Trunnion Corrosion in Total Hip Arthroplasty-Basic Concepts

Abstract

There has been increased interest in the role of corrosion in early implant failures and adverse local tissue reaction in total hip arthroplasty. We review the relationship between the different types of corrosion in orthopaedic surgery including uniform, pitting, crevice, and fretting or mechanically assisted crevice corrosion (MACC). Passive layer dynamics serves a critical role in each of these processes. The femoral head-neck trunnion creates an optimal environment for corrosion to occur because of the limited fluid diffusion, acidic environment, and increased bending moment.

Keywords: Adverse local tissue reaction; Corrosion; Head neck taper corrosion; Passive layer; Total hip arthroplasty; Trunnion.

Figure 1:

Corrosion on a surface. In theory, maximum corrosion occurs if the complete metal surface was exposed on a surface. Corrosion occurs in three basic steps: 1. Metal dissolves on the surface into the aqueous environment and cations are removed (oxidation). 2. Remaining electrons are attracted to a differential charge at another point on the surface where electrons are removed from the metal driving the reaction (reduction). 3. Metal oxide or metal hydroxide form as byproducts of this reaction. Metal oxides and insoluble metal oxides (rust) form an insulating layer on the metal surface almost instantly that inhibit the kinetics of corrosion and insulate the metal from further corrosion.

Figure 2:

Corrosion on a Surface with a passive film – Uniform Corrosion. If a passive film remains homogenous and undisturbed, corrosion is inhibited as the uniform passive film shields the metal surface from the aqueous reaction. Metal oxide forms a stronger barrier to migration of the positive charged metal and electrons as compared to the insoluble hydroxide (rust). The passive film thickness is a balance between reactions that erode and build the film.

Figure 3:

Pitting Corrosion. Slight inconsistencies that develop in the passive film lead to breakdown in small areas, development of a focused anode, and localized galvanic corrosion. A large flow of metal ions occurs at this focused anode. A differential cathode will then develop over a large surface at a distant point. The current flow between these two charges is similar to the voltage difference across a battery or galvanic corrosion.

Figure 4:

Crevice Corrosion. Pitting corrosion that occurs with limited diffusion of ions creates optimal conditions for corrosion. The total hip arthroplasty trunnion is a closed environment that prevents ion diffusion. A water-tight seal is established at a finite point on the neck-head taper and prevents the diffusion of ions. Oxygen depletion and a low pH environment prevent repassivation.

Figure 5:

Fretting Corrosion or MACC. A physical shearing force across the trunnion removes the protective passive film. This creates a larger surface for crevice corrosion to occur.