Polymers - Orthopaedic Biomaterials in Research and Practice - page 148

the ductility and fracture resistance of a material. Small punch
testing uses a die and guide system to constrain small disc-shaped
samples before subjecting the samples to a bending-by-indentation
load (constant displacement) until the failure of the material.
Typically, a load-displacement curve is determined during the
test, from which the modulus of elasticity, ultimate load, ultimate
displacement, and work-to-failure can be derived. Although this
method is actually a shear test, tensile properties can be obtained
by calculation.
Bone cements
The so-called bone cements (BCs) are derived from PMMA materials
that had long been used in dental applications, primarily for construction
and repair of dentures. Sir John Charnley is generally credited with the
first significant use of this material for the support of medullary por-
tions of total hip replacements, beginning in 1958, although there is a
long history of applications in general surgery and, dating back into the
1940s, in orthopaedic surgery. Despite many small changes in the mate-
rial itself and in its use, it remains essentially the same as that used by
Charnley, except for the introduction of barium sulfate (BaSO 4 ) to ren-
der it radiopaque.
The original role intended for BCs was as a “lute”: as a space-filling
material that could be interposed between a metal prosthetic stem and
bone to distribute the stress, thus preventing the local stress concentra-
tion and bony necrosis attendant to early, noncemented device designs.
A second goal was to reduce pain by stabilizing, that is, by “fixing,” the
prosthesis relative to bone. PMMA was not intended to be adhesive and
is not; thus, the continued shorthand term bone cement is a misnomer.
Other, truly adhesive materials have been used experimentally in this
application; they are discussed elsewhere in the consideration of fixation
(see Chapter 11).
There are a number of commercial BCs, all based on PMMA.
Although they differ slightly in composition and properties, it is pos-
sible to speak of them in a generic way. There is no convincing evidence
that these differences, although in some cases requiring alterations of
surgical technique, produce differences in surgical outcome. The resul-
tant material is a dull, opaque pink or white mass. Although (except for
fillers) it is chemically identical to the familiar clear Plexiglas (Rohm &
Haas, Philadelphia, Pennsylvania) or Lucite (DuPont Co, Wilmington,
Delaware), its opacity is due to filler and air inclusions and to a relatively
low crystallinity. Typical molecular weights are 1 × 10 5 to 5 × 10 5 .
The typical PMMA BC is supplied in a sterilized two-part kit, gener-
ally containing a 40 g container of a dry component and a 20 ml vial of a
liquid component. The dry component is usually 10 weight percent (w/o)
BaSO 4 with the balance consisting of a fully polymerized microspheric
(10 -30 μm) powder of PMMA or PMMA-poly(styrene) copolymer. This
component also contains a very small quantity of a free-radical source