Biomedical Engineering Reference
In-Depth Information
depletion from adjacent zones, and therefore minimizes
in vivo
corrosion. Indeed, zones with reduction of Cr content are more
susceptible to intergranular corrosion because of a less stable passive
oxide film [PIL 09]. Since Cr tends to stabilize the weaker body-
centered cubic ferritic phase, the presence of high nickel amounts in
316L counteracts this tendency and stabilizes the face-centered cubic
austenitic phase of steel [DES 08]. The cost, the availability in various
stock forms and the ease to process 316L make it easy to design a
wide range of final implant shapes with a wide range of mechanical
properties. However, stainless steels corrode under highly stressed and
oxygen-depleted environments. Combined with their high modulus,
they are mainly used for temporary devices [DES 08].
The other main disadvantage of 316L is the presence of nickel.
Indeed, Ni has generally a poor biocompatibility: susceptibility to
corrosion via biological fluids, high relative cytotoxicity, hemolytic
behavior in particulate form, genotoxicity, carcinogenicity and
potential mutagenicity [BIE 12, YAN 10]. As a consequence, nickel-
free austenitic stainless steels are considered. BioDur®
1
108 is an
alloy with high nitrogen content which confers higher tensile and
fatigue strength than 316L. Its resistance to corrosion is also superior.
Its properties tend to be the following: density 7.7-8.03 g.cm
-3
, elastic
modulus 200 GPa, ultimate tensile strength 931
-
2, 206 MPa and
3
-
49% elongation. BioDur® 108 is used in bone plates, spinal
fixation, screws, and hip and knee components [CAR 05].
3.2.3.
Cobalt-based alloys
Cobalt-based alloys possess good wear resistance and fatigue
strength. Thus, they are used as prosthesis stems, load-bearing
components in joint replacement devices and dentistry castings. There
are mainly two types of Co-based implants: Co-Cr-Mo casted alloys,
and Co-Ni-Cr-Mo and Co-Cr-W-Ni alloys wrought by hot forging.
Castable alloys are principally used in dentistry and in making joint
components, while the applications of wrought alloys are the stems of
1 BioDur® 108 composition: Fe balancing, 21-24% Mn, 19-23% Cr, 0.5-1.5% Mo,
min. 0.9% N and max. 0.75% Si, 0.25% Cu, 0.10% Ni, 0.08% C, 0.03% P, 0.01% S
[CAR 05].
