Presented at NACOB 98: North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998

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MECHANICAL CHARACTERIZATION OF VERTICAL SHOCK-ABSORBING
PYLONS FOR LOWER-LIMB PROSTHESES

S. Gard
VA Chicago Health Care System, Lakeside Division and
Northwestern University Prosthetics Research Laboratory
Chicago, IL 60611

INTRODUCTION

Vertical shock pylons are designed to function as shock absorbers for lower-limb prostheses by attenuating forces associated with walking and high-impact activities such as running and descending curbs and stairs. Many amputees seem to show a clear preference for walking with these devices, but their function during gait and their influence upon the resulting pattern of walking is unclear. As a preliminary investigation prior to conducting clinical testing, we performed mechanical characterization of three vertical shock pylons.

REVIEW AND THEORY

Vertical shock pylons are intended to replace lost physiological shock absorption and leg compliance, functions normally provided by the anatomical ankle and knee. The pylons increase the comfort of walking on the prosthesis by reducing the forces associated with heel contact during gait and other high-impact activities when shock would normally be transmitted to the amputee's residual limb.

Mooney et al. (1995) examined the differences in the ground reaction forces and moments of force during gait between the sound and prosthetic limbs of a transtibial amputee walking with a Flex Foot Re-Flex Vertical Shock Pylon (VSP), but found only minimal differences between the two limbs both when the pylon was functioning normally and when it was immobilized. Miller (1997) performed mechanical and clinical testing of the Re-Flex VSP, and found that the spring constant and damping coefficient for the Re-Flex corresponded with reported values in the literature for a physiological limb. While the transtibial subjects of the studies clearly preferred walking with the VSP, clinical tests revealed few biomechanical differences in the gaits of the subjects walking with and without the VSP.

The purpose of this investigation was to perform mechanical analyses of three commercially-available vertical shock pylons: Flex Foot's Re-Flex Vertical Shock Pylon, Seattle Limb Systems' AirStance Pylon, and Ohio Willow Wood's Stratus Impact Reducing Pylon. All of the pylons are similar in design-they are spring-like in nature with considerable damping, and they shorten telescopically in response to axial loading. The pylons were tested statically and dynamically to determine their mechanical characteristics in order to gain a better understanding of how these mechanisms might affect amputee gait.

PROCEDURES

The three vertical shock pylons have variable stiffnesses in order to accommodate persons having a wide range of body weights and activity levels. We performed mechanical testing of each pylon at a minimum of four different stiffnesses, and at three different body weights per pylon stiffness.

Static and dynamic testing of the vertical shock pylons were performed using a specially-designed testing apparatus for testing prosthetic feet (Knox, 1996). Static testing was performed by slowly loading and unloading the pylons while recording the linear displacement of the pylons with the applied load. The dynamic testing involved step-loading and step-unloading the pylons to approximately body mass while recording pylon displacement. From the static and dynamic data we are able to determine the spring stiffness, resonant frequency, damping coefficient, damping ratio, and energy efficiency for each pylon.

RESULTS

Figure 1 shows typical data from static testing of the pylons utilizing stiffness elements recommended by the manufacturer for a person having a body weight of approximately 670 N (150 lbs.). All of the pylons demonstrate hysteresis in their force-displacement curves, indicative of energy loss during the loading/unloading cycle. Figure 1 shows that the force-displacement curve for the Re-Flex VSP is fairly linear during loading and unloading, with a relatively constant slope of about 51 × 10 ³ N/m. The curves for the AirStance and Stratus show nonlinear characteristics, and therefore have variable stiffness; we calculated local stiffness values to determine how stiffness varies with applied load and resultant displacement of the pylon. Local stiffness is observed to increase during the initial static loading and unloading in all of the pylons, which is probably due in part to the static friction that must be overcome before displacement occurs.

Figure 1: Static testing sample results.

Typical step-loading and -unloading trials for the Flex Foot Re-Flex VSP (#3) are shown in Figure 2 The damped oscillatory responses suggest that the pylon may be approximately modeled as a 2nd-order underdamped system. We have only recently begun analyzing the dynamic data; to determine the damping coefficient and damping ratio we intend to use local stiffness values as the pylons oscillate about their steady-state positions.

Figure 2: Typical dynamic responses for the Re-Flex VSP (#3).

DISCUSSION

Data reduction and processing of the dynamic data from the mechanical testing of the vertical shock pylons is ongoing. Preliminary results show that the three pylons tested have significantly different mechanical characteristics. In the future, we will perform gait analyses on transtibial amputees walking with these pylons to determine how these prosthetic components affect amputee gait.

REFERENCES

Knox E.H., The Role of Prosthetic Feet in Walking. Ph.D. Dissertation, 1996.

Miller L.A. and Childress D.S., J Rehab Res Dev, 34(1), 52-57, 1997.

Mooney J. et al., Proceedings of the 21st Annual Meeting & Scientific Symposium of the AAOP, March 21-25, p. 24, 1995.

ACKNOWLEDGMENTS

This work was supported by the Department of Veterans Affairs, Rehabilitation Research and Development Service and is administered through the VA Chicago Health Care System, Lakeside Division, Chicago, Illinois.