Biomedical Engineering Reference
In-Depth Information
7.4 (a) Schematic diagram of Al
2
O
3
femoral head on a metal taper. Finite
element model predictions (Affolter et al., 2009) of the regions of maximum
tensile (hoop) stress in a ceramic femoral head loaded in compression on a
metal taper are indicated (on one half of the symmetrical system). (b)
Schematic of Al
2
O
3
±Nb composite femoral head on a metal taper.
processing procedures, clean-room facilities, optimized sintering and hot
isostatic pressing, and rigorous proof testing has resulted in the production of
modern Al
2
O
3
bearings with improved mechanical reliability and wear
(Willmann, 2000). Coupled with these processing and quality control procedures,
the use of alternative ceramics, such as Si
3
N
4
, and ceramic composites, such as
ZrO
2
-toughened Al
2
O
3
(ZTA) can lead to an increase in both K
I0
and K
IC
(De
Aza et al., 2002). For the same pre-existing defect size, these composites can be
stressed to higher loads than the corresponding monolithic ceramics and not
suffer delayed failure.
7.3.2 Squeaking of ceramic bearings
A less serious concern with Al
2
O
3
±Al
2
O
3
THA articulations in vivo is the risk of
`squeaking' (audible noise) which has been recognized within the last 5 years or
so. Squeaking occurs during activities such as walking, bending, and stair
climbing, and it is more common in younger, heavier, and taller patients. Several
suggestions have been put forward to explain this phenomenon, including
mismatched bearing diameters, edge loading due to acetabular component
malpositioning or to microseparation of the femoral head and acetabular liner,
and disruption of fluid film lubrication leading to enhanced friction between the
hard-on-hard bearings (Capello et al., 2008; Walter et al., 2008). While the
causes of Al
2
O
3
±Al
2
O
3
hip bearing noise are unclear, it appears that bearing
noise is multifactorial in origin, and is related to a combination of implant,
patient, and surgical factors.
