Clinical Experience with Total Thermoplastic Lower Limb Prostheses
Vernon R. Rothschild, C.P.O.
John R. Fox, C.O.
John W. Michael, M.Ed., C.P.O.
Russell J. Rothschild, B.S.
George Playfair, Technician
The availability of alloy mixtures of plastics over the past two decades has expanded
the applications for sheet thermoplastics.
While polypropylene has widespread applicability in orthotics, its homopolymer form
has proven to be too brittle for many prosthetic designs. Although the technical feasibility of thermoplastic below-knees (BKs)
was recognized years ago,1,2 fabrication difficulties and particularly problems with breakage prevented widespread use.3,4
Over the past nine years, a practical technique for fabrication of hollow, all-thermoplastic BK prostheses weighing an average of
1.5 pounds (680 grams) has been developed
(Figure 1
). Enthusiastic clinical acceptance
by more than 300 patients fitted by one author (VRR) has encouraged exploration of
above-knee and endoskeletal designs as
well. This preliminary report is intended to
share this concept and encourage other practitioners to investigate the technique.
Although many thermoplastics have been
tried over the years, copolymer alloys of
polypropylene and polyethylene have yielded the best long term results thus far. One-eighth-inch (3 millimeters) copolymer is currently recommended for exoskeletal BKs
and 3/16 inch (5 millimeters) for endoskeletals.
One of the major shortcomings of the
thermoplastics currently used in prosthetics
and orthotics is their tendency to shrink linearly. Since the shrinkage is greatest if the
material is stretched during fabrication, it
must be draped carefully and vacuum
formed quickly for optimum results. Particularly for the exoskeleton, which provides
structural integrity to the prosthesis, drape
molding seems to provide the most uniform
wall thickness throughout the structure. The
polyethylene content creates a sturdy selfadhesive seam which can be buffed nearly
flush; no thermal welding is required.
At first the application was restricted to
somewhat feeble geriatrics with limited ambulation potential. Their enthusiastic response to the reduced prosthetic weight often encouraged more active ambulation. As
experience revealed no catastrophic materials failures in this group, the technique was
cautiously applied to more vigorous individuals. Thus far, no structural failures have
occurred regardless of patients' weight or activity level. One author (VRR), a 235-pound
(107 kilos) BK amputee, has worn a joint
and corset version of this prosthesis daily for
three years without problems.
The exoskeletal BK technique reported
here is a refinement of the method first reported by Wilson and Stills in 1976,5 and is
similar to the thermostat technique taught by
Otto Bock to create a hollow lower limb
prosthesis.6 Creation of a nearly hollow external-keel SACH foot (Figure 2)
is recommended for maximum weight savings.
Although the limited function of the
SACH foot was initially a concern, the
marked weight and inertia reduction has
been so well received by the amputees that
most accept the SACH function without
comment. When fitting more vigorous individuals, the inherent resilience of the copolymer alloy seems to be allowing slight, controlled flexion in stance phase. We now speculate that the use of thermoplastics may impart a measure of dynamic response to the
SACH mechanism, in proportion to the applied loading.
This observation led to investigation of the
merits of an all thermoplastic BK endoskeletal design. As suggested by Valenti previously,7 we believe this approach offers even
more potential for controlled dynamic response. By varying the cross sectional shape
and area of the pylon, it has been possible to
selectively increase or decrease flexibility.
For example, a diamond cross section seems
much more rigid than an equivalent circular
contour. In principle, this may ultimately
lead to the development of selective shapes
for specific biomechanical functions in prosthetics, just as today's certified orthotist routinely varies thermoplastic AFO contours
and thickness to produce the desired biomechanical control.
Endoskeletal designs are particularly useful for higher level amputations. We have
fitted five thermoplastic endoskeletal AKs
to date, using a titanium polycentric knee
(Figure 3)
. Total finished weights have been
as little as 3.5 pounds (1.6 kilos). As in the
exoskeletal BK, it is the terminal pylon/ankle/foot area that is markedly lighter.
This raises the effective center of gravity
of the shank, which increases its rate of
swing, just as raising the counterweight on a
grandfather clock accelerates its function.
This also dramatically reduces the inertial
drag of the shank so that the patient's perceived reduction in effort is greater than the
actual weight savings. This is the same principle utilized by the Flex-Foot and similar.8
The clinical advantages of a lightweight,
low inertia prosthesis are well known.9,10 As
would be expected, patients fitted with this
design have commented favorably on the
ease of swing phase, the reduction in pistoning and a perceived reduction in forces transmitted to the anterior residual limb. All
types of BK amputations and activity levels
have been successfully fitted with the total
thermoplastic design, including a few Symes.
Suspension is obviously much less difficult
with a low inertia prosthesis. Cuff suspension (often without waist belt), rubber
sleeves, joints and thigh corset, or supracondular soft inserts have presented no difficulties; other variants should be equally feasible. Various AK suspension belts as well as
full suction have also been readily achieved.
