Retrieval analysis and in vitro assessment of strength, durability, and distraction of a modular total hip replacement.

We investigated a commercial Co-Cr-alloy head--Ti6Al4V alloy neck and Ti6Al4V stem modular total hip replacement. We assessed the distraction forces after in vitro cycling in bovine serum, fatigue durability, fretting corrosion damage, and load bearing capacity of new implants using fatigue-corrosion, pull-off, scanning electron microscopy, fatigue and compression investigations. In addition, we studied corrosion, fretting damage, and distraction forces on retrievals. For both retrievals and in vitro test samples, the neck-stem interface required the higher distraction force as compared with the head-neck interface. One of 12 retrievals showed strong fretting corrosion at the neck-stem interface which resulted in a high disassembly force of about 16 kN. For in vitro test samples, the neck-stem pull-off force initially increased during cycling and showed a maximum value of 5.704 kN at ∼100,000 cycles, which is equivalent to gait cycles performed in approximately 36 days. Overall, assembly force, initial component settling, and interface corrosion primarily determine the force required to distract the modular components. One million cycles fatigue failure of the neck can be expected at a maximum compression load of -6.5 kN. No component failure was observed during quasistatic compression; rather the neck deformed plastically and the ultimate compression load-bearing capacity was -13 kN.