Metals - Orthopaedic Biomaterials in Research and Practice - page 163

CHaPTer SeVeN
Metals
Metals enjoy wide application in orthopaedics as structural, load-bearing
materials in devices for fracture fixation, partial and total joint replace-
ment devices, instruments, and external splints, braces, and traction
apparatus. The principle reasons for this broad popularity are as follows:
1. Metals have high elastic moduli and reasonable yield points such
that structures may be designed that will bear significant loads
without large elastic deformations or any permanent deformation.
2. Metals have high enough ductility that stresses that exceed the
yield point produce plastic deformation rather than sudden brit-
tle fracture, permitting measures to be taken to modify use or to
replace components before loss of integrity results.
3. Metals also possess sufficient plasticity to have fatigue endurance
limits, thus suiting them for designs required to withstand great
numbers of load-unload cycles, such as bone plates or proximal
femoral components.
4. Metals may be fabricated into parts by a wide variety of conven-
tional techniques and, in most cases, may have their mechanical
properties adjusted before the final shape is attained.
5. When reasonable care is taken in fabrication, surface finishing,
and handling, metal devices have good to excellent resistance to
the variety of external and internal environments encountered in
orthopaedic practice.
Basic metallurgy
The metallic elements of interest for inclusion in metallic orthopaedic
biomaterials are shown in Figure 7.1, a portion of the familiar chemical
periodic table. All have the ability to form interatomic metallic bonds
with each other. These bonds are relatively nondirectional but strong;
one may think of a metal in a simple way as an aggregation of mar-
bles stuck together with very cold molasses. Thus, a very wide range
of metallic material compositions are possible, in distinction to other