Environmental effects on the life of bone-plate-type surgical implants

Assuming satisfactory fixation of metallic bone-plates and no subsequent accidental high impact loading, the major cause of implant failure during the lifetime of the patient will be that of corrosion fatigue. The mechanisms of corrosion fatigue are various and component life depends on a large number of interacting variables. These variables are noted and the particular importance of changing pH in a saline environment for implants is discussed. It is known, for example, that very low pH of body fluids results from shock and/or locally in the crevice between implant and bone. Some in vitro test results for stainless steel type 316, titanium 130, and titanium alloy 318, at a "walking pace" of 1.7 Hz, simulating normal and extreme conditions possibly met in practice, are reported. Further, since it is commonplace for some bone-plates to be drilled for fitting, the combined effect of this procedure and a changing local environment has been investigated. The life of stainless steel 316 was not affected by the hole drilling, but its use as an implant material is questionable because of poor resistance to corrosion fatigues at "lower-than-normal" pH. A small reduction in the life of titanium 130 resulted from the hole drilling operation. However, this material was generally superior to stainless steel at low pH. Titanium 318 showed by far the best resistance to corrosion fatigue but its performance was drastically reduced by hole drilling. Some early failures were also noted as the result of fretting with T318. Good implant design, and cautious modifications when fitting, is required here for the realization of the full potential of T318.