Polymers - Orthopaedic Biomaterials in Research and Practice - page 149

(to be acted on by the initiator in the liquid component); the most com-
mon is benzoyl peroxide. The liquid is methyl methacrylate monomer
(MMA) (≥97 w/o) with an initiator (typically N , N -dimethyl- p -toluidine)
and one or more stabilizers to prevent premature polymerization. The
powder is usually sterilized by cobalt gamma irradiation, whereas the
liquid is sterilized by ultrafiltration. Some BCs contain an antibiotic, in
dry form. This is most typically gentamicin, supplied at 5-10 w/o in the
dry component. In some BCs, the size distribution of the dry compo-
nent is carefully controlled to produce a lower viscosity during the early
stages of polymerization.
When the components are combined, an addition polymerization
process (“curing”) is initiated that produces long-chain polymers that
interpenetrate the previously polymerized microspheres and bind them
together into a single mass. There is also a significant amount of dis-
solution of low-molecular-weight species (from the dry microspheric
powder) and incorporation of them by polymerization. Nonetheless, the
interface between the microspheres and their new matrix remains and is
relatively weak; breaking BC after full curing shows microspheres and
their impressions on the resultant fracture surface. It is also of interest
to note that the newly formed polymer chains are essentially linear. In
addition, there is an almost total absence of cross-linking, either in the
initial powder or in the final compact.
BCs are designed to cure with essentially no change in volume, so
that prosthetic stems may be bedded in them without subsequent loosen-
ing owing to dimension changes. In fact, BCs shrink modestly, perhaps
0.5-1.0 volume percent (v/o) during curing, and then gradually expand
by perhaps 1-2 v/o over the first 30 days in vivo owing to water and lipid
absorption. The shrinkage is partially offset by transient thermal expan-
sion owing to heating during curing but is increased by the presence of
internal porosity.
Internal porosity in BCs has been a subject of some interest. When the
material is mixed traditionally, with a spatula in an open mixing bowl
in the operating room, the result will be 1-10 v/o porosity, depending on
technique and experience. Moderate mixing speeds with care taken not to
stir air into the cement mass seem to produce the lowest porosity. Various
treatments have been suggested, including centrifugation during the early
stages of curing and vacuum degassing of the dry component before mix-
ing, to improve this situation. These methods greatly reduce resultant
porosity and produce modest increases in ultimate tensile strength and
possibly in fatigue life of BCs. However, there is also laboratory evidence
that these treatments increase initial shrinkage by 1-3 v/o. Although
porosity reduction seems a “good thing” on theoretical grounds, there
is as yet no clear evidence of a clinical benefit. Furthermore, with the
addition of antibiotic powders in the BC, questions have also been raised
about the porosity of the BC after drug elution, as well as changes in the
mechanical properties before and after leaching of the antibiotics. Pores
and cracks within the BC have been reported after the elution of the anti-
biotics, although this may be primarily a surface phenomenon. Increasing
amounts of antibiotic powder has been shown to cause gradual decreases