Ceramic Biomaterials - Biomaterials - page 42

FIGURE 2.15 Transmission electron micrograph of well-mineralized bone (B) juxtaposed to the glass-ceramic
(C) which fractured during sectioning (×51,500). Inset (a) is the diffraction pattern from ceramic area and (b) is
from bone area. (From Beckham C.A., Greenlee T.K. Jr, and Crebo A.R. 1971. Calc. Tiss. Res. 8:165-171. With
permission.)
used for making major load-bearing implants such as joint implants. However, they can be used as fillers
for bone cement, dental restorative composites, and coating material (see Table 2.13). A glass ceramic
containing 36 wt% of magnetite in a β-wollastonite- and CaOSiO 2 -based glassy matrix has been synthe-
sized for treating bone tumors by hyperthermia (Kokubo et al., 1992).
2.5 Deterioration of Ceramics
It is of great interest to know whether the inert ceramics such as alumina undergo significant static or
dynamic fatigue. Even for the biodegradable ceramics, the rate of degradation in vivo is of paramount
importance. Controlled degradation of an implant with time on implantation is desirable. Above a criti-
cal stress level, the fatigue strength of alumina is reduced by the presence of water. This is due to the
delayed crack growth, which is accelerated by the water molecules (Park and Lakes, 1992). Reduction in
strength occurs if water penetrates the ceramic. Decrease in strength was not observed in samples which
did not show water marks on the fractured surface (Figure 2.16). The presence of a small amount of silica
in one sample lot may have contributed to the permeation of water molecules that is detrimental to the
strength (Park and Lakes, 1992). It is not clear whether the static fatigue mechanism operates in single-
crystal alumina. It is reasonable to assume that static fatigue will occur if the ceramic contains flaws or
impurities, because these will act as the source of crack initiation and growth under stress (Park and
Lakes, 1992).