Metallic Biomaterials - Biomaterials - page 6

concentration appear to be highly toxic since all viability parameters were altered after 24 h. However,
Cr extract seems to be less toxic than Ni and Co (Granchi et al., 1996).
The modulus of elasticity for the CoCr alloys does not change with the changes in their ultimate ten-
sile strength. The values range from 220 to 234 GPa, which are higher than other materials such as stain-
less steels. This may have some implications of different load transfer modes to the bone in artificial joint
replacements, although the effect of the increased modulus on the fixation and longevity of implants is
not clear. Low wear (average linear wear on the MeKee-Farrar component was 4.2 μm/year) has been
recognized as an advantage of metal-on-metal hip articulations because of its hardness and toughness
(Schmalzried et al., 1996).

1.4 Ti Alloys

1.4.1 Pure Ti and Ti6Al4V
Attempts to use titanium for implant fabrication dates back to the late 1930s. It was found that titanium
was tolerated in cat femurs, as was stainless steel and Vitallium ® (CoCrMo alloy). Its lightness (4.5 g/cm 3 ,
see Table 1.5) and good mechanochemical properties are salient features for implant application.
There are four grades of unalloyed, commercially pure (cp) titanium for surgical implant applications
as shown in Table 1.6. The impurity contents separate them; oxygen, iron, and nitrogen should be con-
trolled carefully. Oxygen, in particular, has a great influence on the ductility and strength.
One titanium alloy (Ti6Al4V) is widely used to manufacture implants, and its chemical requirements
are given in Table 1.7. The main alloying elements of the alloy are aluminum (5.5%-6.5%) and vanadium
(3.5%−4.5%). The Ti6Al4V alloy has approximately the same fatigue strength (550 MPa) of CoCr alloy
after rotary bending fatigue tests (Imam et al., 1983). Titanium is an allotropic material, which exists
as a hexagonal close-packed structure (hcp, α-Ti) up to 882°C and body-centered cubic structure (bcc,
β-Ti) above this temperature. Titanium alloys can be strengthened and mechanical properties varied by

TABLE 1.5 Specific Gravities of
Some Metallic Implant Alloys

Alloys Density (g/cm 3 )

Ti and its alloys 4.5

316 Stainless steel 7.9

CoCrMo 8.3

CoNiCrMo 9.2

NiTi 6.7

TABLE 1.6 Chemical Compositions of Titanium and Its Alloy

Element Grade 1 Grade 2 Grade 3 Grade 4 Ti6Al4V a

Nitrogen 0.03 0.03 0.05 0.05 0.05

Carbon 0.10 0.10 0.10 0.10 0.08

Hydrogen 0.015 0.015 0.015 0.015 0.0125

Iron 0.20 0.30 0.30 0.50 0.25

Oxygen 0.18 0.25 0.35 0.40 0.13

Titanium Balance

Source: Adapted from ASTM. 1992. Annual Book of ASTM Standards , Vol. 13,
Medical Devices and Services , F67-F89, p. 39; F136-F84, p. 55. Philadelphia, PA:
ASTM.
a Aluminum 6.00% (5.50-6.50), vanadium 4.00% (3.50-4.50), and other ele-
ments 0.1% maximum or 0.4% total. All are maximum allowable weight percent.