Biomedical Engineering Reference
In-Depth Information
Kujala et al., 2003; Karageorgiou and Kaplan, 2005; Bose and Tarafder, 2012
), with a specific ori-
entation to match the stress loading conditions and fluid and nutrient transport mechanics (
Bohner
et al., 2011
). Also, what is defined as microporosity, with a diameter ranging between 0.1-10
m
m
(
Bohner et al., 2011; Karageorgiou and Kaplan, 2005; Bose and Tarafder, 2012
) has shown an effect
on the biological response of scaffolds, thus the pores at this scale should be characterized and inte-
grated in the final design. From a structural standpoint, it is also necessary to implement a gradient in
porosity and mechanical properties, from a dense external configuration matching the characteristics
of cortical bone to the highly porous region with interconnected porosity matching the characteristics
of cancellous bone (
Mehrali et al., 2013; Butscher et al., 2011; Porter et al., 2009
). Manufacturing
methodologies that can incorporate the interpretation and implementation of digital data at a macro-
and micropore scale are of concern.
Another issue related to manufacturing the appropriate bone substitute architecture lies in the de-
velopment of appropriate software interpreter design strategies (
Lin et al., 2004
;
Cai and Xi, 2008
;
Sanz-Herrera et al., 2009
) that can convert the desired structural porous morphology and mechanical
properties of the bone to be replaced, into appropriate voxel units that can be fabricated using various
manufacturing platforms. Such voxels may be computed mathematically using topology optimization
algorithms (
Lin et al., 2004; Hollister, 2005
) or numerical simulation (
Sanz-Herrera et al., 2009
).
From a material standpoint, difficulties arise in designing structures that can bioresorb
in vivo
at an
appropriate rate matching bone remodeling. In the context of regenerative medicine, the terminology of
materials with biodegradable, bioresorbable, bioerodible, and bioabsorbable (
Hutmacher, 2000
) prop-
erties are often used. The biodegradation pathway will have an effect on the mechanical, structural, and
biochemical properties of the scaffold, and needs to be fully understood (
Bohner et al., 2011
). Some
of the parameters that affect the degradation rate are pore size, pore interconnectivity, permeability,
scaffold shape, and volume, as well as implantation location within the musculoskeletal system. Fur-
thermore, the long-term native tissue response to the degradation products should also be considered
(
Bohner et al., 2011
). To add to the difficulty of producing an ideal implant, the overall biochemical,
structural, and mechanical properties of the bone substitute should match patient-specific needs such as
age, gender, health, metabolism, implant location, and loading conditions (
Bohner et al., 2011
).
11.4
METALLIC BONE SUBSTITUTES
11.4.1
METALLIC MATERIALS, LIMITATIONS AND OPPORTUNITIES
For a long time, metals were the main material utilized for orthopedic implants. This interest in metals
resulted from the excellent physical and mechanical properties that are intrinsic to metals. At present,
the interest in nonmetallic materials has prompted the fabrication of tissue scaffolds. These materi-
als are mostly polymer or ceramic, and are used to produce biodegradable scaffolds. Biodegradable
scaffolds can be useful for young patients, because they have high growth rates of tissue to restore the
functionality of the damaged area. However, the case is completely different for senior citizens, who
have very low tissue growth rates. When faced with a certain degradation rate of the materials, this may
cause a mismatch in terms of the mechanical properties (
Yarlagadda et al., 2005
). Therefore, permanent
metallic bone substitutes are more appropriate in the case of older patients. Several metals have been
used for implants, such as stainless steels (316L), Co-Cr-Mo, pure titanium, titanium alloys, and tanta-
lum. Each metal has advantages and disadvantages that can either expand or limit its usage.
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