Metal implants provide the bulk of the material used in orthopaedic implants because of their material characteristics, ease of use and relatively low cost.
- Fatigue failure
- Endurance limit
- Creep
- Corrosion
- Galvanic corrosion
- Corrosion due to differing electrochemical properties (electrode potentials)
- Different metals in contact with each other and with a conductive, ion-containing fluid (tissue fluid) will both initially attempt to corrode, but the more reactive metal will act as a sacrificial anode.
- This leads to disproportionate corrosion of the more reactive metal.
- Different alloys of steel, or steel and cobalt-chromium will have different electrode potentials thus leading to galvanic corrosion.
- The boundaries between grains in the crystalline material can be anodic compared to the interior of the grains.
- Crevice corrosion
- When either two surfaces have a slight gap between them (eg poorly-seated screw) or micro-cracks have appeared in a surface from fatigue, these small spaces will have lower concentrations of oxygen and higher concentrations of chloride compared to the surrounding flat surfaces.
- This will lead to a decreased pH within the crack, and corrosion within those cracks through chemical degradation of the material.
- Any irregularities in the surface of a material, deliberate or accidental, can act in a similar manner with a corroded anode at the base, which can be progressively amplified over time as a pit develops.
- Fretting
- Fretting is mechanical abrasion and wear at sites of micro-motion between components.
- This is most often seen at the head-neck junction of hip arthroplasties, and at junctional sites in modular revision implants.
- Galvanic corrosion
Metals and characteristics
- Titanium
- Youngs modulus
- Steel
- Cobalt Alloy
References
Author Contributions
Sean Griffiths, WH Resident, 2020