Examination of failed ex vivo metal-on-metal metatarsophalangeal prosthesis and comparison with theoretically determined lubrication regimes

Replacement of the first metatarsophalangeal (MTP) joint is a relatively uncommon procedure compared with hip and knee arthroplasty. A cobalt chrome-on-cobalt chrome MTP prosthesis, which had a diamond-like carbon (DLC) coating applied to its articulating faces, was obtained for ex vivo analysis. By modelling the ball and socket implant as an equivalent ball-on-plane model and employing elastohydrodynamic theory, the predicted lubrication regimes applicable to this implant design were determined. These calculations were undertaken for a 10–1500 N range of loading values and a 0–30 mm/s range of entraining velocities, for both worn and unworn situations. Calculations showed that the implant would almost always operate in the boundary lubrication regime. The presence of scratches on the articulating faces of the ex vivo sample further implied boundary lubrication. The DLC coating had been removed from the entire face of the phalangeal component and from most of the face of the metatarsal component. From the latter it appeared as if the coating had been scratched and then flaked away parallel to the scratches. In turn this suggested a corrosion based failure of the interface between the DLC coating and the cobalt chrome subsurface.