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-Lite® 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-Lite® 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.