Biomedical Engineering Reference
In-Depth Information
Chapter 14
Isoelastic Polyaryletheretherketone
Implants for Total Joint Replacement
Steven M. Kurtz Ph.D., Judd Day Ph.D., and Kevin Ong Ph.D.
potentially contributing to implant loosening. With
metallic femoral stems in total hip replacement,
long-term bone adaptation occurs along the entire
length of the stem, with bone loss usually adjacent to
the proximal region of the stem (
Fig. 14.1
). Bone
adaptation also occurs behind metal-backed
acetabular components and the femoral components
of total knee arthroplasty.
To reduce stress shielding and associated bone
loss around metallic orthopedic implants,
researchers began investigating more compliant
polymer composites in the 1960s
[4,5]
. fan
implant could be fabricated with an elastic stiff-
ness comparable to bone,
14.1 Introduction
In the middle and late 19th century, there was
a convergence of ideas between engineering and
biology as pioneers such as Julius Ward (1838),
Wertheim (1847), von Meyer (1867), Culmann
(1866), and Rauber (1876) studied the mechanical
properties of bone and noted that the structure of the
trabecular bone followed the lines of stress, exhibit-
ing similarities to engineering structures. This led to
the development of a theory proposed by Julius Woff
(1870, 1892) and refined by Roux (1885) relating
form to the function of bone, wherein the bone
responds and remodels in accordance with its local
stress environment
[1
e
3]
. The theory of bone
remodeling, or “Wolff's Law” as it later came to be
known, established one of the central tenets of bone
biology and remains, to this day, a fundamental
principle of biomechanics.
This theory still has implications for the design of
modern prostheses. When a prosthesis is implanted
adjacent to bone (in the case of a fracture fixation
plate) or inside a bone (in the case of a hip stem), the
medical device provides structural reinforcement,
and, as a consequence of sharing the load, results in
a decrease in the stresses being subjected to the
surrounding tissue. The resulting phenomenon of
“stress shielding” of bone around metallic implants
has long been appreciated and continues to be
researched, especially in connection with orthopedic
implants. In response to stress shielding, the peri-
prosthetic bone will tend to slowly resorb and
redistribute over time, complicating revision and
the so-called “
iso-
elastic
” prosthesis should promote higher stresses
in the bone and thereby decrease stress shielding.
This is extremely important for the treatment of
young, active patients. Since the service life is
finite for any total hip, it is foreseeable that these
patients will need revision during their lifetime.
Revision surgery can be substantially complicated
by lack of bone
stock secondary to stress
shielding.
For many years, the pursuit of isoelastic hip
implants has been a Holy Grail for implant designers.
As we shall see in this chapter, the quest for iso-
elasticity has been fraught with setbacks for hip
stems. Today, isoelastic hip implants are a clinical
reality, but it must be emphasized that they required
decades of development and remain in the interme-
diate stages of clinical evaluation. Polymer
composites, including composites fabricated using
polyaryletheretherketone
(PEEK),
have
been
