By Gordon Zernich, CP

Although many areas of prosthetics have progressed quite a bit, components and techniques that are not in high demand advance slowly. The location of the amputation, its medical necessity and frequency are factors that affect progress in the prosthetic field. A case in point is the design and components of the Symes prosthesis. 

Tomas Dowell, CPO, chief of orthotic and prosthetic lab for the Veterans Affairs Medical Center in Miami, and his staff see many Symes prosthetic amputees due to combat wounds, diabetic foot complications, or gangrene, all of which may lead to progressively higher foot amputations levels. By working with these patients, Dowell and his staff have developed some solutions to the common problems Symes amputees face.

Problems with Symes
The design of the Symes prosthesis frequently depends on the shape of the residual limb’s distal end and its weight-bearing capacity. If it is bulbous, then the socket design may incorporate an expandable pelite liner, or it may use a removable medial window and pelite liner to facilitate donning and doffing. In either case, it may be necessary to add a shoe lift to the sound side to make sure both limbs are of equal length.

Dowell says, “Our choices of components and techniques are limited to the difference in length between the affected leg and the contralateral side. The new generation of some Symes feet allows for some inversion/eversion and plantarflexion/dorsiflexion, but such innovation can affect the scarce room that may be available.” 

The materials used in the newer Symes feet can also create problems. Typically, the newer foot technology mates carbon fiber, titanium, and hard (but not inflexible) resins. However, this newer generation of foot is very rigid compared to the solid ankle, cushion heel (SACH) design it replaced. 

“The newer Symes foot is rigid to accommodate the large amount of force that gets transferred through the rest of the prosthesis during stance phase,” Dowell says. “It functions as a lever throughout the latter part of the stance cycle and its rigidity keeps the construction of the foot itself from failure.” 

“But,” he continues, “The foot’s rigid construction contributes to the biomechanical stress [on] its attachment point. The foot hardly flexes at all. 

“It is so rigid that the wearer rolls over the foot and transfers the stress to the distal socket. The stress created is great enough to fatigue the titanium three-prong adaptor fabricated into the socket, the acrylic resin- carbon fiber laminate holding it there, or both. The heavier or more active the wearer, the more likely something like that will happen.” 

While this problem may be partially solved by strengthening the socket with additional fiberglass or carbon fiber lay-ups, these solutions make an already big and bulbous socket even bigger and harder to fit into a regular shoe. 

The selection of prosthetic feet at high amputation levels is far greater than the limited choices available to the Symes amputee. Such a lack of foot selection restricts the practitioner’s ability to select a foot suitable for different lifestyles and weight levels as well. Invariably, the lack of selection compromises the activity level of most K1 through K4 patients. For level 2 ambulators, the rigidity of the new generation Symes foot is often prohibitive. 

The solution
Under Dowell’s direction, the VA Medical Center’s orthotic and prosthetic lab has developed a Symes prosthosis that is more flexible, less cumbersome, lighter in weight, and as strong as what is commonly fabricated in the profession. (See sidebar) The prosthosis is easily transferable to different shoes of the same heel height as well. 

First, Dowell decided to lessen the rigidity and weight of the current Symes prosthetic socket by using an AFO-type prosthosis design when indicated for short length Symes amputations. 

The Symes AFO Dowell designed incorporates the use of a silicone liner with a pin suspension system, a comparatively lower-profile, non-PTB AFO prosthosis socket, the shuttle lock, a rotating female socket adapter if possible, and a varying choice of semi-flexible or rigid carbon fiber footplate(s). 

The materials and fabrication technique used to make it are the same as what a technician would use to make any laminated prosthetic socket that incorporates a locking pin suspension unit. Its construction, lay-up and resin depend on the patient’s weight and activity level. 

The foot components are selected with the same criteria by using a number of carbon footplates for the amount of energy storage and release needed. The shuttle lock is fastened to the carbon fiber footplate(s) via its packaged hardware, and urethane bonding resin is applied there for additional strength. 

The result is an AFO that is strong enough to provide support during the first half of the stance cycle, and then provide energy storage and release during the second half. The distal reciprocal of the AFO prosthosis socket is deep enough to contain 75 percent of the frequently bulbous distal residual limb. 

The silicone gel of the liner adds immeasurably to its comfort and use, while the pin keeps things locked tight. Finally, dynamic alignment adjustments are easily made with a 4mm Allen key. 

“The design lessens the stress on both the patient’s gait and the prosthosis during the heel and toe-off part of the gait cycle, [because] it allows some give between the distal, posterior residual limb and its corresponding area on the prosthosis,” Dowell says. 

His criteria for using this new Symes AFO prosthosis include: 

• a well-healed distal residual limb, with no skin lacerations, ulcers, skin slough, adherent scars, or other complications,
• minimal volume fluctuation,
• an intact Achilles tendon, and
• the ability to bear weight distally. 

Two more innovations
Dowell and his staff also use the following innovations, depending on the length of Symes amputation present. 

When indicated, they use a much less rigid carbon fiber footplate(s) in the AFO Symes prosthosis than what is currently available with the new generation of Symes feet or the footplate available for the Chopart prosthesis. The longer the limb length, however, the more likely the carbon fiber footplate(s) will be attached directly to the AFO prosthetic socket after statically aligning the patient’s toe out, his or her sagital and frontal weight line, and degree of socket flexion. 

The innovation here is the use of the AFO prosthosis design, a silicone cushion liner, an extremely lower-profile, flexible foot and weight savings. It is indicated for the long length Symes residual limb. Its design eliminates the likely limb length discrepancy generated by a traditional Symes socket, pelite liner and the newer generation rigid foot. 

“When we place a more flexible footplate on the distal end of an AFO Symes prosthosis, it provides for a greater capacity for energy storage and release,” Dowell says. “When this carbon graphite footplate and its (high density) plastizote cosmetic cover are placed inside the shoe, the sole of the shoe also contributes to the support provided through the first half of the stance gait cycle and its energy storage and release thereafter.” 

Second, space permitting, instead of fabricating the shuttle’s lock into the AFO prosthosis socket, Dowell attaches the shuttle lock directly to the footplate(s). The pin’s reciprocal is laminated into the distal socket. The pin locks directly into the foot. It may be used for medium length Symes residual limbs. 

This design, which Dowell describes as a Symes floor-reaction prosthosis, uses fewer components than the Symes AFO and the lowest profile pin suspension and shuttle lock on the market. As per foot orthotics, forefoot and hindfoot posts tweak the alignment if necessary. 

The results
Generally, Symes patients at the VA Medical Center in Miami are favorably impressed by the innovations Tomas Dowell and his staff have incorporated into the prosthetic socket and foot designs developed for them.  Most patients there have felt improved gait and cadence responsiveness. There is far less likelihood of achieving those same outcomes with the newer generation of Symes feet coupled with exoskeletal sockets and pelite liners. 

Gordon Zernich, CP, works for the VA Medical Center in Miami, Fla.