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The Lange Silicone Partial Foot Prosthesis

Lawrence R. Lange, CPO

Introduction

Though partial foot amputations are seldom seen in most practices, and often do not require the use of a prosthesis, our office has found a need for an improved prosthesis for this amputation level. Currently, several prostheses are available for this level, but they have design problems. Foot plates with toe fillers provide no means of securing the residual limb to the prosthesis and require shoes with lace closures (frequently high top) to supplement this feature. Supramalleolar varieties are usually fabricated from semirigid materials and are difficult to don and doff, uncosmetic in appearance and do not provide for residual ankle motion. This article will offer an alternative to these supramalleolar varieties and correct some of their detriments.

Development

Before 1984, partial foot prostheses were primarily rigid devices of conventional design fabricated from leather and metal or plastic laminate with a foam toe filler. These devices did not replace anatomical motion lost by amputation, but they did retain rollover in the toe section. In more distal amputation levels, the prosthesis was incorporated as part of the shoe, and no true socket existed. This allowed wasted motion between the residual foot and the device. There was no total contact in these devices, and retaining the heel in the shoe often required the use of a lace closure and high top shoes. The emphasis was always toward rigidity. Prosthetic failure-device or skin breakdown-resulted from the inability of these devices to follow residual anatomical joint motion. For more proximal levels of amputation, trimlines extended to the knee to distribute the anterior pressure over a greater surface area. These devices became even more rigid due to increased breakdown problems between the prosthetic socket and toe lever.

Literature in this field before 1984 is scant in reference to treating these levels of amputation. With increased surgical successes in these distal amputation levels, and improved limb salvage techniques, more patients now have healed partial foot amputations. This has resulted in the development of more orthotic and prosthetic devices to treat these patients, and consequently, the O&P field has become more concerned with preserving motion and flexibility. The impact of energy-storing materials has produced significant change in the treatment of amputees and the expectations for their performance. Better interface materials that behave like soft tissue supplements have emerged. All of these improvements have resulted or played a part in the development of the silicone partial foot prosthesis.

The work on prototype flexible partial foot prostheses began while the author was working at a pediatric orthopedic institution. We treated some patients with congenital partial foot anomalies. Designs known through education, experience and literature were rigid, prone to failure or uncosmetic. Cosmesis was a primary concern to the parents of the patients. Concerns also existed about growth, fit and function. Attempts were made to fabricate semirigid devices incorporating clear and fleshtone flexible plastics with foam toe fillers. The devices' fit and function were fine but their appearance was lacking. Plastic sockets were then implanted into juvenile Symes SACH feet. This resulted in better cosmesis but failure of the device became the primary consideration. A more flexible, comfortable material as the socket interface material was decided upon. More flexibility did not appear to hamper gait, and patient complaints and proximal trimlines became fewer and lower, respectively. The interface material, leather with a cowhide liner, was comfortable but did not survive well with children. It soiled quickly, and odors could not be eliminated.

The final evolution of the basic device was complete when silicone sockets were laminated to attach to the hollowed foot shells. The socket was adhered to the foot shell with silicone sealant, and Velcro closures provided the means for donning and doffing the device. Improvements over time included performing the second lamination over the foot shell and initial socket to permanently combine the two, and the use of a zipper in the posterior of the socket to provide a more cosmetic closure. With the introduction of energy-storing feet, cosmetic, hollow foot shells became available.

The present silicone partial foot prosthesis requires two laminations and incorporates the foot shell between the two. This permanently bonds the foot shell to the socket and produces an elastic, resistive toe lever. This toe lever can be further enhanced by adding a variable durometer RTV foam in the hollow toe section. Thus, while more natural ankle motion is achieved, there is resistance to drop off past the midstance part of gait, allowing for more normal ambulation. The zipper closure allows very easy donning and doffing, and the trimlines can be varied to enhance suspension and control of the device for different levels of amputation.

Description

A supramalleolar negative impression is taken of the patient in a weightbearing or nonweightbearing position, depending on the integrity of the ankle joint and practitioner preference. A positive model is developed in the usual manner, and a parting lacquer is applied. The desired thickness of sock to be worn with the prosthesis is placed over the model, and a PVA inner bag is applied and capped. Three to four layers of perlon-tricot stockinette are sewn and attached to the model, and another PVA bag is applied. Silicone resin is poured into the sleeve and laminated into the stockinette. We laminate with Otto Bock silicone for soft inserts. This resin requires curing over a two-hour period at greater than 100° F. This can be accomplished most easily by placing a heating pad over the laminated positive model. Once this inner lamination has cured, the outer PVA bag is retained and sealed smoothly over the distal end. This will act as a separator during the second part of the fabrication.

