Polymers - Orthopaedic Biomaterials in Research and Practice - page 133

increase in activity levels of patients and their expectations. The drive to
improve this record must be much more cautious than John Charnley's
pioneering efforts were. Now, there is a solid foundation of experience
and expectation, which is placed in hazard when material and design
changes are made in pursuit of further improvement.
In particular, it must be emphasized that selection of materials is not
analogous to choices among ice cream flavors. A proven design may
acquire improved performance through a superior value of a key prop-
erty of a new material. However, this material possesses a different com-
bination of properties than the one previously used, and a simple change
of materials will not produce an optimized design. A change of mate-
rials should be accompanied by a complete re-evaluation of the over-
all design and may result in a different final configuration. Therefore,
changes of materials should always be regarded as significant and not be
undertaken lightly.
We can place materials on a generic basis into four classes: metals,
polymers, ceramics, and composites. These class distinctions are based
both on external appearances and on related internal chemical composi-
tion and molecular structure differences. This chapter is the first of three
that discuss the properties of present and emerging orthopaedic biomate-
rials with these classes as a central organizing feature.

Comparative properties

It is difficult to make many generalizations about the properties of each
class of materials since exceptions may be cited to any such broad con-
clusion. This is particularly true of composite materials that, ab initio,
are intended to have properties adjustable over wide ranges by control of
internal details of design.

Table 6.1 Comparison of properties of materials by classes

Property Highest a Intermediate Lowest

Tensile modulus (Y) C M P

Yield strength (σ y ) M — P b

Ultimate strength (σ u ) C c M P

P M C

Strain to failure (ε u )

Toughness M C P

Hardness d C M P

Resistance to in vivo attack C P M

Local host response (bulk) M P C

Systemic/remote response
(degradation products)

M P C

a

C, ceramics; M, metals; P, polymers.

b

Ceramics do not have appreciable plastic properties at 37°C.

c

Theoretical; practically metals, as a class, are superior.

d

Strongly dependent on processing.