The chief disadvantage of this approach is
that linear, rotary and angular adjustments
are eliminated; only minor socket modifications are possible following final fabrication.
For that reason, meticulous attention to detail in fitting and alignment is critical.
On the other hand, it is relatively simple to
refabricate an all thermoplastic exoskeletal
BK, if alignment changes become necessary
(Figure 4)
. Pouring rigid polyurethane transfer foam back into the cavity of the prosthesis will capture the alignment and allow the
exterior shell to be split via cast saw and
removed. Necessary alignment changes can
be made by cutting, wedging, sliding, or inserting polyurethane foam spacers as necessary.
After minor smoothing, the model is
ready for a new vacuum/drape molding to
create the external shell. Hollowing out the
foam, preparing the keel and rebonding the
sole material completes the refabrication.
One recurrent problem in the early years
of development was delamination of the rubber sole material from the thermoplastic
keel. Development of a proprietary bonding
process seems to have eliminated this aggravation.
One final concern, expressed by some patients, is the limitation of thermoplastic colors to pink, brown, or white. If a custom
finish is required, a "beauty coat" consisting
of two layers of perlon stockinette laminated
with polyester resin will create a conventional external appearance. Application of a
heat-shrink Pe-LiteŽ cover11 or spray finish
such as New Skin11 is also possible. All such
techniques will add a few ounces to the finished result.
In summary, successful long-term experience with all thermoplastic lower limb prostheses for a broad variety of amputees is
being reported. Although similar to previously published techniques, the refinement
noted here uses readily available materials
and conventional thermoforming techniques
to create a lightweight, low inertia, water-resistant prosthesis. Since durability and patient acceptance have been encouraging, this
technique may have more widespread application than previously thought.
Vernon R. Rothschild, C.P.O., is president of
Rothschild's of Forestville, 7832 Parston Drive,
Forestville, MD 20747, (301) 736-9350.
John R. Fox, C.O., is vice president of Rothschild's of Forestville.
John W. Michael, M.Ed., C.P.O., is assistant
clinical professor and director of the Department of
Prosthetics & Orthotics at Duke University Medical Center, Box 3885, Durham, NC 27710, (919)
684-6890. He is also a member of the JPO Journal
of Prosthetics and Orthotics editorial board.
Russell I. Rothschild, B.S. Florida International University, is Vice- President of Rothschild's of
Forestville.
George Playfair, is a technician at Rothschild's
of Forestville.
References:
- Wollstein, L.V., "Fabrication of a Below-Knee Prosthesis Especially Suitable in Tropical
Countries," Prosthetics International, 4:2, 1972,
pp.5-8.
- Wilson, A.B. and M. Stills, "Ultra-Light
Prostheses for Below-Knee Amputees," Orthotics and Prosthetics, 33:2, 1979, pp.45-50.
- Reed, B., A.B. Wilson and C. Pritham,
"Evaluation of an Ultralight Below-Knee Prosthesis," Orthotics and Prosthetics, 33:2, 1979,
pp.45-50.
- Convery, P., D. Jones and J. Hughes, "Potential Problems of Manufacture and Fitting of
Polypropylene Ultralightweight Below-Knee
Prostheses," Prosthetics and Orthotics International, Vol. 8, 1984, pp.21-28.
- Wilson, A.B. and M. Stills, "Ultra-Light
Prostheses for Below-Knee Amputees," Orthotics and Prosthetics, 30:1, 1976, pp.43-47.
- Schroeder, F.K. and J.R. Hendrickson, "The
Otto Bock All Plastic AK Prosthesis for the Geriatric Amputee," Orthotics and Prosthetics, 22:1,
1968, pp.29-32.
- Valenti, T.G., "Experience with EndoFlex,"
AOPA Assembly, Washington, D.C., 1988.
- Michael, J.W., "Energy Storing Feet: A Clinical Comparison," Clinical Prosthetics & Orthotics, 11:3, 1987, pp.154-168.
- Vachranukunkiet, T., H. Lawall and M. Torres, "Multipurpose Prosthetic System for Bilateral Geriatric Lower Limb Amputees," Archives of
Physical Medicine and Rehabilitation, Abstract
65, 1984, pp.644-645.
- Leimkuehler, J.T., "A Lightweight Laminated Below the Knee Prosthesis," Orthotics and
Prosthetics, Vol. 36, 1982, pp.46-49.
II Pe-LiteŽ leg covers, Durr-Fillauer Medical,
Inc., P.O. Box 5189, Chattanooga, Tenn. 37406,
1-800-251-6398.
- New Skin, New Life Laboratories, Inc.,
12221 Wilshire Blvd., Los Angeles, Calif. 90025.
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