Biomedical Engineering Reference
In-Depth Information
on titanium surfaces as long as oxygen is
present and protects it from corrosion by
forming a thin fi lm, the so-called passive fi lm.
This passive fi lm regenerates immediately after
mechanical destruction and thus protects the
surface instantaneously [
tantalum can be designed as a porous struc-
tural scaffold with the same advantages as
those of titanium fi ber mesh. The orthopedic
applications of porous tantalum are diverse
and include primary and revision joint-
reconstruction implants, spinal interbody
fusion devices, and trauma void-fi lling struc-
tural applications [
].
Another advantage of a metal such as tita-
nium is strength, which makes it very useful for
bone replacement. Further, the fl exibility of a
scaffold material, with reference to stress shield-
ing, is an important factor. The work of Jansen
et al. demonstrated a relationship between fl ex-
ibility of the mesh structure and tissue response.
Flexibility presumably eliminates focal stresses
by distributing the stresses between implant
and tissue over a larger area [
28
].
Cobalt-based alloys are generally described
as nonmagnetic and resistant to wear, corro-
sion, and heat. Cobalt-based alloys are used for
surgical applications, including orthopedic
prostheses for the knee, shoulder, and hip, as
well as for fracture fi xation devices. Unfortu-
nately, cobalt-based alloys are diffi cult to fab-
ricate, which limits their use as nondegradable
porous scaffolds. Furthermore, the properties
of these cobalt-based alloys are less desirable
than those of stainless steel and titanium [
8
].
Finally, the tissue-engineered scaffold should
have porosity and interconnectivity to allow
tissue ingrowth and stabilization. These
requirements limit the number of available
candidates. In view of this, titanium fi ber mesh
is a possible candidate material. Porous and
nonporous titanium implants with varying
geometrical properties have been produced
and their properties investigated. During the
fabrication of the porous surfaces, porosity,
pore size, and pore shape can be varied, which
infl uences the amount of bone ingrowth into
the porous surfaces. Comparison of porous
metal with conventional solid metals used in
the manufacture of orthopedic devices shows
that porosity allows a more normal restoration
of the bone than occurs with the use of non-
porous implant materials. Proper bone growth
requires initial stability. The frictional proper-
ties of porous titanium fi ber mesh in contact
with bone exceed those of the solid-metal mate-
rials that are available today. In the early post-
operative period, these frictional and structural
properties provide the construct with a high
initial. In the long term, the porous metal
serves as a scaffold for bone while allowing
proper loading and maintenance of vascularity
in surrounding and ingrown bone [
16
].
Stainless steel, like tantalum and titanium,
has several advantages including uniformity,
structural continuity, strength, low stiffness,
high porosity, and high coeffi cient of friction.
The advantages of using stainless steel rather
than titanium are its lower cost and higher
fracture toughness [
21
]. However, Paquay et al.
showed that porous stainless steel
29
L released
signifi cantly more corrosion products than
did titanium mesh, which explained the better
performance of titanium mesh, as judged by
the good tissue reaction to the titanium meshes
when they were placed subcutaneously [
316
].
Further observations showed that meshes
fabricated of fi bers of small diameter released
signifi cantly more corrosion products than
did meshes with of larger fi ber diameter. This
phenomenon was attributed to the larger
surface area of the meshes that contained fi bers
of small diameter. These results can be related
to the results of an in vivo study [
27
] that found
a correlation between the tissue behavior and
the fi ber diameter of the various
27
L stainless
steel meshes. This experiment showed that
although the bulk material was the same in
the various mesh materials, the amount of
corrosion products surrounding the implant
markedly infl uenced the behavior of the
tissue.
In summary, porous titanium fi ber mesh
offers several advantages over other materials
by virtue of its uniformity and structural con-
tinuity, as well as by its strength, low stiffness,
high porosity, corrosion resistance, and high
coeffi cient of friction [
316
]. In
summary, porous titanium fi ber mesh offers
several advantages over other materials owing
to its uniformity and structural continuity, as
well as to its strength, low stiffness, high poros-
ity, and high coeffi cient of friction [
7
7
].
5.3.2 Other Nondegradable Metals
Tantalum is an elemental metal that is biocom-
patible and corrosion resistant. Like titanium,
7
].
