ADDITIVE MANUFACTURING FOR BONE LOAD BEARING APPLICATIONS - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 226

Table 11.1 Range of mechanical properties for human cancellous and cortical bone
( Porter et al. 2009; Yang et al. 2001 )

Tensile strength
(MPa) Bone type Compressive strength (MPa) Young's modulus (GPa)

Cancellous bone N/A 4-12 0.01-0.5

Cortical bone 60-160 130-225 3-30

resorbing the matrix ( Karageorgiou and Kaplan, 2005 ). From a structural standpoint, in designing a
bone substitute, it is necessary to consider 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 ).
This means that an ideal bone substitute must have a heterogeneous porous structure, with varying
physical and mechanical characteristics. In addition, the implant must also be designed to have an
anatomically accurate three-dimensional shape in order to maintain a natural contact load distribution
post implantation ( Koh, 2004 ).

11.2.3 MECHANICAL PROPERTIES OF BONE AND REQUIREMENTS
FOR BONE SUBSTITUTES
The high level of porosity and pore interconnectivity that is ideal for a bone substitute may be limited
by the mechanical strength requirements for that specific implant, especially in the case of load bearing
applications. The bone substitute should provide physical support, starting from the seeding process in
vitro until the tissue is remodeled in vivo . Furthermore, the implant must provide sufficient mechanical
support to endure in vivo stresses and load bearing cycles ( Hutmacher, 2000 ). Table 11.1 summarizes
the range in mechanical properties of human cancellous and cortical bone.

11.3 DIFFICULTIES IN ACHIEVING AN IDEAL BONE SUBSTITUTE
Manufacturing of optimal porous bone substitutes from a biochemical, structural, and mechanical
properties point of view is highly complex due to a collection of factors. From an architectural stand-
point, the bone substitute supports biological and mechanical functions ( Bohner et al., 2011 ), which
may be in conflict. For example, for increasing the load-bearing property of the material, a denser
material is needed, which conflicts with the requirement of having a highly porous matrix to encour-
age bone ingrowth and fluid permeability ( Karageorgiou and Kaplan, 2005 ). What is generally defined
as an optimization of scaffold properties is likely a tuning of a single parameter with little regard to
how other scaffold properties are modified. Furthermore, characterizing, digitizing, and manufactur-
ing the scaffold architecture are difficult tasks. Using characterization methods to reveal pore surface,
pore volume, pore shape, interconnectivity, and volume porosity in bone tissues ( Bohner et al., 2011 ),
and furthermore translating such data into a digital format that can be interpreted into fabrication
methodologies in a continuous or discrete fashion can be a challenge. Typically, the interconnect-
ed macroporosity should be > 50 m m ( Bohner et al., 2011; Yang et al., 2001; Chang et al., 2000;