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Process of Development and Application of Porous Plastic to Prosthetic Sockets

Toshiro Nakamura, O.A.
Eiji Hatano, M.D.

Introduction

The majority of materials used today for artificial limbs and accessories are non-porous plastics. The prescribers and manufacturers of artificial limbs are primarily concerned with the functional and structural characteristics of prostheses, whereas the majority of users express a decided preference for improvement in the feeling of compatibility and comfort when wearing these artificial limbs. Perspiration and body odor emanating from the socket in contact with the skin present an acute problem, particularly during Japan's summer with its high temperatures and high humidity.

The insides of sockets made from nonporous plastic material become quite similar to a steam bath, because there is no ventilation for the heat and perspiration generated.

In the past, prosthetic sockets were made of porous materials such as wood and Tuflite.^1,2 In recent years, however, the majority of sockets are made of non-porous materials.

One company received recommendations from Ichiro Kawamura, president of Kawamura Artificial Limbs, Co., Ltd., Osaka, Japan, about the crucial requirements for porous material sockets in 1984. Efforts were immediately directed towards the realization of such sockets through research and development.

Tetsuya Watanabe and Masayoshi Hatano, et al., of our product development division, recognized that polyester hydrates approximated the porosity and moisture absorbing property of wood while still retaining the heat hardening property of plastics and, despite many repetitive trials and errors, progress was achieved toward producing a socket made from porous material (Figure 1) .

Selection of Material

1. Sintering molding of polyethylene and popypropylene: Sintering molded olefin resin products are widely employed as filtration materials; and depending upon the requirements, various materials can be produced with differing degrees of porosity. These materials are advantageous due to the differing degrees of porosity and ventilation which can be achieved over a wide area. However, for correct molding, metal molds and special engineering knowledge of temperature control during the molding process are required. The sintering molding of sockets thus incurs considerable cost.
2. Secondary processing of sintering molded olefin resin products: Secondary processing experiments were attempted using the molded products in sheet and tubular form.
• Heat was applied to sheet material and it was drawn over socket forms. However, as the sheet was elongated, the porosity was likewise deformed and extended beyond a practical degree.
• Forming tests were tried with finer porosity material, but sintered molded finer material was not as easily molded. Drawing into socket shapes invariably caused the material to rupture.
Therefore, the material was judged inappropriate for sockets.
3. Ceramics: Costs incurred for the necessary metal molds and the specialized engineering for molding were prohibitive.
4. Polyester hydrates: Ready availability of the material and the required basic engineering knowledge made this material particularly appropriate for socket production.

Material for the Inner Socket (Liner)

The production of artificial limbs using porous material demands that porous material also be employed for the inner socket. To satisfy this requirement, the following materials were considered:

1. Polyethylene foam: Non-porous, separate cellular polyethylene foam has been widely used for the inner sockets, but when formed into continuous cellular foam, its rigidity is weakened (inferior creeping characteristic) and is liable to readily collapse.
2. Polyurethane foam: This material, which recently became available on the market, can be heat processed, so we commenced with testing.

Physical Properties of Polyester Hydrates

The unsaturated polyester resin (commercially branded NakaresinŽ ), a product developed by Kayaku Nouilly Co., Ltd., permits the absorption of water into the resin. During production of the socket, first the unsaturated polyester is subjected to high-speed agitation and water is gradually added for an emulsified distribution. A prescribed amount of hardening agent is then added to this emulsion and is injected into the molds where it gels in approximately 10-15 minutes. An open cell socket is formed from a plastic which contains water distributed .throughout its structure (Figure 2) .

When this socket is dried at temperatures of 60° - 80°C, the water contained in the plastic evaporates and the areas where water was contained become cavities, resulting in the formation of a porous and permeable socket.

Figure 3 is a cross section Scanning Electron Microscope configuration of the plastic employed. The distribution of circular air cavities measuring in microns (0.5 03m) is easily discernible.

Characteristics which can be enumerated are: (1) moisture absorbency and permeability; (2) breathes moisture; (3) has the texture of wood; and (4) apparent specific gravity is light. Also it can be adjusted by changing the water/plastic mix ratio.

Porosity can be determined by the apparent changes in weight caused by changes in moisture content during processing, as indicated by Figure 4 .

The relationship between the amount of water added and the strength of the material can be judged according to the increasing amount of water, which reduces the strength of the material and results in a relative reduction in weight (Figure 5) .

Comparison of Polyester Hydrate and Wood

To investigate the ease of processing, strength, permeability or porosity, and moisture absorbent qualities, and in order to produce a socket that most closely resembles wood in preferred texture and feel, trial sockets made of 200cc polyethylene and varying amounts of water were produced and subjected to simple experiments.

Water Permeability

About l50cc of water was placed in the various sockets for permeability comparison. To ensure identical conditions, the distal ends of the sockets were made non-porous with "Rigolac" (Figure 6) . It is, therefore, apparent that the disappearance of water is proportionate to the thinness of the wood. In resin sockets more water can be contained in the socket wall. However, little difference in water loss was apparent between test sockets numbers 2, 3 and 4. In socket numbers 5 and 1, water permeability (water leak) was apparent to the naked eye.

