Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 175

100

T max

90

80

70

60

T set

50

40

30

T amp

20

Working Time

10

Dough Time Setting Time

0 0

500 1000 1500 2000 2500

Time (secs)

Fig. 8.3 A typical curing curve for acrylic bone cement where T max is the maximum temperature
reached, T set is the setting temperature and T amb is the ambient temperature

bone canal prior to implanting the stem. As the monomer begins to polymerise, the
cement hardens around the orthopaedic implant, holding it in place. This reaction is
highly exothermic; an example of a temperature plot of bone cement during
polymerisation is shown in Fig. 8.3 .
The heat energy produced during polymerisation is 57 kJ per mole MMA, result-
ing in temperatures, which can exceed 100 °C. These elevated temperatures can
cause cellular bone necrosis and contribute ultimately to aseptic loosening [ 20, 47,
89 ]. It should be noted though that the polymerisation temperatures experienced
in vivo are much lower (between 40 and 47 °C) at the bone interface [ 93 ] . This is due
to reduced thickness of bone cement mantle, the presence of blood circulation and
the dissipation of heat through the implant and surrounding tissue [ 47 ] . It has been
shown that volumetric shrinkage can occur due to thermal contraction on cooling and
the changing density as polymerisation progresses [ 30, 47 ] . Gilbert et al. [ 30 ] reported
that volumetric shrinkage as a result of density variation, due to the exothermic
polymerisation, was between 5.1 and 6.5 %, depending on mixing method employed
and type of cement. Both shrinkage mechanisms have been identified as factors
which influence the levels of residual stresses within the cement [ 30, 69 ] .
As illustrated in Fig. 8.3 , the time that has elapsed after initial mixing when the
cement takes a homogeneous dough-like state is known as the “ dough time ”. This
point may be identified with temperature or average molecular weight of the polymer.
However, as specified in the British Standard BS 7253 (ISO 5833:2002) [ 11 ] , it is the
point at which the cement will no longer stick to powderless surgical gloves (typically
2-3 min after initial mixing). The time from the end of dough time until the cement
can no longer be manipulated is defined as the working time. During an operation this