final
cushion.
Further crushing specific areas of the negative impression 35 may not be
required if a different support theory is applied to construct the cushion,
for
example the equal pressure distribution theory. In those circumstances where
the
support theory does not adjust the negative impression, the user can evaluate
the
feeling of the final cushion from the impression 35 in the foam block 30. In
those
circumstances, the negative impression 35 will become the support contour 33
of
the finally constructed cushion 31 (Fig. 18), and the user can evaluate that
support
contour from the negative impression 35 directly created in the foam block.
Next, the relieved negative impression 35a, is used to create a positive
mold 89 (Fig. 12) of the relieved negative impression 35a, as shown at step 88
in
Fig. 3. The positive mold 89 (Fig. 12) is an accurate simulation of the
anatomical
portion of the user, as influenced by the range of motion, and as influenced
by any
adjustments to create the relieved negative impression 35a. The positive mold
89
is created as shown in Fig. 11 by first placing the relieved negative
impression 35a
into a mold box 90. Thereafter, fluid molding material 92, such as plaster of
Paris,
is added to the mold box 90 to cover and completely fill the relieved negative
impression 35a. In addition, enough molding material 92 is added to the mold
box
90 to create enough substance in the resulting positive mold 89 so that the
positive mold structure will not inadvertently break into pieces, crack or
crumble.
After the mold box 90 has been filled with the fluid molding material 92, the
molding material is solidified or hardened. The solidified molding material 92
captures the shape of the relieved negative impression 35a (Fig. 10). The foam
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CA 02475638 2008-03-27
block 30 is removed from around the solidified molding material, as shown at
94 in
Fig. 3 and in Fig. 12, to reveal the resulting positive mold 89. The shape of
the
positive mold 89 is identical to the shape of the anatomical portion of the
user,
which has been moved through the range of movement (Figs. 7 and 8) and
adjusted (35a) to provide greater pressure and clearance relief in certain
areas 91
where the negative impression 35 was further indented in accordance with the
support theory as discussed in conjunction with Fig. 10. (The added clearance
in
the perineal area is not shown.) The shape of the positive mold 89 is
complementary in shape to the relieved negative impression 35a (Fig. 10).
To the extent that the positive mold 89 may have slight rough surfaces or
slight irregularities, the shape of the positive mold 89 may be smoothed and
otherwise contoured. The amount of smoothing or contouring required depends to
some degree on the consistency of the fluid molding material which solidified
around it and the ability to remove trapped air bubbles in the fluid molding
material.
In addition, material is removed from areas 95 of the positive mold 89 to
enhance the support characteristics and offloading of bony prominence.
Removing
material from the areas 95 of the mold 89, as shown at 96 in Fig. 3 and in
Fig. 13,
will create complementary areas of the support contour 33 of the finally-
constructed cushion 31 to protrude or extend more into the anatomy of the
user,
because removal of the material from the mold 89 in the areas 95 causes more
support material to be formed in the completed cushion at those areas. The
removal of material from the mold 89 in the areas 95 creates a protrusion-
enhanced mold 89a, as shown in Fig. 13.
The protrusion-enhanced mold 89a therefore defines configurations where
the finally-constructed cushion will provide areas of enhanced or projected
support
and areas 91 of greater relief. Of course, the areas of enhanced support 95
may
be employed only to the extent required by the support theory, just as with
the
areas 91 of greater relief.
CA 02475638 2008-03-27
The protrusion-enhanced positive mold 89a is thereafter placed in a form
98, as shown at 100 in Fig. 3 and in Fig. 14. Moldable material 102 capable of
forming the cushion 31 (Fig. 19) is thereafter added into the interior of the
form 98
to completely engulf and surround the positive mold 89a, as shown in Fig. 15.
The
moldable material 102 is confined within the form 98 by placing a lid 104 on
the
form 98, as shown in Fig. 16. Thereafter, with the moldable material 102
confined
within the form 98 and engulfing and surrounding the positive mold 89a, the
moldable material 102 is allowed to cure as shown at 106 in Fig. 3.
Moldable material 102 capable of forming the cushion will typically be a
synthetic foam material which is fluid when surrounding the positive mold 89a
but
which later cures into a more rigid but nevertheless resilient support
structure 108
(Fig. 18) from which the cushion 31 (Fig. 19) is formed. There are a number of
different types of synthetic resilient plastic foam materials which meet these
requirements. The cushion manufacturer selects the type of synthetic resilient
plastic foam material to be used in forming the cushion, according to the
resiliency
characteristics of the plastic foam material desired.
The type of moldable material 102 preferred for use in the present invention
is generally circular polyethylene beads. The polyethylene beads are poured
into
the cushion-creating form 98 as shown in Fig. 15. Each of the polyethylene
beads
is formed with an exterior coating which is activated by heat. Once activated,
the
coating of each bead adheres to the coating of its adjoining beads, thereby
linking
all of the beads together in a single matrix-like structure which forms the
resilient
support structure 108 (Fig. 18) from which the cushion 31 (Fig. 19) is formed.
