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Viscoelastic Properties of Plastic Pediatric AFOs

The viscoelastic behavior of pediatricsized polypropylene ankle-foot orthoses (AFOs) has been investigated by measuring the stiffness and buckling of AFOs before, during and after cyclic loading. Cyclic loading simulates normal gait by forcing the AFOs into dorsiflexion (10 degrees) and plantarflexion (15 degrees). Mechanical properties were measured after three 24-hour periods of 187,200 cycles. After cyclic loading, mechanical properties were again measured at six discrete time intervals to determine if the plastic recovered or remained permanently degraded. The cycling rate simulated a relatively rapid walking pace of 130 steps per minute. The AFOs exhibited a significant reduction in mechanical properties—30 percent reduction in stiffness and 4.4 percent increase in malleolar diameter— after just one 24-hour period of cycling. Within 15 minutes after cycling had ceased, however, the AFOs began to recover their mechanical integrity and within an hour had recovered fully. Microfracturing (crazing) was observed in the sagittal walls of the AFO specimens. This was attributed to excessive stress as the plastic curved outward to accommodate the malleoli prominences.