Biomedical Engineering Reference
In-Depth Information
Biocompatibility
It has been found that elements such as vanadium (often alloyed with titanium)
have corrosion and cytotoxicity risks (Wapner, 1991). As a result, toxic-free
titanium alloys such as Ti-13Nb-13Zr and Ti-15Mo have been developed to
achieve higher biocompatibility properties since the 1980s (Okazaki et al.,
1996). Beside the composition of titanium alloys, the outermost oxide layer of
titanium-based orthopedic implants is a main factor influencing biocompatibility
properties. Titanium dioxide is the most stable titanium oxide and is resistant to
most chemical events. The excellent corrosion resistance of titanium and its
alloys is to a large extent due to this oxide layer (containing titanium dioxide and
other oxides like Al 2 O 3 , Nb 2 O 5 , etc.). For this reason, titanium is usually
considered biocompatible when used as an orthopedic implant (Textor et al.,
2001). Numerous in vitro and in vivo studies have confirmed the bio-
compatibility properties of titanium and titanium alloys (Bordji et al., 1996;
Larsson et al., 1996; Long and Rack, 1998; Sittig et al., 1999; Shin et al., 2005).
In addition, numerous clinical studies on different cementless titanium hip
prostheses with various surface treatments confirmed a high survival rate. For
example, Zenz et al. (1995) observed a 97.9% success rate after 10 years of
implantation for rough grit-blasted Ti-6Al-4V and Ti-6Al-7Nb stems.
McLaughlin and Lee (1997) found a 96% survival rate after 10 years for
titanium plasma-sprayed Ti-6Al-4V stem implants.
Though titanium and titanium alloys generally perform well as hip implant
materials (in the scale of 5 to 10 years), it is still a challenge to obtain an
effective longevity over 20 years. One of the problems with current titanium
alloy implants (regardless of chemistry and crystallinity) is that their surfaces
lack enough bioactivity to promote and sustain bone cell functions necessary for
quick and strong osseointegration, i.e., there is a lack of direct structural and
functional connections between living bone and the surface of load-bearing
titanium-based implants. As a result, numerous researchers have focused on
improving titanium implant surface properties to prevent interposition of non-
bone (fibrous) tissue and to support better new bone growth.
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2.3.2 Cobalt±chromium alloys used in hip and knee
replacements
Mechanical and tribological properties
There are four cobalt±chromium alloys commonly used for hip and knee
replacements today (Table 2.4). Among them, F75 has been widely used in the
fabrication of femoral stems because of its corrosion resistance in chloride
environments due to the oxidized surface. As mentioned before, large grain sizes
develop during the casting process, which results in decreased yield strength. In
addition, casting defects may cause fatigue fracture of a femoral stem. To
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