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Technical Note: The Anatomical Above-Knee Suspension Belt

Vern M. Swanson, CP

Introduction

Prosthetists often find suspension systems for above-knee prostheses are not as functional or comfortable as needed, especially on geriatric patients. Unfortunately, not every patient is a candidate for suction suspension. The patient may have a femoral bypass that is sensitive. The patient may have difficulty donning a suction socket. He or she may be sitting most of the time, making it difficult to tolerate suction.

Alternative suspension systems such as Silesian belts and A/K neoprene belts are more comfortable for patients. However, in some cases, these belts do not suspend the socket well enough or maintain proper alignment.

Other suspension systems use leather belts with plastic pelvic joints and bands, such as polypropylene or ortholene (1). Flexible plastic pelvic bands are more comfortable for patients but sometimes do not function well in suspension and maintaining alignment. Occasionally the plastic pelvic band reverts to its original flat shape, losing the contour of the pelvis. This occurs more frequently when the plastic band is heated unevenly with a heat gun rather than in an oven (2).

Another standard system uses leather belts with metal pelvic joints and bands. This system is more functional, maintaining alignment and providing stability. However, in some instances, patients experience discomfort when sitting because the pelvic band is rigid metal.

This new suspension belt system is called anatomical because it has the actual shape and contours of the pelvis (see Figure 1 ). The patient is casted for the belt, and fabrication occurs on the positive model.

A soft interface is fabricated on the model and then a flexible acrylic belt is laminated over that. The positive model is also used to set the pelvic joint in proper alignment. A typical prescription for this system would read, "Pelvic joint with custom acrylic anatomical belt and soft interface liner."

The advantages of the anatomical A/K suspension belt are:

• increased patient comfort. Bony prominences may be built up on the positive model.
• reduced irritation due to the soft interface.
• better hygiene. Leather belts deteriorate. Ipoform may be cleaned daily with rubbing alcohol (3).

The technique also helps prosthetists set the pelvic joint (4).

The only disadvantage of the anatomical suspension belt is that it takes time for prosthetists to become technically proficient.

Casting Technique

1. It is assumed that the proper fit with the test socket has been established, and the static length and alignment are set.
2. Apply one layer of eight- to 10-inch cotton stockinette over the abdomen and ribs.
3. Mark the bony prominences (see Figure 2 ). The center of the trochanter should be marked on the test socket and on the stockinette so that it transfers to the cast. When the test socket is filled, transfer the location of the center of the trochanter onto the positive model. This will be used as a reference point to locate the pelvic joint. As an option, the lower pelvic joint may be installed on the test socket before the patient is casted. The A/K manual places the joint center one inch proximal and 1/2-inch anterior from the center of the trochanter (5). This will place the joint center very close to the axis of rotation of the patient's hip joint. These numbers serve as a guideline for average-size people. If the patient has large bone structure, the joint center may be up to 1 1/2 inches above the trochanter. If the patient has smaller bone structure, the joint center may be as little as 1/2-inch above the trochanter. If the lower joint is installed on the test socket, it will define the joint center's exact location on the cast.
4. Place a piece of rubber tubing on the anterior midline where the cast will be cut.
5. Instruct the patient to stand straight with a normal base, with equal weight on each limb.
6. Cast the patient around the waist with one roll of six-inch regular plaster bandage.
7. Apply a splint of eight layers of six-inch plaster bandage to the lateral side encompassing the proximal socket.
8. Apply a second roll of six-inch regular plaster bandage around the waist to reinforce the splint. Draw plumb lines for reference.
9. Key mark the cast, and cut the anterior opening over the rubber tubing.
10. Reinforce the impression with plaster, bandage, and fill with plaster.

Cast Modifications

1. Remove all sock marks.
2. Remove any irregularities created by the impression, except over bony prominences (see Figure 3 ).
3. Build up the bony prominences, two to three mm depending on the patient (see Figure 4 ).

Fabrication

1. Place an appropriate size of five-mm ipoform in the oven at 250° F for about three minutes. Place one end of the heated ipoform on the anterior midline then stretch the other end around the cast to overlap.
2. Wrap the ipoform with an Ace bandage.
3. Cut off the excess, and staple the ipoform where the anterior opening will be.
4. Apply one perlon stockinette for vacuum and a PVA bag (6).
5. Add one layer of perlon stockinette.
6. Apply four layers of nyglass stockinette (7).
7. Add one layer of three-inch carbon tape (see Figure 5 ) (8). The carbon tape should start one inch from the sound side anterior superior illiac spine and go across the anterior and lateral sides. Stop the carbon tape one inch from the posterior superior illiac spine on the residual side. Secure the edges of the carbon tape to the nyglass with double-stick tape (9).
8. Apply one layer of carbon acrylic felt approximately three inches by nine inches over the three-inch carbon tape where a pelvic band would be located (10). Secure the edges of the felt with double-stick tape.
9. Apply a second layer of three-inch by nine-inch carbon tape over the acrylic felt where a pelvic band would be and where the upper joint will be riveted (see Figure 6 ). Secure the edges of the carbon tape with double-stick tape.
10. Add four layers of nyglass stockinette.
11. Apply one layer of perlon stockinette.
12. Add the outer PVA bag with the seam where the anterior opening will be.
13. Mix approximately 1,500 grams of 100 percent flexible acrylic resin and laminate the belt (11,12,13,14).
14. After the lamination has set, turn off the vacuum and let the lamination cure overnight.

