Biomedical Engineering Reference
In-Depth Information
Lateral condyle
Porous cancellous bone
Femoral shaft
Patellar
surface
Dense cortical
bone outer layer
Medical condyle
Figure 9.1
Photograph of a human femur. For a better understanding of the figure,
refer to the colour section (Figure 11).
polymeric), which are separated by an interface. In bone, collagen is
a triple helix of protein chains, which has high tensile and flexural
strength and provides a framework for the bone structure. Bone mineral
is a crystalline calcium phosphate ceramic (similar to hydroxycarbonate
apatite, HCA) that contributes the stiffness and compressive strength
of bone. The macrostructure of bone is also organised in a hierarchical
fashion. For example, long bones like the femur have cortical bone,
which is a dense structure with high mechanical strength, around the
outside for stiffness. Within the long bone is trabecular bone, a network
of struts (trabeculae) enclosing large voids (macropores) with 55-70%
interconnected porosity, which is a supporting structure (Figure 9.1).
There is a great demand for synthetic bone substitutes that can be
used in place of transplanted bone. Porous materials are also designed
and manufactured to act as a
scaffold
for the growth of new bone
tissue in order to restore the natural state and function, this being the
fundamental aim of the tissue engineering discipline. The structure and
properties of these scaffolds are pertinent to the tissue concerned and
the mechanical loads it will experience
in vivo
(Chapter 12). The generic
requirements for ideal synthetic bone grafts are listed in Table 9.1. An
ideal synthetic bone graft would mimic the hierarchical structure of
bone. However, bone structure is too complex to mimic exactly. All that
can be done with current material technologies is to take inspiration
