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Technical Note: An Alternative Below-Knee Ultra Lite Technique

Vern M. Swanson, C.P.

Introduction

As prosthetists, one of our goals is to design and fabricate lightweight below-knee (BK) prostheses. With this goal in mind, an alternative to the Otto Bock ultra lite technique is presented here for the practitioner to consider (1,2).

This procedure may be done to fabricate a new BK prosthesis or to duplicate an existing prosthesis. However, it is more advantageous to perform this procedure on an existing prosthesis. With an existing prosthesis, the prosthetist has had the opportunity to dynamically establish the optimum socket fit, length, and alignment, particularly if the prosthesis was fabricated with an adjustable endoskeletal system.

This ultra lite technique uses wax to provide the shape of the shin section. The shin is laminated with carbon acrylic resin and a reinforced lay-up. The wax is melted out, leaving a hollow, lightweight exoskeletal socket and shin.

The indications for this procedure are as follows:

1. The patient would benefit from a lighter weight prosthesis or a more durable acrylic laminated prosthesis, instead of one with a soft cover.
2. The socket will require minimal grinding to relieve for pressure areas.
3. All adjustments to the length and alignment are optimum on an existing prosthesis.
4. The ultra lite procedure is discussed with the patient, and the patient would like to have it done.
5. The ultra lite procedure is recommended to the prescribing physician, and a prescription is written.

The contraindications are the following:

1. The prosthetist determines that the BK socket will require much grinding and relieving.
2. The patient has a contracture that may be correctable through physical therapy.
3. The patient may soon become a bilateral amputee.
4. The patient prefers to have a soft endoskeletal covering.
5. The prosthetist does not have a prescription authorizing the procedure.

Fabrication Technique

Let's assume the procedure is being performed on an existing BK endoskeletal prosthesis:

