Clinical Perspectives on Prosthetic Ankle-Foot Designs

Terry J. Supan, CPO, FAAOP, FISPO

Prosthetic foot development over the last 30 years has attempted to mimic the anatomical foot. Early prosthetic feet designers tried to create a foot shape that was reasonably natural in appearance and provided a stable base of support. As post–World War II biomechanical understanding and manufacturing capabilities evolved, component designers moved toward simulating additional functions of the foot-ankle complex. The original design concept for articulated feet was to try to mimic sagittal plane motion only. Civil War-era amputees either used a "peg leg" or a foot shaped to match the shoe but with little function. As materials changed, different bumper materials evolved and passive flexing of the distal end of the foot would occur at the metatarsal level. One problem with early articulated feet was the substantial amount of maintenance necessary to maintain their function. In the 1950s, studies on the biomechanics of walking resulted in the creation of the patella tendon bearing, transtibial prosthesis including the concurrent development of the SACH (solid ankle, cushioned heel) foot.¹

This foot's rigid keel provided a stable weight-bearing platform. The cushioned heel softened the impact of weight transfer during loading response. The flexible belting material of the toe allowed smoother rollover at the end of stance. To accommodate to uneven ground, a number of multi-axial feet were designed. In theory, they also helped reduce unwanted ground reaction forces from being transmitted up through the prosthesis to the limb. The designs attempted, in part, to mimic some of the functions of the subtalar joint of the natural foot. These designs often had the same problems with wear and durability that many single axis feet had.

During the late 1970s, Vietnam-era veterans and other younger, active amputees stimulated the development of both the Seattle Foot™ and the Flex-Foot,™ designed to allow faster ambulation and possibly assist in running. These functional goals were very difficult to achieve in either the singleaxis foot or the solid ankle cushioned heel (SACH) foot. Originally these two designs were mislabeled by many as "energy saving" feet. Subsequent analysis has not shown that these feet or any other versions introduced since the early 1980s have reduced metabolic cost at normal walking speed.

A parallel development during the early 1980s focused on simulating passive subtalar joint motion within the prosthetic foot. The SAFE™ foot and the College Park™ foot are examples. The former is based on University of California at Berkeley Laboratory studies and the latter is based on podiatric theories.

In subsequent years, prosthetists, engineers and manufacturers expanded the use of the new foot designs. It seemed that every 6 months another foot design was marketed, often with the promise of unrealistic functional benefits. Often scientific evidence did not support these new designs. Still, sports opportunities for amputees grew as did the athletic, leisure and professional accomplishments of prosthetic users. During the late 1980s and early 1990s, several evaluation studies were conducted by prosthetic residents at Southern Illinois University^2–3 and physical therapy students at the University of Iowa^4–8 to clarify the functions of emerging foot designs. Despite limited numbers and less-than-perfect science, these evaluations did provide insight into the functional gains and functional limitations offered by these new feet.

So how can prosthetists use the literature that is available to them? It is recommended that the clinic team that evaluates the patient review the literature regularly to be aware of the specific limitations that a particular foot is intended to address. Biomechanical studies and comparisons of foot function in gait labs are among the most common studies done on the topic of prosthetic feet. All should be readily available and reviewed by clinicians. At a minimum, manufacturers provide specifications, offer training courses, and in most cases will supply test data.

Historically, the SACH foot is being used when a condition of midstance stability is required, but that may not be the best option for the sedentary, geriatric amputee. A foot that is too stiff can be just as debilitating as one that is too flexible. The same holds true of the higher forces necessary to create the returned energy in the dynamic response style foot. The desire to have the function of the subtalar joints also may be more applicable to the sedentary patient than has been understood in the past. Arguably, for some patients, power to advance their leg, a more accommodative ankle, or dynamic stimulation for balance may be just as desirable as a rigid forefoot for stability from mid to late stance. From this author's viewpoint, the Functional Category K-levels used by Medicare have not truly addressed the multifaceted functional needs of individual amputees. For example, a sedentary Category I individual may not be a community ambulator, but could possibly benefit from a more responsive foot than the traditional SACH foot or single-axis foot. A category II community ambulator who typically walks only at a very controlled and defined speed, may still need to vary cadence at critical moments (such as when crossing a busy intersection) and, therefore, might benefit from a more dynamic foot. To make such decisions with greater confidence, it is critical that more clinically relevant studies be performed that might form the basis of a more accurate functional classification scheme for the provision of prosthetic components. Development of an improved tool for assessing the potential functional capabilities of amputees that could be used consistently across the country would help the clinic team make more objective and consistent choices about components based on how effectively they enhance outcomes for individual amputees.

Correspondence to: Terry J. Supan, CPO, FAAOP, FISPO, Orthotic & Prosthetic Associates of Central Illinois, 355 W. Carpenter, Suite B, Springfield, IL, 62702–4945; e-mail: .

TERRY J. SUPAN, CPO, FAAOP, FISPO, is affiliated with Orthotic & Prosthetic Associates of Central Illinois, Springfield, IL.

References:

Radcliffe CW, Foote J. The Patellar-Tendon-Bearing Prosthesis. Berkeley, San Francisco: University of California; 1961.
Barth DG, Schumacher L, Thomas SS. Gait analysis and energy cost of below-knee amputees wearing six different prosthetic feet. J Prosthet Orthot 1992;4:63–75.
Wagner J, Sienko S, Supan T, Barth D. Motion analysis of SACH vs. Flex-Foot in moderately active below-knee amputees. Clin Prosthet Orthot 1987;11:55– 62.
Hsu MJ, Nielsen DH, Yack HJ, Shurr DG. Physiological measurements of walking and running in people with transtibial amputations with 3 different prostheses. J Orthop Sports Phys Ther 1999;29:526–533.
MacFarlane PA, Nielsen DH, Shurr DG, Meier K. Gait comparisons for below-knee amputees using a Flex-Foot versus a conventional prosthetic foot. J Prosthet Orthot 1991;3:150–161.
MacFarlane PA, Nielsen DH, Shurr DG, Meier K. Perception of walking difficulty by below-knee amputees using a conventional foot versus the Flex-Foot. J Prosthet Orthot 1991;3:114–119.
MacFarlane PA, Nielsen DH, Shurr DG. Mechanical gait analysis of transfemoral amputees: SACH foot versus the Flex-Foot. J Prosthet Orthot 1997;9:144–151.
Nielsen DH, Shurr DG, Golden IC, Meier Ko. Comparison of energy cost and gait efficiency during ambulation in below-knee amputees using different prosthetic feet. J Prosthet Orthot 1989; 1:24–31.