Effects of shear force reduction during mechanical testing and day-to-day variation on stiffness of commercial prosthetic feet: a technical note

Abstract

Background: Mechanical testing is the principal method used to quantify properties of commercial prosthetic feet in a controlled and standardized manner. To test feet in a mechanical testing machine without overconstraining the system, tangential shear forces must be minimized. However, there is scant published information comparing techniques for reducing shear forces during mechanical testing. Furthermore, there are no data on variability in linear stiffness across testing sessions.

Objectives: To compare techniques for reducing shear forces during mechanical testing of prosthetic feet and to evaluate variation in linear stiffness across testing sessions.

Study design: Repeated measures.

Technique: Force-displacement data were collected at two pylon progression angles, one for the forefoot and one for the heel, and compared across three conditions: roller plate (RoPl), low-friction interface on the shoe (SB), and no method for reducing shear forces (NoSB). Data were collected for a range of commercial prosthetic foot models and sizes. Select data were collected over multiple days to assess variation over test sessions.

Results: Differences in stiffness between RoPl and SB test conditions ranged from -0.9% to +2.6% across foot models. By contrast, differences between RoPl and no method for reducing shear conditions ranged from -2.9% to +14.6%. Differences in linear stiffness between test sessions ranged from -2.2% to +3.6%.

Conclusions: Methods for reducing shear force in this study demonstrated roughly equivalent effects. Thus, a low-friction interface may be used as a less expensive and less complex method for reducing shear force in prosthetic foot testing. In addition, mechanical testing results were relatively consistent across multiple test sessions, lending confidence to test consistency.