Vacuum Forming Of Plastics In Prosthetics And Orthotics
A. Bennett Wilson, Jr. *
The first reference to vacuum forming of plastics applied in orthotics appeared in 1968, in an article in "Orthopaedics: Oxford" by Dr. Gordon Yates of England . In this well-illustrated article, he described the end products, lower-limb orthoses of polypropylene and ABS (acrylonitrile-butadiene-styrene) but left much to the imagination concerning the fabrication procedure. Included were Helfet heel-cups (similar to the University of California at Berkeley shoe inserts) , ankle-foot orthoses (AFO), and knee-ankle orthoses (KAO).
Inspired, apparently, by Yates, the Ontario Crippled Children's Centre and the George Brown School of Applied Arts and Technology, both of Toronto, Canada, began in 1970 a program in the application of vacuum-forming techniques in orthotics, at all levels, and to some degree in limb prosthetics . The result of this program was the design and development of a machine especially suited to needs of orthotists and prosthetists (Fig. 1 ). Eventually machines of this type were made available commercially by VAF Industries, Ltd.* also of Toronto.
In 1969 Snelson and Mooney of Rancho Los Amigos Hospital, Downey, California, received a grant from the Social and Rehabilitation Service, Department of Health, Education, and Welfare to develop a practical method for fabrication of a transparent socket. Methods of making clear sockets had been developed previously but none was sufficiently practical, even for use in research programs. After some experimentation, Snelson and Mooney turned their attention to vacuum-forming polycarbonate sheet stock in spite of being told by suppliers of the materials that "it won't work." The result was a method of producing plastic sockets, both clear and other-
wise, using very simple equipment (Fig. 2 ) . Further experience has shown that this simple method is also suitable for production of AFOs. Success with this method of production has persuaded Orthomedics, Inc.** to make a simple compact machine available commercially (Fig. 3 ).
In 1969 the Arkansas Cerebral Palsy Equipment Center began the use of individually molded wheelchair-seat inserts for severely involved cerebral palsied patients . The first sixty units were laid up in the conventional manner using fiberglass and resin, but since 1972, the seat inserts have been produced by vacuum-forming ABS over a mold using a machine commercially available from the EMC Co.*** for fabrication of plastic signs and other items. By use of the vacuum-forming technique, the production rate has been increased about 300 percent. In addition to the cerebral palsied patients, the Center is providing seat inserts for paraplegic and quadriplegic patients in order to spread the load over the weight-bearing areas when the patient is seated.
(An interesting facet of the Arkansas procedure is the method used for taking the impression, which consists of the revival of dilatancy , a process also employing the use of vacuum, and one which has been tried from time to time in taking impressions of amputation stumps. It may be time to initiate new experiments in dilatancy in view of the new knowledge gained and new processes developed over the years.)
As part of its evaluation program, the Committee on Prosthetics Research and Development ordered a prototype machine from VAF (Fig. 4 ) and sent it to the Veterans Administration Prosthetics Center for trials. Primarily because of electrical problems, it was difficult to obtain experience sufficient to evaluate either the equipment or the process. Meanwhile, Moss Rehabilitation Hospital in Philadelphia, because they could not obtain early delivery from VAF, purchased a large commercially available machine from Plastic Vac, Inc.,+ primarily to make lower-limb orthoses (Fig. 5 ).
Later the J.A. Pentland Co. and Rancho Los Amigos Hospital purchased VAF machines similar to one bought by CPRD.
Natresources, as a consultant to NOPCO, designed a special machine for prosthetics and orthotics practice for use in Boston. Massachusetts. Others, including a group in Winnipeg, were experimenting with vacuum forming.
Because it seemed beneficial to bring the various groups together and exchange information and then develop recommendations for future work, CPRD convened a meeting of representatives of all groups in North America known to have experience in vacuum forming of components for prostheses and orthoses at Moss Rehabilitation Hospital, June 3, 1973 .
It was the consensus of the participants that vacuum forming of plastics offers great potentials for improving not only prosthetics and orthotics services, but also should be very useful in educational programs.
