Design Innovation Paper

Development of a Bimodal Ankle-Foot Prosthesis for Walking and Standing/Swaying

[+] Author and Article Information
Andrew H. Hansen

Director, Minneapolis VA Rehabilitation
Engineering Research Program
Minneapolis VA Health Care System,
Associate Professor, Program in Rehabilitation Sciences,
Department of Physical Medicine and Rehabilitation,
University of Minnesota,
One Veterans Drive (MS 151),
Minneapolis, MN 55417
e-mail: Andrew.Hansen2@va.gov

Eric A. Nickel

Research Biomedical Engineer
Minneapolis VA Health Care System,
One Veterans Drive (MS 151),
Minneapolis, MN 55417
e-mail: Eric.Nickel@va.gov

Manuscript received August 2, 2012; final manuscript received April 18, 2013; published online July 3, 2013. Assoc. Editor: William K. Durfee.

J. Med. Devices 7(3), 035001 (Jul 03, 2013) (5 pages) Paper No: MED-12-1099; doi: 10.1115/1.4024646 History: Received August 02, 2012; Revised April 18, 2013

The human ankle-foot system conforms to a circular effective rocker shape for walking, but to a much flatter effective shape for standing and swaying. Many persons with lower limb amputations have impaired balance and reduced balance confidence, and may benefit from prostheses designed to provide flatter effective rocker shapes during standing and swaying tasks. This paper describes the development and testing of an ankle-foot prosthesis prototype that provides distinctly different mechanical properties for walking and standing/swaying. The prototype developed was a single-axis prosthetic foot with a lockable ankle for added stability during standing and swaying. The bimodal ankle-foot prosthesis prototype was tested on pseudoprostheses (walking boots with prosthetic feet beneath) for walking and standing/swaying loads, and was compared to an Otto Bock single-axis prosthetic foot and to able-bodied data collected in a previous study. The height-normalized radius of the effective rocker shape for walking with the bimodal ankle-foot prototype was equal to that found earlier for able-bodied persons (0.17); the standing and swaying effective shape had a lower height-normalized radius (0.70) compared with that previously found for able-bodied persons (1.11). The bimodal ankle-foot prosthesis prototype had a similar radius as the Otto Bock single-axis prosthetic foot for the effective rocker shape for walking (0.17 for both), but had a much larger radius for standing and swaying (0.70 for bimodal, 0.34 for single-axis). The results suggest that the bimodal ankle-foot prosthesis prototype provides two distinct modes, including a biomimetic effective rocker shape for walking and an inherently stable base for standing and swaying. The radius of the prototype's effective rocker shape for standing/swaying suggests that it may provide inherent mechanical stability to a prosthesis user, since the radius is larger than the typical body center of mass’s distance from the floor (between 50–60% of height). Future testing is warranted to determine if the bimodal ankle-foot prosthesis will increase balance and balance confidence in prosthesis users.

Copyright © 2013 by ASME
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Fig. 1

Ankle-foot effective rocker shapes for walking (light gray), standing and swaying (dark gray), and quiet standing (black) (figure adapted from Ref. [8]). Effective rocker shapes of the ankle-foot system are found by transforming the center of pressure of the ground reaction force into a shank-based coordinate system.

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Fig. 2

CAD renderings of the final design in the locked standing/swaying mode (left) and in the unlocked walking mode (right). Major parts include the foot plate (1), slider (2), pillow block (3), ankle yoke (4), male pyramid (5), posterior bumper (6), anterior bumper (7), ankle shaft (8), small steel shaft (9), and slider springs (10). The slider is pushed and pulled by a small actuator located within the pillow block (see Fig. 3).

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Fig. 3

Exploded view of the working parts of the bimodal ankle-foot system, labeled as per Fig. 2, with the linear actuator (11) shown. The ribbon cable from the actuator is cut short in the drawing. In the prototype, the ribbon cable folded into a track along the bottom of the foot plate (1–not shown) and came out a small slot in the posterior section of the foot plate. The slot is visible in the CAD renderings in Fig. 2.

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Fig. 4

Section view of the bimodal ankle-foot system showing the fully-assembled position of the parts, including the linear actuator which is located within a hollow space in the pillow block

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Fig. 5

Photographs of the pseudoprostheses connected to the Otto Bock single-axis prosthetic foot (left) and the bimodal ankle-foot prosthesis prototype (right). The wireless receiver, relay, and battery pack were taped to the pseudoprosthesis for testing of the prototype.

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Fig. 6

Vertical ground reaction forces (VGRF) on bimodal ankle-foot prosthesis prototype (BM–black) and single-axis foot (SA–gray) for walking (top) and standing/swaying (bottom). The dark gray line in the standing/swaying plot (bottom) is the sum of the forces on both feet during standing/swaying.

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Fig. 7

Effective shapes of the single-axis prosthetic foot (top) and bimodal ankle-foot prosthesis prototype (bottom) for walking (white) and standing/swaying (gray). The bimodal ankle-foot prosthesis prototype appears to have a more distinct difference in effective shapes compared with the single-axis prosthetic foot, consistent with the distinct differences found earlier in able-bodied persons (Fig. 1). (Note that the top image has been flipped horizontally to facilitate comparison with able-bodied data (Fig. 1) and the bimodal ankle-foot prosthesis prototype.)

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Fig. 8

Best fit radii to the effective rocker shapes of able-bodied ankle-foot systems compared with effective shapes measured for the bimodal ankle-foot (AF) prosthesis prototype and the Otto Bock single-axis prosthetic foot. Data for the able-bodied ankle-foot system are medians with error bars drawn between the first and third quartiles [8]. A picture of a human is drawn to indicate scaling to height as well as an indicator of the body center of mass, which is typically between 0.5 and 0.6 times body height. A rocker radius that is greater than the height of the body center of mass is mechanically stable.




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