Biomedical Engineering Reference
In-Depth Information
leads to a combination of cross-linking and chain scission. cross-linking is
beneficial for reducing wear. Chain scission produces a decrease in molecular
weight, with a concomitant reduction in wear resistance and mechanical
properties. thus, it is very important to control the conditions under which
gamma irradiation is used. For instance, when irradiation is conducted in
the presence of oxygen, scission predominates over cross-linking. However,
when conducted in an inert environment, such as nitrogen, cross-linking
predominates over scission.
22
the cross-linking of UHMWPe increases the wear performance of the
material. However it adversely affects uniaxial ductility and the uniaxial failure
strain of UHMWPe decreases linearly with increasing radiation dosage.
23
even though there are a few disadvantages of cross-linked UHMWPe, it
is now by far the most widely used alternative to conventional UHMWPe,
especially in the UsA.
8.3 Acrylic polymers as bone cement
the acrylic polymers, or polyacrylates, are compounds with a high molecular
weight. their chemical and physical properties depend on the lateral
substituents of the polymeric chains. these materials have an excellent light
transmittance, making them good materials for intraocular lenses and contact
lenses, both hard and soft.
the most common acrylic compound used for bone repair is poly(methyl
methacrylate) (PMMA), also known as 'Plexiglas' (Fig. 8.3). PMMA is a
hydrophobic, linear chain polymer that is transparent, amorphous and glassy
at room temperature. it is obtained by an addition polymerization (chain
reaction) of methyl methacrylate (MMA) in the presence of an initiator
of the radical polymerization and an activator for the formation of free
radicals.
24
the PMMA is used as bone cement in joint arthroplasty. Bone cements
are used for the fixation of artificial joints. The cements fill the free space
between the prosthesis and the bone and constitute a very important zone.
owing to their optimal rigidity, the cements can evenly buffer the forces
acting against the bone. the close connection between the cement and the
CH
3
CH
3
CH
3
H
2
H
2
C
C
R
C
C
C
R
CH
2
n
COOCH
3
COOCH
3
COOCH
3
Methyl methacrylate
(MMA)
8.3
Chemical structure of monomer MMA and PMMA.
Poly(methyl methacrylate)
(PMMA)
