Computer-Aided Thigh Corset Pattern
Generation
Marc St-Georges, Eng.
Claude Levesque, C.P.(c)
Carole St-Jean, C.P.(c)
Introduction
A lower limb prosthesis is essential if upright mobility is to be restored to those who
have suffered the loss of a lower limb. An
artificial leg is basically composed of a series
of components that respectively imitate and
simulate the appearance and the function of
the lost anatomical parts. Since leg design
depends on the geometrical relationship between these components and the interface
with the amputee, the prosthetic socket, its
alignment on the limb and the means employed to provide suspension are therefore
the most significant elements in the limb design.1
To provide the weight support and control
so essential for standing and walking, the
prosthetic socket cannot be simply a positive
mold of the shape of the remaining limb. It
must be designed specifically to support the
pressure developed between the residual
limb and the prosthesis under dynamic as
well as static loading conditions.
The main function of the residual limb is
that of a lever arm that is used to power and
control the prosthesis through the socket interface. The more intimate and accurately
defined the socket and the more precise the
fit on the residual limb, the more efficient
will be the force transfer. Furthermore, the
greater the surface area in contact between
the residual limb and the socket, the better
the fit and thus the better the control of the
prosthesis. For these reasons, maximum
contact area at residual limb and socket interface is the most desirable.
At the Institut de Réadaptation de Montrial, a type of socket which is often used is
the weight bearing thigh corset. This type of
thigh corset, used with trans-tibial amputees, is an adjustable system providing additional weight support and knee stability.
The thigh corset is usually fabricated of
heavy, seven- to eight-ounce molding leather, a material sufficiently rigid to maintain a
proper shape in accordance with biomechanical principles of weight support in the thigh
region.2 This shape is preferred, since the
weight is borne mostly on the proximal region of the thigh region.
Prosthetic socket design consists of four
major interrelated considerations: support,
control, suspension and alignment. All these
considerations make proper thigh corset pattern production a tedious task. Precise patient measurements are taken in order to
produce the technical drawing which is used
as the pattern for thigh corset creation. This
design procedure requires approximately 30
minutes and design methods vary from practitioner to practitioner. A tool which lends
itself very well to doing this task is computer-aided drafting and design (CAD). It allows
for standardized, accurate and rapid pattern
generation. Furthermore, the widespread
availability of computers, CAD software
and computer peripherals and their ever decreasing costs, make computer-aided pattern
generation more and more feasible and
worthwhile.3
Materials and Method
A computer program has been written using the computer aided drafting and design
language (CADL) of the CAD software
CADKEY, version 3.02 ($500 US), which
runs on an IBM XT or AT (or compatible),
with a hard disk, a CGA screen and a graphic
display card. The patterns are plotted on a
Hewlett Packard model 7475A plotter (Figure 1)
. The program requires only three input measurements from the user: proximal
thigh circumference, distal thigh circumference and finally, thigh corset height. The
software is user friendly and the system is
easy to use. The user is prompted by the
computer for these three measurements.
Once these values are entered, the program
runs, and automatically displays in a matter
of seconds, the desired thigh corset, before
the user decides to plot out the final product.
Thigh corsets are plotted out on standard
11" x 17" plotting paper in less than two minutes. Thigh corsets that are larger than 16" in
circumference or 10" in height are easily plotted in two halves on two separate sheets.
Plotting time for these types of corsets is
approximately four minutes. Extra large
thigh corsets can even be plotted on three
separate sheets. Reference points ensure a
good alignment of separate sheets when this
is necessary. As shown in Figure 2
, different
information such as patient name, date, etc.,
is recorded on the pattern produced. It is
important to note that the thigh corset pattern shown in Figure 2
is scaled down in order to include it in its entirety on an 8.5" x 11"
sheet of paper. Furthermore, set-up positions are indicated, as are eyelet holes. The
program is written in such a way that the
number of eyelet holes is determined according to thigh corset height. There is the possibility of four, five or seven holes. Also, three
cubic spline functions are used in the program to respectively draw up the proximal
and proximo-lateral curves of the thigh corset and the postero-distal curves of the thigh
corset. The cubic spline functions allow for
optimum curve production for varying thigh
corset dimensions. For example, the postero-medial region of the pattern varies according to thigh corset height. In the end, a
complete thigh corset pattern is produced.
No further adjustments or operations are
necessary. The leather thigh corset can be
cut from the pattern once proper fit and
overlap are assured.
Discussion and Conclusion
The use of computer-aided drafting and
design for automatic thigh corset pattern
generation has produced preliminary results
that are very promising. It is presently used
at our amputee clinic to produce half the
thigh corset patterns prescribed. Time required to draw up the thigh corset pattern
has been reduced from approximately 30
minutes to four minutes, which is the time
required by the plotter to plot a complete
pattern comprised of two halves. This timesaving method not only is more cost effective
(faster fabrication and service delivery), but
ensures better quality control for all thigh
corset patterns, because each pattern is
drawn the same for identical input measurements and patterns are easily corrected or
redrawn in the event of an error. Furthermore, automatic pattern generation ensures
standardization of the design and drawing
processes and the plotting step ensures maximum accuracy of shape reproduction.
The only shortcoming of the method is the
relatively high cost of the computer equipment and peripherals used to produce the
thigh corset pattern. It could run between
$6,000 and $7,000 if one does not already
have a computer. On the other hand, acquisition of such equipment will certainly prove
to be a useful tool in a variety of other applications. For example, at the Institut de Readaptation de Montreal," the CAD software
(CADKEY version 3.02) is used as a simulation tool in wheelchair seating to effectively
design seating systems in wheelchair structures at our seating clinics. Also, a new pilot
project has been initiated to use CAD for
producing precise technical drawings to fabricate lower limb prostheses. This allows for
better job definition and task delimitation
between the prosthetists and the technicians.
In conclusion, computer-aided drafting
and design is a worthwhile and cost-effective
tool in thigh corset pattern generation. It
reduces pattern fabrication time by 85 percent and improves quality control. Volume
of client technical files and dossiers is reduced and access for continued consultations
and modifications is made easy. Overall,
specialists and clients will benefit.
Editor's note: Readers who want a copy of a
sample program should contact the authors at the
above address.
Claude Lévesque is a certified prosthetist at the
Montréal Rehabilitation Institute, 6300 Ave Darlington, Montreal, Quebec, Canada H35 2J4,
(514) 340-2080.
Carole St-Jean is a certified prosthetist at the
Montréal Rehabilitation Institute.
Marc St-Georges is a rehabilitation engineer at
the Montréal Rehabilitation Institute.
References:
- Kottke, F.J. et a!., Krusen's Handbook of
Physical Medicine and Rehabilitation, W.B.
Saunders Company, Third Edition, Philadelphia,
PA, 1982, 1023 pages.
- American Academy of Orthopaedic Surgeons, Atlas of Limb Prosthetics; Surgical and
Prosthetic Principles, The C.V. Mosby Company,
St. Louis, MO, 1981, 668 pages.
- Souter Glass, D., "Leatherwork Pattern
Generation by Computer" (Technical Note), Orthotics and Prosthetics, Summer 1987, 41:2, 32
pages.
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