It was also the consensus that not enough is known about materials and techniques. While general knowledge exists in industry where plastic shapes are mass-produced for a large variety of products, specialized knowledge is needed in tailoring special shapes in the prosthetics and orthotics field. In order to assure satisfactory results, there is a need to determine specifics in process variables for different materials and end products. Until the variables unique to the fabrication of prosthetic and orthotic components are documented, the process cannot be taught effectively.
The participants from educational institutions felt that equipment and processing details for vacuum-forming orthotic components such as lower-limb appliances were not well enough defined to consider opening courses immediately. Teaching the vacuum forming of larger components, such as spinal or other torso-fitting configurations, was even further off in time.
The state of the art in vacuum-forming sockets, however, appeared to be ready for use in formal educational programs. Accordingly, it was proposed that a two-day instructional course be convened in Downey, California. Orthomedics agreed to teach the course without reimbursement. It was held July 2 and 3, 1973, and was attended by representatives from New York University, Northwestern University, University of California at Los Angeles, and the University of Washington.
METHODS AND MATERIALS
There are many methods of using air-pressure differential to form heated plastics in sheet form into a given shape. In some instances, female molds are used; in others, male molds, and elaborate schemes have been developed for the mass production of items to provide uniform wall thickness and intricate shapes (Fig. 6, Fig. 7, Fig. 8 and Fig. 9 ). For prosthetics and orthotics, the use of the method known as "drape" molding, or forming (Fig. 9 ), seems to be the most appropriate, although it is sometimes difficult to obtain uniform wall thickness by drape forming. All of the machines used in prosthetics and orthotics to date use the drape method.
Five materials have proven to be useful in prosthetics or orthotics: polycarbonate, acrylonitrile-butadiene-styrene, cellulose acetate butyrate, polypropylene, and polyethylene. Their physical characteristics are shown in Table 1 .
No single material has an ideal set of properties for use in prosthetics and orthotics. Polycarbonate is transparent and has high impact resistance, but its fatigue strength is low and the cost is high. ABS seems to be the preferred material for spinal supports and seats because of its rigidity. CAB is used rarely because of certain properties such as rate of elongation, but should not be overlooked for special cases. Polypropylene's resistance to fatigue and its low cost makes it very useful for limb orthoses, although it is not available in a transparent form. Polyethylene shows a lot of promise because when formed over a layer of Plastazote, which is itself expanded polyethylene, a perfect bond is formed. This readily permits the installation of a cushion between the soft tissues and the external supporting structure when necessary.
Other materials, new and old, are being tried at various places in an effort to find even more suitable materials for each application.
STATE OF THE ART
The use of vacuum forming in prosthetics and orthotics is increasing progressively. The following examples are those known to the author, but by no means include all that are being used in North America.
At the OCCC, vacuum forming is used routinely in the supply of virtually all lower-limb orthoses, back panels for orthoses, and in many seating and support devices. Polypropylene is generally used for limb orthoses, polycarbonate for back panels, and ABS for seating and support devices (Fig. 10, Fig. 11, Fig. 12).
At Orthomedics, polycarbonate sockets are used routinely as check sockets in lower-limb prosthetics, and polypropylene lower-limb orthoses are used widely (Fig. 13) .
At the Veterans Administration Prosthetics Center approximately 75 percent of the ankle-foot orthoses are being vacuum-formed of polypropylene. Experiments to develop practical methods of fabricating knee-ankle orthoses, orthoses for the upper limb, and limb prostheses are under way.
In Arkansas the Cerebral Palsy Center is routinely providing vacuum-formed seat inserts of ABS for cerebral palsy patients and patients with spinal-cord injuries.
Moss Rehabilitation Hospital is providing on a routine basis lower-limb orthoses molded from polypropylene and is experimenting with other materials. This group at MRH is also developing methods to check the performance of any given orthosis in order to develop a method for correlation of prescription, fabrication, and fitting (Fig. 14, Fig. 15 and Fig. 16 ).
The Prosthetics Education Program at the University of Washington is using the Ortho-medics technique in their prosthetics education program. Clear sockets have been found to be very useful for laboratory instruction.
A firm, Plasthetics, Inc.,++ has been established to provide vacuum-forming services, especially for prosthetists and orthotists.
