O&P Library > Orthotics and Prosthetics > 1983, Vol 37, Num 3 > pp. 15 - 23

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A Below Knee Vacuum Casting Technique

Drew A. Hittenberger, C.P. *
Kenneth L. Carpenter *

INTRODUCTION

Accurate casting is thought by many to be the single most important factor in the successful fabrication and performance of the below knee prosthesis. Fitting is becoming increasingly accurate with improved socket materials (polyester resins, acrylic resins, and silicone compounds) and more sophisticated fitting techniques (total contact sockets, and check sockets). This article presents a below knee casting technique which utilizes vacuum pressure to draw the plaster against the patient's skin. This procedure produces negative impressions which duplicate residual limb contour and shape with a high degree of accuracy. The purpose of this research is to obtain the best possible method of casting for improved socket fit and increased comfort and control of the prosthesis for the patient.

HISTORY

At present, the most common casting technique involves wrapping the residual limb with plaster of paris and then molding it against the skin by hand until the desired negative impression is obtained.1,9 To distribute the pressure equally around the residual limb, Gardner (1968)³ described a technique using a pneumatic sleeve while Murdock (1968)6 developed the Dundee socket which used fluid pressure to compress the plaster against the skin. Fillauer (1971) developed the two and three stage casting technique which duplicates the boney anterior prominences and soft posteriolateral tissue areas separately.² Other multistage techniques were developed by Gleave (1972)⁴ and Rice (1979).⁸ In 1971, Zettl and Traub reported a premodified casting technique10, and, in 1982, Hanger presented a technique which used positive pressure provided by a controlled environment treatment (CET) machine to compress a bag over the plaster wrapped residual limb.⁵ All of these techniques have attempted to accomplish the same result; that of creating as accurate a negative impression as possible.⁷

VACUUM CASTING

The below knee vacuum casting technique uses vacuum to draw the plaster against the patient's skin. This process ensures equal distribution of pressure around the residual limb, thus eliminating the need to mold the negative impression by hand. The amount of vacuum pressure applied to the extremity can be varied depending on the amount of detail desired. This technique is compatible with, and improves the accuracy of, existing casting techniques such as the two and three stage procedures of Fillauer. The vacuum casting technique utilizes standard equipment which is widely available in prosthetics facilities.

The materials and equipment required for this technique are as follows (Fig. 1 ):

1.  Portable vacuum (Hosmer Cat. No. 51277)
2.  1/4 inch wound tubing
   (Heritage Cat. No.430-04)
   (Zimmer Cat. No. 1500-13)
   (Depuy Cat. No.5420-78)
3.  Universal connector
   (Heritage Cat. No.460-01)
   (Zimmer Cat. No. 1500-15)
   (Depuy Cat. No. 5420-95)
4.  1 roll, 1 inch Microfoam tape
   (3M Cat. No. 1528-1)
5.  1 roll, 2 inch Microfoam tape
   (3M Cat. No. 1528-2)
6.  1 roll, 1 inch double-faced tape
   (3M Cat. No. 950)
7.  1 1/2 inch x 14 inch Plastisol sealing band
8.  Cast sock
9.  Indelible pencil
10.  1 to 2 rolls standard plaster of paris bandage
11.  10 inch x 14 inch Zip-Lock® plastic bag
12.  1 Yates clamp
13.  One pair scissors
14.  Plaster parting cream
    (Otto Bock Cat. No.64025)

Procedure

The below knee vacuum casting technique is as follows:

