Biomedical Engineering Reference
In-Depth Information
Table 3. Zirconia or high-density glasses are added for achievement of increased strength
and increased radio-opacity. The glasses are used preferentially in dental applications where
translucency is an additional desired feature. The high radiopacity of zirconia or other
heavy-element containing phases means that the physician during the injection can follow
the paste penetration in bone tissue without risking any possible leakage of the material into
the surrounding tissue.
Compound
Formula
Function
Amount
(wt-%)
Mean particle
size
Ca-aluminate
CaOxAl
2
O
3
Cement binder
50-70
< 5 m
Zirconium
dioxide
ZrO
2
Radiopacier
20-40
< 1 m
µ-Silica
SiO
2
Expansion and
viscosity controller
< 10
<50 nm
Table 3. Typical composition of an injectable biomaterial cement powder.
Typical processing agents are accelerators/retarders, dispersants, viscosity agents to control
reaction rate, temperature and the cohesiveness, and in general the rheology. Examples are
lithium chloride, polycarboxylate polymers and cellulose, as well as glass poly-alkeonates.
For the CS-system Ca-chloride at high concentrations is normally used as an accelerator. For
cements as injectable biomaterials, the reaction rate must be controlled with respect to
working time, setting time, curing time and the maximum temperature during hydration.
Typical data are presented in Table 4. The cement reactions are all exothermic and the
temperature raise is controlled by the specific cement phase selected, and the hydration rate
and the amount of material injected. For dental application the temperature raise is limited
to a few
o
C above 37
o
C. For orthopaedic applications where larger amounts (2-10 cm
3
) are
used the temperature raise is more pronounced but lower than that of the conventional
PMMA-based materials (Lewis, 2006).
System
Working time at 23
o
C, min
Setting time at 37
o
C,
min
Max reaction temperature,
o
C
Ca-aluminate
Approx. 5
8-12
< 60,
(for dental applications <
40)
Ca-silicate
Approx. 10
15-18
< 45
Ca-phosphate
5
10-12
< 40
PMMA
5-10
11
< 90
Table 4. Typical working and setting times and maximum reaction temperature of the
systems discussed.
The on-going precipitation of hydrates and the reduction of the amount of liquid phase
result in the formation of a material skeleton. This repeating reaction is fast at the beginning,
resulting in a hardened product within 4-20 minutes depending on intended application.
Strength corresponding to load carrying capacity is reached after approximately one hour.
