New plastics for forming directly on the patient
J. Compton *
J. E. Edelstein *
Several newly available plastics become malleable when immersed in warm water or subjected to warm air; they return to a more rigid state when cool. Because fabrication requires that they be heated only to temperatures tolerated by the skin, one can mould them directly on the patient to produce custom-designed and individually-fitted devices. The new thermoplastics are modified readily upon reheating with warm water or air. Although varying in colour and texture, these materials all present a comparatively pleasing appearance. As a group, however, they are less rigid and less durable than other plastics or metals used in prosthetics and orthotics. While the initial cost of the new thermoplastics exceeds that of other materials, the finished appliance generally is less expensive because less time, skill, and equipment are needed for fabrication. Each plastic is analyzed in terms of its chemical and physical characteristics; commercial forms, whether marketed in sheet, tape, or precut splint form; and fabrication requirements. Although their principal use is in upper-limb orthotics, especially hand and forearm appliances, these plastics are suitable for selected application in spinal orthotics, such as collars and body jackets; lower-limb orthotics, particularly shoe inserts; and for the sockets of temporary prostheses.
Greater utilization of plastics is one of the most important trends in modern orthotics. In the past, orthoses were made almost exclusively of metal, plaster of Paris, or leather. These materials are still used, but, as in prosthetic practice, plastics are employed with increasing frequency.
Plastics are synthetic materials which may be moulded, extruded, laminated, or otherwise formed into various shapes. These materials may be categorized as thermoplastics and thermosets, the latter being formed by moulding liquid plastic over a positive model. Once solidified, thermosets cannot be reshaped, except to a minor extent, by subsequent application of heat; they are very suitable, however, where great rigidity is required.
Thermoplastics, in contrast, become formable when heated, and rigid when cooled. After they have cooled, they may be reheated and reshaped many times. This useful feature permits repeated alterations of the orthosis. Many newly marketed thermoplastics become malleable when heated to temperatures low enough to be tolerated by the skin and therefore can be applied directly to the patient.
Plastics which require no more than 80°C (180°F) to become workable may be termed low-temperature thermoplastics. Some foams must be heated to a higher temperature initially, but, since air convection cools their surfaces rapidly and they have low conductivity, they can be formed safely on the patient; such materials also may be considered as low-temperature thermoplastics. Since no cast is needed, low-temperature thermoplastics hasten provision of orthoses to the patient and are so relatively easy to work that most rehabilitation personnel can form devices with minimal practice and very little equipment. Even the most modestly furnished department should be able to supply hot water, scissors, and a source of hot air such as a heat gun, toaster oven, or electric frying pan with which to fabricate orthoses from most of the low-temperature materials.
Thermoplastics for orthoses are generally manufactured in sheets. A few are sold as rolls of plastic-impregnated fabric. Such bandage can be wrapped easily around the body segment or can be applied as overlapping, layered splints.
Plastics provide a very wide range of choices in physical characteristics, especially strength, rigidity, and appearance. Strength is indicated by the amount of force necessary to cause failure, either by breakage or stretching. Rigidity is indicated by the force needed to cause deflection of the material for a given distance. These properties depend on the thickness, as well as the chemistry of the material. Conse-quently, orthotic design should include strategically-placed reinforcements, curves, corrugations, and rolled edges to increase rigidity selectively.
It is very easy to cut a pattern in low-temperature thermoplastics with scissors, particularly when the material is warm. The very process of cutting the heated plastic, in most instances, produces smooth edges without need for sanding or polishing. Some of the stiffer plastics, however, can be cut better with a coping saw, while others are easier to cut with scissors when cool.
Although all materials have shrinkage and elastic memory, some display one or both of these properties to a marked extent. Shrinkage is reduction in size due to thermal stress; heat modifies intermolecular bonds which reduce their length. A material which shrinks markedly requires that it be cut from a pattern which is large enough to compensate for shrinkage so that the orthosis, once completed, will not fit too tightly. Low-temperature thermoplastics have á relatively long working time, adequate to form complex shapes without resorting to repeated heating or the use of expensive equipment such as vacuum formers. Those materials which exhibit considerable elastic memory can be stretched a great deal when heated, yet will return to the original dimensions when cool. Elastic memory generally refers to the return to the original manufactured form after a material is mechanically stressed; when stress is relieved, the intermolecular bonds return to their relaxed state.