1.  Apply plaster parting cream on the patient's residual limb to facilitate easy removal of the cast (Fig. 2 ).
2.  Apply a wet cast sock to the residual limb. Draw the sock tightly against skin to eliminate any wrinkles (Fig. 3 ).
3.  Secure the cast sock with one inch Microfoam tape three to four inches above the Patella (Fig. 4 ).
4.  Cut off the excess stockinette and seal the entire proximal edge against the skin with two inch Microfoam tape. One or two layers is sufficient. This will create an airtight seal against the skin (Fig. 5 ).
5.  Mark the boney prominences with an indelible pencil.
6.  Feed the vacuum tube through the Plastisol sealing band until the perforated holes are all on the distal side of the band (Fig. 6 & Fig. 7 ). Then place the sealing band on the two-inch Microfoam tape with the perforated side projecting distally. Lay the tubing down the residual limb and trim off the excess at mid-patella (Fig. 8 & Fig. 9 ).
7.  Cut a small hole in the stockinette just superior to the patella and feed the perforated tubing underneath the stockinette to the middle of the patella. The tubing must be under the stockinette to create an air passage during vacuum evacuation. Wrap the Plastisol band snugly around the Microfoam tape to create an air-tight seal (Fig. 10 ).
8. Apply plaster of paris bandage over the patient's residual limb, being careful not to cover the Plastisol sealing band. Avoid wringing the water-saturated plaster bandages. They must remain wet to allow the vacuum pressure to draw the plaster against the skin. Wrap the bandage loosely to avoid distortion from roping. Standard bandages rather than elastic plaster bandages are used due to their greater strength. Three to four layers are sufficient. An excess of five layers cannot be drawn against the skin accurately because of the excessive vacuum pressure required (Fig. 11 ).
9. Apply the Zip-Lock® plastic bag over the plaster wrapped limb, keeping the fastener in contact with the Plastisol sealing band (Fig. 12 ). Using the Zip-Lock® fastener, seal the bag and secure with a Yates clamp (Fig. 13 ). An effective seal is important; without it the vacuum casting technique will not work.
10.  Apply vacuum pressure to the bag, drawing between 20 to 30 inches of mercury (Fig. 14 ). While under vacuum, the negative impression may be molded by hand if desired, but this is not necessary. If molding is required, the vacuum casting technique has a unique tendency to hold the specified shape molded into the negative impression in position (Fig. 15 ).
11.  Once the plaster has hardened, the negative impression can be removed following removal of the bag, Plastisol sealing band, Microfoam tape, and finally the negative impression (Fig. 16).

Fig. 2, Fig. 3, Fig. 4, Fig. 5,Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11 , Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16

If a multistage cast (Fillauer's two and three stage technique) is used, the vacuum is applied after each wrap (Fig. 17 ). Once the plaster has hardened, the vacuum bag is removed and the patient is ready for the next stage (Fig. 18 ). Again, it is important that the wrap or splints (anterior or supracondylar) not exceed five layers in thickness. When using the three stage technique for supracondylar suspension, it is important that the Plastisol band be repositioned proximally to allow sufficient room for the proximal splint.

CONSIDERATIONS

While the below knee vacuum casting technique ensures equal pressure distribution, certain precautions should be taken:

1.  Always use a clean tube. If a clean tube is not used, there is a greater chance that it will become clogged and the vacuum will be insufficient to draw the plaster against the skin.
2.  Placing the tubing underneath the stockinette will function as: (a) a filter to prevent large plaster particles from being drawn into the tube and, (b) to keep the plastic vacuum bag from sealing directly against the tubing and thus blocking the vacuum holes.
3. Seal the vacuum bag tightly against the Plastisol band. A good seal must be obtained or maximum pressure will not be achieved. If sealing does become a problem, wrap double-faced tape around the Plastisol band before applying the vacuum bag.
4. For improved posteriodistal vacuum pressure on the residual limb, the tubing may be wrapped proximally around the extremity and connected back to itself using a universal connector (Fig. 19 ,Fig. 20 , &Fig. 21 ).

Fig. 19, Fig. 20, Fig. 21

DISCUSSION

The vacuum casting technique is more efficient, and actually takes less time, than more conventional techniques when considering setup, application, and removal.

1.  Less time is needed to mold the plaster in place because the vacuum creates the general contour.
2.  The plaster sets quicker because vacuum removes water from the mold, which promotes setting and reduces the time required on the patient.

