Chapter 17B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles
The Syme Ankle Disarticulation: Prosthetic Management
Richard Voner, C.P.O.
John W. Michael, M.Ed., C.P.O.
Disarticulation at the ankle has challenged the prosthe-tist since the procedure was first introduced by Syme in 1842. Surgical modifications have been introduced to make the stump neater and less bulky, allow more room for the ankle joint, and produce a more cosmetic prosthesis.
A satisfactory end-bearing Syme limb demands a prosthesis with the following characteristics:
- Transmission of body loads
- Light enough to wear comfortably
- Ability to supply the equivalent of foot and ankle function
- Lengthening of the limb to adjust for loss of the talus and os calcis
- Distribution of the high forces developed in the ankle area
- Provision of rotary stability about the long axis
- Provision of shock absorption
- Suspension during swing phase
- Readily donned without requiring multiple non-cosmetic, difficult fasteners
- Adjustability to relieve pressure along a sensitive scar line
Despite the advantages provided by the long lever arm of the essentially intact tibia and fibula and the virtually full end-bearing capabilities of the heel pad, multiple problems still exist in the design of the "ideal" prosthesis. Reports in the literature of "new prosthetic approaches" attest to the fact that the final perfect prosthesis has not yet been designed.
In addition to producing the artificial limb, the pros-thetist may aid in postoperative management by applying walking casts. These protect the tissues during the healing phase and hasten contouring of the stump (see Fig l7A-18).
SOLUTIONS TO PROBLEMS INHERENT IN THE DESIGN AND MANUFACTURE OF SYME PROSTHESES
Weight and Bulkiness
Until 1940, the usual prosthesis was a leather socket reinforced with steel straps and with an anterior tongue and lacer. The ankle was frequently a single-axis joint with bumpers. Early use of polyester-fiberglass laminate with an opening for entry of the residual limb materially reduced bulkiness. However, to increase the strength of the socket, it was necessary to substitute ep-oxy resins for the polyesters before adequate strength was obtained. A prosthesis of this type was developed and is being used by the Canadian Department of Veteran Affairs. It is commonly called the "Canadian Syme prosthesis" (Fig 17B-1.).
Reproduction of Ankle Joint Motion
The limited space available between the distal portion of the residual limb and the floor severely constrains the design of foot mechanisms for the Syme prosthesis. Although articulated joints have been used in the past, they were plagued by chronic wear and tear and have been largely abandoned. As a result, almost all contemporary Syme prostheses utilize a nonarticu-lated foot.
For many years, a modified solid-ankle, cushion-heel (SACH) foot was the only available option. Although clinically successful, the rigidity of the wooden keel added significant stresses to the prosthesis and was not suitable for some vigorous activities, while the limited space for the heel cushion limited the shock absorption at heel strike as compared with the SACH foot designed for higher-level amputations. In an effort to overcome these shortcomings, several more flexible foot options are now available.
A special version of the stationary-ankle flexible-en-doskeleton (SAFE) foot has been used with the Syme prosthesis for the past decade with good results. It has a flexible anterior keel that allows an easier rollover and reduces the ground reaction forces on both prosthesis and residual limb. More recently, several dynamic-response feet have become available in a low-profile style suitable for the Syme amputee. Carbon Copy II and the Seattle Litefoot both have plastic spring keels that add a measure of dynamic response to the prosthesis while incorporating an abbreviated cushion heel to simulate plantar flexion following heel strike.
Flex-Foot and the similar Springlite design are also available for the Syme level. Both designs utilize a carbon composite spring heel and keel to simulate ankle motion. The Quantum foot from England provides similar function by using fiberglass-reinforced spring keels/ heels and has also been adapted for Syme prostheses. Patient acceptance of these newer alternatives has been favorable thus far, but reliability and specific indications have yet to be determined.
Provision for Donning
Provision for donning is necessary to allow the bulbous distal end to pass the narrow shank portion of the prosthesis.
The following are some of the methods of providing the different diameters:
- Older prostheses had an anteriorly opening corset that could be laced.
- Plastic prostheses have windows either medially, posteriorly, or posteromedially (Fig 17B-2.).
- Closed double-wall prostheses with flexible inner walls allow expansion so that the bulbous end is inserted past the expandable portion. The elasticity is sufficient to close about the end and provide suspension (Fig 17B-3.,A and C). A double wall with an elastic panel also provides enough expansion (Fig 17B-3.,B).
- A flexible inner socket of Kemblo rubber, Silastic foam, or similar material bridges the narrow portion of the stump above the heel pad and maintains a total-contact, stump-socket wall relationship (Fig 17B-4.).
Distribution and Absorption of Stresses Developed During Stance Phase
Uniform distribution of loads along the tibia is necessary during push-off or rollover. Careful molding is necessary along the tibial crest. High forces in the ankle area require sufficient material to absorb the stress. Because of the bulbous form, there will be a certain bulky appearance to any prosthesis design. There is a constant problem then between thickness requirements for strength and thinness required for appearance. Strength is most easily obtained by using high strength-to-weight ratio plastics that can be molded easily over a plaster positive model.
