Fracture analysis of retrieved orthopedic wires

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.