Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 281

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Corrosion and degradation
Materials may react chemically with their environment. These reactions
may produce changes in the properties of materials as well as altera-
tions in the environment. When the materials are metallic, composed of
atoms coupled by metallic bonds, the reaction processes are collectively
termed corrosion . In exceptional cases, bacteria and cells, such as mac-
rophages, can mediate corrosion processes enzymatically; this phenom-
enon is properly termed biocorrosion . In all other cases, corrosion in
vivo is identical to that elsewhere, depending on the material involved
and the conditions of environmental exposure.
The results of corrosion include release of free metallic or metal-
containing ions and production of ionic and covalently bound metallic
compounds as well as alteration of pH and of the concentration of other
chemical species near the corroding interface. In orthopaedic applica-
tions, gross (or bulk) properties of metallic materials are rarely affected,
but local attack may produce dramatic consequences.
When the materials are ionically or covalently bound, as in the case
of ceramics and polymers, the result of chemical reaction with the
environment, when present, is most generally called material degrada-
tion or sometimes biodegradation when the effect is observed in vivo .
Biodegradation , although a term in general use, should be reserved to
describe effects that are explicitly cell mediated, that is, degradation pro-
duced by cellular enzymatic action that would not occur under the same
physiologic conditions in the absence of cells and cellular products.
Biodegradation, although rare, may occur occasionally in orthopaedic
applications.
The chemical results of nonmetallic degradation of materials include
simple release of compounds and molecules by dissolution or bulk diffu-
sion (elution) and reaction with the bulk material to produce rearrange-
ments in bond structure and occasionally in bond type. These processes
introduce components of materials into the environment and may result
in profound changes in the mechanical, electrical, and optical properties
of the bulk materials. In the extreme case, materials may be deliber-
ately designed to dissolve, as in the case of resorbable sutures or resorb-
able internal fixation devices. Although this process is generally termed