ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 228

Table 11.2 Comparison of the mechanical properties of different metals ( Krishna et al. 2007 )

Material Bone Magnesium Co-Cr-Mo and alloys Ti and alloys Stainless steels

Density (g cc − 1 ) 1.8-2.1 3.1 8.3-9.2 4.4-4.5 7.9-81

Compressive
strength (MPa)

130-180 65-100 450-1896 590-1117 170-310

Elastic modulus
(GPa)

3-20 41-45 200-253 55-117 189-205

Toughness
(MPam 1/2 )

3-6 15-40 100 55-115 50-200

Stainless steel is considered one of the first metals used in the orthopedic field as plates and screws
for bone fixation in the early twentieth century ( Stevens et al., 2008 ). The most popular stainless steel
alloy used in prosthesis fixation is (316L), with moderate strength and toughness in comparison to other
metals as shown in Table 11.2 . This alloy is distinguished by good corrosion resistance in comparison
to other steel alloys, since the 12% Cr in its content forms a corrosion protective layer Cr 2 O 3 on the
surface ( Navarro et al., 2008 ). This metal is both widely available and economically effective in terms
of processing and manufacturing ( Long and Rack, 1998; Dabrowski et al., 2010 ). However, as the wear
resistance of stainless steel is very low, its usage in hip replacement was stopped ( Navarro et al., 2008 ).
Nowadays, stainless steel is rarely used for orthopedic implants. Instead, stainless steel is used for
temporary fixation devices such as nails, screws, and plates due to the superiority of other metals such
as Ti, Ti alloys, and Co-Cr alloy in terms of mechanical properties and corrosion resistance ( Navarro
et al., 2008 ).
Another widely used metal in the orthopedic industry is Co-Cr-Mo alloy that is characterized by a
high level of mechanical strength, fatigue strength, wear resistance, and low cost of production ( Re-
claru et al., 2005; Navarro et al., 2008; Dabrowski et al., 2010 ). While these advantages are considered
beneficial for some applications, this is not generally true for orthopedic implants, since this metal's
high strength and elastic modulus has caused a stress shielding between the implant and the bone due
to the mismatch of mechanical properties. Moreover, the high level of metal ions that are released and
the nanoparticle debris which is caused by the wear, negatively affect the biocompatibility of this metal
( Billi and Campbell, 2010; Dabrowski et al., 2010 ). Despite these drawbacks, this metal is the most
preferable in hip replacement due to its high level of wear resistance compared to the other metals. In
particular, this material is used to fabricate the femoral head and acetabular cup which is an area of
high friction ( Navarro et al., 2008 ). In addition, dentists have some interest in this metal as a coating
for some dental devices ( Long and Rack, 1998 ).
NiTi (nitinol) is a shape memory alloy, which means it can restore its original shape after a plas-
tic deformation by using a heat treatment. This alloy was introduced in 1960 and is characterized
by high strength, superelasticity, and good corrosion resistance ( Shishkovsky et al., 2008; Michiardi
et al., 2006; Navarro et al., 2008 ). However, the release of the Ni ions limits its usage as an orthopedic
prosthesis due to the possibility of toxicity and the inflammation effect of the surrounding tissue. This
has led researchers to treat the surface through exposure to the oxidization process in an attempt to
create a protective layer free of Ni ( Wataha et al., 2001; Navarro et al., 2008; Michiardi et al., 2006 ).