Trunnion Corrosion in Total Hip Arthroplasty - Basic Concepts

There has been an increased interest in the role of corrosion in total hip arthroplasty. This is based on reports of early implant failures and adverse local tissue reaction resulting from excessive corrosion at the modular interfaces of some implant designs. Orthopedic alloys are not selected based solely on their mechanical properties of strength, but rather because they possess the best balance between material properties of corrosion resistance and mechanical properties of strength. The passive layer of a metal oxide that develops on a surface of a metal serves a critical role in preventing corrosion. However, this protective layer is a dynamic structure. Aggressive corrosion occurs on implants when the kinetics of this oxide layer's destruction dominates over its generation. There is a spectrum of different types of corrosion defined by the environment and stability of the passive layer. Pitting is the localized dissolution of this protective metal oxide film. Crevice corrosion occurs with a similar mechanism but in an isolated environment that promotes corrosion. Fretting corrosion or mechanically assisted crevice corrosion (MACC) occurs with the addition of mechanical oscillating loads at the modular junction that disrupts this protective layer. Understanding this process is important to improve implant designs, surgical technique, and assessment of patients.