Biomedical Engineering Reference
In-Depth Information
of cystic lesions and augmentation of screws, (d) more recently, for the
treatment of spinal fractures and vertebroplasty (with or without the aid of
a kyphoplasty balloon).
18-20
This last application is common with abCs
and even if the nature and properties of the two types of cements are very
different, some results reported in the literature about
ex vivo
biomechanical
studies show no significant differences between the two materials.
21-26
CPCs are also used to fabricate pre-set granules and blocks, which have
some advantages in comparison to sintered ceramic granules or blocks. The
advantages arise from the low-temperature and wet processing method intrinsic
to CPCs. indeed, the apatite CPC products are micro/nanocrystalline, have
high specific surface area and, therefore, are much more reactive than sintered
ceramics. in addition, CPCs enable the fabrication of low temperature calcium
phosphates, such as brushite, octacalcium phosphate or calcium deficient
hydroxyapatite, which cannot be obtained by high temperature sintering and
which are much closer to the calcium phosphates found in living tissues.
10.3 Acrylic bone cements
10.3.1 Chemical composition
abCs are based on PMMa which is accepted as a biocompatible polymer
when cured. The bone cement is prepared by mixing two components, a
liquid phase and a powder phase.
∑ Liquid component: transparent, volatile and with a characteristic smell. its
viscosity is low and its boiling temperature is approximately 100°C at 760
mm Hg. Its density is 0.94 g cm
-3
. It contains three basic ingredients:
- MMA monomer (CH
2
C(CH
3
)COOCH
3
): 97% v/v approximately;
- n,n-Dimethyl-
p
-toluidine (
p
-N(CH
3
)C
6
H
4
CH
3
): 2.7% v/v
approximately; acts as accelerant of the polymerization reaction
which activates the initiator mixed with the powder;
- Hydroquinone (OHC
6
H
4
OH): 750 ppm; acts as an inhibitor, which
prevents the premature polymerization of the monomer.
∑ Solid component: it contains three basic ingredients:
-
PMMa ([CH
3
C(CH
2
)COOCH
3
]
n
): 89% w/w approximately. In some
instances, instead of PMMa beads, other polymers or copolymers
are used. The diameter of most PMMa particles in the cement
ranges between 30 and 150 µm and their shape depends on the
manufacturing process used;
- Benzoyl peroxide (COC
6
H
5
OOCC
6
H
5
O)(BP): 0.75% w/w approxi-
mately; acts as initiator, producing free radicals when it reacts with
the amine n,n-dimethyl-
p
-toluidine (DMT) of the liquid phase;
- Barium sulphate (BaSO
4
) or zirconium dioxide (ZrO
2
): 10% w/w;
added in order to obtain radiopacity.
