Polymers - Orthopaedic Biomaterials in Research and Practice - page 157

going through a conservative adoption period. Isoelastic hip stems made
of PEEK have been cleared by the US Food and Drug Administration,
and PEEK acetabular cup designs have received CE mark approval and
have had limited clinical use. At this time, more long-term data are nec-
essary to fully understand the clinical performance of these systems.
resorbable polymers
The polymers discussed so far are intended to retain their shape and their
essential properties after implantation. However, there are a number of
applications in which it would be desirable to have properties change or
even to have the material completely disappear with time. This principle
has been long recognized in the use of absorbable sutures in deep tissue
sites. One of the most challenging of these potential applications is in
internal fracture fixation. It would be ideal to have a device that would
slowly weaken and eventually disappear, transferring load to the healing
bone and encouraging maximal Wolff's law remodeling.
There have been a number of attempts to produce such materials. The
most promising absorbables today are alloys of poly(glycolic acid) (PGA)
and poly(lactic acid) (PLA), particularly copolymers poylglycolide-
co-polylactide and poly-l-lactide-co-dl-lactide. PLA is naturally degraded
by hydrolysis in vivo , whereas PGA persists unchanged for long periods.
Thus, alloys of these two materials may be produced that lose structural
integrity over periods varying from a few weeks to 6 months. PLA-PGA
alloys are very attractive as resorbable implant materials because their
degradation products are indistinguishable from naturally occurring
organic molecules and the evidence is that these products are handled
through normal catabolic pathways.
Intentional degradation of the implant takes place through random
hydrolysis of the polymer chains, enhanced by enzymes. Degradation can
be affected by factors such as polymer molecular weight, molecular ori-
entation, monomer concentration, presence of low-molecular-weight com-
pounds, geometric isomerism, crystallinity, conformation, surface area/
weight ratio, porosity, and site implantation. Generally, amorphous copoly-
mers will disintegrate more rapidly than more crystalline compositions.
As the polymeric implant begins to fragment, its molecular weight and
strength properties are diminished. During this process, lactic acid and
glycolic acid are metabolized into water and carbon dioxide. Eventually,
macrophages will fully digest the polymeric debris. Complete resorption
of commonly sized implants is highly variable depending on the polymer
and may take place in between 8 months and 5 years.
The primary advantage of using resorbable material for the fixation
of bone fractures is the elimination of a permanent metallic foreign
implant. This is advantageous for reasons including reducing the risks
of implant migration, stress shielding of the underlying bone, inter-
ference with computed tomography and magnetic resonance imaging
scanning, and other aesthetic implications associated with removal of
the devices. Unfortunately, the current perception is that bone plates