Carbon Nanotubes in Acrylic Bone Cement - Biologically Responsive Biomaterials for Tissue Engineering - page 173

Fig. 8.1 Number and
fixation type of primary
THRs preformed in Sweden
from 1979 to 2007 [ 43 ]

Number of primary THRs
per type of fixation, 1979-2007

15,000

Resurfacing implant
Reversed hybrid
Hybrid
Uncemented
Cemented

14,000
13,000
12,000
11,000
10,000
9,000
8,000
7,000
6,000
5,000

4,000
3,000

1992-2007:
Male.......39.8%
Female...60.2%

2,000
1,000
,0 79 81 83 85 87 89 91 93 95 97 99 01 03 05 07

mixed immediately prior to implantation during surgery and delivered directly to
the implant site. The compositions of the main commercial bone cements are sum-
marised in Table 8.1 , showing variations in chemical composition.
Other cements may also contain antibiotics (e.g. gentamicin sulphate [ 34, 52 ] ) in
order to improve the body's response to the implant, reducing the risk of subsequent
infection and implant rejection.

8.2.2 Polymerisation Reaction

PMMA is an amorphous polymer, which is plasticised on the addition of the monomer
methyl methacrylate (MMA). When bone cement is mixed two processes occur. Firstly
the monomer is absorbed by the PMMA beads and secondly, a free radical polymerisa-
tion reaction occurs [ 47 ]. This reaction is shown schematically in Fig. 8.2 .
During this reaction the Dimethyl P Toluidine (DmpT) causes the benzoyl per-
oxide (BPO) to decompose leaving a benzoyl radical, and a benzoyl anion (Fig. 8.2a ).
These benzoyl radicals then initiate the polymerisation of the MMA by combining
and forming an active centre (Fig. 8.2b ). These active centres then combine with
multiple molecules to form a polymer chain (Fig. 8.2c ). This reaction forms a vis-
cous fluid allowing the polymerising cement to be moulded as required. This is the
stage when the orthopaedic surgeon would inject the bone cement into the prepared