Biomedical Engineering Reference
In-Depth Information
Chapter 3
Compounds and Composite Materials
Stuart Green Ph.D.
3.1 Introduction
In addition to matching the stiffness of bone with
carbon compounds, CFR PEEK with continuous
fibers has excellent strength, fatigue resistance, and
durability. For example, research has shown that this
material is resistant to the effects of steam, gamma
irradiation, and boiling saline solution with no
significant effect on transverse flexural strength (the
weakest direction) [5] . Additionally, PEEK polymer
carbon composites have excellent compression
strength durability following conditioning in physi-
ological saline [6] . Importantly, research has shown
that CFR PEEK polymer is a strong and durable
composite material in the extremely aggressive
environment of the human body [7] . CFR PEEK
Carbon fiber-reinforced PEEK (CFR PEEK)
polymer has been of interest to the medical implant
community since at least the late 1980s [1] ,
substantially because of the material's versatility,
compatibility with modern imaging technologies,
excellent mechanical properties, and biocompati-
bility. The material is of particular interest to those
manufacturers who develop applications that interact
with bone, such as compression bone plates, spinal
fusion cages, intramedullary nails, and hip pros-
theses, which usually are made from metallic alloys
with a stiffness significantly higher than that of the
adjacent cortical bone by a factor of 10 e 20 times.
This stiffness mismatch can lead to a process of
bone resorption as a result of “stress shielding” [2] ,
and in extreme examples in the case of orthopedic
bone plates, it can lead to a reoccurrence of fracture
after the device is removed. In addition, composite
hip design and simulation studies show that
compared with cobalt chrome and titanium alloys,
composites with a more closely matched stiffness to
that of the surrounding bone offer a way of gener-
ating more favorable stresses and deformations [3,4] .
The Bradley hip, as shown in Fig. 3.1 , takes advan-
tage of the matched stiffness between bone and CFR
PEEK by combining the strength of a metal insert
with the stiffness of CFR PEEK to provide a tailored
hip prosthesis. This stiffness-matched solution offers
reduced bone resorption around the hip stem, which
can in the case of fully metallic stems lead to bone
weakening and eventual fracture. It is largely because
the stiffness of CFR PEEK can be tailored to more
closely match that of cortical bone that this material
is being used in applications for which stress
shielding may be an important clinical consideration.
Figure 3.1 “Bradley hip”dcarbon fiber-reinforced hip
prosthesis with hydroxyapatite coating on proximal
fixation position. Photo courtesy of Invibio.
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