Metals - Orthopaedic Biomaterials in Research and Practice - page 164

II A III A

12
Mg

13
Al

VIII

IV B V B VI B VII B

22
Ti

23
V

24
Cr

25
Mn

26
Fe

27
Co

28
Ni

40
Zr

41
Nb

42
Mo

46
Pd

73
Ta

74
W

77
Ir

78
Pt

FIGUre 7.1 (See color insert.) elements of interest in orthopaedic alloys.

types of chemical compounds, in which the nature of other bond types
(ionic, covalent, etc.) dictates the inclusion of the constituent elements
only in well-defined ratios. For instance, in sodium chloride, an ionically
bonded compound, the combination of the two elements is only stable
when they are present in a 1:1 ratio.
Metal “compounds,” or more properly mixtures, are called alloys.
The usual route to the development of an alloy of commercial value is
to select a base element and then add small portions of other elements
to change and enhance the properties of this base metal. The most com-
mon composition groups, that is, alloy systems, used in orthopaedics are
summarized in Table 7.1.
Although metallic bonding leads to mixtures over wide possible
ranges of composition, there may be preferred ratios of different ele-
ments that have some stability. These are called intermetallic com-
pounds . The presence, deliberate or inadvertent, of carbon produces a
range of very specific carbon-metal compounds called carbides . Since
the metallic atoms are of different sizes, they may take up preferred min-
imum volume spatial arrangements when present in particular elemental

Table 7.1 Common orthopaedic alloy systems

Principal alloying
elements Base element Generic name Typical applications

Fe C a + Cr, Ni, Mn, Mo, V Stainless steels Fracture hardware, braces,

surgical instruments

Co Cr, Mn, W, Mo, Ni, Nb, Ta Super alloys or cobalt-

base alloys

Joint replacement
components

Ti Al, V, Fe, Nb, Ta, Zr Titanium-base alloys Fracture hardware, joint

replacement components

Pt lr, Pd Precious alloys Electrodes

a

Carbon is not a metallic element; its role in determining alloy properties will be discussed later, but its
controlled presence is what distinguishes steels from other iron alloys. Metallic elements are given in the
approximate order of frequency of use as alloying agents.