Biomedical Engineering Reference
In-Depth Information
Table 10.3
Processing parameters that affect the properties of a bone
cement
Powder phase
Chemical composition
Relative proportion of the constituents
Additives (seeds, accelerants, retarders, etc)
Particle size distribution of the powder
Liquid phase
Additives (accelerants, retarders)
pH
Mixing parameters
Liquid/powder ratio
Mixing protocol (time, speed, etc)
Delay time before implantation
Environmental factors
Temperature
Humidity
pH
T
max
is the maximum temperature reached by the polymerizing cement
in °C.
27-29
The peak temperature is produced by exothermic propagation
reactions which take place during polymerization. The cement sets before
the peak temperature is reached. The time at which the mixed cement mass
no longer adheres to a surgically gloved finger is known as the dough time.
Finally, the difference between the setting time and the dough time is called
the handling time and corresponds to the period of time during which the
cement is workable and has to be implanted.
For the cement to be produced, it is necessary for the liquid monomer to
wet the powder particles of PMMa, which should then swell and allow the
diffusion of the liquid into the organic matrix of the particles. Simultaneously,
it is necessary for the initiator contained in the powder (bP) to be completely
diluted into the liquid. Monomer evaporation during these early stages of
mixing occurs and the clinical consequences have been considered.
30, 31
The
chain entanglement of the polymer powder and the polymerized monomer at
the interface ensures good adhesion between the polymerized monomer and
the PMMa particles in the set cement. Several physical processes, such as
swelling of the powder, diffusion of the monomer, dilution of the BP into the
liquid and polymer-polymer interdiffusion determine the setting dynamics
and the final properties of the ABC.
The bP-DMT reaction produces other free radicals or by-products different
from the benzoyl peroxide free radical. The kinetics of the free radicals
and their slow decay have been well studied.
32
It has been suggested that
unreacted radicals and residual components in the set mass may affect the
biocompatibility of the cement and its eventual degradation.
33
another very important residual component is the monomer which is left
unpolymerized after the cement has set and which excludes the unreacted
monomer that evaporated before the cement sets and cools down to its
