Forty-three (43) trans-tibial prostheses with a mean period of use of 33 months were evaluated in terms of utilisation and durability. The majority of the prostheses (80%) were worn by amputees with demanding occupations, such as farmers, fishermen and tradesmen. The prostheses were in use approximately 9 hours per day. No major or frequent breakdowns of the polypropylene prosthetic components were found. The suspension belts were the parts most frequently affected; a total of 32 needed replacement after an average 11 months of use. Eleven (11) prostheses were completely replaced, more than half at least partly because of socket-fitting problems. In all, socket-fitting problems were found in 15 prostheses, causing pain and consequently limitation of use. While the prosthetic polypropylene components were satisfactory, the rubber foot was a major cause of early breakdown. A total of 40 feet were replaced; their mean period of use before breakdown was 9 months. In practice, parts were frequently replaced at a later stage than desirable, meaning that there was frequent “overuse” of prostheses with worn parts. Measures were taken to increase the life span of the prosthesis: change in the design of the foot; issuing a spare foot with the prosthesis; strengthening the suspension belt. Additional evaluations are necessary to confirm the degree to which the findings are representative.

This paper presents some of the heuristics that a system architect could use to preserve the full operational potential during the system's intended life span, and beyond. The proposed heuristics prevent a single subsystem from severely limiting the evolution and maintenance potential of the whole system. The durability of the system's function ought to have precedence on the subsystems' own evolution, resulting in improved system robustness. The scope of function durability is first exposed. Side topics are then examined and the way they relate to the system viability is discussed. The second part introduces the proposed heuristics. Rationale and illustrations are given for each of them.

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