Analysis of Corrosion Deposit on Hip Explant
Analysis of the light green and off-white corrosion deposit on the CoCrMo alloy hip implant shown above on the femoral stem just below the taper fitted to the head was carried out by XPS to determine its chemistry. The deposit was primarily a combination of chromium phosphate and chromium oxide with bone minerals and polypeptides, though many other compounds of chromium, molybdenum, and cobalt were also identified.
In most environments, chromium oxide concentrations at the surface provide good corrosion resistance, but chromium is not as resistant to phosphate attack as one would like it to be. Cobalt and molybdenum are in the CoCrMo alloy also and are also subject to forming phosphates. The phosphates of cobalt and molybdenum are not as soluble as are those of chromium, so the corrosion product which accumulates in clumps around the hip implant has much more chromium in it than it has cobalt or molybdenum.