Biomedical Engineering Reference
In-Depth Information
with significant deformation of the zirconium substrate and associated cracking
of the ZrO 2 -based surface layer.
7.5
Future developments in ceramic bearing
materials
The ideal orthopedic bearing material in THA and TKA must be able to
withstand high cyclic loads for a few decades, without major corrosion or
fretting at modular connections to metal implants, with reliable biocompatibility
and material stability in vivo, and with low wear rates. In addition to the
experimental validation of these characteristics, clinical studies are necessary to
determine how the bearing performs in the in vivo environment, where multiple
unpredictable variables can affect material behavior.
Modern bioceramics meet or exceed many of the requirements of the ideal
orthopedic bearing material, but catastrophic failure in vivo, while rare, is a
persistent concern. Future developments in ceramics for THA and TKA are
aimed at maintaining the low wear of the articulating surfaces but providing the
safety associated with ductile metal bearings. Systems under development
include ceramic±metal composite bearings, nanoceramics, and ceramic nano-
composites.
7.5.1 Ceramic±metal composite bearings
As described earlier (Fig. 7.4a), when a compressive stress is applied to the
femoral head during physiological activity, it gives rise to a tensile (or hoop)
stress component in the taper bore which is concentrated at the contact area to the
metal taper and at the bottom of the taper bore. Ceramic±metal composite
femoral heads (Fig. 7.4b) are under development by Columbia Biomaterials
LLC, Columbia, MO, in which the articulating surface of the femoral head is
dense, fine-grained Al 2 O 3 , whereas the taper bore is a biocompatible metal (Nb
or Ta) (Huang et al., 2009; Rahaman et al., 2010). In this design, tensile stresses
in the taper bore are dissipated by plastic deformation of the ductile metal phase,
so they cannot lead to catastrophic failure by slow crack growth as would occur in
monolithic ceramic femoral heads. This design differs from that of oxidized
zirconium in that the ceramic surface layer is considerably thicker (2±3mm), and
has a dense, fine-grained microstructure, similar to that of modern Al 2 O 3 femoral
heads. The composite femoral head is intended to provide an alternative to Al 2 O 3
articulating against UHMWPE, as well as Al 2 O 3 ±Al 2 O 3 (hard-on-hard) bearings.
￿ ￿ ￿ ￿ ￿
7.5.2 Nanoceramics and ceramic nanocomposites
The mechanical properties of ceramics for bearing applications, in particular
wear, can be further improved by using ceramics with ultrafine or nanoscale
Search WWH ::




Custom Search