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Silicone-Only Suspension (SOS) for the Above-Knee Amputee

Louis J. Haberman, CPO
Robert A. Bedotto, CPO, LPT
Ellen J. Colodney, MD

Introduction

For several years now, prosthetists have used various silicones to make socket inserts. The new generation of silicone liners replaces the gel/leather liners that did little more than cushion the residuum. These new liners, as described by Pritham and Fillauer, are applied directly over the residuum, without a sock (1). The silicone liner creates a negative atmosphere and an adhesive bond to the skin, so it moves with the tissue.

Uillauer; Dietzen, Harshberger and Pidikiti; and others developed systems for prosthetic suspension in concert with the silicone liner (2,3). These systems are mechanical in nature, offering a distal means of suspension (see Figure 1 ). Some inherent weaknesses in such designs are

• limited space may not permit installation
• donning may be difficult due to deficiencies in upper limb strength, dexterity, sensation and/or impaired vision
• their additional weight, fabrication time and cost
• their disproportionate distal shrinkage
• their increased complexity

Stasis dermatitis, which is linked to extreme proximal tightness and an absence of distal contact, creating negative pressure, can be eliminated in several weeks with use of a new, well-fitting socket and a conventional silicone liner.

The total-contact silicone interface creates a "second skin" effect. Since it is mildly compressive in nature, the soft tissue and bone become firmly integrated. The SOS design creates suction suspension between the outer surface of the silicone liner and the inner socket wall. The soft tissue is never subjected to the stresses created in other suction socket designs. This offers a level of comfort, control and susspension heretofore not possible.

The authors advocate the use of the silicone liner for the great majority of lower limb amputees. It seems many prosthetists use a silicone liner only when offering a 3Stype prosthesis. Distal, mechanical connections for suspension, particularly for below-knee amputees, should not be the only criterion used for applying silicone liners. The SOS offers wearers the benefits of silicone technology along with an uncomplicated, yet superior means of attachment/suspension.

Casting Techniques

Standard

The residuum may be casted in a traditional manner. It is suggested that pantyhose be employed since they offer an appropriate level of compression, protect the tissue and improve the prosthetist's view of the involved side and surrounding structures. Apply elastic and conventional plaster bandages without roping. A hand cast is recommended for ischial containment socket designs although the Shamp AK Casting Brim has, on occasion, been used quite successfully (8). Maintain and mark the adduction and flexion angles on the cast model for later orientation for pouring, modification and alignment.

Pour the negative cast. A copy of the newly created positive model should be made over a five-ply residual limb sock and Saran Wrap or other protective film. This copy will be used in producing the socket model. The model should be reduced slightly during the smoothing process. The proximal brim can be modified according to Fillauer guidelines or the prosthetist's experience. The original model will act as the model for the silicone liner.

Double Casting

Since all silicone liners distort and compress the soft tissues they contain when applied, it can be assumed that an entirely new shape emerges. If the differences are substantial, the socket may be too large or improperly contoured for the appropriate fit required for suction suspension.

The authors have achieved excellent results, requiring minimal guesswork, by using the double cast technique. Take an initial cast in a traditional manner, then design, fabricate and apply a silicone liner to the residuum. Place a separator over the silicone and make a second cast to reflect the controlled distortion of the residuum as it now appears (see Figure 3 ). The resulting check socket will closely approximate the contours and volume created by applying the silicone liner to the residuum. Ischial containment, flexible Surlyn^TM sockets have been used most often in applying the SOS system since they provide the best suction seal. Surlyn seems to suffer less shrinkage after molding than other thermoplastic products. The SOS design has also been used with laminated versions of the ischial containment and quadrilateral shapes with excellent results. It is possible to convert an existing semi-suction, above-knee prosthesis to a full suction design (with consideration given to socket contours) with the introduction of the SOS system.