ADVANTAGES AND DISADVANTAGES
At this point, it appears that vacuum forming of sheet plastics offers prosthetists and orthotists an opportunity to provide improved, more functional prostheses and orthoses more rapidly than can be done with present practice. Orthoses, especially, can be tailored to fit the needs of the patients more adequately with thermoplastics than with either metal or plastic laminate. The cosmetic factors should not be overlooked. Color, even transparency, and snugly fitting orthoses provide less conspicuous devices.
Vacuum forming has the potential of providing the clinical prosthetist prompt service when required, especially for patients fitted for the first time. The use of check sockets in lower-limb prosthetics becomes practical. Needed to be developed are simplified means for coupling the vacuum-formed socket to a pylon for extended use.
For the first time now, research groups have an inexpensive way of forming sockets, transparent as well as translucent, so that extensive experiments in casting techniques, heretofore considered to be too costly, can be carried out. At the present time, just such an experiment is being conducted jointly by Rancho Los Amigos Hospital and CPRD in determining the relative differences between the conventional technique for casting a below-knee stump and two other recently developed methods.
It would seem that vacuum-formed sockets, especially transparent ones, would prove to be extremely valuable in an education program. Present-day prosthetist students do not so to school to learn lamination, yet they must laminate sockets over casts they have taken and modified in order to determine if they have taken the cast and modified it satisfactorily. The use of vacuum forming to provide sockets would permit the students to spend their time more fruitfully than making a lay-up.
THE FUTURE OUTLOOK
At this time, it appears that vacuum forming has a great future in prosthetics and orthotics. There is reason to believe that materials with even more appropriate physical properties will be emerging from the plastics industry, and ingenious prosthetists, orthotists and engineers will devise practical means of taking the maximum advantage from these new materials.
* 330 Bay Street, Suite 1001, Toronto 1, Canada.
** Orthomedics, Inc., 8332 Iowa Street, Downey, California 90241.
*** P.O. Box 9331, Dallas, Texas.
+ P.O. Box 5543, Charlotte, North Carolina.
++ 5640 Enterprise Drive, Lansing. Michigan 48910.
- Artamonov, Alex, Vacuum-forming techniques and materials in prosthetics and orthotics, Inter-Clin. Inform. Bull., 11:10:9-18, July 1972.
- Artamonov, Alex, Vacuum forming of sheet plastics, ISPO Bulletin, No. 4, October 1972.
- Bowker, John H., and B.A. Reed, A vacuum-formed plastic insert seat for neurologically handicapped wheelchair patients, Inter-Clin. Inform. Bull., 12:10:7-12, July 1973.
- Committee on Prosthetics Research and Development, Workshop on vacuum-forming equipment, a report of a meeting, National Academy of Sciences, June 1973.
- Hamontree, Samuel, and Roy Snelson, The use of check sockets in lower-limb prosthetics, Orth. & Pros., 27:4:30-33. December 1973.
- Henderson, W.H., and J.W. Campbell, UC-BL shoe insert: casting and fabrication, Bull. Pros. Res., BPR 10-11: 215-234, Spring 1969.
- Inman, V.T., UC-BL dual-axis ankle-control system and UC-BL shoe insert: biomechanical considerations. Bull. Pros. Res., BPR 10-11:130-145, Spring 1969.
- Lyons, Christopher, Vacuum-formed upper-extremity splints. Inter-Clin. Inform. Bull., 11:10:19-23, July 1972.
- Mereday, C., C.M.E. Dolan, and R. Lusskin, Evaluation of the UC-BL shoe insert in "flexible" pes planus, New York University, September 1969.
- Mooney, Vert, and Roy Snelson, Fabrication and application of transparent polycarbonate sockets, Orth. & Pros., 26:1:1-13, March 1972.
- Murphy, E.F., The fitting of below-knee prostheses, Chapter 22 in Klopsteg and Wilson's "Human Limbs and Their Substitutes," McGraw Hill, 1954, reissued by Hafner Press 1968.
- Povey, R.W., A vacuum splint for use in orthopaedic operations, J. Bone and Joint Surg., 52-B:3: 535-539, August 1970.
- Snelson, Roy, Fabrication of vacuum-formed sockets for limb prosthetics, Orth. & Pros., 27:3:-13, September 1973.
- Yates, Gordon, A method for the provision of lightweight aesthetic orthopaedic appliances, Orthopaedics: Oxford, 1:2:153-162, 1968.