Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 306

that element in an alloy; i.e., the elements of the released ions should not
be expected exclusively from the composition of the alloy.) To be effec-
tive, a passive layer must be nonporous, stable, and resistant to mechani-
cal stress, and must have a structure that will impede ion transfer. The
stability of passive layer is affected by structure, thickness, presence
of defects, and chemical composition. Oxide layer morphology can be
affected by surface treatments, electrochemical history, and immersion
time.
Because the oxide layer is constantly under attack, there is a poten-
tial for the local breakdown of the layer. Most commonly, the integrity
of oxide film is breached through mechanical abrasion (fretting, wear),
chemical reactions, or biological effects. Consequences of breakdown of
the passive layer may lead to increased corrosion rates, possibly causing
an inflammatory response in bone or tissue surrounding the implant.
Corrosion by-products may also lead to osteolysis.
Beyond the oxide layer, corrosion of a particular material depends on
composition, electrode potential, stress, and surface roughness. Stainless
steel is relatively more susceptible to pitting and crevice corrosion owing
to the inclusion of dissimilar materials (e.g., manganese, included for
ease of machining). Impurities may initiate defects at grain boundaries,
making them ideal sites for corrosion. Stainless steel is also relatively
more vulnerable to high-stress environments via a stress corrosion
mechanism. The inclusion of chromium in stainless steel improves cor-
rosion resistance by facilitating a chromium oxide layer on the surface.
Co-Cr alloys have good corrosion properties but are relatively more sus-
ceptible to crevice and fretting corrosion after scratching or third-body
wear. Of the major metals in use, titanium is the most resistant because
of the stability of its oxide layer. This layer can be modified through
anodization to enhance its corrosion resistance. For a given alloy com-
position, after preparation and postfabrication heat treatment, the fabri-
cation technique has little effect on uniform corrosion rates. Thus, cast
and forged components of the same general alloy type tend to have very
similar uniform corrosion rates.

Modular devices As a general rule, the potential for corrosion is relatively more severe in

modular devices, such as plate and screw combinations or hip implants,
than in one-part/monoblock devices. Uniform corrosion must be occur-
ring, but the primary physical evidence suggests that crevice, pitting,
and fretting corrosion are the most common forms of attack in such
situations. Pitting tends to occur on the undersides of screws, whereas
crevice corrosion occurs in the crack between screw head and plate,
or within the fittings of modular components. The situation in total
joint replacement components, such as the femoral portion of a total
hip replacement (THR), is far more complex. Restricted environments
common in complex orthopaedic designs can increase likelihood for
corrosion as discussed above, but there is some concern mechanically
assisted corrosion processes such as fretting corrosion, especially owing
to the increase in popularity of modular implants. Modular implants
are intended to provide surgeons with greater flexibility in adapting