Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 284

Corrosion may produce a concentrated attack on metals leading to the
initiation of other degradation processes. The stainless steel Kuntscher
IM rod shown in Figure 12.2 was retrieved after 4.75 years of implanta-
tion. It had been inserted in the femoral medullary canal to stabilize
a transverse shaft fracture, and a transcortical screw with its point in
the rod groove had been used to limit rotation (Figure 12.2, left). On
retrieval, the screw body was missing (completely corroded), but a local
form of corrosive attack at its contact point on the rod had served as an
initiation site for a nearly complete transverse fatigue crack.

Chemistry of
corrosion

Metals as used in implants and surgical instruments are said to be fully
reduced . That is, they have neutral or zero electronic valence, permitting
bulk parts to be made entirely of metal atoms bound together by electron
sharing in nondirectional metallic bonds (see Chapter 3). This renders
them highly opaque to light and reflective, if smooth, giving the charac-
teristic shiny metallic appearance that we associate with metals.
There are four types of reactions that change the valence of atoms
in reduced metals by electron transfer processes and produce metal-
containing ions and compounds. These reactions, shown in Table 12.1,
are collectively responsible for corrosion. They occur because, in chemi-
cal terms, the total energy content of the products (on the right side of
each equation) is less than that of the reactants (on the left side of each
equation). Thus, they are an expression of the general tendency of the
universe to “run down”; of all physical systems to seek their lowest pos-
sible energy content (level).
Oxidation and hydroxylation produce surface films of products that
may or may not be adherent to the bulk metal. These films may also,
themselves, be able to dissolve and release ions into solution, as ioniza-
tion does, or may be subject to solution reaction, also resulting in ionic
release.

Screw

FIGUre 12.2 Partial failure of im rod.