Mechanism of fretting corrosion at the modular taper interface of hip prosthesis

Modularity of total hip arthroplasty (THA) has been linked to various forms of adverse local tissue reaction (ALTR). ALTR is often a result of metallic particles and ions released from corroding implant materials to the peri-prosthetic tissues and blood stream of the human body. More so, it is a consequence of several fretting and crevice-induced corrosion mechanisms occurring simultaneously. Fretting corrosion and fatigue damage of the modular taper is initiated and sustained by micromotions at the taper interface through the multi-directional loads applied onto the prosthesis during daily living activities. Subsequent to the failure of the implant, analysis of retrieved explants generally offer information regarding the mode of wear and to an extent, the types of corrosion damage. However, retrieval studies are limited in that, they do not provide a holistic insight into the in-vivo degradation mechanisms which ultimately led to the early failure of the implant. On the other hand, controlled in-vitro studies proves useful for replicating the evolution of wear and corrosion in-situ. In addition, the role of multiple individual factors which contribute to fretting corrosion can be elucidated through in-vitro methods. In this study, metal – metal and ceramic – metal fretting interfaces were investigated. Advance microscopy and spectroscopy techniques were employed in the characterisation of passive films, corrosion products and metallurgical transformations of CoCrMo and Ti6Al4V alloys. Both mechanical and electrochemical data consisting of interfacial energy, open circuit potential (OCP) and fretting corrosion currents were measured using an in-situ tribocorrosion cell. Other surface analytical techniques were used to quantify wear and obtain surface topography. The results showed that CoCrMo and Ti6Al4V display independent characteristic behaviours when in a metal – metal or ceramic – metal fretting contact. For example, fretting contacts involving Ti6Al4V experienced higher contact compliance than those of CoCrMo. The higher interfacial compliance vii thus led to a significant proportion of wear being redistributed at the contacts involving Ti6Al4V alloys. It was subsequently observed that the redistributed wear at the interface leads to a mixed fretting regime whereby, the contact appear to be ‘cold-welded’ during a partial-slip regime and subsequently transitions to a gross slip fretting regime when the ‘welded’ interfacial material fractures. The phenomena was evident in both metal – metal and ceramic – metal interfaces involving Ti6Al4V. The subsurface transformation in CoCrMo alloy when subjected to fretting was observed to be typically strain-induced twinning and loss of nano-crystalline region. On the other hand, Ti6Al4V alloy was observed to be strain-induced recrystallization, mechanical mixing, crack initiation and propagation. It was also observed that the chemical composition of fretting corrosion products at the metal – metal interfaces were dependent on the contact condition and the specific area within the contact that the wear product is located. For example, metal–oxides and chlorides were prominent within creviced regions whilst precipitation of metal-phosphates and metal–oxides were present in well aerated regions. Evidence of Cr6+ and pitting corrosion products were also identified at the most severe creviced environment of the CoCrMo – Ti6Al4V fretting contact. This research also showed that the evolution of fretting corrosion current (specifically in self-mated CoCrMo contact) can be linked to the surface history of wear and corrosion. Furthermore, studies conducted on realistic taper components revealed that the use of ceramic bearings in order to eliminate one of two conductive components only resulted in the reduction of static corrosion currents but not fretting corrosion currents. Rather, it was deduced that interference fit at the ceramic – metal interface (relative to metal – metal) increases the surface area that is susceptible to passive oxide abrasion under fretting conditions. Therefore, the use of a ceramic bearing did not reduce fretting current at the modular taper interface.