Ceramics and composites - Orthopaedic Biomaterials in Research and Practice - page 208

toughness and ductility (from the matrix). This is a common practice
with ceramics owing to their brittle nature.
Although any materials may be combined into a composite, the most
successful generic combination has been the ceramic reinforced poly-
mer matrix composite (PMC). PMCs are usually referred to in the engi-
neering literature as fiber-reinforced polymer composites or FRPs. This
is undesirable as the reinforcing phase may be in other forms (gran-
ules, microspheres, whiskers, etc.) than fibers. These are widely used in
industrial applications, appearing in consumer products such as tennis
racquets, automobile body parts, and sailboat hulls. PMCs have been
widely adopted in military applications, despite their high cost, because
the combination of their high strength and toughness with their being
lightweight can radically improve the performance of vehicles and air-
planes historically fabricated almost entirely from metals.
Hydroxyapatite-reinforced high-density polyethylene is one example
of a composite material that has been used clinically. A major concern
with composite materials is the quality of mechanical properties found
at the interfacial bond between the hydroxyapatite-reinforcing particles
and the polyethylene. In nonmedical applications, various sizing or
adhesive agents are used to improve the mechanical behavior of this
interface. However, most of such agents provoke unsatisfactory local
host responses; thus, the direct use of composites previously developed
for other applications may not necessarily be suitable for biomedical
tissue-contacting applications.

Properties of PMCs

General
considerations

Composites gain their mechanical properties from a combination of
the properties of the individual components (filler[s], matrix) and from
interactions between these components. Although there are traditional
combinations, such as glass fibers in epoxy, there are essentially an infi-
nite number of possible composite materials. This complexity is further
increased by the ability of the designer and engineer to vary compos-
ite properties in different parts of a component by altering the filler-to-
matrix ratio and, in some cases, the internal orientation of the filler.
For this reason, and for two others, there are no “off the shelf” com-
posites waiting for application to orthopaedic problems, as there have
been in the case of metal alloys and polymer compositions. These addi-
tional reasons are as follows:

1. Most industrial composites are not designed for continuous ser-
vice immersed in water.

2. Industrial composites contain large amounts of chemical agents
(for priming, bonding, mold lubrication, sealing, etc.) such that
might produce adverse host responses.

However, a number of matrix-filler combinations have been tried to
a limited extent in implant applications (Table 8.2). As previously noted,