Explant analysis of a chromium nitride coated metal-on-metal total wrist replacement: A case study

Explant analysis can provide important understanding of how artificial joints perform in the human body. The articulating surfaces of the metacarpal head and the radius cup from a chromium nitride coated metal-on-metal Motec wrist implant were analysed. Due to bone resorption and aseptic loosening, the implant was removed after 6 years in the patient, and metallosis was observed during removal. Visually, some areas of the articulating surfaces appeared polished, others were dulled. A chemical composition analysis of the metacarpal head showed that the polished surfaces were chromium rich, implying this surface was the original chromium nitride coating, whereas the dulled surfaces were cobalt rich, indicating the underlying cobalt chromium substrate. In addition, the underlying cobalt chromium substrate was an order of magnitude rougher than the polished surface, indicating the scale of damage to it. It is speculated that the loss of the coating, and the subsequent damage to the underlying substrate due to a third-body wear process, led to osteolysis and the metallosis seen at revision surgery.