Development and testing of thermoplastic structural components for modular prostheses

The wider use of thermoplastic structural components in modular artificial limbs would enable their general properties of low density, corrosion resistance and mouldability and more specific properties of certain thermoplastics such as shock absorption, fatigue and wear resistance to be used to the advantage of patients and manufacturers. They provide an alternative to metal and carbon fibre reinforced resin systems. Emphasis has been placed on the development of rotationally moulded Nylon 11 shank sections, using Philadelphia recommended load levels as the design criteria for structural integrity. Laboratory testing underlined the importance of fatigue testing of thermoplastic components since structural deterioration due to creep--a time dependent mechanical property of thermoplastics--can be ascertained in fatigue testing but would not be evident on the shorter timescale of the static test. Experimental below-knee prostheses incorporating suitably designed plastic shanks and alignment devices can withstand high static loads and exhibit long fatigue lifetimes in excess of 2 million cycles. The shank design offered an opportunity for testing under service conditions the validity of the Philadelphia Static Load level (2.5 kN) since shank failure loads are around this figure. Patient trials of experimental prostheses based on various combinations of plastic shanks and alignment devices and conducted over 33 months indicate that the Static Load Level along with fatigue testing is a satisfactory test criterion for general service use of thermoplastic prosthetic components.