IN-VIVO CORROSION OF MODULAR HIP-PROSTHESIS COMPONENTS IN MIXED AND SIMILAR METAL COMBINATIONS - THE EFFECT OF CREVICE, STRESS, MOTION, ANDALLOY COUPLING

One hundred forty-eight retrieved modular hip prostheses of both mixed (Ti-6Al-4V/Co-Cr) and similar (Co-Cr/Co-Cr) metal combinations were e xamined and positive evidence of corrosive attack was found in the con ical taper region between head and stem. Significant corrosion was obs erved in both mixed and similar metal combinations with 16% of necks a nd 35% of heads (for mixed-metal cases), and 14% of necks and 23% of h eads (for similar-metal cases) showing moderate to severe corrosive at tack. There was a significant correlation between the percentage of pr ostheses with moderate to severe corrosion and the duration of implant ation for both mixed and similar metal cases, indicating that this cor rosion process is progressive in time. Moderate to severe corrosion wa s seen as early as 2.5 and 11 months (mixed and similar metals, respec tively). Scanning electron microscopy and x-ray analysis identified se veral forms of corrosive attack in the cobalt-based component of the t aper. These included, for both mixed and same metal combinations: pref erential dissolution of cobalt, fretting, and pitting; mixed metals on ly: the formation of a Ti-Cr-Mo interfacial phase and interdendritic c orrosion; and for similar metals: intergranular attack adjacent to gra in boundaries enriched in molybdenum and silicon. It is hypothesized t hat the restricted crevice environment, coupled with high cyclic stres ses which cause repeated fracture of the passive oxide films in the ta per, result in an unstable electrochemical environment within the crev ice for both the cobalt alloy and Ti-alloy passive films. The passivit y of these alloys is subsequently lost and active attack of the taper results. Also, the repeated fracturing of the passive films will resul t in large amounts of corrosion products being formed. This corrosion and particulate accumulation could result in loss of mechanical integr ity of the implants in vivo, create particles for third body wear, and release particles into the surrounding tissues. (C) 1993 John Wiley a nd Sons, Inc.