Biomedical Engineering Reference
In-Depth Information
7.4
Development of ceramics for bearing
applications
7.4.1
`Single-phase' ceramics
Following the pioneering work of Boutin (1971) on the use of ceramic bearings
for THA, the first generation Al
2
O
3
bearings suffered unacceptably high fracture
rates in vivo. However, advances in materials processing, quality control, and
implant design in the 1980s and 1990s resulted in a marked improvement in the
strength and reliability Al
2
O
3
bearings. Modern Al
2
O
3
bearings, such as femoral
heads manufactured by CeramTec and sold under the trade name Biolox forte,
have a failure rate reported to the manufacturer of 0.02% (Garino et al., 2006).
Whereas Al
2
O
3
bearings are by far the most widely used ceramic bearings, the
low fracture toughness (or low resistance to brittle fracture) of Al
2
O
3
remains a
concern.
Zirconia-based femoral heads were introduced in the 1980s in response to
concerns about Al
2
O
3
femoral head fractures in THA (Christel, 1989; Piconi and
Maccauro, 1999). Zirconia stabilized in the tetragonal phase with yttria (Y
2
O
3
),
referred to as yttria stabilized tetragonal zirconia polycrystals (Y-TZP), has a
flexural strength and fracture toughness that are 2±3 times the values for Al
2
O
3
(Table 7.2), making it one of the most fracture-resistant ceramics. It was
anticipated that ZrO
2
would reduce the risk of femoral head fracture while
retaining the excellent wear performance of smaller diameter heads against
UHMWPE. However, the degradation of Y-TZP in a moist atmosphere leads to
microcracking and loss of strength. Following an unusually high rate of clinical
failures which resulted from alteration of the manufacturing variables, Y-TZP
was withdrawn from clinical use (Masonis et al., 2004). The addition of small
quantities of Al
2
O
3
(0.15±3 wt%) to Y-TZP reduces the degradation. The
stabilization of ZrO
2
with ceria (CeO
2
), to form Ce-TZP, has lower mechanical
properties than Y-TZP, but the resistance to degradation is superior. Zirconia
partially stabilized with magnesia, referred to as Mg-PSZ, has superior
smoothness and fracture toughness than Al
2
O
3
, but its strength and fracture
toughness are inferior to those for Y-TZP. Mg-PSZ does not suffer from the
degradation problems of Y-TZP in a moist atmosphere. Mg-PSZ femoral heads
are presently available for use in ceramic±UHMWPE articulations in THA, and
Mg-PSZ femoral bearings are currently undergoing clinical trials for use in
ceramic±UHMWPE articulations in TKA. Zirconia-based bearings articulating
against UHMWPE have a similar wear rate to that of Al
2
O
3
±UHMWPE
bearings, but they are not approved for use in ceramic-on-ceramic bearing
couples.
Si
3
N
4
bearings, manufactured by Amedica Corp., Salt Lake City, UT, under
the trade name MC
2
, are currently undergoing trials for THA and TKA applica-
tions (Fig. 7.5). Si
3
N
4
has been the subject of considerable research and
development for decades, for high temperature structural applications such as
