Biomedical Engineering Reference
In-Depth Information
include beta titanium alloys based on the Ti-Mo system (Ti-12Mo-6Zr-2Fe, Ti-
15Mo-5Zr-3Al, and Ti-15Mo-3Nb-3O) (Steingemann et al., 1993; Wang et al.,
1993a,b). These alloys have a Young's modulus around 80 GPa (compared with
110 GPa for Ti-6Al-4V). Since Mo is still considered potentially detrimental,
Mo-free titanium alloys such as Ti-15Zr-4Nb-2Ta-0.2Pd and Ti-15Sn-4Nb-2Ta-
0.2Pd have been developed (Okazaki et al., 1993). However, elements such as
Sn and Pd may still lack biocompatibility. Finally, titanium alloys using only
biocompatible elements such as Nb, Ta, and Zr have been developed. One
example is Ti-13Nb-13Zr which is considered to be completely biocompatible
and possess a Young's modulus of 79 GPa (Kovacs and Davidson, 1993; Mishra
et al., 1993). Another example is Ti-35Nb-5Ta-7Zr, which has a Young's
modulus as low as 55 GPa (Ahmed et al., 1995). The improved biocompatibility
of these Ti-Nb-Zr alloys is closely associated with their corrosion resistance.
Electrochemical measurements of Ti-13Nb-13Zr revealed that Nb and Zr helped
to form and tended to be incorporated into a highly protective passive layer on
the alloy surface. As a result, this alloy has a much lower potential electro-
chemical interaction than Ti-6Al-4V and has no adverse effect resulting from
dissolved metal ions as Al and V do.
2.4.2 Porous metal prostheses
Compared with solid metal implants, porous metal prostheses, either only
surface or total bulk porosity, may have advantages in the following fields: first,
a tailored modulus that can match the modulus of bone and avoid stress
shielding; second, improved fixation by allowing bone ingrowth, i.e., osseo-
integration. In this light, a variety of fabrication techniques have been developed
over the years and the interaction between porous metals and tissues have been
investigated (Hirschhorn et al., 1971; Bobyn et al., 1990, 1992; Ryan et al.,
2006).
Obviously, porous metals may have a few challenges when serving as hip
implants. First of all, a porous metal matrix might have reduced fatigue strength.
For instance, researchers found that the high cyclic fatigue strength of porous
coated Ti-6Al-4V is about one-third of the solid alloy (Wolfarth and Ducheyne,
1994). Therefore, a partly or fully porous coated solid metal substrate instead of
a fully porous metal is more appropriate for hip applications to provide sufficient
mechanical strength; porous coated metals are usually avoided at highly stressed
surface areas. Furthermore, to address this concern, optimization of the porosity,
pore size and shape, pore distribution, and interconnectivity can be critical.
Wolfarth and Ducheyne (1994) predicted a doubling of fatigue strength when
optimizing these porous geometries using finite element analysis. Heat treatment
is another method to increase the fatigue properties of porous metals. Cook et al.
(1988) showed a 15% improvement in fatigue strength of porous Ti-6Al-4V via
post-sintering heat treatments that produced microstructures more resistant to
