Biomedical Engineering Reference
In-Depth Information
length and the geometry of the crack, and
A
and
m
are constants which
depend on the material and the environment.
10.3.4 Some factors affecting the microstructure
and mechanical properties of acrylic
bone cements
ABCs can be interpreted as multi-phase materials consisting of PMMA
beads, polymerized monomer and radiopacifier particles, as represented in
Fig. 10.2(a). In addition, there are some factors related to mixing technique,
the presence of blood, grease or body fluids, or possible delaminations
produced when introducing the cement in the bone cavity, which will affect
the bulk and the interfacial microstructure of the cements and consequently
their mechanical behaviour.
Porosity
The presence of porosity in bone cements depends strongly on the mixing
technique used. it is convenient to reduce the porosity of bone cements in
order to improve the mechanical performance of abCs and, in fact, the
improvement of the mixing techniques has undergone a fast and spectacular
evolution. The techniques used to reduce porosity include mechanical or
ultrasound mixing, pressurization of the cement, centrifugation of the mixture
and vacuum mixing. All these techniques result in a reduction of porosity
from about 8% which is achieved by conventional hand mixing to values
below 1% for vacuum mixing. Good reviews and discussions about the
results on the fight against porosity have been published.
63, 64
Although most of the reported results show that mechanical properties
improve as porosity is decreased, some authors find that certain mechanical
properties do not improve at all, mainly fracture toughness and the fatigue
crack propagation rate.
63, 65
The main argument used to explain the detrimental
effect of porosity is that pores act as stress concentrators. However, it has
to be borne in mind that under a tensile state of stress, polymers have a
maximum inherent flaw size which will control the fracture load of the
material. On the other hand, the main deformation mechanism of PMMA
in tension is crazing and therefore, the crack will propagate with a Dugdale
plastic region ahead of it, which will account for the crazed region. This
model of crack propagation in bone cements would explain that the size of
the largest pore in the plane of the advancing crack is more important than
porosity. A pore can blunt the tip of the crack, depending on the Dugdale
zone size and the size of the pore.
