Biomedical Engineering Reference
In-Depth Information
become the main reasons necessitating revision surgery [6]. In other words, based
on clinical data collected by the Canadian Joint Replacement Registry (CJRR) in
2004, the most common reasons for revision of a total hip replacement were loos-
ening of the implant (55%), followed by osteolysis (33%), implant wear and tear
(30%), instability/dislocation (17%), bone and implant fracture (12%), and infec-
tion (10%) [7]. Since some patients have more than one reason necessitating
orthopedic implant revision surgery, the aforementioned percentages do not add
up to 100%.
7.2.2.1 Loosening of the Implant.
Generally, one crucial criterion for the
long-term success of orthopedic implants is the forming of suffi cient bonding of
the implant to juxtaposed bone—a process known as osseointegration which was
initially described as a direct structural and functional bone-to-implant contact
under load by Per-Ingvar Branemark [8]. Osseointegration plays an important
role in minimizing the motion-induced damage to surrounding tissues. Insuffi -
cient bonding to juxtaposed bone may lead to a mismatch of mechanical proper-
ties between implant materials and surrounding bone tissues [9].
As a result, a variety of implant-loosening conditions originating from stress
and strain imbalances may frequently take place. Several factors such as surface
roughness of the fi xture, overloading and infection [10,11] can be related to loss
of acquired osseointegration and even failure of osseointegration. Most com-
monly, surface properties of implant materials have been intimately related to the
success of osseointegration between an endosseous implant and bone. There are
numerous studies elucidating that surface roughness of implant materials affects
the rate of osseointegration and biomechanical fi xation [12 - 17] . Since traditional
orthopedic implant materials have been chosen based on their mechanical prop-
erties and biological inertness, there has not been enough concern about their
chemical structure and surface properties. Thus, loosening of implants (such as
for the socket or femoral component) has been frequently observed when con-
ventional micron-structured materials are used in total hip replacements.
On the other hand, the formation of a fi brous soft tissue capsule which origi-
nates from excessive secretion of fi brous tissue from infl ammatory cells is another
vital factor that leads to insuffi cient osseointegration and eventually causes loos-
ening of implants (Figure 7.1) [9,19].
After implanting biomaterials into a surgical site, it is inevitable to damage
the surrounding tissue and cause injury. Then, various host responses such as
infection and infl ammation are triggered (Figure 7.1). Initially, neutrophils and
macrophages arrive at the injury site and begin to attempt to phagocytose the
implant material; when they fail to engulf this large mass, they fuse together
to form giant cells and interrogate again the implant to wall it off from the
surrounding tissue. At the same time, giant cells send out chemical messengers
(cytokines) to recruit other cells to “help” them. In response to cytokines, fi bro-
blasts (which are in charge of secreting and synthesizing collagen into the extra-
cellular matrix comprising the soft tissue capsule [18]) arrive and generate a
collagen capsule, the fi rst sign of a new tissue. The fi nal stage of the process is
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