Biomedical Engineering Reference
In-Depth Information
Macrophages are believed to play a key role during osteolysis (shown in
Figure 7.1). But there exists a critical number of active macrophages necessary for
osteolysis since only one disturbed macrophage and its neighboring osteoclast
would not cause osteolysis. For instance, clinical experience indicates that oste-
olysis cannot happen if the wear rate of the polyethylene cup is below 0.05 mm
per year. In contrast, osteolysis frequently occurs if the wear rate exceeds 0.2 mm
per year [30]. Moreover, studies have also illustrated that there exists a critical
size for bioactive particles that can trigger cellular responses and fi nally result in
osteolysis. Macrophages have evolved to detect and phagocytose bioactive parti-
cles in the size range of 0.5 - 10
μ
m. Particles larger than 10
μ
m or less than 0.5
μ
m
are relatively less active for macrophages. Particles larger than 10
m will cause
giant cell responses which lead to the formation of fi brous tissue (see details in
7.2.2.1) without osteolysis. In addition, particles with sharp edges are more active
than spherical particles [20,29].
It is apparent that minimizing the generation of wear particles in the interfa-
cial tissues is a basic approach to preventing osteolysis. Therefore, there has been
much effort to design alternative wear-resistant bearing materials in order to
reduce the production of wear particles from the bearing area. The metal-on-
metal, ceramic - on - ceramic or ceramic - on - polyethylene implants have been
developed to prevent osteolysis and have been shown to produce less wear
particles in vitro or in vivo than the conventional metal- on - polyethylene materi-
als [138-139]. For example, a fi ve-year prospective randomized study in the U.S.
revealed that osteolysis greatly decreased from 14.0% of patients with metal-
on-polyethylene bearings to 1.4% patients with alumina-on-alumina ceramic
bearings [139]. In addition, a variety of nanocomposites (such as zirconia-tough-
ened alumina nanocomposites [140] , ceria - stabilized tetragonal zirconia - alumina
nanocomposites [141] or hydroxyapatite coated ceria-stabilized tetragonal
zirconia-alumina nanocomposites [142]) have shown promise as bearing materi-
als with excellent cytocompatibility and low wear rates. Furthermore, metal-on-
metal bearings also exhibit low wear rates (such as 0.004 mm per year compared
with 0.1 mm per year for polyethylene [138]).
But there still exists many concerns about the generation of metal implant
ions or corrosion for metal combinations, which may lead to implant failure in the
long term. Recent studies reported that special surface coatings (i.e., TiN, CrN or
diamond-like carbon coatings) on titanium alloys may improve the performance
of metal - on - metal bearings [143 - 144] . Moreover, except designing novel artifi cial
joint materials with more wear-resistant features, there are other possible strate-
gies that may address the problem of osteolysis, including preventing debris from
accessing the bone-implant interface through extensive bone growth and, thus,
decreasing cellular reaction to such wear debris [26].
Some preliminary results of the effects of nanoparticle wear debris on osteo-
blasts and on other cells have been reported [31-33]; such studies showed a less
adverse effect on osteoblast viability for nano compared to micro particle wear
debris. Although the effects of nanoparticulate wear debris at the bone-implant
interface are not totally understood, to date, the biologically-inspired features of
μ
Search WWH ::
Custom Search