Biomedical Engineering Reference
In-Depth Information
4.5.2
Coatings
For many years, the clinical application of calcium orthophosphate-
based bioceramics has been largely limited to non-load bearing
parts of the skeleton due to their inferior mechanical properties.
One of the major innovations in the last ~30 years has been to coat
mechanically strong bioinert and/or biotolerant prostheses by
calcium orthophosphates [525-527]. For example, metallic implants
are encountered in endoprostheses (total hip joint replacements)
and artificial teeth sockets. The requirement for a sufficient
mechanical stability necessitates the use of a metallic body for such
devices. As metals do not undergo bone bonding, i.e., do not form a
mechanically stable link between the implant and bone tissue, ways
have been sought to improve contacts at the interface. The major
way is to coat metals with calcium orthophosphate bioceramics that
exhibit a bone-bonding ability between the metal and bone [194,
205, 339, 525, 528-533]. Thickness of the coatings varies from
submicron dimensions to several hundreds microns (Table 4.4)
and this parameter appears to be very important. For example, if a
calcium orthophosphate coating is too thick, it is easy to break. On
the contrary, if the coating is too thin, it is easy to dissolve, because
resorbability of HA, which is the second slowest to dissolve among
calcium orthophosphates (Table 1.1), is about 15-30 μm per year
[534]. One should stress, that calcium orthophosphate coatings are
not limited to metals only; they can be applied on carbon, bioinert
ceramics and polymers as well [535]. Furthermore, multi-walled
carbon nanotubes can be functionalized by deposition of calcium
orthophosphate coatings [536]. The list of most important coating
techniques is comprised in Table 4.4, while the main advantages and
drawbacks of each coating technique, as well as the major properties
of the deposed calcium orthophosphates, are discussed in details
elsewhere [194, 240, 295, 525, 526, 537-551]. Unfortunately, none
of these methods is able to provide the perfect covering because each
coating always contains cracks, pores, second phases and residual
stresses that reduced their durability and might lead to a partial or
complete disintegration of the coating in body fluids. The biomedical
aspects of osteoconductive coatings for total joint arthroplasty have
been reviewed elsewhere [552].
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