Biomedical Engineering Reference
In-Depth Information
Physiological environment
The influence of body fluids and body temperature (37°C) properties is
significant in different properties of the cement. Uptake of liquids has been
shown to affect some of the properties of the cement after implantation.
Sorption of water generally lowers its mechanical properties.
61, 66-68
However,
fracture mechanics studies show that the crack velocity is slower in water
than in air and that fracture toughness is about 15-20% higher in water than
in air.
69
Other results show that the work to fracture increases with the time
of storage of the cement in liquids such as water, Ringer's solution and lipids,
although in each case the work to fracture after storage at 21°C is higher
than after storage at 37°C.
70
Water in abCs acts as a plasticizer.
Additives
The main additives usually found in bone cements are radiopacifier particles like
BaSO
4
and ZrO
2
, and antibiotics. It has been shown that these radiopacifying
agents have a significant effect on the mechanical properties of acrylic
bone cements, which depend on their size and morphology. Indeed, whilst
the addition of BaSO
4
produces a decrease in the tensile strength of about
10%,
34, 43, 71
the addition of ZrO
2
does not affect this parameter. On the other
hand, it has been reported that the fracture toughness, which is unaffected
by the inclusion of BaSO
4
, is increased by around 20% by the addition of
ZrO
2
.
72
Moreover, both inorganic radiopacifiers enhanced the fatigue crack
propagation resistance of the cement.
62, 72
This behaviour can be explained
in terms of the morphology of the radiopacifying agent particles and their
interaction with the polymeric matrix. Since there is no chemical adhesion
between the inorganic particles and the PMMA, it could be interpreted
that the filler particles behave like pores when a tensional state of stress is
applied to the cement. However, at this point the morphology of the particles
plays a determinant role with respect to the mechanical behaviour of the
cement. Indeed, the ZrO
2
particles, which have a cauliflower-like shape,
can anchor mechanically to the matrix, reinforcing it to a certain extent.
This effect does not take place in the case of the smaller and more regular
BaSO
4
particles.
72
Other studies have also shown that the addition of radiopaque agents to
PMMA enhances the macrophage-osteoclast differentiation and therefore they
may contribute to bone resorption and aseptic loosening.
73
Furthermore, these
agents evoke a significant pathological response in the surrounding tissue.
Barium sulphate has been shown to intensify the release of inflammatory
mediators in response to PMMa particles [74]. There is also evidence that
the release of the radiopacifier particles in the surrounding tissues can cause
damage to the articulating surfaces and a marked increased in the production
