Electrochemical Testing of Modular Taper Junctions: Effect of Assembly Force and Head Offset

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Abstract

Examinations of retrieved orthopedic implants have demonstrated corrosion; however, the extent of corrosion damage can be quite variable. There are a number of factors that may influence this corrosion, including device design and materials, surgical technique, and patient factors such as lifestyle, activity, and comorbidities that affect the local physiological environment around the hip prosthesis. This study evaluated the effect of a number of those factors on the corrosion of the head-stem junction of a modular hip using an electrochemical method. Several groups of head-stem taper coupons were immersed in phosphate-buffered saline and held at a constant anodic potential in an electrochemical cell while being mechanically loaded. Femoral head size, diameter, offset, and assembly force were varied. Incrementally increasing cyclic loads were applied to the femoral head. After the short-term incremental test was completed, some specimens were subjected to a long-term 4-kN cyclic load (R = 0.1) that was applied for 106 cycles. Current was continually monitored, and displacement sensors monitored the relative micromotion of the assembled components. Test specimens assembled with less than 4 kN of load experienced a statistically significant increase in corrosion current during both the short-term and long-term tests compared to specimens assembled with higher loads. The larger +12-mm head offset exhibited a statistically significant increase in corrosion current compared to the +8.5-mm offset in the long-term test when assembled with the 1-kN load. It is recommended that taper assembly force and head offset should be controlled in future corrosion tests of taper junctions. Although there are measurable differences in corrosion current reported in this laboratory experiment, the values for corrosion current have not been correlated with clinical performance or outcomes. This study has shown that assembly technique and head offset are factors that affect corrosion current measured in the laboratory.