Design and Construction

Usually three layers of conventional nylon stockinette are used for the matrix material. The nylon can be impregnated, under light to moderate vacuum, with either Durr-Fillauer or IPOS silicone (see Figure 4 ) (9,10). Both produce satisfactory results. If the IPOS silicone is used, a 5:1 proportion of Component A to Component B is recommended. The Otto Bock silicone Orthosil may be substituted (11). The authors have no experience with this material and cannot offer suggestions for its use.

Nylon matrix materials must be impregnated thoroughly with care to avoid the formation of air bubbles. If air bubbles occur, particularly at the distal end, the suction pull will extend through the liner to the distal residuum, causing a pooling of fluid, capillary eruption and discoloration. For consistent results:

1. Heat nylon matrix materials in an oven at 1500 F for three to five minutes to eliminate any moisture in the nylon (12).
2. Place the prepared silicone mix into a de-bubbler device for five to 10 minutes to release trapped air (see Figure 5 )(13).
3. Apply silicone paste to the distal end of the model to ensure a complete seal and apply nylon matrix (see Figure 6 ).
4. Pour silicone mix with care to avoid introducing additional air.
5. Tie off the proximal bag and invert the model as described by Otto Bock and DAW Industries. This will allow any remaining bubbles to rise easily above the liner trimlines.

Donning Procedure

Of great importance to amputees, particularly geriatric above-knee amputees, is ease of donning a prosthesis. Most geriatric above-knee amputees like to apply the prosthesis when seated (see Figure 7 ). The SOS system permits a seated application, which takes less effort and eliminates the requisite balance and proprioceptive skills normally associated with applying suction sockets.

For convenience and ease of unrolling the inverted silicone liner onto the residuum, apply a light layer of talcum powder. The liner may be indexed with an indelible marker or punch hole to indicate its proper orientation. Clean the powder from the liner with a damp cloth, and dry the liner. Apply Nivea cream to the outer surface of the silicone liner and smooth it over its entire surface (see Figure 8 , and Figure 9 )(14). Less Nivea is required at the proximal two to three inches of the liner since any excess cream will spread and cover this area during the donning process. Nivea cream creates a true seal between the silicone liner and the inner socket wall. Seven creams were tested for the seal requirement, and only Nivea yielded consistent results. Nivea contains a silicone oil, which may explain its effectiveness in this application (15).

The prosthetic socket is applied to the residuum while the valve is open (see Figure 10 ). This will create a secure suction suspension before standing. Upon standing, the patient applies weight to the prosthesis while intermittently pressing the suction valve (see Figure 11 ): An adjustable leak rate valve may be used to further assist the patient during the donning process by letting the residuum settle into its appropriate configuration within the socket (16). It has often been observed that more trapped distal air can be released through a conventional valve after several minutes of ambulation.

Patients have reported that the Nivea seal is maintained throughout the day. Several patients had to apply a second application late in the day; however, this may be related to an insufficient first application since follow-up and reinstruction resulted in a single application of Nivea cream for this patient group as well.

Contraindications

There are no absolute contraindications in SOS use. Relative contraindications include:

• Upper limb dysfunction. The absence of good upper limb control may or may not be a contraindication for providing SOS. While a reasonable level of upper limb dexterity and strength is required to don the silicone liner independently, such upper limb involvement may preclude the use of more conventional systems as well. The decision here to provide the SOS is not clear-cut. The prosthetic team may have to make a "best guess" in these instances.
• Distal redundancy will challenge the patient to roll the silicone liner above this tissue mass. The tissue tends to widen substantially when subjected to vertical traction forces associated with applying silicone liners. It is suggested that the distal third of the cast model not be reduced for compression as is standard in such instances. It has been necessary, in one instance, to increase the distal circumference greater than the original cast.

