Polymers - Orthopaedic Biomaterials in Research and Practice - page 156

implants, as was the case with early silicone rubber heart valve poppets
(balls). Although evidence for such absorption has been found, there is
no convincing evidence that it has contributed to device failure.
Commercial silicones routinely use initiators containing toxic metals
such as lead and tin. As a result, all silicones used in biomedical applica-
tions must be custom compounded to assure purity and nontoxicity; all
of these materials are, de facto, medical grades.
PeeK
Polyarlyetheretherketone (PEEK) is a biocompatible polymer that has
generated interest in orthopaedic, trauma, and, particularly, spinal appli-
cations. PEEK is associated with few toxic, inflammatory, or allergic
reactions, and there is no expected cytotoxicity, mutagenicity, or immu-
nogenicity. It also has a very high chemical resistance, being insoluble
in common solvents and is resistant to attack by all substances except
sulpheric acid.
The turning point with the use of PEEK for medical applications is
captured within its stiffness and strength properties. PEEK is often used
to capitalize on its “isoelastic” potential, creating implants with stiffness
close to matching that of bone and potentially reducing adverse effects
of stress shielding. While unmodified PEEK has a flexural modulus of
only 4 GPa and a strength of 93 MPa, its stiffness and strength can be
modified through annealing and carbon fiber-reinforced (CFR) filling,
hydroxyapetite (HA) filling, and HA coating. Although there are many
variations to CRF PEEK, and alterations to mechanical properties can
be quite complex, the virgin material can be modified to have a flex-
ural modulus of up to 21 GPa and a tensile strength of 225 MPa. There
is some anisotropy that is introduced through the fiber reinforcement.
When compared to other high-performance thermoplastic materials of
polybutylene terephthalate and PS, PEEK has been found to have the
highest fracture toughness and bending fatigue resistance, properties
found to be insensitive to preconditioning and thermoforming. Its supe-
rior fatigue resistance is understood to be derived from its intimate bond
with carbon fibers, and the matrix-fiber interfacial bond is enhanced by
the creep resistance of PEEK.
PEEK is thermally stable at sterilization temperatures and will not
degrade during either electron beam or gamma irradiation. PEEK will
not hydrolyze and the degree of water uptake can be reduced by increas-
ing the volume fraction of fiber-reinforced material as carbon fibers do
not absorb water and do not degrade under physiological conditions. If
sterility is to be achieved through gamma radiation, neither carbon fibers
nor virgin PEEK degrade under gamma irradiation. In general, PEEK is
resistant to all common sterilization techniques.
While its use in arthroplasty and trauma applications has been limited
thus far, PEEK has found more widespread use in spinal interbody fusion
cages, posterior dynamic spinal stabilization devices, arthroscopic suture
anchors, and cranial defect repair. PEEK in orthopaedics is currently