The vacuum casting technique is very accurate. Unlike existing hand molding techniques, the vacuum casting technique was developed for the specific purpose of eliminating impression variations, such as those encountered in serial casting a patient by the same practitioner and casting a patient by different practitioners. The prosthetist can now regulate and visually monitor the amount of pressure applied to the plaster negative. The resulting accuracy makes this technique easy to teach.

As many prosthetists are aware, exact duplication in conventional hand molding techniques is difficult because casting requires distorting tissue in some areas, and not in others, while maintaining total contact. However, the vacuum casting technique does exactly this. It provides total contact under equal pressure, and allows the prosthetist to mold the impression in particular areas if necessary. The vacuum casting technique has proven so far to be the most efficient system to offer both of these capabilities. This system can also be modified for use in orthotics, which will be discussed in a subsequent article.

CONCLUSION

The success or failure of any casting technique depends on its accuracy, but accuracy cannot be obtained without the ability to regulate and monitor the amount of pressure while casting. Under existing casting techniques, hand molding takes place which is dependent on the subjective judgment of the practitioner. This explains why variations occur between the same patient, different patients, and between practitioners.

The vacuum casting technique was developed for the specific purpose of minimizing impression variations by allowing the prosthetist to regulate and monitor the amount of vacuum pressure to achieve the detail desired. This technique also enables the prosthetist to modify certain areas simultaneously if desired. The vacuum casting technique appears to reduce time, be more efficient, and increase the comfort and control of the prosthesis due to the improved accuracy of this fitting system.

ACKNOWLEDGMENTS

This research was supported by the Veterans Administration Contract V663P-1172, Rehabilitation Research and Development funds.

Dr. Ernest M. Burgess*, Timothy Staats**, and the staff at Prosthetics Research Study Center are specifically thanked for their participation in the development and preparation of this report.

BIBLIOGRAPHY

Fajal, Guy, "Stump Casting for the PTS Below-Knee Prosthesis, Prosthese Tibiale Supra Condylienne," Prosthetics International, 3:4-5:22-24, 1968.

Fillauer, C, "A Patellar-Tendon-Bearing Socket with a Detachable Medial Brim," Orthotics and Prosthetics, Volume 25, Number 4, pp. 26-34, 1971.

Gardner, Henry, "A Pneumatic System for Below-Knee Stump Casting," Prosthetics International, 3:4-512-14, 1968

Gleave, J.A.E., Moulds and Casts for Orthopaedic and Prosthetic Appliances, Charles C. Thomas, Publisher, Springfield, IL, 1972.

McQuirk, A.W., "Vacuum Casting, The Advanced Course on Below Knee and Through Knee Amputations and Prosthetics," (I.S.P.O.) May 10-13, 1982, Koge, Denmark.

Murdoch, George, "The Dundee Socket for the Below-Knee Amputation," Prosthetics International, 3:4-5-15-21, 1968

Quigley, M.J. and A. Bennett Wilson, "An Evaluation of Three Casting Techniques for Patellar-Tendon-Bearing Prostheses, Selected Reading—A Review of Orthotics and Prosthetics, report prepared on behalf of the Subcommittee on Evaluation, Committee on Prosthetics Research and Development, National Academy of Sciences.

Rice, V., "Technical Note: Casting Technique for Below-Knee Prostheses," Orthotics and Prosthetics, Volume 33, Number 4, pp. 51-54, 1979

Wilson, Leigh, and Erik Lyquist, "Plaster Bandage Wrap Cast, Procedure for the Below-Knee Stump," Prosthetics International, 3:4-5:3-7, 1968

Zettl, J.. and J.E. Traub, "Premodified Casting for the Patellar Tendon Bearing Prosthesis," Artificial Limbs, Vol. 15, No.1 pp. 1-14, Spring 1971.

*Director and Principal Investigator, Prosthetics Research Study Center, Seattle, Washington

**Director, University of California at Los Angeles Orthotics and Prosthetics Education Program

O&P Library > Orthotics and Prosthetics > 1983, Vol 37, Num 3 > pp. 15 - 23