Case Histories

Case 1

A 72-year-old female required an above-the-knee amputation due to peripheral vascular disease. Previous vascular bypass surgery contraindicated abdominal/pelvic compression, thereby requiring a socket-only suspension system. Traditional suction suspension, via Ace wrap donning, was not practical due to the patient's advanced age, diminished strength and dexterity, and poor balance. A shuttlelock/silicone liner was impractical inasmuch as the additional distal length requirements would have resulted in a substantial disparity between the anatomical and prosthetic knee joint axes. Additionally, it was felt that the patient could not don the liner/pin in the same orientation consistently. The SOS design was well-received by the patient (see Figure 12 , and Figure 13 ). The patient maintains full suction suspension, is comfortable and uses a standard cane for independent ambulation.

Case 2

A 55-year-old male with vascular occlusion and subsequent gangrene required an above-knee amputation. After acute care hospitalization, he wass transferred to The Rehabilitation Institute of Morristown Memorial Hospital. The rehabilitation team scheduled early post-operative ambulation due to the patient's excellent healing, strength, balance and absence of co-morbidity. A silicone liner was used with the temporary prosthesis to protect sutures. The reduction of traction and shear forces, associated with silicone liners, protected the suture line and may have accelerated the residuum maturation process.

The patient experienced no skin irritation during the temporary prosthesis phase of rehabilitation. It was necessary to fit two silicone liners during this period due to rapid maturation and substantial shrinkage. The patient has requested the SOS for the permanent prosthesis.

Case 3

A 79-year-old female vascular diabetic was amputated above the knee and subsequently received a conventional temporary prosthesis. Residuum shrinkage was consistent and appropriately managed with the increasing application of residual limb socks.

After several months the patient developed two bone spurs at the distal femur, which produced severe pain whenever the tissues were pulled taut against the spurs. The patient had only partial relief with multiple local anesthetic and steroid injections. This highly painful condition required discontinued use of the temporary prosthesis. The surgeon believed revision surgery would be inappropriate and suggested finding a prosthetic solution. SOS was recommended and provided. The distal end of the ischial containment socket was extended to create a distal air chamber. It was hypothesized that the silicone liner would reduce the painful traction forces over the bone spurs and that the distal air chamber would reduce firm contact pressure in the region as well. It should be noted that a total contact fit is present via the silicone liner despite the extended socket, thus eliminating distal edema problems.

Although mild, intermittent discomfort remains, the patient is more comfortable when wearing the prosthesis. The SOS design was much easier for the patient to manage than residual limb socks and pelvic suspension. She also says she can better control the prosthesis and enjoys better freedom of movement.

Other Considerations

Allergic reactions to medical-grade silicones are rare, occurring most often with improper skin/liner hygiene. Folliculitis, an inflammation of one or more follicles, may develop if skin hygiene is inadequate since sweat accumulation enhances bacterial buildup. Folliculitis can be effectively treated by cleansing the residuum daily with an antibacterial scrub and by washing the silicone liner daily with soap and water. More severe cases in susceptible patients may require an oral antibiotic to combat streptococcus and staphylococcosis.

Liquid Ivory soap has not caused skin eruptions when used as a liner cleanser. Initial use of Dial Antibacterial Liquid soap as a liner cleanser caused a significant dermatitis condition, resembling diaper rash, in almost 30 percent of our patients. After washing and rinsing well, the inner surface of the liner should be wiped with an alcohol pad to remove any lingering soap residue.

Results

As of this writing, 16 above-knee amputees have been provided with SOS. They have been closely monitored at The Rehabilitation Institute of Morristown Memorial Hospital by prosthetists, physicians and physical therapists. There have been no rejections. All the patients report they are comfortable and confident with the SOS design. An enhanced level of prosthetic control has been observed in those patients who previously wore alternative prosthetic suspension systems.

New amputees fitted with the SOS system have progressed rapidly through the training process, as reported by physical therapy. Several patients in the group required additional instruction about the donning procedure. Intermittent slight loss of suction in three instances for two patients in the group was due to inadequate application of Nivea cream.

The patient population differs in a variety of ways, including age, activity level and level of amputation (see Table 1 ). Despite the many variables in the group, all have progressed rapidly through the rehabilitation process and have become independent ambulators.

