Selection and Adoption

Prior to 1960, lower-limb amputees chose articulated single-axis prosthetic feet, which inhibited dorsiflextion and plantar flexion, along with other complications. Although there were other options, the single-axis fit the most diverse number of amputees and doctors were more comfortable fitting and prescribing the type of prosthetic. The articulated prosthetic has a cleft at a point corresponding to the anatomic ankle.[1]

[1] Comparrison of four energy storing prosthetic feet discussed below:

A. SAFE, B. Seattle Foot, C. Carbon Copy II, D. STEN Foot

The single-axis articulated unit has been manufactured longer than any other modern prosthetic foot. The foot is usually made out of wood with rubber or felt in the toe section and a vertical bolt joint connecting the foot and the leg. The joint along with an axle allows for dorsiflextion and immediate plantar flexion after heel-strike. However, rubber bumpers restrict the range of both flexions, which makes the articulated unit only affective on level ground. The rubber bumpers are adjustable according to the amount of walking the wearer does and are sometimes replaced by rough felt. Unfortunately, the single-axis foot is not aesthetically pleasing with a large gap between the bolt joint and the leg. The gap also allows water and dirt to enter the prosthetic that make it more prone to wear and hinder the prosthetic’s function. The metal parts and the heavy rubber add to the weight of the lower-prosthetic and, although it has been proven that weight does not affect stride, it is cumbersome to the wearer. [2]

There are also multi-axis articulated prosthetics. However, the Greissinger multi-axis assembly is not as commonly prescribed to amputees because of the price and the weight. Regardless, there are advantages over the single-axis models. Most significantly, the multiple-axis foot allows further rotation of the foot enabling the wearer to walk on uneven ground. [3]

Overcoming the popularity of the articulated prosthetic foot is the non-articulated model. The non-articulated foot uses a fully connected external structure from the leg to the toes.[4] The non-articulated foot currently surpasses the articulated foot in popularity. The classification includes the Seattle Foot and the SACH (Solid Ankle- Cushioned Heel) foot.[5] The foot consists of a “central rigid keel that terminated at the point corresponding to the metatarsophalangeal joins, a posterior wedge of resilient material, and a covering of slightly resilient synthetic rubber.”[6] The non-articulated foot can bend slightly to adjust to uneven ground and absorb stress from many different angles. The non-articulated foot has the most number of different models. There are prosthetics that have contoured toes and ones designed for a 9-cm high heel. The Dynamic Foot is a variation that has a short keel that adjusts automatically to the alignment needed for flat-soled and low-heeled shoes, along with a space in between the first and second toes designed for thong sandals. The non-articulated SACH feet are light weight, more durable, environmentally resistant, more variable, and significantly more aesthetically pleasing. However, the foot allows for little dorsiflextion and plantar flexion and does not have an effective ankle axis which requires the wearer to apply great force to flatten the foot and stabilize the knee.[7] Despite the SACH foot’s problems, it is the most widely manufactured design in the United Stated and Canada.[8]

An alternative to the SACH foot is the SAFE foot, or the Stationary Attachment Flexible Endoskeleton Assembly, manufactured by John Campbell and Christopher Childs in Oregon during the mid-1970s.[9] The SAFE foot features a “stationary ridged polyurethane bolt block joined to a polyurethane elastomer section” at a 45-degree angle to the stagittal plane (the plane that runs from the bottom to the top of the body). [10] The polyurethane bolt block is designed with settings that allow the amputee to adjust the height according to various heel heights. The SAFE foot is better than the SACH foot on rough terrains, but it is slightly more expensive and heavier (but lighter than the Greissinger foot).[11] Acceptance of the SAFE foot was delayed because of structural problems, but is now more popular than the SACH foot in spite of the price and weight.[12]

Non-articulated prosthetic feet also vary greatly in keel design. The Stored Energy (STEN) foot added a necessary function of prosthetics, energy storing in the keel. The prosthetic devise’s keel contains three wooden pieces with cylindrical dense rubber in between at the two main keel joints. It also has a cushioned heel, similar to the SACH and SAFE feet. The keel and cushion are surrounded by a foam rubber mold and features toes and toe nails. The STEN foot adjusts to uneven grounds and adds a spring in the wearer’s step. The cost and weight are moderate when compared to the SAFE foot and the SACH foot.[13]

