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Corrosion

Frank J. Frassica, MD
Chairman, Department of Orthopedic Surgery
Robert A. Robinson Professor of Orthopedic Surgery
The Johns Hopkins Hospital;
Professor of Oncology
Sidney Kimmel Comprehensive Cancer Center
Baltimore, Maryland

Galvanic Corrosion

Galvanic corrosion is caused by an electrochemical potential that is created between two metals that are located in a conductive environment, such as body fluids.

Examples of galvanic corrosion include:

Screw heads and a plate
Femoral head screw and barrel of a dynamic hip screw
Interlocking screws and an intramedullary nail

Galvanic corrosion can also occur within a metal if there are impurities (intergranular corrosion).

Fretting Corrosion

Fretting corrosion occurs when micromotion exists between two metals in contact. One of the most common examples of fretting corrosion is micromotion between a modular femoral head and the tapered neck junction. Modular components, such as the S-ROM system (DePuy Orthopaedics Inc., Warsaw, Ind), are subject to fretting corrosion at each of the junctions.

Techniques to minimize fretting corrosion include:

Making sure the head-neck junctions are dry and clean
Eliminating micromotion but having an exact fit (ie, not mixing manufacturers)

Crevice Corrosion

Crevice corrosion occurs at the sites of a surface defect in a metal implant. At these defects, changes in pH and electrolyte concentrations cause corrosion.

Common examples of crevice corrosion include:

At the interface between an uncemented acetabular component and the cancellous screws (at the contact point where the head of the screw is countersunk into the acetabular shell)
At the interface between a screw head and the plate at the point where the screw head contacts the plate

Oxidative Degradation

One of the most important examples of corrosion is the breakdown of ultra-high density polyethylene. Wear particles result in osteolysis and bone loss. When UHMWPE is sterilized in air, free radicals are generated and lead to oxidative degradation of the UHMWPE.

Other important terms to recognize include:

Ram extrusion: Manufacturing method for UHMWPE
Compression molding: Manufacturing method for UHMWPE
Direct molding: Manufacturing method for UHMWPE
Sterilization with ethylene oxide: Alternative

Bibliography

Wright TM, Maher SA. Biomaterials. In: Einhorn TA, O’Keefe RJ, Buckwalter JA, eds. Orthopaedic Basic Science. 3rd ed. Rosemont, Ill: American Academy of Orthopaedic Surgeons; 2007:65-85.

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