Written instructions are provided to all patients receiving the SOS system. The instructions offer information on donning, care of the silicone liner and socket, appropriate application of Nivea, valve maintenance and skin precautions.

The residuum should be volumetrically stable before casting for the SOS system. Moderate shrinkage can result in a loss of suction, requiring replacement of the inner socket.

Based on the successes with these patients, the SOS design has already been applied to upper limb pediatric and adult amputees. The authors have achieved suction suspension, protection and comfort for the residuum and full range of motion by eliminating the supra-epicondylar socket design. There have been promising results in the development and application of the SOS system for the myoelectric prosthesis. A new miniature thermoplastic suction socket valve has been designed by the first author for use in upper-limb prostheses.

Conclusion

A new application of the silicone liner has been devised which provides true suction suspension yet eliminates the need for any mechanical attachments to the socket. The use of Nivea cream has proven effective in maintaining a suction seal between the outer silicone liner and the inner socket for a full day. An alternative casting technique has been suggested to facilitate the proper SOS fit.

Among SOS's benefits:

• The residuum is protected from friction and shear forces.
• The soft tissue is not torqued, as is the case with conventional suction sockets.
• The donning process is simple, convenient and is performed initially in a seated, safe position.
• Due to reduced fabrication time, the system is also more cost-effective.

Acknowledgments

The authors thank The Rehabilitation Institute of Morristown Memorial Hospital (RIMM) for its support in this effort.

Appreciation is also expressed to Arthur Sirdofsky, CO, for his excellent photography and to Jack Mitchell, R(A)P for his technical contributions.

Louis J. Haberman, CPO, is president of Garden State Orthopaedic Center Inc., Oakland, N.J., and Garden State Alliance Orthopedic Services Inc., Morristown, N.J. He is director of prosthetics and orthotics at The Rehabilitation Institute of Morristown Memorial Hospital (RIMM). He can be reached at (201) 538-4948.

Robert A. Bedotto, CPO, LPT, is vice president of Garden State Orthopaedic Center and Garden State Alliance Orthopedic Services. He can be reached at (201) 337-5566.

Ellen A. Colodney, MD, is director of inpatient services at The Rehabilitation Institute of Morristown Memorial Hospital (RIMM).

References:

1. Fillauer CE, Pritham CH, Fillauer KD. Evolution and development of the silicone suction socket (3S) for below-knee prostheses. Journal of Prosthetics and Orthotics 1989;1:2:92-103.
2. Ibid.
3. Dietzen CJ, Harshberger J, Pidikiti RD. Suction sock suspension for above-knee prostheses. Journal of Prosthetics and Orthotics 1991 ;3:2:90-93.
4. Bennett L, Lee BY. Vertical shear existence in animal pressure threshold experiments. Decubitus 1988;1: 1:18-24.
5. Bennett L. Transferring load to flesh: 11, analysis of compressive stress. Bulletin of Prosthetics Research Fall 1971 ;10:45-63.
6. Bennett L. Transferring load to flesh: V, experimental work. Bulletin of Prosthetics Research Fall 1973;10:88-103.
7. Bennett L. Transferring load to flesh: VIII, stasis and stress. Bulletin of Prosthetics Research Spring 1975;10:202-210.
8. Shamp AK Casting Brim, Orthomedics Inc., California.
9. Durr-Fillauer Inc., Fillauer Silicone Products, Chattanooga, Tenn.
10. IPOS Inc., Ipocon Silicone, Niagara Falls, N.Y.
11. Orthosil Silicone, Otto Bock Orthopedic, Minneapolis, Minn.
12. Mitchell, John. Technical suggestion, Morristown, N.J.
13. De-bubbler Device, The New Generation, Cliffside Park, N.J.
14. Nivea, Beiersdorf Inc., Norwalk, Conn.
15. Comtesse, Jean Paul. Personal communication, March 1991.
16. Suggestion from Charles J. Courtney of Universal Artificial Limb, Washington. D.C.

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