Biomedical Engineering Reference
In-Depth Information
Traditionally, the differences in the property requirements of these compo-
nents have warranted the use of very different materials and processing routes for
these components resulting in the unavoidable formation of abrupt interfaces
between types of materials. Localized wear at these interfaces may cause debris
formation which is a great concern from the health and safety point of view.
For example, while the femoral stem is often made of titanium alloys, such as
Ti-6Al-4V, the poor wear resistance of this alloy prevents its use for femoral head
applications. Therefore, more wear-resistant materials are used for the femoral
head, such as ceramics (e.g., alumina or zirconia) or cobalt-chromium alloys.
However, while alumina or zirconia ceramic femoral heads offer excellent wear-
resistance, these ceramics do not have the same level of fracture toughness as
their metallic counterparts leading to problems such as fracture of these heads in
use. This has even lead to the recall of hip implants using zirconia femoral heads
[12-14]. Furthermore, the use of a ceramic femoral head attached to a metallic
femoral stem also leads to an undesirable abrupt ceramic/metal interface in the
hip implant.
From the materials perspective, while the use of titanium-base alloys had
been quite benefi cial for such implants, most of the materials that are currently in
use, such as Ti-6Al-4V, were originally developed for other applications such as
for aircraft engines. Consequently, in general the biomaterials that have been
used to date possess a less than optimum combination of biocompatibility and
mechanical properties, representing a fairly unfavorable compromise regarding
the necessary balance of properties for application in orthopedic implants. The
ideal biomaterial for orthopedic implant applications, especially for load-bearing
joint replacements, is expected to exhibit excellent biocompatibility with no
adverse cytotoxicity, excellent corrosion resistance, and a good combination of
mechanical properties such as high strength and fatigue resistance, low modulus,
good ductility, and good wear resistance [1,15]. The two primary Ti base alloys
used in implants today are commercially pure (C.P.) titanium and Ti-6Al-4V ELI
(extra low interstitial impurity content). Recent studies show that the release of
both V and Al ions from Ti-6Al-4V might cause long-term health problems, such
as peripheral neuropathy, osteomalacia, and Alzheimer diseases [16,17].
Another issue with the existing Ti-base orthopedic alloys is that their modulus
is signifi cantly higher than that of bone tissue (
10 - 40 GPa), leading to
stress-
shielding
that can potentially cause bone resorption and eventual failure of
the implant [18]. Thus, there is currently a substantial thrust directed towards
the development of completely biocompatible low modulus implant alloys.
∼
- Ti
alloys exhibit a substantially lower modulus as compared with stainless steels,
cobalt-based alloys and also conventional
β
α
β
alloys, such as Ti - 6Al - 4V. Recently
developed biocompatible
-Ti alloys developed for this purpose include Ti-12Mo-
6Zr - 2Fe [19] , Ti - 15Mo - 5Zr - 3Al [20] , Ti - 15Mo - 3Nb - 3O [21] , Ti - 13Nb - 13Zr [22] ,
and Ti-35Nb-5Ta-7Zr [23]. Interestingly, while the role of microstructure in deter-
mining the properties has been well-recognized and critically addressed in the
fi eld of structural materials, in the case of implant alloys, the role of the micro-
structure and the infl uence of processing on the microstructure has been sparsely
β
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