Most low-temperature thermoplastics are self-adherent when warm. Straps and other attachments can then be interlayered between folds of plastic. Some of those which are not readily self-adherent may be bonded by removing a thin portion of the surface with a solvent, then pressing the two sheets of plastic together. Others require the use of contact, epoxy, or solvent cements. An alternate method for securing attachments is by the use of rivets or similar fasteners. This type of hardware is not suitable for use in plastics which tend to creep, that is, deform slowly under sustained load; in such materials, fasteners may withdraw eventually from their holes.
These plastics permit rapid fabrication of custom-fitted orthoses. One may either trace a pattern from a pattern book or other source onto the plastic, or may design a pattern on paper toweling, cutting and piecing until the required contour is achieved; the resulting pattern is then traced onto the plastic. For those materials which are very elastic when hot, one need cut only general contours from the plastic, leaving broad margins, then shape and trim the plastic while it is draped on the patient. For standard applications, such as a wrist orthosis to prevent palmar flexion, it may be expedient to purchase a pre-cut splint. The flat plastic is then heated, shaped, and trimmed to suit the individual.
The principal use of low-temperature thermoplastics is in upper-limb orthotics where rapid provision of assistive or protective orthoses is often desirable. One can maintain the arches and interdigital spaces of the paralyzed hand with simple thermoplastic orthoses. Less rigid plastics, especially those which are foamed, may need to be reinforced in areas of high stress by interleafing metal or polyethylene strips. Greater rigidity is also necessary in hand orthoses which will incorporate outriggers. For support of the wrist, elbow, trunk, or other body segments which present relatively smooth curves, one should select stronger, more rigid plastic, perhaps adding a thin lining of polyethylene foam to improve pressure distribution. Foamed polyethylene is also useful for custom-made inner soles for patients with deformed feet, as well as for cervical collars. Many of the more malleable plastics are ideal for forming custom-contoured handles on utensils for patients with impaired grasp. Low-temperature thermoplastic, particularly that manufactured in tubular form, is appropriate for fabrication into sockets, especially for below-knee and below-elbow prostheses.
Low-temperature thermoplastics are not suitable for all orthotic applications. Where high stress is anticipated for example, in an orthosis to be worn for a long period, or one to be applied to a severely spastic patient who will exert great force on it, thermoplastics which require higher heat for forming, thermosetting plastics, or metal may be needed for the requisite strength. Although sheets of low-temperature thermoplastics are generally much more expensive than other plastics, metal, or leather, the relative reduction in fabrication time and technical skill needed to complete an appliance usually results in an appreciable net economy.
Table 1 indicates pertinent information on the characteristics of plastics which can be formed directly on the patient. The comments are intended to highlight the distinctions of each plastic, rather than to provide the detailed analysis or fabrication instructions which can be secured from the manufacturer.
Low-temperature thermoplastics are an important addition to the professional armamentarium which may permit faster provision of custom-made orthoses to speed patient rehabilitation.
AliMed, 138 Prince Street, Boston, Massachusetts 02113 (Aliplast).
BRB Industries, Inc., 467 11th Street, Hoboken, New Jersey 07030 (Surgical Orthopaedic Splinting).
Hexcel Medical Products, 11711 Dublin Boulevard, Dublin, California 94566 (Hexcelite).
Johnson and Johnson, New Brunswick, New Jersey 08903 (Orthoplast).
Merck Sharp and Dohme Orthopedics Company, Inc., 2990 Red Hill Avenue, Costa Mesa, California 92626 (Lightcast).
Rolyan Medical Products, Post Office Box 555, 14635 Commerce Drive, Menomonee Falls, Wisconsin 53051 (Polyform).
Fred Sammons, Inc., Box 32, Brookfield, Illinois 60513 (Bioplastics, Kay-Splint).
Smith and Nephew Limited, 2100 52nd Avenue, Lachine 620, Quebec, Canada (Plastazote, Glassona, San-Splint).
Thermo-Mold Medical Products, Inc., Route 130, Post Office Box 36, Burlington, New Jersey 08016 (Warm 'n Form).
United States Manufacturing Company, 623 South Central Avenue, Post Office Box 110, Glendale, California 91209 (Polysar).
WFR Corporation, 199 Gregory Boulevard, Norwalk, Connecticut 06855 (Aquaplast).