Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 176

is the time during which the surgeon must insert the stem and adjust its position.
Finally, the setting time is the time from the onset of mixing until the surface tempera-
ture reaches one-half of the maximum temperature, as described in ISO 5833:2002.

8.2.3 Thermal and Chemical Necrosis

As mentioned previously, in vivo temperatures during the exothermic polymerisa-
tion of bone cement can potentially cause thermal necrosis (tissue death) of the bone
and impaired local blood circulation, which can lead to early failure through aseptic
loosening of the implant [ 37 ]. It has been reported that for epithelial cell death to
occur, an exposure time of 1 s is required for temperatures above 70 °C, 30 s for
temperatures greater than 55 °C and approximately 5 h for temperatures greater
than 45 °C [ 90 ]. Collagen protein molecules are denatured at 45 °C, and experience
irreversible damage at 60 °C if held at these temperatures for an hour. It has also
been reported that thermal necrosis occurs in bone tissue when exposure is greater
than 1 min for temperatures above 50 °C and denaturation of sensory nerves occurs
for temperatures above 45 °C if exposure exceeds 30 min. The amount of heat gen-
erated during polymerisation is dependent on the amount of reacting monomer;
however the maximum temperature reached is also dependent upon the rate of heat
dissipation. In vitro testing completed by Stanczyk and van Rietbergen [ 89 ] sug-
gested that the tips of bone trabeculae protruding into setting cement may experi-
ence temperatures in excess of 70 °C. In the 1960s, upon the first use of bone cement
in TJR surgery, Charnley believed that while temperatures of ~100 °C could be
reached during polymerisation, in the presence of a metallic prosthesis, which
would act as a heat sink, there was a reduction in the peak temperature experienced
in vivo [ 13 ]. Since then, numerical simulations and in vitro studies of thermal necro-
sis and peak exotherms in TJR have helped to establish two methods which may
assist the reduction of thermal necrosis: (a) the use of thin cement mantle layers and
(b) pre-cooling of the bone surface [ 14, 27 ] .
An additional potential adverse consequence of using standard acrylic bone
cements is the leaching of residual liquid monomer into the surrounding tissue, which
may cause inflammation, chemical necrosis and even death. Average levels of resid-
ual monomer can be as high as 5 %; however local concentrations may be as high as
15 %, increasing the likelihood of chemical necrosis [ 89 ]. Vacuum mixing of acrylic
bone cement has been associated with reduced levels of residual monomer as mixing
bone cement at reduced pressures increases monomer polymerisation [ 20 ] .

8.2.4 Mechanical Properties

The main role of bone cement is to transfer load between bone and the orthopaedic
implant. Several studies have shown that the composition of acrylic bone cement
significantly influences the mechanical properties of the cement [ 33, 51 ] . It is during