The Carbon Copy II prosthetic foot uses double carbon-fiber material in the keel and cushion heel molded in polyurethane foam with toes and toe nails. The benefits of carbon material rather than steel parts are mainly carbon’s light-weight and durable characteristics. Also, the Carbon Copy II is an energy-storing prosthetic and is designed to store more energy in a running gait than the previous prosthetics discussed. However, the foot provides less lateral movement when compared to the SACH foot, does not adjust to uneven ground, and is the most expensive of the options discussed. [14]

The Seattle foot is the result of the collaboration between Boeing Company and the Prosthetics Research Study and funded mainly by the Veterans Administration. The goal of the invention was to improve prosthetic devices for athletic amputees, as well as the rest of the population. Inventors, which included most notably Ernest Burgess, used the SACH foot and improved it greatly.[15] The foot incorporated a Delrin keel, Kevlar padded toe, a cushioned heel, plus a polyurethane mold with the most anatomic depiction of any other prosthetic foot.[16] The flexible Delrin, a DuPont company product, keel stored energy during heel-lifting and released the energy to push the foot off the ground and supports both the horizontal (transverse) and vertical (saggital) planes.[17] The result of the large energy-storage is a more natural and springing step when compared to any other prosthetic. Research has shown, “Below- knee amputees have shown preference for ankles offering more flexibility.” [18] The Seattle Foot gives amputees the benefits of a dynamic response foot during activity, which they did not previously enjoy. Therefore, the design is “appreciated by a significant proportion of the moderately active group of lower-limb amputees as well as athletes.”[19]

The prosthetic device is designed to fit industry standard fittings, which makes the foot adaptable and easily fit to the stump. The Seattle foot includes a space between the first and second toe and is available in a smooth mold as well. The Seattle foot is highly adaptable according to the amputee’s needs with three different keel designs depending on shoe size and desired spring in the step. The Seattle foot’s energy storage is significantly larger than the Carbon Copy II and the STEN foot.[20] Also, the patent claims that the Seattle Foot is “highly efficient in accommodating the necessary loads and does so with a minimal number of components, for reduced manufacturing costs, and a minimum number of internal joints, for reduced risk of mechanical failure.”[21] The weight is not notably greater than the SACH foot, and less than other energy-storing prosthetics in competition with the Seattle Foot.

In addition to inventing the actual foot, Burgess and his team invented computer software, called the “Shape Maker,” which designs custom limbs at a faster and cheep rate. Burgess gave the technology to prosthetic teams in developing countries, like Vietnam, who hold a high amputee population.[22] With the technology to make cheap and effective prosthetic devices, the Seattle Foot flourished.

[2] Left to Right: SACH, Seattle Foot, Carbon Copy II


[1] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 43.

[2] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 43.

[3] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1875.

[4] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1875.

[5] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 44.

[6] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1875-6.

[7] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1877.

[8] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 45-46.

[9] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 47.

[10] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1877.

[11] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1877.

[12] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 47.

[13] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1877.

[14] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1878.

[15] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 48.; see also the “Invention” page that includes a far more in depth description of the invention.

[16] Joan E. Edelstein, “Prosthetic Feet: State of the Art,” Physical Therapy 68, no. 12 (December 1988): 1878.

[17] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 48.

[18] Paul Klopsteg, et al, Human Limbs and Their Substitutes, (New York: McGraw-Hill Book Company, Inc., 1954) 165.

[19] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 48.

[20] A. P. Arya, A. Lees, H. C. Nirula, L. Klenerman. “A Biomechanical Comparison Of The SACH, Seattle And Jaipur Feet Using Ground Reaction Force,” Prosthetics and Orthotics International 19, (1995):44.

[21] “US Patent 4645509: Prosthetic Foot Having a Cantilever Spring Keel,” Patent Storm, http://www.patentstorm.us/patents/4645509.html (accessed March 30, 2009).

[22] Carole Beers, “’Seattle Foot’ Inventor Dies,” Seattle Times, October 1, 2000. http://community.seattletimes.nwsource.com/archive/?date=20001001&slug=4045489 (accessed March 25, 2009).

Figure Sources:

[1] A. Bennett Wilson Jr., Limb Prosthetics, 6 ed. (New York: Domos Publications, 1989), 47.

[2] Valarie S. Bodeau, “Lower Limb Prosthetics,” E Medicine, http://www.jackgoldsurgical.com/PRODUCTS/eMedicine%20-%20Lower%20Limb%20Prosthetics%20%20Article%20by%20Valerie%20S%20Bodeau,%20MD.htm (accessed April 12, 2008).

Leave a Reply