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Research Papers

Prosthetic Ankle-Foot System That Adapts to Sloped Surfaces

[+] Author and Article Information
Eric A. Nickel

 Minneapolis VA Health Care System, One Veterans Dr. (Research 151), Minneapolis, MN 55417Eric.Nickel@va.gov

Andrew H. Hansen

 Minneapolis VA Health Care System, One Veterans Dr. (Research 151), Minneapolis, MN 55417Andrew.Hansen2@va.gov

Steven A. Gard

 Northwestern University Prosthetics-Orthotics Center, 680 N. Lakeshore Dr. Suite 1100, Chicago, IL 60611sgard@northwestern.edu

J. Med. Devices 6(1), 011006 (Mar 13, 2012) (6 pages) doi:10.1115/1.4005787 History: Received July 20, 2011; Revised December 14, 2011; Published March 12, 2012; Online March 13, 2012

The anatomical ankle is capable of providing adaptation to sloped surfaces, a function that is not available in most traditional lower limb prostheses. Commercial prostheses that are claimed to adapt to slopes are limited by high cost, delay in response, reduced stability, and loss of energy through damping. The purpose of the present work was to develop a prototypical prosthetic ankle unit that adapts to sloped surfaces and is sufficiently durable for short-term take-home trials. The prototype switches between low and high rotational impedances by means of a wrap spring clutch mechanism. The clutch is held in a disengaged position when unloaded and deflection of a compressible pylon under axial load rotates a control collar and engages the clutch. The prototype was subjected to 100,000 cycles of mechanical endurance testing based on ISO 10328 standards to determine the suitability for two-week take-home testing. Three persons with unilateral transtibial amputations were recruited to test the prototype in the laboratory, providing subjective feedback through a survey and participating in a motion analysis study to confirm the performance of the slope adaptation function. Translation of the ankle moment-angle curves for all subjects along the ankle angle axis demonstrated a change of the ankle alignment when subjects walked with the adaptable ankle on surfaces of different slopes. The ankle moment-angle curves had a lower slope than the subjects’ usual prostheses, and some subjects had distinct flat regions in the moment-angle curves when using the adaptable ankle. The arbors of the clutch demonstrated significant wear when tested to 100,000 cycles based on ISO 10328 standards, yet the adaptable ankle continued to hold testing loads. The alignment change observed for sloped surfaces suggests the prototype was providing slope adaptation. The flat regions on the ankle moment-angle curves suggest the clutch may have been slipping. Refinement of the clutch engagement mechanism and continued development to reduce the weight and size of the prototype is needed prior to take-home testing.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Prototype of adaptable ankle with foot shell removed. This prototype was fatigue tested based on ISO 10328 standards. Adaptation was tested by evaluating the ankle moment-angle curves calculated from gait analyses of three persons with unilateral transtibial amputations.

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Figure 2

Model of adaptable ankle. During early loading, the clutch is disengaged, and the low stiffness element dominates (a). Upon reaching “foot flat” and sufficient loading (b), the wrap spring clutch (triangle with “c”) is engaged, transferring displacement in the direction of dorsiflexion (c) to the high stiffness element, bypassing the low stiffness element.

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Figure 3

Theoretical function of the stiffness elements. The low stiffness element has a low slope (klow ) on the ankle moment-angle curve and is generally deflected in plantarflexion during early stance. The high stiffness element has a greater slope (khigh ) and is only compressed when the wrap-spring clutch is engaged and the ankle moves in the direction of dorsiflexion.

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Figure 4

As the adaptable ankle adapts to the surface slope, the low stiffness elements are compressed based on the ankle angle at foot flat ((a) and (b)). Thus, the high stiffness element would begin to be compressed at different ankle angles dependent on the surface slope. The point on the ankle moment-angle plot in the high stiffness region where the net ankle moment is zero (no net moment being applied to the user) is referred to as the “equilibrium point” and is expected to shift along the ankle axis for surfaces of different slopes (c).

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Figure 5

Adaptable ankle broken down to structural groups with non-structural parts removed. The shank group (a) transferred loads from the user’s socket to the driving arbor of the wrap spring clutch. The intermediate group (b) received moments from the clutch (when engaged) and applied them to the high stiffness bumper. The foot group (c) supported the shaft and held the foot plate.

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Figure 6

Side view of the high stiffness bumper with the socket side of the base structure removed. The high stiffness bumper and torque transfer cap rested within the socket. The torque transfer cap pressed down on the top of the bumper when loaded, while the bottom of the bumper rested against a rib projecting from the socket wall (rib visible in Fig. 5).

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Figure 7

The space between the wrap spring clutch arbors contained the low stiffness bumpers. One aluminum block was bolted to each arbor such that when the arbors rotated with respect to one another, a low stiffness bumper was compressed. The arbors are shown with the clutch spring. The bumpers were cast in sections then assembled into arcs.

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Figure 8

The engagement mechanism of the adaptable ankle. When unloaded, the linkage pulled up on the string to rotate the control collar of the wrap spring clutch, forcing the clutch open. When loaded, the shock absorbing pylon compressed, propagating displacement through the linkage such that the clutch spring was permitted to reengage, effectively locking the shank and intermediate structures together against motion in the direction of dorsiflexion. The ability to rotate was blocked out of the shock absorbing pylon.

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Figure 9

Plots of mean ankle moment versus ankle angle during single-limb support for the subjects tested (n = 3) using their usual prostheses (a) and the adaptable ankle prototype (b). The translation of the moment-angle curves when using the adaptable ankle prototype suggests the prototype was changing set-point on sloped surfaces. The lower slope indicates the adaptable ankle was also less stiff than the subjects’ usual prostheses. One subject had plateaus when using the adaptable ankle prototype, suggesting the clutch was slipping, possibly due to miscalibration of the engagement mechanism.

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