Biomedical Engineering Reference
In-Depth Information
been the subject of theoretical and experimental investigations (Chevalier et al.,
1999; Wan et al., 1990). The K
I0
corresponds to crack equilibrium with zero
crack velocity, so crack propagation does not occur. It determines the safe range
for using ceramics in total joint replacement. A higher K
I0
value is indicative of
a higher reliability and, hence, a longer lifetime of the material. The K
I0
value
represents a more intrinsic property for a material when compared with the
fracture toughness K
IC
, which applies to fast crack growth.
Failure of ceramic bearings can originate from flaws introduced into the
bearing during fabrication or post-fabrication surface finishing, or flaws pro-
duced as a result of in vivo corrosion or degradation. Flaws due to inadequate
fabrication or surface finishing include porosity, large grains, and microcracks.
Observations of bearings retrieved from patients often show a variety of flaws
on the articulating surface resulting from corrosion or degradation, due to a
dislocated bearing rubbing against the acetabular component (Bal et al., 2007).
These flaws include enhanced porosity, metal staining from a metallic
acetabular component, scratches, pits, or grooves. Clinically, ceramic bearing
failures have been found to occur in the absence of any identifiable risk factor or
explanation (Barrack et al., 2004; Hannouche et al., 2003), which is sympto-
matic of failure caused by slow crack growth. Patient obesity and strenuous
activity may not contribute significantly to the catastrophic failure (fast fracture)
of ceramic bearings because the loads applied are well below the fracture
strength of the material, but they may contribute to failure by slow crack growth.
Tensile (or hoop) stresses, such as those generated when a ceramic femoral
head is impacted on the metal taper of a femoral implant or during impaction of
an eccentrically seated ceramic acetabular insert in a metal shell, are relevant to
the risk of bearing failure. Until the mid-1970s ceramic femoral heads were
attached to femoral stems with suboptimal methods such as gluing, screwing,
and brazing. Introduction and optimization of the Morse taper design for
ceramic bearings led to a decline in the incidence of ceramic femoral head
failures (Hannouche et al., 2003). Finite element analysis of the stress
distribution in ceramic femoral heads subjected to a standard rupture test (ISO
7206-10) shows that the areas of maximum tensile (or hoop) stress occur in the
contact area to the metal taper and at the bottom of the taper bore (Fig. 7.4a)
(Affolter et al., 2009). As ceramic taper bores can have large and numerous
surface flaws from drilling, slow crack growth of these surface flaws, particu-
larly in the areas of maximum tensile stress, can provide an important failure
mechanism.
According to Equation 7.2, two ways of improving the strength of ceramics
are: (1) decreasing the presence of flaws, as well as the size a of the flaws, by
careful processing and quality control, and (2) increasing K
IC
, by alloying or by
forming the ceramic into a composite. Advances in materials processing, quality
control, and implant design in the 1980s and 1990s have markedly improved the
reliability and lifetime of ceramic bearings. The use of tightly controlled ceramic
