Corrosion and degradation - Orthopaedic Biomaterials in Research and Practice - page 311

processes. The primary mechanical consequences are due to changes
in plasticization (see Chapter 6). However, extreme degrees of absorp-
tion may produce a local swelling and stress-cracking phenomenon
called crazing . Polymers that exhibit crazing are generally not suited for
implantation, but an exception is the polyamides, which absorb lipids
avidly and swell up to 5 volume percent. They are useful as locking
inserts for screws in multipart internal fracture fixation devices.
Additionally, several processes actually change the molecular struc-
ture of polymers. Chain scission, either by free-radical depolymeriza-
tion or by hydrolysis, reduces molecular weight and polymer structural
integrity. These are the inverses of the two primary polymerization
mechanisms: addition and condensation polymerization, respectively.
Additionally, a number of agents, including oxygen free radicals and ion-
izing radiation, can produce cross-linking between molecules.
Most implant-grade polymers are selected for their resistance to
these degradation phenomena, in addition to other properties. However,
as implantation times increase, relatively low degradation rates may
become important. Even a stable material such as ultrahigh-molecular-
weight polyethylene (UHMWPE) exhibits molecular weight reduc-
tion, primarily associated with the presence of free radicals, after long
implantation times. Thus, an increased sensitivity to the possibility of
chemically mediated polymer degradation is necessary.

Biodegradation Biodegradation is a much overused term, frequently applied to any deg-

radation seen in vivo . In the limited, more correct use to describe degra-
dation promoted by enzyme systems, it is well recognized in materials
science but is a rare phenomenon in the patient. There is evidence for
biodegradation of PMMA, which produces naturally occurring cata-
bolic products, but it has not been suggested that this can produce sig-
nificant degradation of physical properties or any clinical consequences.
However, rapid bacterial degradation of resorbable polymers has been
reported; this may be secondary to local pH effects, but its true mecha-
nisms or effects are not entirely clear. Apparently, neither metals nor
ceramics undergo biodegradation in vivo , understood in this limited
sense, although metal-binding bacteria are well known and used indus-
trially in ore beneficiation (enrichment).

additional problems

PROBLEM 12.4
Immunity (select the phrase that best completes the sentence)

A. May be produced by vaccination against stainless steel

B. Is inherent in a metal in a given environment and may provide
improved fatigue life

C. Results in the loss of metal from an implant

D. May cause implant rejection