Fabricating an Expandable Inner-Socket Prosthesis

MARCUS E. PHELPS, CP. JAMES W. STANFORD, CP.


For purposes of simplicity this report describes only the fabrication procedures used in the preparation of a prosthesis for the Syme's amputation. However, the same fabrication principles have been used successfully also in constructing prostheses for knee-disarticulation, elbow-disarticulation, and wrist-disarticulation amputees, as well as for patients with certain types of congenital deformities.

Cast Preparation

    1. The positive cast is prepared in the usual manner with any necessary reliefs incorporated, including provision for a patellar bar, if indicated.

    2. The circumference of the largest part of the bulbous distal end of the stump is recorded together with the circumference at the smallest portion of the shank which is usually just proximal to the bulb.

    3. If these circumferences differ by more than 1 ¼ in. to 1 ½ in., it will be necessary to build up the cast at the narrowest portion so that the circumferential differences will be within the l ¼-in. to 1 ½ in. range ( Fig. 1 ).

When impregnated with silicone rubber, nylon stockinet does not have very much stretch. A greater variance in the two circumferences would result in a socket too tight to get into or, worse yet, too tight to take off. Definition of the flare should only be enough to hold the prosthesis on the stump-no more.

  1. Several coats of parting agent are applied to the positive cast.

  2. Three nylon stockinets sewn at the end are pulled over the cast. The stockinet should not be stretched too tightly over the flares. If 3-in. nylon would normally be used, 4-in. nylon should be used for this job.

  3. A capping procedure may be used as the first lamination. A PVA sleeve is pulled over the distal half of the bulbous end and taped off with pressure tape ( Fig. 2 ). The end is then laminated with 4110 polyester resin.

  4. After the 4110 has set, the cap sleeve is removed and a PVA bag is applied over the entire lay-up.

  5. The circumference at the widest part of the bulb is measured and the tape is then moved proximally on the cast until a dimension equal to the recorded measurement plus 1/2 in. is reached.

  6. At this point a band of pressure-sensitive tape is applied and pulled tightly to the cast.

Lamination of the Inner Socket

Silicone rubber (385 Elastomer Dow Corning) is used in the lamination of the inner socket but, as it comes in its container, this material is too thick to work into the nylon stockinet. It must be thinned out to a more workable consistency for stringing. To do this, approximately 30- to 50-per-cent thinner (360 Medical Fluid) should be added to the silicone rubber and mixed well.

For 100 grams of workable rubber any one of three formulae may be used:

70 gr. silicone

60 gr. silicone

50 gr. silicone

30 gr. fluid

40 gr. fluid

50 gr. fluid

Any one of these formulae will work and the fitter's own experience will dictate which consistency he prefers. The larger the bulbous end, however, the more flexibility is needed.

The average Syme's lay-up will take 300-400 gr. of material.

Any of the above mixtures should be promoted at the rate of 25 drops of promoter per 100 gr. To do this, about 5 gr. of 360 Medical Fluid are poured into a 1-oz. cup and the required number of drops of promoter added. This promoter mixture should be well mixed and then poured into the silicone-thinner mixture and well spatulated for 30 sec. The process is completed by pouring this mixture into the PVA sleeve and stringing it down.

The lamination of the flexible portion of the socket should then be done quickly. The above formulae will permit about 7-10 min. of working time. It must be remembered that working with silicone is somewhat tricky since the material behaves erratically and is affected by atmospheric conditions.

Vacuum should not be used on this lamination since the vacuum will pull the Silastic beyond the area you have taped.

The Silastic must be carefully worked into the nylon to be certain of full saturation. The end of the PVA bag should be tied close to the end of the cast and the lamination allowed to set for several hours ( Fig. 3 ).

  1. When the Silastic has set, the PVA bag and tape are removed and powder is sprinkled on the Silastic portion of the lay-up.

  2. A PVA bag is prepared for the next lamination and the inside of this PVA bag is powdered also. (Unless powder is used the PVA bag will not slip over the Silastic.)

  3. The PVA bag is applied over the complete lay-up.

  4. The lay-up is then laminated using 4110 polyester resin under vacuum this time. (A slight film of 4110 will remain on the Silastic portion of the lay-up but this film will readily peel off when it has hardened. Care should be taken to string as much 4110 off the Silastic as possible.)

  5. When the plastic has hardened, the PVA bag is removed and the Silastic cleaned. This completes the fabrication of the inner socket ( Fig. 4 ).

Wax Buildup

  1. As much wax buildup as is necessary for good shape is used, beginning at the widest point of the bulb and extending proximally to a point just above the Silastic.

  2. The area proximal and distal to the wax buildup is roughed up to help provide a mechanical bond for the last lay-up ( Fig.5 ).

  3. For the final lay-up 4 or 5 sewn nylon stockinets (depending on the strength desired) are pulled over the cast, and are laminated with 4110 resin under vacuum.

  4. When the lamination has set thoroughly, a ½-in. hole is bored in the posterior portion of the lay-up to allow the wax to melt out. (Care should be exercised when boring the hole not to penetrate the silicone lay-up.) The wax is melted out in a 240-deg. oven.

  5. After an appropriate cooling time, the lay-up can be trimmed and the cast released, as a rule intact ( Fig. 6 ).

J. E. Hanger, Inc., Atlanta. Georgia