1. Make arrangements with the patient to have the prosthesis dropped off for the day, or schedule a long appointment. Tell the patient it will take two-and-a-half hours.
2. Remove the foot and install a polyvinyl acetate (PVA) bag or a balloon over the soft cover (Figure 1) . Spray silicone on the bag as a separator.
3. Apply a two-stage split cast to the endoskeletal cover (Figure 2) . The cast will be used as a form for the wax. After the anterior panel sets, apply petroleum jelly to the edges. Then apply the posterior splint. Key-mark the cast and remove it.
4. An alternative procedure is to pick a prosthesis that has the shape that is desired and cast it with the same technique (Figure 3) .
5. Duplicate the socket without the insert, using duplicating plastic (Figure 4) . Duplicating directions and a list of materials can be found in Otto Bock Technical Information (3). After the second coat of duplicating plastic has set for 15 minutes, remove the duplicate mold from the socket.
6. Apply petroleum jelly to the outside of the duplicate mold as a separator, and place the duplicate back in the socket. Position the prosthesis in the vertical jig for alignment duplication (Figure 5) .
7. Foam the distal half of the duplicate mold with standard polyurethane foam to lock it onto the pipe (Figure 6) .
8. After the foam sets, remove the duplicate from the socket. If the upper walls of the mold appear weak, reinforce them with three layers of plaster bandage to prevent the foam from expanding the upper half of the mold. Then finish filling the socket duplicate with polyurethane foam (Figure 7) .
9. Fabricate an ankle block that is 2 1/2-inches in height. Shape the ankle block on the foot. Remove 2 mm from the surface of the shaped block to allow for the thickness of the lamination (Figure 8) . Mark the toe out reference line on the block, and then seal the surface of the block with acrylic resin.
10. Reassemble the prosthesis, and return it to the patient. The laboratory may now fabricate the ultra lite shin at its convenience.
11. Apply a nylon for vacuum and a PVA bag to the duplicate mold. Apply one perlon, two nyglass, then the outer PVA bag (4,5). Laminate the shell with 80-20 carbon acrylic resin (Figure 9) .
12. Trim the shell half an inch proximal to the widest point of the mold; discard the proximal portion and sand the outside of the distal section. Cut a piece of 1 1/2-inch diameter polyvinyl chloride (PVC) pipe to extend from the end of the distal socket shell to the top of the ankle block (Figure 10) . Bond the PVC pipe to the shell with acrylic paste. Tape the proximal edge of the shell to the model with one layer of pressure-sensitive tape to prevent rotation. Place two screws in the top of the ankle block (Figure 11) . The screws and PVC pipe are used as anchors to lock the wax in place.
13. Place the plaster split cast over the socket and ankle. Mark where the cast should be trimmed. The trimline should be half an inch below the posterior wall. If needed, trim the cast and check the fit again.
14. Spray silicone on the inside of the cast and apply a light layer of petroleum jelly as a separator. Reinforce the sides of the cast with three layers of plaster splints, and wrap the ankle with a four-inch plaster bandage to prevent the wax from leaking (Figure 12) .
15. Melt three pounds of beeswax in a medium-sized pot (6). Pour the wax into the cast, fill to about four inches from the top. Slide the mold back down into the wax and lock it in. Let the wax set overnight (Figure 13) .
16. Peel off the plaster cast in the morning. If the cast sticks, use a heat gun to soften the wax. Shape the wax with a sharp Sureform file. Smooth the wax with screen, and fill any holes with melted wax. Check to make sure there is a minimum of one inch at the proximal edge of the shell that is not covered with wax to allow for bonding (Figure 14) . After the shaping is completed, reseal the ankle with acrylic resin so the ankle will not absorb wax during the melt out.
17. Apply one layer of pressure-sensitive tape to secure the laminated shell to the duplicate mold. Apply a thin nylon for vacuum, a PVA bag, one perlon, two nyglass, one carbon fiberglass stockinette (Figure 15) , two nyglass, one perlon and then the outer PVA bag (7). Laminate the shin with 80-20 carbon acrylic resin (Figure 16) .
18. After the lamination sets, trim it flush with the bottom surface of the ankle block. Place the prosthesis in the oven with a pan under it. Heat at 250 degrees Fahrenheit for 20 minutes (Figure 17) . Remove the prosthesis. Install an ankle bolt and pull the ankle block out with vise grips (Figure 18) . Place the BK prosthesis back in the oven for about 2 1/2 hours at 250 degrees Fahrenheit until all the wax has melted and drained out of the shin.
19. After the wax is all out, trim the proximal socket and pull the prosthesis off the duplicate mold and let it cool (Figure 19) . Cut off the PVC pipe and sand the distal end. Bond the distal socket shell to the laminated shin section with acrylic resin (Figure 20) . Rough up the sides of the ankle block and bond it in place with acrylic resin.
20. The ultra lite shin is now ready (Figure 21) . When the patient returns, remove the suspension component, insert and the foot from the existing prosthesis, and install them on the ultra lite shin.

Summary

Twelve patients have been fit with this ultra lite technique. All the patients reported a significant reduction in the weight of the prosthesis, less fatigue and increased endurance. The average weight of a man's finished prosthesis is two pounds and five ounces. The average weight of a woman's finished prosthesis is two pounds and one ounce.

Consistent patient results will be achieved when this ultra lite technique is combined with comprehensive patient management. The technique may be used with various feet that are attached with standard ankle bolts.

Vein M. Swanson, Sr., C.P., is manager prosthetist of Swanson Prosthetic Center, Inc., 3102 Sylvania Ave., Toledo, Ohio 43613. He is the immediate past president of the Ohio Chapter of the American Academy of Orthotists and Prosthetists, Inc., 4355 N. High St., #208, Columbus, Ohio 43214.

References:

1. Schroeder FK, Hendrickson JR. The Otto Bock all-plastic AK prosthesis for the geriatric amputee. Orthotics and Prosthetics 1968 ;22:1:2932.
2. Research and Development Department of Otto Bock Orthopedische Industrie, Duderstadt, Germany. Fabrication of a lightweight below-knee prosthesis. Otto Bock Technical Information 1;2:1:1-28.
3. Research and Development Department of Otto Bock Orthopedische Industrie, Duderstadt, Germany. Otto Bock pedilen duplicating plastic 617H37. Otto Bock Technical Information
4. Perlon stockinette #623T3-10, Otto Bock, Minneapolis, Minn.
5. Nyglass stockinette #623T11-9, Otto Bock, Minneapolis, Minn.
6. Beeswax #199067, Pel Supply, Cleveland, Ohio.
7. Carbon fiberglass stretch stockinette #09505, IPOS, Niagara Falls, N.Y.

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