Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 188

static mechanical properties, in addition to significantly reducing the exotherm of
the polymerisation reaction. Ormsby et al. [ 68 ] stated that the dispersion of the
MWCNT is critical in improving the mechanical properties of the bone cement
composites. This is thought to be a significant finding as mechanical failure of the
bone cement mantle remains a major problem in joint replacement surgery [ 94 ] . As
is the case with typical fibre-reinforced composites, mechanical failure of PMMA
bone cement is thought to take place in three phases:

• Crack initiation due to an initial imperfection in material stability,

• Slow crack growth and/or

• Rapid propagation to fracture

MWCNT addition to the liquid monomer by magnetic stirring was shown to
have an overall negative effect on the mechanical performance of the bone cement.
This may be attributed to poor MWCNT dispersion within the liquid monomer,
resulting in MWCNT agglomerations within the cement matrix. Such agglomera-
tions acted as stress concentrations within the cement, providing a mechanism for
premature failure of the cement when subjected to a loading regime. In contrast, dry
blending of the MWCNT into the polymer powder or disintegrating the MWCNT in
the liquid monomer using ultrasonic agitation increased the disentanglement of the
nanotubes and allowed for a more homogeneous dispersion of the MWCNT in the
resulting nanocomposite. Andrews and Weisenberger [ 4 ] reported that ultrasonic
dispersion was an effective method for dispersion of MWCNT at low levels
(<5 wt.%) of concentration [ 4 ]. However, Marrs et al. [ 56 ] stated that care is needed
when dispersing MWCNT within a polymer matrix. Furthermore, they went on to
detail the adverse effects of inadequately dispersed clumps of MWCNT, particu-
larly at levels of loading greater than 5 wt.%.
The presence of well-dispersed MWCNT within the PMMA bone cement
resulted in a percentage of the MWCNTs being orientated with their longitudinal
axis perpendicular to the crack wave. Such MWCNTs were effective in bridging the
initial crack and preventing crack propagation, with the potential, in theory for fur-
ther enhancing the longevity of the cement mantle. These improvements reported
by Ormsby et al. [ 67 ] are clinically beneficial for the use of reinforced PMMA bone
cement in total joint arthroplasties due to a reduction in the rate of crack propaga-
tion. This improvement may be most important for improperly placed femoral
implants with thinner cement mantle layers, which continues to be cited as a factor
that may reduce joint replacement longevity [ 60 ] .
Previous studies have reported reductions in the thermal properties of PMMA
cement on addition of 5-15 wt.% steel fibres [ 45 ]. The importance of minimising
the exothermic reaction is significant, as it may result in a permanent cessation of
blood flow and bone tissue necrosis, which shows no sign of repair after 100 days
[ 23, 25, 58, 59 ]. The cumulative thermal necrosis index (TNI) has been used previ-
ously to assess the level of irreparable damage bone cement caused by heat genera-
tion [ 20, 59 ]. If the TNI value exceeds one, then there is the possibility of thermal
damage to the living tissue cells. The TNI was calculated for two temperatures,