Provision of Rotary Stability About the Long Axis
A patellar tendon-bearing shape of the proximal part of the brim will stabilize against the mediolateral flares of the tibia. Flattening of the posterior portion of the brim adds to a triangulation effect.
Provision of Pressure Relief Along a Sensitive Scar Line
Direct end bearing can be reduced by proximal loading of the prosthesis along the tibial flares.
Provision of Suspension During Swing Phase
The bulbous distal end and the flare of the tibia and fibula provide sufficient surface. The contour of the distal part of the socket must be in intimate contact with the residual limb in its most bulbous portion. If the distal end is especially narrow and has minimal flare, a suprapatellar suspension strap may be added.
Provision of Shock Absorption
The cushioned heel of the SACH foot is smaller than in a transfemoral or transtibial prosthesis. A good share of the impact force is dissipated in the heel, but some load must be assumed by the knee, which flexes just after heel contact.
Sweating in a Plastic Prosthesis
Porous plastic laminates have been introduced but have not proved completely satisfactory to date. Substitution of several layers of prosthetic socks may provide better moisture distribution.
Plastic laminates provide a thinner wall. Air-cushion types that require no window and double-walled types with an inner elastic panel are less unsightly because they require no straps, buckles, or other outside paraphernalia for closure. However, they are thicker just above the ankle (Fig 17B-3.,C and Fig 17B-4.,C).
Correction of Limb Length Discrepancy
A thinner SACH foot must be used for the Syme ankle disarticulation than that used for transtibial or trans-femoral amputations. Because of this, it is not always possible to use as thick a heel cushion as desired for shock absorption. In bilateral amputees, this is not a problem, and such improvements as a five-way ankle may be used.
Comfortable Transmission of Stump-Socket Forces Through a Satisfactory Socket
Socket design must provide the following:
- Stabilization against rotary forces about the long axis. Three-point or triangular stabilization against the flares of the tibia plus a flattening across the posterior portion of the gastrocnemius will provide a good share of stability. In the two-stage method, the distal portion of the stump is squared slightly. This possibly aids in preventing rotation about the long axis.
- Weight support can be distributed between the end of the prosthesis and the proximal portion of the socket brim. Dispersion of forces against the proximal surface of the leg at push-off can be accomplished through careful fitting against the mediolateral surfaces of the tibia.
- Dispersion of force encountered at heel contact is accomplished through contact from the heel to the upper gastrocnemius. The cuplike contour for the stump end must extend superiorly enough to prevent motion between the socket and the stump in an anteroposterior direction.
Indications for each of the described types of prostheses will depend primarily on the physical characteristics of the residual limb. Most patients can be fitted with a closed double-wall prosthesis with attached flexible inner walls fabricated with expandable material. The closed rigid shell with a flexible removable inner socket may allow even earlier donning of the limb without undue difficulty. The closed prosthesis presents a much neater appearance and is particularly desirable for women.
When the amputation stump has a bulbous and irregular distal end, often seen in older amputees and after trauma, it may be necessary to fabricate one or more windows in the prosthetic shell to allow the terminal tissues to slide past the smaller and narrower proximal area. The prosthetist may need to innovate and deviate from standard designs in certain unusual circumstances. Almost without exception, all prostheses will be fitted with the SACH foot described earlier.
Surgeons performing the Syme ankle disarticulation can often simplify prosthetic considerations by careful attention to bone contour and heel pad positioning. Improvements in design and materials have allowed amputees using old-style prostheses to convert successfully to using the modern limbs now available.
The Syme ankle disarticulate has usually walked in a plaster cast with a rubber heel or artificial foot prior to delivery of the definitive prosthesis. Virtually no training is required during the cast period except for occasional use of crutches or a pickup walker at the outset.
After delivery of the prosthesis, instructions are given for donning and doffing. In addition, the patient is taught stump hygiene, use of prosthetic stockings, and daily maintenance of the prosthesis. Most patients state that it will take a day or two to get used to the limb.
Modern plastic materials and construction techniques permit manufacture of Syme prostheses that are improved in appearance and durability, lighter in weight, free from malfunction of mechanical components, and simpler and less costly to manufacture.
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- Marx HW: An innovation in Syme's prosthetics. Orthot Prosthet 1969; 23:131.
- Mazet R Jr: Syme's amputation. JBone Joint Surg [Am] 1968; 50:1549.
- Michael JW: Component selection criteria: Lower limb disarticulations. Clin Prosthet Orthot 1988; 12:99-108.
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- Radcliffe CW: The biomechanics of the Syme prosthesis. Artif Limbs 1961; 6:76.
- Romano RL, Zettl JH, Burgess EM: The Syme's amputation: A new prosthetic approach. Inter-Clin Info Bull 1972; 11:1.
- Sarmiento A, Gilmer RE Jr, Finnieston A: A new surgical-prosthetic approach to the Syme's amputation. A preliminary report. Artif Limbs 1966; 10:52.
- Warner R, Daniel R, Leswing AL: Another new prosthetic approach for the Syme's amputation. Inter-Clin Info Bull 1972; 12:7.
- Wilson AB: Prostheses for Syme's amputation. Artif Limbs 1961; 6:52.
Chapter 17B - Atlas of Limb Prosthetics: Surgical, Prosthetic, and Rehabilitation Principles