Materials Investigation Of Failed Plastic Ankle-Foot Orthoses
E.P. Lautenschlager *
S.C. Bayne *
R. Wildes *
J.C. Russ *
and M.J. Yanke *
In April 1974 the Department of Biological Materials received for analysis the Ortholen ankle-foot orthosis (AFO) that had failed as shown in Fig. 1. This device was one of two worn bilaterally by an active man who weighed 175 lb., and was 5 ft. 11 in. tall. The failure began as a small crack at the base of the right angle, and then propagated slowly for several months until the integrity of the device became questionable.
The purpose of the investigation reported here was to determine the reason for failure and to make suggestions for the improvement of the orthosis.
METHODS OF INVESTIGATION
After gross observations as to the influence of loading the device were made (Fig. 2), the orthosis was sectioned in the area of the failure, and scanning-electron micrographs (SEM) were taken of the fractured surface (Fig. 3 and Fig. 4 ).
In addition, four specimens for mechanical testing were cut from the device and tested in three-point bending in an Instron machine at a strain rate of approximately 0.4 per minute.
Fig. 2 shows that downward loading near the ball of the foot places the right-angled area in a state of tension and opens up the crack.
Because tension opened the crack and the propagation was reported to be slow, fatigue was suspected as the mode of failure. Fatigue can occur with cyclic loads which are well below the ultimate or maximum load-bearing characteristics of the material. Because fatigue progresses in a stepwise fashion with a crack that is opened up a small additional amount during every cycle, it can be detected from a fracture surface characterized by small steps or striations. This is the case as shown in Fig. 4 for the SEM of the fracture surface of the AFO supplied.
The bending strength, elastic modulus, and testing strain rate are listed in Table 1 as the average ± one standard deviation, as measured for four specimens. The formulae used in the calculations are as follows: Equ. 1
When compared to other orthotic materials listed in Table 2, the strength of the material is among the weaker normally employed.
SUGGESTIONS FOR IMPROVEMENT
Although one might consider utilizing a stronger material, the basic change should result in a reduction of the high stress concentration produced by the sharp right-angle bend (i.e., the fracture site).
Reduction in stress at the radial edges could be provided by a larger radius, as shown in Fig. 6, left, but this would change the resistance to both dorsiflexion and plantar flexion. However, failure occurs by fatigue, which is primarily a tensile phenomenon (plantar flexion). Therefore, when an orthosis is required to resist only plantar flexion (high tensile forces), and need not necessarily rigidly counteract compression forces, an orthosis of the type shown in Fig. 6, right, would suffice. Here a webbing strap provides the resistance to tension during plantar flexion but folds or collapses during compression, thus preventing fatigue failure.