Biomedical Engineering Reference
In-Depth Information
Ti-6Al-7Nb can be forged into hip implant components at temperatures between
900 and 950 ëC (Windler and Klabunde, 2001). This is because hot forging or
other thermo-mechanical treatments can inherently close the pre-existing voids
and improve mechanical and fatigue properties of the same material.
Strength and elasticity
Titanium alloys usually have higher strength and elasticity than bone. A low
Young's modulus enables more physiological transmission of loads to the femur
and can avoid proximal stress shielding and bone resorption in the proximal
femur. Although titanium alloys already have a relatively low elasticity
(110 GPa) compared with CoCr alloys (210 GPa), recent research has focused on
developing novel titanium alloys which possess even lower elasticity (Kuroda et
al., 1998). This issue will be discussed in detail in later sections.
Fatigue strength
The fatigue properties of titanium alloys used for hip implants are very
important because they will be subjected to high dynamic loading of millions of
cycles during its lifetime. The fatigue strength of titanium alloys is significantly
higher than pure titanium and is affected by its microstructure and surface
treatment. For example, forged and polished Ti-6Al-7Nb has a fatigue strength
of about 600MPa which is excellent compared with its static strength (Windler
and Klabunde, 2001). However, it is known that titanium alloys are sensitive to
fatigue induced by notches. Therefore, an appropriate hip design as well as
choice of material is critical here. In other words, the design should minimize
excessive stress due to construction notches in order to avoid hip fracture.
Wear resistance
Untreated titanium alloys usually do not have high enough wear resistance for
use in articulating situations which appear in both hip and knee replacements.
Therefore, research has improved the tribological properties of titanium alloys
through various surface treatments. For example, a layer of hard coatings of TiN
(Coll and Jacquot, 1988; Liu et al., 2003) and diamond-like carbon (McNamarta
et al., 2001; Platon et al., 2001) up to several microns can be deposited onto
titanium alloy surfaces by chemical or physical deposition. However, the main
challenge for hard coating methods lies in the insufficient adhesive strength of
the coatings and the potential presence of wear particles. Hence, so far, such
coatings are not a practical choice for femoral heads of hip implants and
articulating parts of knee implants. Some other techniques to enhance titanium
and cobalt±chromium alloy wear resistance will be discussed in later sections.
