Joint Replacement Implants - Biomechanics of Hard Tissues - page 205

Table 8.1

Typical mechanical properties of metals used for joint replacement implants.

Material Young's modulus Ultimate tensile Fatigue strength @10

(GPa) strength (MPa) million cycles (MPa)

Stainless steel 193 1100 583 [7]

Cobalt chrome
molybdenum alloy

200 655 290 [3]

Titanium alloy 114 860 500 [8]

All values from Brown [9], unless indicated otherwise.

molybdenum alloy, and titanium alloy are all used for joint replacement implant
stems, and their typical mechanical properties are shown in Table 8.1. Various
grades of stainless steel are used with a common one known as Ortron 90 ,
while Ti-6Al-4V is the typical titanium alloy [3]. Each metal has a national or
international standard associated with it to ensure consistency in composition and
mechanical properties. For example, BS 7252-9 [4] is for stainless steel, BS 7252-4 [5]
is for cobalt chrome molybdenum alloy, and BS 7252-3 [6] is for titanium alloy.
Metals are also commonly used as bearing materials in joint replacement
implants. The metals used for bearing surfaces are stainless steel and cobalt
chrome molybdenum alloy. Titanium alloy is not used as a bearing material as it
has been shown to have poor wear characteristics on its own. However, there are
a range of coatings, such as titanium nitride, that can be applied to titanium alloy
to ensure that it has suitable mechanical properties to act as a bearing material [3].
Oxidized zirconium, commercially known as Oxinium , is a metal with a ceramic
surface that is used as a bearing surface in some joint replacement implants [10].
The ceramics alumina and zirconia have been increasingly used as bearing
surfaces in hip joint replacement implants, as they have excellent wear resis-
tance. Alumina has a Young's modulus of 380GPa and a fracture toughness of
4MPam 0 . 5 . The Young's modulus and fracture toughness of zirconia are 210GPa
and 8 MPa m 0 . 5 , respectively. The standards for alumina and zirconia are BS 7253-2
[11] and BS 7253-6 [12], respectively. As ceramics have a much lower fracture tough-
ness compared to metals, this did lead to some fractures in the bearing surfaces
in the early days of their use. However, improved manufacturing processes for
ceramics have generally prevented this problem and led to their increased use [3].
The polymer ultrahigh molecular weight polyethylene is most commonly used
as one of the bearing surfaces in joint replacement implants [13]. It has been
shown to be a material that has low friction when articulated against a metal or
ceramic. Ultrahigh-molecular-weight polyethylene has a typical Young's modulus
of 677MPa, an ultimate tensile strength of 43MPa, and a typical fatigue strength at
10 million cycles of 18MPa [14]. Various standards for ultrahigh-molecular-weight
polyethylene exist including BS ISO 5834-2 [15]. A newer variation of the material is
highly cross-linked polyethylene, where radiation is used to increase the cross-links