Air Permeability or Porosity

The exterior surface areas of the respective sockets were made approximately iden tical and external pressure was applied. As indicated in Figure 7 the 5:5 ratio of socket number 1 is the most approximate equivalent to a wood socket.

Percentage of Water Content (Absorption)

Investigations were made to determine the capacity of the respective sockets for water absorption. Sockets were immersed in water for 24 hours and weighed (value "a"). The dried weight of the socket (value "b") after 10 hours at 500 was subtracted from value "a. The percentage of evaporated water or moisture ("a - b") is shown in Figure 8 . The average absorption rate of the two wood sockets is also indicated.

Considerable fluctuation occurred in the various factors considered, all of which depended upon the condition, pressure, and thickness of the emulsion at the time of molding. Our experiments showed that the closest approximation to wood was achieved when the water/plastic ratio was 5:5. However, at a 5:5 ratio, separation of water and plastic occurs during molding which makes processing difficult. In order to maintain ex tra strength, a ratio of 7 plastic parts: 3 parts water should be employed.

Manufacturing Process for Sockets

The laminating method simulates the conventional method, but requires a special external barrier of a particular silicon film our company produces and PE or PVA film to mix the water into the plastic. Figure 9 illustrates the kit contents. The production process is illustrated .in Figure 10 and the method is shown in Figure 11 .

Users Survey

By the end of 1987, our company produced artificial limbs, which were worn by iS subject above-knee amputees and about 35 subject below-knee amputees. In comparison with conventional sockets, perspiration at the amputated limb surface was negligible and users were satisfied with the artificial limbs. The longest term subject has worn a limb constructed of conventional materials for 35 years.

The results of the question of durability and general wear were satisfactory and no skin abrasions or sores arose. Survey results conducted on seven below-knee amputee subjects and two above-knee amputee subjects are listed in Table 1 .

Evaluation

Polyester hydrate sockets are constructed of a fine, lightweight material, which simulates the texture of wood. These sockets provide the amputated residual limb with a totally clean, more comfortable environment. The exterior appearance, however, may be slightly inferior and requires considerable care in controlling the hardening time. In some cases, the socket may require special reinforcement. The development of polyester hydrate sockets now makes it possible for amputees to overcome some of the discomforts associated with wearing prostheses. Another benefit is the low production cost which is possible using conventional lamination methods.

Summary

When wearing artificial limbs, residual limbs must enjoy conditions allowing the skin to breathe freely. Polyester hydrate resin sockets are designed to improve this situation. Inherent problems remain, however, concerning durability, hygiene, etc. which must be addressed in future research. Under climatic conditions similar to Japan's where high moisture and temperatures prevail, this product is rated highly.

Toshiro Nakamura, O.A., is President of Nakamura Brace Co. Ltd., Ohmori, Ohda Shimane, 694-03 Japan.

Eiji Hatano, M.D., is a Lecturer at the Orthopaedic Surgery Department at the University of Hiroshima, 1-2-3 Kasumi, Minami-ku, Hiroshima 734, Japan.

References:

1. Barnes, H.B., "Skin and Stump Hygiene," Artificial Limb, 3(4), 1956.
2. Yoshimura, Suano, et al., "Experience with Wooden Sockets," General Rehabilitation, 12(12), 1984, pp.916-964.
3. Aoyama, Takashi, et al., "Lightweight Moisture Absorbent Socket With Urethane Plaster Bandage," Japan Prosthesis Institute Bulletin, 19, 1981, pp.14-23.
4. Faulkner, V., "A Below-Knee Prosthesis With a Porous Sorous Socket," Orthotics and Prosthetics, 27,1973, pp.1-5.
5. Hatano, Eiji, et al., "Trial Manufacture of Artificial Porous Limb Sockets," Seikei Geka, 11, 1987, pp.230-233.
6. Nippon Kayaku, K.K., "Kayaresin WNC," Unpublished Technical Materials.
7. Nasayoshi, Hatano, et al., "Guide of Porous Socket," Nakamura Brace Co., Ltd., 1987, Japan.
8. Tetsuya, Watanabe, "Choice of Porous Socket Materials," Nakamura Brace Co., Ltd., 1988, Japan.
9. Wu, Y., et al., "Scotchcast P.V.C. Interim Prosthesis for Below-Knee Amputees," Bull. Prosthet. Res., 10(40), 1981.
10. Kiyoyuki, Oji, et al., "Polyester Hydrate," Plastic Material, 4(40), 1972.
11. Rubin, C. and J.L. Byers, A Porous Flexible Insert for the Below-Knee Prosthesis," Orthotics and Prosthetics, 27(3), 1973.
12. Scheinhaus, Arthur, et al., "A Modification of the Porous Below-Knee Soft Socket Insert," Orthotics and Prosthetics, 32(1), March, 1978.