Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 178

allow a more accurate prediction of load transfer and in-service conditions in the
cement mantle [ 66, 76 ]. Stresses due to shrinkage exist in cement surrounding the
stem in both the longitudinal and the hoop direction [ 76 ]. These stresses will be at
their greatest immediately post-operatively, with a reduction occurring with time as
stress relaxation and creep occur [ 66, 91, 95 ]. This may create more favourable
stress distributions at the cement-bone and cement-prosthesis interfaces, as finite
element modelling of bonded and un-bonded stems predicts that an increase in com-
pressive stresses at these regions may occur [ 95 ]. The presence of pores at either
interface has been attributed to volumetric shrinkage of the cement during polymeri-
sation. Some polymerisation shrinkage will occur while the cement is still viscous
and hence can be accommodated by flow [ 69 ]. However, as polymerisation pro-
gresses, this flow reduces shrinkage and cracks can initiate in high-stress areas as a
mechanism of stress relief. While residual stresses do exist in fully polymerised
bone cement, the additional presence of porosity, high stress concentrations or
excessive heat generated during polymerisation may still be required for large cracks
to initiate, as residual stresses alone may not be enough to generate cracks [ 49 ] . The
ultimate tensile strength of various bone cements ranges between 24 and 49 MPa
[ 50 ], whereas residual stresses of between 2.5 MPa [ 66 ] and 12.6 MPa [ 69 ] have
been reported. The direction in which the cement will shrink is of great significance.
Orr et al. [ 69 ] reported that this has a direct relation to the levels of micro-cracking
that may occur as a result of shrinkage stress, although the use of acoustic emission
has provided evidence for the shrinkage of the cement onto the femoral implants
[ 76 ]. Furthermore, shrinkage is known to be affected by the volume fraction of
monomer content [ 30 ] .

8.2.7 The Role of Porosity

Lewis [ 50 ] identified four main reasons why porosity occurs in bone cements:

1. The entrapment of air between the polymer powder and monomer liquid as the
powder is wetted by the monomer upon mixing
2. Evaporation of the liquid monomer during polymerisation
3. Entrapment of air during mixing
4. Entrapment of air upon transfer of the dough into the cement gun (depending on
mixing method)

Reduced porosity allows for improved compressive, flexural and fatigue properties
of acrylic bone cement [ 20, 21, 50, 62 ]; therefore the level of porosity (both macro-
and micropores) should be minimised. Pores of diameter ³1 mm are deemed macro-
pores, and are generally introduced during the mixing process when air is trapped
within the cement mixture. These macro-pores are often cited as being the cause of
low fatigue life for test specimens as crack initiation is often associated with a single
pore. Micropores have diameters £1 mm, and may be established due to the evapo-
ration of the liquid monomer during the polymerisation process and/or entrapment