NANOMATERIALS FOR IMPROVED ORTHOPEDIC AND BONE TISSUE ENGINEERING APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 215

TABLE 7.3. Coeffi cient of Friction for Different Bearing Couples Used in Total Hip
Replacements

Bearing couples Coeffi cient of friction

Cartilage - cartilage 0.002

CoCr - UHMWPE * 0.094

Zirconia - UHMWPE 0.09 - 0.11

Alumina - UHMWPE 0.08 - 0.12

CoCr - CoCr 0.12

Alumina - alumina 0.05 - 0.1

Data obtained from Park and Lakes, 1992 [132] and Streicher et al., 1992 [133], [74].
* UHMWPE, ultra - high molecular weight polyethylene.

Currently, the most widely used bearing couples in hip replacements are
metal-on-polyethylene components. However, as we know, wear debris originat-
ing from polyethylene causes foreign body reactions around the implant that
frequently infl uence the longevity of orthopedic implants. Consequently, many
researchers have focused on evaluating alternative materials for articulating
surfaces and lowering the wear rate of articulating components in order to reduce
wear-related complications to improve the life expectancy of bone implants [64].
Generally, low wear rates can be achieved by using materials with high hardness.
Clearly, ceramics have rather high hardness compared to other implant materials
(see previously shown Table 7.2). For this reason, ceramics such as alumina are
widely evaluated as wear-resistant articulating materials considering their high
hardness, chemical inertness, respectable strength, and fracture toughness [65].
For instance, metal-on-metal implants with ceramic coatings (such as nanostruc-
tured diamond), ceramic - on - ceramic, and ceramic - on - polyethylene couples have
been studied (Table 7.3). It has been shown that the wear rates of ceramic-on-
polyethylene and further ceramic-on-ceramic systems [66] are noticeably lower
than for metal-on-polyethylene systems of total hip prostheses.
Moreover, since grain size and porosity can be closely related to wear behav-
ior of ceramics, many investigations on the wear resistance of alumina ceramics
have focused on the role of nanophase ceramics. By reducing the grain size as well
as decreasing porosity, the wear characteristics can be signifi cantly improved [67-
71] . For example, Al 2 O 3 -SiC nanocomposites have been shown to have much
greater resistance to surface fracture than conventional ceramics [65]. Polycrys-
talline phased-stabilized zirconia from nanoparticles has improved ductility, frac-
ture toughness and wear-resistance compared to current-generation zirconia [72].
In this light, nanostructured diamond and diamond-like carbon coatings on
cobalt-chrome and titanium alloys can be considered for potential orthopedic
applications [54] due to their excellent mechanical properties compared to
conventional micronscale implant materials. They are one of the hardest,
strongest, and most wear-resistant materials, and also have suitable biocompati-
bility and corrosion resistance. Through using chemical vapor deposition (CVD)