Durability Testing of Artificial Feet in the Laboratory

R. L. Daher, M.Sc J. B. Heath

This study was supported by a National Health Grant #606-7-258, Department of Health and Welfare, Canada.

Although observations made during the clinical use of commercially available artificial feet are valid, the accumulation of this type of data is time-consuming. Moreover, these data tend to leave the observer with impressions rather than hard facts. It is more instructive to expose each specimen foot to a facsimile of walking while applying periodic tests for deformation and altered qualities of elasticity. Any flaws in materials or construction can be clearly demonstrated by surface inspection, x-rays, or serial sectioning of the foot. Using the cycle-test equipment developed by the Prosthetics/Orthotics Research and Development Unit, random samples of commercially available SACH feet are subjected to periodic loads simulating those in effect during walking. The different SACH feet are compared on the basis of durability and function. The present report outlines the purpose of the test, the design and function of the equipment (Cycle-Tester), and the methods of recording results.

General Test Procedures

As shown in Fig. 1 , the equipment is capable of testing two feet simultaneously. A sock and a shoe are applied to each foot for the cycling test in order to simulate normal wear. The standard "Hush Puppy" shoe manufactured by Greb is used in all tests. At heel-contact (Fig. 2 ) the foot is loaded rapidly by an injection of high-pressure air into the air cylinder. By means of a timed cam arrangement the pressure inside the cylinder is switched to a reserve tank (shown below the force plate in Fig. 1 ) again at a controlled pressure to maintain a relatively constant load until just before unloading, prior to "toe-off." Heel-contact occurs at a point 20 deg. before the shaft of the foot holder reaches the vertical position and toe-off occurs at 35 deg. past the vertical as shown in Fig. 2 and Fig. 3 .

The Test Apparatus

Four cantilever beams having two strain gages, each wired in a Wheatstone half-bridge circuit, provide individual vertical-load signals to a Budd Model 350 strain indicator by means of a Vishay SB-1 switch and balance unit. The signal is recorded on one channel of a Model 322 Sanborn dual-channel DC amplifier recorder. The second channel records the time period (angle) of the cycle by means of a General Electric H 13 A1 photoelectric interrupter module. For the recorder, an interrupter disk mounted on the shaft which provides the rocking action to the foot gives precise photolight interruptions every 10 deg. Fig. 4 presents a typical recording obtained for one strain-gage beam. Note that the top channel records the strain level while the lower channel records the 10-deg. increments in the form of a spike. A wider slot in the interrupter disk at 20 deg. before the vertical (heel-contact point) indicates the start of the cycle. The top channel of the recorder is precalibrated in terms of microstrain levels thus providing a direct microstrain readout for the entire cycle. Prior to mounting the strain-gage beams, calibration curves of force in pounds versus microstrain are plotted as shown in Fig. 5 .

By means of the strain-gage-beam calibration curves, the microstrain levels at specified intervals are converted individually into readings of pounds force for each of the four beams. Summation of the four forces gives the actual vertical load at the specified instant. Fig. 6 shows a plot of total vertical load versus angle of the driving pin or time for one cycle. The zero position is at heel contact. Fig. 7 is a plot of vertical load versus actual foot angle for the identical cycle. In Fig. 7 , the 90-deg. angle occurs when the attachment face of the foot is in the horizontal position.

The Overall Program

A program of subjecting all commercially available SACH feet to the cycle test is now under way. Basically the criteria used for the test procedures are those outlined by the Veterans Administration Prosthetics Center, New York, in their "Standards and Specifications for Prosthetic Foot/Ankle Assemblies." Prior to cycling, a durometer test (load versus deflection) is applied. The durometer testing equipment establishes the distance between the point of foot contact and a base reference point. After cycling, permanent deformation is automatically recorded as a result of the contact point shifting along the x-axis. Also, any change in the resistance of the foot foam is recorded for the entire loading cycle. Heel durometer readings are taken at an angle of 15 deg. and sole readings at 30 deg. using the attachment face of the foot as a reference. In addition, x-rays of the foot taken prior to and upon completion of the cycle testing indicate any structural breakdown of the keel or of the belt attachment to the keel. Unless prior breakdown occurs in terms of cracking or a serious loss of resistance is indicated by the durometer testing, the foot is cycled for a total of 500,000 cycles. A complete report on the results of these tests will be made available upon completion of the current program.

Health Sciences Centre, Rehabilitation Centre Winnipeg, Manitoba, Canada