Even though orthopedic wire is commonly used in orthopedic surgery, very little is known about its fatigue properties. This article describes the development of a simple, effective testing machine that simulates the in situ fatigue loading of orthopedic wire. A mathematical analysis has predicted the maximum stress that is generated in wires in the fatigue machine. The fatigue resistance of intact monofilament wires made from different materials was studied. It was found that F90 was more resistant than F562, which in turn was more resistant than F138, grade 2. Also studied was the effect of cold-forming different notch sizes on the wire surface. It was found that even the slightest notch (1% of wire diameter) reduced the fatigue life by 63%.

Ten retrieved samples of 316L stainless steel wires, implanted for times from 11 months to 11 years, were examined fractographically and metallurgically to evaluate the effects of a physiological environment on their fracture. Seven samples were from L-rod instrumentation, and three samples were from trochanter reattachment. Of 16 breaks in the 10 samples, 94% were caused primarily by cyclic loading (fatigue), and 6% were caused by tensile forces. Two factors were found to influence the effective life of an implanted wire. First, the surface condition (deformation) appeared to be most important, and second, corrosion pits present on most wire surfaces could possibly be an influence in initiating a crack. Biomechanical forces on wires enveloping L-rods tended to promote metal deformation and faceting, which decreased time to fracture. This decrease in wire life was evident from a substantial hardness increase in faceted areas.