For the second step of this procedure, a Quantum (see Figure 1 ), Carbon Copy System III or similar foot shell of the desired overall length and width is selected. The attachment plate is cut out of the foot, and the interior dimensions are enlarged to accommodate the newly laminated socket. The socket, still on the cast with the previous outer PVA bag intact, is fitted to the foot shell until the toe out is correct, and the foot shell is not constrictive around the socket. The foot shell may be made more or less rigid along the sides and posterior by varying the thickness of the remaining shell. The toe of the foot shell is filled with RTV foam (available from SPS), varying the durometer of the foam based on the length of the residual limb. The greater the length of the residual limb, the lower the durometer necessary for a toe lever. The outer layer of PVA is now removed from the inner lamination. Otto Bock silicone adhesive is applied to the interior of the foot shell and over the cured RTV foam. The model is then introduced into the shell, and the adhesive is smoothed along the edges to provide a cosmetic transition from shell to socket. Once this has cured, the foot shell is prepared for the second lamination.

A tie-off ring is carved into the shell at the toe break using a Dremel-type tool. This ring allows the stockinette used in the final lamination to be tied off at this point, providing a smooth transition from the lamination to the foot shell (see Figure 2 ). The outside of the foot shell is abraded proximal to the lamination ring. After this is done, silicone adhesive is applied over the entire outside of the first lamination and the foot shell as far distal as the toe break (see Figure 3 ). A zipper is positioned on the socket's posterior. The zipper should be metal with fabric sides. Polyester fabric is preferred so small holes can be burned through the fabric to anchor the adhesives through the material (see Figure 4 ). After positioning the zipper on the socket, it should be pressed into place and adhesive allowed to extrude through the holes burned into the fabric. Care should be taken to keep adhesive from getting onto the tracks of the zipper (see Figure 5 ). Two to three final layers of stockinette are applied and pressed into the adhesive around the zipper. The last layer or two should be of a cosmetic shaded nylon stockinette to match the patient's skin tone (see Figure 6 ).

By using this technique, instead of pigmenting the silicone, curing problems have been reduced. Once this has been done, the outside PVA sleeve may be applied, and a final lamination performed. The lamination should be kept as thin and wrinkle-free as possible to provide a light cosmetic finish.

After curing the completed prosthesis, the outer PVA bag may be removed. The laminated silicone over the center of the zipper may be cut. Do not trim silicone from the attachment straps. The zipper may be opened at this point. Using a very sharp knife or scalpel, cut a straight line extending from the base of the zipper to the proximal trimline. Once this is done, the prosthesis may be removed from the positive model and trimmed. This fabrication procedure will produce a light, soft, flexible, cosmetic alternative to previously used varieties of partial foot prostheses (see Figure 7 , Figure 8 and Figure 9 ).

For more distal partial foot amputation levels, the height of the prosthesis may be lowered to the height of the shoe counter. This allows the finished prosthesis to be donned and doffed without the use of a zipper. With this exception, the fabrication is the same as is described above. The greater the height of the prosthesis, the greater its suspension and stabilization qualities.

Discussion

To provide better medio-lateral stability, we have experimented with the addition of a form of high-density polyethylene, Subortholene, as a sandwiched splint material. This addition may be important for treating ankle joints that suffer ligamental instability or loss of segmental integrity. This can naturally be the consequence of previously untreated neuropathic dysvascular feet. This addition need not interfere with normal tibial-talar motion but would interfere with what has become abnormal subtalar motion.

To date, more than 20 of these devices have been fitted to patients and follow-up maintained. Only two devices were rejected. One rejection was due to uncontrollable edema, the other to the patient's preference for something more solid. (see Table 1 ).

All levels of partial foot amputation have been treated with this device. Modifications may include higher trimlines or the incorporation of semirigid materials for increased support at the more proximal amputation levels.

Contraindications include the more proximal levels of amputation - i.e., Lisfranc and Chopart-for patients who may wish to participate in very strenuous sports. Patients with severe plantar surface ulcers secondary to Charcot joints in the foot or those with radiological evidence of bony disruption of a severe nature would probably not benefit from this treatment, but rather should have weight relief of the lower extremity as their primary care.

Summary

This article describes the rationale and fabrication procedures for a lightweight, flexible, partial foot prosthesis fabricated using silicone resin. It was developed in response to the need for a prosthesis that would provide qualities not present in previously described, less functional devices. The above-mentioned prosthesis allows for ankle motion, interchangeability of shoes, good cosmesis, comfort and excellent suspension.

Acknowledgements

I would like to acknowledge and thank the following individuals for their assistance in the development of the silicone partial foot prosthesis and the collection of the data for this article: Monte Chambers, RT (P); Michael O'Donnell, CPO; Neil Slavkin, CP; and Ron Kidd, CPO.


Lawrence R. Lange, CPO, is vice president of Hanger Orthopedics, 95 Bridge St., Wheeling. W.Va. 26003.

References:

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