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Design Innovation

Prosthetic Ankle-Foot Mechanism Capable of Automatic Adaptation to the Walking Surface

[+] Author and Article Information
Ryan J. Williams

 Plexus Technology Group, 5511 Capital Center Drive, Suite 600, Raleigh, NC 27606ryan.williams@plexus.com

Andrew H. Hansen

 Northwestern University, Jesse Brown VA Medical Center, 345 East Superior Street, Room 1441, Chicago, IL 60611-4496a-hansen@northwestern.edu

Steven A. Gard

 Northwestern University, Jesse Brown VA Medical Center, 345 East Superior Street, Room 1441, Chicago, IL 60611-4496sgard@northwestern.edu

J Biomech Eng 131(3), 035002 (Jan 06, 2009) (7 pages) doi:10.1115/1.3005335 History: Received March 01, 2008; Revised August 21, 2008; Published January 06, 2009

A conceptual design has been generated for a prosthetic ankle-foot mechanism that can automatically adapt to the slope of the walking surface. To help prove this concept, a prototype ankle-foot mechanism was designed, developed, and tested on three subjects with unilateral transtibial amputations walking on level and ramped surfaces. The mechanism is capable of automatically adapting to the walking surface by switching impedance modes at key points of the gait cycle. The mechanism simulates the behavior of the physiologic foot and ankle complex by having a low impedance in the early stance phase and then switching to a higher impedance once foot-flat is reached. The “set-point” at which these changes in impedance occur gets reset on every step in order to reach the proper alignment for the walking surface. The mechanism utilizes the user’s bodyweight to help switch impedance modes and does not require any active control. It was hypothesized that the ankle-foot mechanism would cause the equilibrium point of the ankle moment versus the ankle dorsiflexion angle curves to shift to accommodate the walking surface. For two of the three subjects tested, this behavior was confirmed, supporting the contention that the design provides automatic adaptation for different walking slopes. Further work is needed to develop the prototype into a commercial product, but the mechanism was sufficient for illustrating proof-of-concept.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) Data from Hansen (7). This behavior could be mimicked by two sets of efficient springs. (b) Design concept to mimic the behavior shown to the left. NSs are low impedance to mimic the initial stance portion. The TS spring has a high impedance to mimic the behavior between foot-flat and toe off. A locking mechanism is used to engage the TS spring at foot-flat and to disengage this spring at toe off.

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Figure 2

Operation of the design concept. In the diagrams, the locking mechanism is considered locked when shown in gray and unlocked when shown in white. See text for details of operation.

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Figure 3

Hypothesized adaptation of the ankle moment versus ankle dorsiflexion curves. An adaptable ankle-foot mechanism should shift its set-point into dorsiflexion when walking uphill and into plantarflexion when walking downhill.

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Figure 4

Side view of the prototypical ankle-foot mechanism developed for this investigation

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Figure 5

CAD model of the ankle-foot mechanism developed for this investigation

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Figure 6

Exploded view of the CAD model of the ankle-foot mechanism. This figure is intended as a guide in understanding the different parts of the mechanism.

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Figure 7

Side view of the CAD model with the closer arm removed. (a) When loading is below a threshold on the ankle-foot mechanism, the cam is held off of the base by the compression springs and its connection to the link. (b) When the weight applied to the ankle-foot mechanism is above a threshold, the compression springs are compressed and the cam is forced into the base via the link. This weight-activated cam locking mechanism is used to engage the stiff bumpers, thus switching the ankle-foot mechanism between the two impedance modes: low (a) and high (b).

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Figure 8

Prosthetic-side ankle flexion/extension curve for Subject A walking on the five surfaces with (a) the prototype and (b) his usual prosthetic ankle-foot system. Positive angles are for dorsiflexion whereas negative are for plantarflexion. Data for Subject A walking with the mechanism and with his usual prosthesis have been averaged for ten walking trials.

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Figure 9

Prosthetic-side knee flexion/extension curve for Subject A walking on the five surfaces with (a) the prototype and (b) his usual prosthetic ankle-foot system. Positive angles are for flexion whereas negative angles are for extension. Data for Subject A walking with the mechanism and with his usual prosthesis have been averaged for ten walking trials.

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Figure 10

Prosthetic-side hip flexion/extension curve for Subject A walking on the five surfaces with (a) the prototype and (b) his usual prosthetic ankle-foot system. Positive angles are for flexion whereas negative angles are for extension. Data for Subject A walking with the mechanism and with his usual prosthesis have been averaged for ten walking trials.

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Figure 11

Single-limb-stance prosthetic-side ankle moment versus ankle dorsiflexion angle curves for representative trials of Subject A walking with the ankle-foot mechanism (a) and with his usual prosthesis (b)

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