Trimming and Finishing

1. Establish trimlines by placing two inches of cotton webbing around the socket for men or 11/2 inches of cotton webbing for women, inferior to the illiac crests, and mark the trimlines (see Figure 7 ).
2. Mark a one-inch spread on the anterior midline for tightening the belt. Cut the lamination on anterior midline where the opening will be. Do not cut the trimlines of the belt until it is removed from the ipoform insert.
3. Place the partially trimmed belt back on the ipoform insert. Mark a 1/2-inch relief area on the anterior distal trimline to accommodate thigh flexion while sitting and bending forward (see Figure 8 ).
4. Finish trimming and buffing the edges.
5. Make a 11/2-inch-wide Velcro closure strap for men, a one-inch Velcro closure strap for women. Rivet the strap to the acrylic belt with two rivets on each side to prevent rotation.
6. Mark the location of the joint center on the cast (see Figure 9 ). Notice the ipoform under the joint has been cut away. This represents the lateral wall of the socket. Use the trochanter as a reference point.
7. Contour the lower joint to the cast. It will be necessary to remove some plaster under the head of the joint. Contour the upper joint to the pelvic belt, and mark the upper joint's length.
8. Key mark the upper joint. Cut off excess metal and rivet upper joint. If additional ML stability or strength is needed, rivet a 1 1/2-inch by 4-inch band of polypropylene between the upper joint and acrylic belt (see Figure 10 ).
9. Stabond the acrylic belt to the ipoform while on the cast (15). Trim the ipoform 3/4~ inch longer than the plastic all the way around the belt and buff the edges.
10. Place the completed belt on the cast. Heat the ipoform and roll the trimlines of the ipoform away from the cast (see Figure 11 ).
11. Locate the lower pelvic joint on the A/K socket using the trochanter reference point, as was done in Step 6 of Trimming and Finishing, and attach the joint.

Conclusion

The Anatomical A/K Suspension Belt is suitable for A/K geriatric patients (see Figure 12 ). It also may be used for more active patients who are not candidates for suction suspension (see Figure 13 ).

Ten patients have been fitted with the anatomical suspension belt. All are wearing their prostheses full-time and are comfortable with the suspension systems.

Five of the 10 patients fitted had previously worn other suspension systems. They preferred the anatomical suspension belts for comfort and support. They no longer complained about the prostheses turning inward or pistoning. This was encouraging since the only change to their prostheses was the installation of the anatomical suspension belts.

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

References:

1. Fischer E. Above-knee polypropylene pelvic joint and band. Orthotics and Prosthetics December 1976;30:4:41.
2. Clover WM Jr. Vacuum-forming: turning art into science. Seminar, Ohio Chapter American Academy of Orthotists and Prosthetists, Warren, Ohio, June 2, 1990.
3. Ipoform, IPOS #04004, Niagara Falls, N.Y.
4. Pelvic joint, USMC #P02-23R-G000, Pasadena. Calif.
5. Anderson MH, Bray JJ, Hennessy CA. In Sollars RE (ed). Prosthetic Principles-Above-Knee Amputations. CC. Thomas, Springfield, Ill. 1960:258-259.
6. Perlon, Otto Bock #623T3-25, Minneapolis, Minn.
7. Nyglass, Otto Bock #623T9-25, Minneapolis, Minn.
8. Carbon graphite tape, DAW #FITG-C3, San Diego, Calif.
9. Double-stick tape, Pel Supply #807446, Cleveland, Ohio.
10. Acrylic felt, DAW #FCA-001, San Diego, Calif.
11. Flex-acryl, DAW #FPR-FA1, San Diego, Calif.
12. Hardening powder, DAW #FPR-H1, San Diego, Calif.
13. Caucasian color pigment, DAW #FPP-PC, San Diego, Calif.
14. Negroid color pigment, DAW #FPP-PN, San Diego, Calif.
15. Stabond contact cement, Ohio Willow Wood #3F-140-1, Mount Sterling, Ohio.