Energy-Storing Feet

FRANCIS J. TROST, MD


The human foot acts as a bumper at heel contact, cushioning the forward fall of the body and absorbing the decelerating forces generated by the initial placement of the foot on the walking surface. In stance phase, the foot acts as an elastic support and stabilizer for the body and, as the heel leaves the walking surface, the foot propels the body forward.

Until recently, prosthetic feet provided the first two functions of the normal foot, but had a negligible effect in propulsion. This lack interfered with the amputee's ability to run and jump. The deficiency caught the attention of investigators who developed prosthetic feet which, by reason of their materials and construction, could propel the body forward at heel-off. Basically, this was accomplished by using materials that stored energy when compressed by the body during early stance phase, and released that energy when the compression forces were removed at heel-off.

History and Development

Attention was first drawn to the limitations of prosthetic feet by Bernice Kegel. This led to a cooperative effort between the Seattle Prosthetic Research Study Group and engineers from the Boeing Aircraft Company, resulting in the development of a prosthetic foot that would store and release energy. After its introduction in 198 1, the Seattle foot underwent testing and revisions. It fulfills the designers' goals, allowing wearers to run. It also provides other functional and cosmetic advantages. The foot showed such promise that soon there appeared on the market other energy-storing feet of various designs and materials.

All energy-storing feet have one thing in common. An energy-inert material has been replaced with material that will deform with pressure and then resume its original shape when pressure is removed with a consequent release of energy.

Fittings at the Shriners Hospital

Fifty-two juvenile amputees were studied. The group included 36 unilateral below-knee, 2 bilateral below-knee and 14 above-knee amputees. Fifty-four energy-storing feet were provided, including Flex-Feet, Carbon Copy 11 feet, Seattle feet, and STEN feet. We sent evaluation forms to each child and received 31 replies. Most liked the new foot better than their old one which, in almost all the cases, was a SACH foot. In addition, most felt the energy-storing foot looked better, felt lighter and livelier, and walked smoother. Nineteen stated they could walk farther with the energy-storing foot, but twelve wrote that it made no difference.

When questioned regarding any activities that they could do with the new foot that they could not do with the old one, the group's overwhelming answer was that there were none. Many qualified this, however, by saying that the activities with the new foot were easier, especially running. One girl found she could now play jumping jacks with the energy-storing foot and another was enthralled because she could now paint her toenails.

In evaluating specific activities, most amputees responded that running, jumping, climbing stairs and ramps, walking, and negotiating uneven surfaces were easier with energy-storing feet.

Twenty-three had no trouble getting used to the foot, although eight who did have problems mentioned tripping, difficulty kneeling, residual limb irritation (usually due to alignment problems), and the different forward propulsion of the foot.

Many experienced foot breakage. Children are particularly hard on prosthetic devices. Most breakage occurred early in the series. As improvements were made in the feet, breakage declined somewhat but it still remained an issue. Serious breakage led us to discontinue the use of STEN feet. The toes of the Seattle feet, and, to a lesser extent, of the Carbon Copy 11 feet occasionally failed. The second most common failure was at the forefoot, especially in those feet that have a spring-leaf construction, namely Seattle and Carbon Copy 11. Ten children had problems fitting the feet into shoes because of the forefoot width.

Overall, 26 preferred the energy-storing feet and 5 chose the SACH foot. In general, the children's comments indicated they like the livelier foot but wanted it to be more durable. Some complained of skin irritation when the foot was changed; our impression was that this was usually due to malalignment.

Problems exist in fitting children with energy-storing feet. It is impossible to fit young children with many of these feet because they are not made in sizes small enough. A corollary to this is that the energy-storing system will also have to be altered for small, lighter children. In this regard, the Flex-Foot may be advantageous because it is essentially custom made. It is very costly and the test model could not be fitted on extremely long below-knee or Syme's limbs. Breakage is a more persistent problem in children than it is in most adults; therefore, the feet should be more durable if children are to wear them on a large scale.

Minneapolis Veterans Affairs Hospital Experience

The amputee clinic at the Veterans Affairs Hospital has made wide use of energy-storing feet for four years. They have prescribed them for new amputees who have never worn a prosthesis and for those who have worn conventional feet, such as SACH and single-axis. Adults in the clinic are older dysvascular amputees or long-term post-traumatic veterans.

In 1987, we prescribed 33 feet: 23 Carbon Copy 11, 5 Flex-Feet, 3 Seattle and 2 STEN feet. These are typical annual prescriptions,

The popularity of Carbon Copy II feet was due to their good energy storing characteristics, light weight, minimal breakage, and competitive price. Although we were very happy with the performance and durability of the FlexFoot, because of its high cost we tended to reserve it for patients with special needs. In spite of the fact that the Seattle foot was developed in part by grants from the Department of Veterans Affairs, we fitted few of them. The performance characteristics of these feet were good but, at first, we had a problem with supply and had significant breakage, especially of the toes. Severe breakage problems with the STEN foot limited our use. We are advised that later models are more durable.

Although we do not have as detailed data for the adults as we do for the children, there was overwhelming acceptance of the feet. Typical comments from adults converted from older feet included the impression that the new feet were lighter, livelier, quicker, provided increased thrust, increased walking tolerance, decreased fatigue, and made running and jumping easier.

Negative comments included complaints of instability and poor knee control, the need to relearn walking patterns, increased problems with slippery surfaces and going downstairs, and a sensation of falling off the front of the foot on toe-off. One bilateral amputee rejected the feet as being "too unstable. "

Gait Laboratory Studies

Four amputees participated in laboratory analysis including measurement of the determinants of gait and oxygen consumption using their prosthesis with a SACH foot and, after the necessary modifications, with a Carbon Copy 11 foot. Subjects included Syme's, below-knee and high above-knee amputees, and an individual with proximal femoral focal deficiency who was fitted as a long above-knee amputee.

Although the results are preliminary and the number of patients small, the following results are of interest:

  1. Energy expense (milliliters of oxygen/kilogram/meter) is greater in the higher level amputees, as one would expect. Energy-storing feet were associated with decreased oxygen consumption for the lower level amputees at fast cadence but made no difference at normal cadence. For higher level amputees, no difference in oxygen consumption between the conventional and the energy-storing foot existed at either cadence.
  2. Energy-storing feet affected gait kinematics; users had a mild deviation toward hip flexion and pelvic forward tilt and a tendency to increased knee flexion in swing phase.
  3. Why there is the discrepancy in energy cost between high and low level amputees is subject to conjecture. Additional testing will have to be done to resolve this question.

Conclusions

Energy-storing feet seem to be a valuable addition to the prosthetic armamentarium. They provide an element of propulsion present in anatomic feet, but previously lacking in prosthetic ones. They benefit almost all amputees regardless of age or activity level, from young athletes to the aged, most of whom realize improved function and appearance.

Problems with energy-storing feet remain, particularly cost, durability, and availability for small children.

Although patient acceptance of these feet has been excellent, further testing and evaluation should be done. Of interest is the work being done in gait laboratories to assess the effect of energy-storing feet on the determinants of gait and on energy consumption.

Shriners Hospital for Crippled Children, 2025 East River Road, Minneapolis, MN 55414

References:

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