A Prosthesis To Restore Balance And Prevent Pressure Ulcers After Partial Amputation Of The Foot

W. A. Wigney, F.Ch.A.V., S.R.Ch


Reprinted with permission from The Medical Journal of Australia,I

After conservative surgery of the foot for gangrene or infection, it is often impossible to avoid disturbing the natural balance and stability. In particular this is so when one is dealing with the extensive infection of the plantar fascia which may be encountered in the diabetic patient, who in addition may often have an impaired peripheral circulation. If a metatarsal resection has been necessary or exposed bone has to be removed to allow soft tissue to fall together, malfunction of the foot will result, and inevitably some interference with balance can be expected. Even the removal of the fifth toe can give the patient a sense of insecurity. Further, abnormal pressure areas are often produced, which may in turn hasten the development of perforating ulcers or small areas of gangrene.

In these circumstances, some means of restoring function and balance to the foot is required which is both acceptable and nontraumatic to the patient. This paper describes a prosthesis made from inert silicone rubber for this purpose.

Material and Methods

Silicone rubber, which is commercially available in Australia, is used extensively in industry and to a lesser extent in medicine. The Dow Corning "Silastic" silicone rubbers, which vulcanize at room temperature, are known as "RTVs." The two-part silastic RTV products consist of uncatalyzed fluid silicone rubber plus a catalyst or curing agent. These must be blended before room temperature curing of the fluid rubber will take place. By the use of a "Silastic" RTV thinner (Dow Corning, 1963a), which is also an inert silicone fluid, the consistency, density, and softness of the product can be controlled over a wide range, and by variation of temperature and humidity, together with the amount of catalyst, the setting time can be controlled so that working time can be varied from five to 90 minutes without difficulty. The material used in this study was the medical "Silastic 382," which is inert and highly resistant to body fluids. It has been used widely for tissue implantation (Dow Corning 1963b).

The catalyst "M," which is stannous octotate, is also relatively inert and produces very little tissue reaction; but care should be taken that it does not come into contact with the eyes. The cured silicone rubber is water-repellent, retains its flexibility over a long period of time, and can be autoclaved or sterilized in a hot oven without damage. Further, if required, the material can be readily pigmented to match the natural skin color closely.

The method of manufacture of a prosthesis is as follows: The patient is referred for a prosthesis when the foot is well-healed and the skin is supple and in good condition. A negative cast is then made of the patient's foot and from this a positive cast is obtained.

Then, with the use of a whole foot (corresponding in size to that of the patient), a negative cast of the forepart of the foot or of the particular digit or amputated area in question is made with an alginate impression material ("Kromopan"). Melted dental wax is then poured in and out of the negative cast of the forefoot several times until a wax model approximately three-eighths inch thick has been obtained. The "Kromopan" mold is then broken and the wax model pressed onto the positive cast of the patient's foot, extra wax being added to cover the plantar, dorsal, medial, and lateral aspects to one-eighth-inch thickness up to the ankle, or as far as is desirable. In this way, the appearance of the original foot is restored.

Next, a negative cast of this reconstructed foot is made with the exception of the posterior fifth, and into this negative cast, suitably catalyzed and tinted "Silastic 382," silicone rubber is poured to fill it to three-quarters capacity. While the "Silastic" rubbers remains in its liquid form, the original positive cast of the amputated foot, from which all wax has been removed, is forced into the negative cast, so that the liquid silicone rubber is forced over all aspects of the foot to form a bucket into which the patient's foot will fit snugly.

After curing is completed, the silicone prosthesis is trimmed where necessary and lined with chamois leather. In order to strengthen the prosthesis, nylon fibres can be incorporated into the silicone rubber while it is in its liquid state.

Results

This type of prosthesis has been successfully fitted to four patients, who have used it constantly for six to twelve months. In this period no complications have arisen, and the durability appears to be very good. In particular a normal shoe can be worn, and each of the patients has been satisfied with the sense of balance and stability achieved. The following case report illustrates the type of problem encountered.

Mr. L., aged 69 years, a diabetic for 25 years with proven pernicious anemia, has been followed for the last 16 years. Some loss of peripheral sensation was noted in March, 1950, when pernicious anemia was recognized, and this has been progressive, despite treatment with "Cytamen" and good control of his diabetes with insulin. From 1955 to 1960 he had four hospital admissions associated with infection and gangrene of the toes of his foot. The peripheral circulation was poor, and no arterial pulses could be palpated below the femoral. In December 1961, a left transmetatarsal amputation was performed by Mr. Rowan Webb, and after a suitable interval a silicone rubber prosthesis was made and fitted in late 1962 (Fig. 1 and 2 ). This has been worn with an ordinary shoe since early 1963. In July 1964, he developed an infection and gangrene of the right great toe, and a similar amputation was performed on the right foot. It is planned to fit a similar prosthesis once the skin is completely healed and in good condition.

Discussion

In recent years, the importance of a good prosthesis as an adjunct to successful conservative surgery of the foot has been realized. The moccasin technique, as described by Charlesworth (1951) and modified by Downie and O'Connor (1956, 1957), was a definite improvement on earlier attempts to use foam rubber in the shoe, but required special modification and fitting of the shoe. The present technique, which has previously been described in brief (Wig-ney, 1964), and in which inert silicone rubber is used, overcomes this problem, and such a prosthesis has proved durable in up to 12 months' use. The simplicity of the technique allows it to be used in a standard chiropodial laboratory at low cost. It is important that the prosthesis fit the patient's foot exactly so as to give a firm grip, and in this regard close cooperation with the surgeon is of great practical advantage. This report is written in the hope that it will merit wider application, which may lead to improvement in the present technique.

Acknowledgements

I wish to acknowledge the willing assistance of Dr. F. I. R. Martin and Mr. Rowan Webb in the preparation of this report, and to thank them and the honorary medical staff of the Royal Melbourne Hospital for permission to report their cases.

W. A. Wigney, F.Ch.A.V., S.R.Ch is the Senior Chiropodist, Diabetic Clinic The Royal Melbourne Hospital Melbourne, Australia