The plastic beads are available in different shapes, sizes, densities and
materials. For polyethylene spherical beads, the typical diameter is in the
range of
0.1875 to 0.25 inches, and the typical density is in the range of 12 grams per
liter
to 27 grams per liter. The cushion may be formed with an upper human interface
portion that presents the contour 33 and a lower base portion which completes
the
cushion. In that case, the size and density of the plastic
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CA 02475638 2004-07-23
beads may be different for each of the upper and lower portions, to impart
different
resiliency characteristics to each different portion. For example, spherical
polyethylene beads of approximately 0.25 inches in diameter and 12 grams per
liter may be used for the human interface portion and spherical polyethylene
beads of approximately 0.1875 inches in diameter and 27 grams per liter may be
used for the base portion. In those circumstances, the upper human interface
portion will have somewhat more resiliency while the base portion will have
somewhat less resiliency. When square or pillow-shaped polypropylene beads
are used, the size may be in the range of approximately 0.1875 inches on the
side
to approximately 0.09375 inches on the side, with a density of approximately
29
grams per liter.
By adjusting the size and density of the beads added into the cushion-
creating form 98 (Fig. 15) and the relative amount of compression of the beads
created when the beads are held in the form 98 by the lid 104 (Fig. 16) prior
to
heating to fuse them together at the contact points, the resiliency
characteristics of
the support structure 108 (Fig. 18) and the cushion 31 (Fig. 19) are adjusted
in
accordance with desired characteristics.
Because of the generally circular nature of the beads and the fact that the
beads are fused together at contact points, the resulting matrix-like
structure of
adhered beads has porosity which allows air and liquid to pass through the
matrix-
like support structure 108 (Fig. 18). This is a particular advantage in
wheelchair
cushions, because the ventilation of air to the areas of skin which are at
risk for
pressure ulcers generally decreases the incidence of such pressure ulcers.
After the moldable material 102 has cured within the cushion-creating form
98 (Fig. 15), and the shape of the support structure 108 of the cushion has
been
defined by the positive mold 89 (Fig. 14), the form 108 is removed, as shown
in
Fig. 17. The positive mold 89 remains within the support structure 108 as
shown
in Fig. 17. Next, as shown at 110 in Fig. 3 and in Fig. 18, the positive mold
89 is
removed from within the support structure 108, revealing the cushion support
contour 33 created in the support structure 108. Of course the support contour
33
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has the shape of the positive mold 89a, which created the areas 91 and 95 of
greater relief and enhanced protrusion, respectively.
The completed cushion 31 is created by trimming or otherwise shaping the
support structure 108 into the desired exterior shape of the cushion 31, as
shown
at 112 in Fig. 3 and in Fig. 19. Only that portion of the support structure
108
outside of the support contour 33 is trimmed away, since the configuration of
the
support contour 33 has been previously established as a result of using the
impression foam in the manner described above. After trimming the excess
amount of support structure 108 to create the cushion 31 shown in Fig. 19, the
matrix-like structure of fused-together polyethylene beads may be encased
within
a covering (not shown) to complete the construction of the cushion.
As an alternative to trimming the excess support structure 108 to form the
cushion 31, the shape of the cushion-creating form 98 may be configured to
establish the final cushion shape around the positive mold 89. In that case,
the
resulting support structure 108 becomes the cushion 31.
As is apparent from the previous discussion, the use of the impression foam
having the characteristics described allows a very accurate negative
impression of
the anatomical portion of the user to be obtained. This very accurate negative
impression 35 is obtained without the use of expensive and sophisticated shape
simulators. After adjusting the negative impression 35, if desired, the
adjusted
negative impression 35a is used directly to create the positive mold 89 from
which
the support contour 33 of the cushion 31 is formed. No measurements are
required, no translation of those measurements into a sophisticated
mathematical
shape simulating algorithm is required, and no separate creation of a positive
mold
based on a simulating algorithm is required. The block 30 of impression foam
retained within the container 32 may be readily transferred to the location of
the
user, and transferred from the user to the manufacturer. The crush resistance
characteristics of the impression foam also permit adjustments of the negative
impression to accommodate a range of user movement. The negative impression
formed in the foam block can be tried by the user before the completed cushion
is
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constructed. The cost of the custom cushion is reduced as a result of these
and
other factors, while still obtaining a better degree of fit and support in the
resulting
cushion. Many other advantages and improvements will be apparent after gaining
a full appreciation of the present invention.
Presently preferred embodiments of the invention and many of its
improvements and advantages have been described with a degree of
particularity.
This description is of preferred examples of implementing the invention, and
the
description of the preferred examples is not necessarily intended to limit the
scope
of the invention. The scope of the invention is defined by the following
claims.
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