The Strip-Casting Technique for the Syme Prosthesis

IAN COCHRANE C.P.


The Syme amputation, when done properly, produces one of the most successful end-bearing stumps known. The amputee often can walk on the bare stump for short distances without discomfort, and this capability is extremely valuable to the amputee. However, to retain this function it is essential that the amputee be provided with the best possible prosthesis for end-hearing. An ideal prosthesis is possible only through the use of a good casting procedure that will result in a well-contoured distal cup.

The strip-casting technique has been used at this Centre for the past three years, and we have found that the technique is not only a timesaver but also produces an extremely accurate fit. The procedure is carried out while the stump is suspended in the Northwestern casting ring. Although this equipment was originally designed for below-knee stumps, we have found it to he excellent for stabilizing distal tissues while casting Syme amputees.

Because the plaster is applied in narrow, vertical strips that are not subjected to undue tension or stress during application, the resultant positive model is an accurate reproduction of the stump. The positive model requires only smoothing and, in a few cases, light build-up of the proximal tibial crest. Plaster should not be added to the positive model indiscriminately; only extreme bony prominences or trigger points should be relieved.

Preparation Prior to Casting

A measurement is taken from the tibial tubercle to about 7.5 cm (3-in.) beyond the distal end of the stump. This measurement will determine the length of the plaster strips required for casting. The plaster may he cut from either 10-cm (4-in.) roll bandage or 10-cm plaster slab five-ply. The average adult casting requires approximately 24 single lengths of the required measurement. This is equivalent to three rolls or five lengths of the five-ply slab.

After the lengths have been cut, they are folded two at a time longitudinally leaving a 2-cm (3/4-in.) border with a two-ply thickness (Figure 1 ). This arrangement provides a feathered edge which reduces bulk as the vertical strips are applied edge upon edge during the casting procedure. A 15-cm (6-in.) circle of five-ply is cut for the distal hemisphere of the bulbous end.

A heavyweight cast sock is clamped in the Northwestern casting ring. The stand is adjusted to the patient's required height. The casting procedure is best done with the patient standing and placing partial weight on the cast sock (Figure 2 ). However, in the event that the amputee cannot stand for the time required, the procedure may be carried out in the sitting position. Since the casting ring is adjustable on the Berkeley brim stand, the ring can he lowered to the sitting position comfortable for the amputee. Also, it can be positioned horizontally to apply the required pressure to the end of the stump (Figure 3 ). The sock should be placed on the ring so that it is in good contact with the stump from the tibial tubercle to the distal end. Prior to casting, certain areas should be indicated on the stump with indelible pencil. The apex of the tibial tubercle is marked with a cross. The proximal 10-cm (4-in.) portion of the tibial crest is defined. Any sensitive areas or trigger points should he indicated with an indelible mark. These indications are all the markings required, and the application of plaster can now begin.

The Distal Cup

The circular slab for the distal cup is centered under the distal stump and worked up around the equator of the bulb using a cupping motion of the palm. The plaster is worked well into the cast sock to avoid displacement while carrying out the remaining plaster work. Just before this plaster sets, the anterior edge is turned down to form a ridge at the level of the equator, from the medial midline to the lateral midline. The posterior aspect of this slab is left flat and projecting proximally as it was applied (Figure 4 ).

Vertical Strips

The first plaster strip may now he applied on the posterolateral aspect with the thick edge projecting 13 mm (1/2 in.) anterior to the lateral midline. After the plaster is worked smooth, the leading edge is turned hack 13 mm to the midline and worked into a ridge similar to the ridge prepared on the anterior portion of the distal cup (Figure 5 ). This ridge will form an anterior margin of the posterior cast section for removal from the stump upon completion of the casting. The vertical strips all start at the tibial-tuhercle level proximally and project distally, overlapping the distal cup to form one structure. Successive strips are applied, thick edge upon thin edge, until the opposite midline is reached. The final posterior strip is folded back and ridged to complete the medial border of the posterior section of the cast (Figure 6 ).

Anterior Panel or Opening

When the plaster has set sufficiently, the remaining portion of the exposed cast sock and the edges of the plaster are greased well with Vaseline (Figure 7 ). The remaining plaster strips are now laid into the opening in a similar manner to complete the enclosure. The only difference in this procedure is that the portion of the bandage that hangs beyond the equator of the bulb is folded back up to reinforce the distal border of the panel (Figure 8 ). While the plaster is setting, register lines are struck around the opening to assure accurate refitting of the panel after removal from the stump (Figure 9 ). When the plaster has set, the anterior panel is removed (Figure 10 ). The sock may be cut anteriorly for easy removal from the stump (Figure 11 ). After the cast sock is removed from the cast, the panel is replaced immediately and plastered in position prior to pouring the positive model.

Conclusion

Because the ring-casting technique provides good supportive suspension to the tissues and because the plaster is laid on rather than wrapped circumferentially, there is literally no distortion of tissues; and the result is an accurate, negative replica of the stump. For this reason we have fOund that taking length and circumference measurements prior to casting is redundant.

You will note that with this procedure there is no reference made to additional weight-bearing at the patellar tendon or the medial tibial flare. We have found that these areas must be utilized only on rare occasions when there is a lack of heel-pad tissue to provide comfortable end-bearing. Because the Syme amputation was originally intended for total end-hearing, we have found it desirable to take advantage of this capability and reduce the length of the socket as much as possible. Accurate contouring of the distal cup is the key to a good end-bearing socket.