Metals - Orthopaedic Biomaterials in Research and Practice - page 185

Table 7.8 Platinum and platinum-base alloys, nitinol, tantalum, and magnesium:
composition and mechanical properties

55-Nitinol
(austenite)

55-Nitinol
(martensite)

Property Pt (99.85%) Pt101r Tantalum Magnesium

Tensile modulus
(Y, GPa)

147 # 120 50 186 41

Ultimate strength
(σ u , min, MPa)

152 a 275 a 690-1380 - 285 a 86.8 ac

485 cw 650 cw

Strain to failure
(ϵ u , min, %)

35 a 35 a 13-40 <60 30 a 13 ac

5 cw

Note: a, Annealed; cw, cold worked; ac, as cast; #, not reported, similar to Pt.

spinal correction rods, fracture fixators, and compression staples, while
other widely used applications include cardiovascular stents.

Tantalum

Tantalum is a refractory metal that has found some use as a medical
material in vascular ligation clips, arterial stents, wire mesh, and sutures.
It is the material's physical resemblance to cancellous bone in terms of
porosity and stiffness that has shown the greatest potential for orthopae-
dic use. The material is an excellent surface for integration with bone and
soft tissue. Zimmer currently markets a tantalum “Trabecular Metal”
as a surface material to promote bone ingrowth for total joint arthro-
plasty components and as a scaffold for biologic ingrowth (Figure 7.9).
Trabecular metal is a metallic strut configuration that is similar to tra-
becular bone, which ideally makes it conductive to direct bone apposi-
tion. Tantalum's high strength-to-weight ratio, low stiffness in a porous
state, and high coefficient of friction facilitate physiologic load transfer
and minimize stress shielding. It is noted that tantalum is not the only
alloy used for porous coatings, as titanium and titanium alloys, magne-
sium and magnesium alloys, and cobalt-chromium alloys have all found
use as porous metal matrices.
Tantalum has a body-centered cubic α-phase and a meta-stable tetrag-
onal β-phase that rarely exists in the bulk form. In its α-phase, tantalum
is characterized as being hard, dense, and very ductile. It has a passive
oxide surface (Ta 2 O 5 ) that has a high resistance to chemical attack in the
physiological environment, leading to a very high resistance to corro-
sion. It is also considered good in the soft tissue environment as it is non-
ferromagnetic and thus will not displace, rotate, or heat up during MRI
diagnostic scanning. Tantalum coatings are typically fabricated through
pyrolysis of a thermosetting polymer foam. Pure tantalum deposited on
the surface of a vitreous carbon skeleton using chemical vapor deposi-
tion. The resulting composition of the structure is 99% tantalum, 1%
vitreous carbon by weight. Average pore sizes of 400 μm to 600 μm are
typical.