Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
FIGURE 1.5
Microstructure of Ti alloys (all are ×500). (a) Annealed α-alloy. (b) Ti6Al4V, α-β alloy, annealed. (c)
β-alloy, annealed. (d) Ti6Al4V, heat-treated at 1650°C and quenched. (Adapted from Hille, G.H. 1966.
J. Mater.
1,
373-383; Imam, M.A. et al. 1983.
Titanium Alloys in Surgical Implants
. Philadelphia, PA: ASTM Special Technical
Publication 796, pp. 105-119.)
aged, which showed high corrosion resistance with low modulus (
E
= 79 MPa) (Davidson et al., 1994).
The formation of plates of martensite induces considerable elastic distortion in the parent crystal struc-
ture and increases strength (Figure 1.5d).
The mechanical properties of the cp titanium and its alloys are given in Table 1.7. The modulus of
elasticity of these materials is about 110 GPa except 13Nb13Zr alloy. From Table 1.7, one can see that
the higher impurity content of the cp-Ti leads to higher strength and reduced ductility. The strength of
the material varies from a value much lower than that of 316 stainless steel or the CoCr alloys to a value
about equal to that of annealed 316 stainless steel of the cast CoCrMo alloy. However, when compared
for the specific strength (strength per density), the titanium alloys exceed any other implant materials as
shown in Figure 1.6. Titanium, nevertheless, has poor shear strength, making it less desirable for bone
screws, plates, and similar applications. It also tends to gall or seize when in sliding contact with itself
or other metals.
Titanium derives its resistance to corrosion by forming a solid oxide layer to a thickness of 10 nm. Under
in vivo
conditions, the oxide (TiO
2
) is the only stable reaction product. However, micromotion at the cement-
prosthesis and cement-bone are inevitable and consequently, titanium oxide and titanium alloy particles are
released in cemented joint prosthesis. Sometimes this wear debris accumulates as periprosthetic fluid col-
lections and triggers giant cell response around the implants. This cystic collection continued to enlarge and
aspiration revealed a “dark” heavily stained fluid containing titanium wear particles and histiocytic cells.
Histological examination of the stained soft tissue showed “fibrin necrotic debris” and collagenous, fibrous
tissue containing a histiocytic and foreign body giant cell infiltrate. The metallosis, black staining of the
periprosthetic tissues, has been implicated in knee implant (Breen and Stoker, 1993).
The titanium implant surface consists of a thin oxide layer and the biological fluid of water molecules,
dissolved ions, and biomolecules (proteins with surrounding water shell) as shown in Figure 1.7. The
microarchitecture (microgeometry, roughness, etc.) of the surface and its chemical compositions are
important due to the following reasons:
1. Physical nature of the surface either at the atomic, molecular, or higher level relative to the dimen-
sions of the biological units may cause different contact areas with biomolecules, cells, and so on.
