Biomedical Engineering Reference
In-Depth Information
Al
2
O
3
±Al
2
O
3
bearings provide an attractive option, particularly for younger
patients.
In addition to the normal (low) wear associated with the articulating surfaces,
observations of unbroken Al
2
O
3
±Al
2
O
3
bearing couples retrieved from patients
showed that some bearing couples had an unusual wear pattern, described as
`stripe wear' (Nevelos et al., 1993, 1999, 2001; Shishido et al., 2003; Walter et
al., 2004; Yamamoto et al., 2005). This stripe wear pattern consists of a long
narrow area of damage on the femoral head and a matching area of wear near the
rim of the acetabular liner (Fig. 7.2), resulting from contact between the femoral
head and the rim of the acetabular liner. Stripe wear has been associated with
steep acetabular cup angles, young patients, and revision surgery (Nevelos et al.,
2001). Observations with well-fixed and well-positioned acetabular components
indicated that another phenomenon, microseparation of the femoral head and
acetabular liner during the swing phase of walking, could also contribute to
stripe wear (Lombardi et al., 2000; Nevelos et al., 2000; Walter et al., 2004;
Yamamoto et al., 2005). Hip simulator studies have shown that stripe wear leads
to higher wear rates, depending on the severity of the microseparation. However,
even for bearings with stripe wear, the wear rate and volume of wear debris are
so small that the incidence of osteolysis (implant loosening) for Al
2
O
3
±Al
2
O
3
bearings should be far lower than for bearing couples with UHMWPE acetabular
liners. Further studies are necessary to understand and reduce the occurrence of
stripe wear, particularly under conditions corresponding to severe
microseparation conditions found in hip simulator experiments.
7.3
Limitations of ceramics for bearing applications
7.3.1
Failure of ceramic bearings
As in most load-bearing applications, particularly those subjected to a tensile (or
hoop stress), the inherent brittleness of ceramics is a major concern. Cata-
strophic failure of ceramic bearings in vivo (Fig. 7.3), while rare, is a serious
complication, with profound consequences for the patient, surgeon, and ortho-
pedic bearing manufacturer. Failure of ceramic bearings in vivo commonly
results from slow crack growth under the static or repetitive loading experienced
in the body. Under an applied tensile stress , the stress at the tip of a crack can
be described by the stress intensity K
I
given by
K
I
p
a
7:1
where a is the length of the crack (Lawn, 1993). It is generally assumed that
fast failure occurs in brittle solids if the stress intensity at the crack tip,
represented by K
I
in Equation 7.1, becomes equal to, or greater than, the critical
stress intensity factor, K
IC
, more commonly called the fracture toughness. The
fracture strength
f
of a brittle material can be written as
