Mechanical testing of prosthetic feet utilized in low-income countries according to ISO-10328 standard

This report summarizes the results from 1132 ISO-10328 standard tests performed on 21 different prosthetic foot models commonly utilized in the developing world. None of the tested feet passed the strictest ISO testing protocol. All but one failed at the initial Static Proof test, which simulates a single momentary overload, due to permanent forefoot deformation. In addition, all tested feet had significant internal failures that were visible when sectioned longitudinally. Static Proof testing revealed average permanent deformation of the forefoot of all feet that exceeded the optional 5 mm ISO requirement. Forefoot deformation for non-Jaipur rubber feet came closest to meeting the standard at 8.3+/-3.4 mm; deformation of the various types of rubber Jaipur feet was the greatest at 22.5+/-5.4 mm. Forefoot deformation for polyurethane (PU) feet was 13.6+/-5.5 mm. Forefoot deformation of the ethyl-vinyl-acetate (EVA) feet was slightly greater than the Jaipur feet at 22.8+/-5.7 mm. After the Static Strength test, which simulates a higher momentary overload, permanent deformation of the feet increased. The average maximum deformation for rubber SACH forefeet varied from 17 - 30 mm, and 11 - 26 mm for the heel; Jaipur forefeet 47 - 60 mm and heels 13 - 19 mm; PU forefeet 20 - 44 mm and heels 20 - 33 mm; and EVA forefeet 33 - 50 mm and heels 16 - 31 mm. After completion of the Cyclic Test the prosthetic feet were sawn in half and closely examined visually. All feet revealed internal derangements: (i) Deformation of rubber or PU foam under the keel of forefoot and/or heel: HCMC, VI, EB1, BAVI, HI Cambodia, Myanmar, Angola, TATCOT, Kingsley and CR; (ii) Delamination from the keel: Mozambique, PHN, and Pro-cirugia; and (iii) Delamination between foam layers: BMVSS, NISHA, MUKTI, and OM. The influence of the two environmental factors tested was minimal for rubber feet with respect to deformation and inconsistent for the polymer feet; in particular for the forefeet. Creep increased with humidity exposure in some feet of natural rubber. However, creep decreased with ultraviolet (UV) exposure for these natural rubber feet, as was also the case for EVA feet, whereas the creep increased for two PU feet. Comparison of the effect of humidity and UV exposure generally showed less creep with UV exposure. In conclusion, ISO-10328 testing prior to release of a new foot construction for amputee use appears to be useful in the developing countries as well as in the developed world, even though it does not simulate the wear on the plantar surface that is seen clinically in barefoot walking. Inspection of the internal structures after the laboratory testing has been shown to reveal occult failure mechanisms in all tested feet.