Metals - Orthopaedic Biomaterials in Research and Practice - page 187

Its corrosion resistance and mechanical properties can be improved
through alloying and surface treatments. Mg alloying elements include
Zn, Mn, Al, Ca, Li, Zr, Y, Cu, GD, Ni, Nd, and rare earth. Corrosion
can be increased when elements Fe, Ni, and Cu are present, all of which
are common impurities found within refined Mg. Alloying with Zn, an
essential trace mineral, can increase its yield strength and potentially
reduce the risk of hydrogen gas evolution. Copper can increase the
strength of an Mg alloy but it is unclear about the cytotoxicity of exces-
sive amounts of copper. Manganese can improve corrosion resistance by
reducing corrosion potential caused by impurities such as Fe (Mn sepa-
rates Fe and Mg). Like Mg, Mn is an essential mineral, though excessive
amounts may lead to neurological disorders.
There still remain a number of challenges to the widespread use of
Mg for orthopaedic applications. Controlling the corrosion rate in elec-
trolytic, aqueous environments when chloride is present can be difficult.
Excessive corrosion can cause any implant to lose mechanical integ-
rity early. In some cases, the implant may not retain sufficient strength
through the healing process. Compounding the problem of strength,
alloys currently in use tend to be relatively brittle. Alloying is one avenue
to affect the corrosion rate, though there are limits in element selec-
tion owing to potential systemic affects. New processing techniques also
continue to be developed to improve upon the materials' ductility.
Perhaps more problematic is the production of hydrogen gas as a by-
product of the reaction with aqueous solution, attributed to the high oxi-
dative corrosion rates of Mg. This may increase the pH locally and lead
to the formation of potentially harmful hydrogen pockets, which can
overwhelm the host response and increase the potential for gas gangrene
when corrosion is too fast. The clinical effect of hydrogen evolution on
the local tissue response is still uncertain.

Mechanical
properties

Magnesium is an exceptionally lightweight material with approximately
one-third the density of titanium. As a bioabsorbable material, it has a
distinct advantage in that it is on average over twice as strong as most
polymers. Its primary advantage is in its reduced stiffness which may
minimize risks from stress shielding in load-bearing implants, as it has
a modulus that is about half that of titanium. Table 7.8 gives the basic
mechanical properties for magnesium. Material properties for porous
Mg are discussed in Chapter 13.

Platinum-base alloys

Platinum-base alloys are not used for device fabrication because of their
high cost, but they are used as electrodes in electrical stimulation appara-
tus, for example, in faradic stimulation of bone growth. They are popular
for this application since they are highly corrosion resistant with good
mechanical properties. Platinum may be used by itself or alloyed with
1-10 w/o of rhodium or iridium. The composition and mechanical prop-
erties of pure platinum and of a common alloy are given in Table 7.8.