Biomedical Engineering Reference
In-Depth Information
All coatings must adhere satisfactorily to the underlying substrate
irrespective of their intended function. Specifically, a mechanical
stability of calcium orthophosphate coatings should be high enough
to maintain their bioactive functionality after a surgical implantation.
Generally, tensile adhesion testing according to standards such
as ASTM C633 is the most common procedure to determine the
quantitative values for calcium orthophosphate coating adhesion to
the underlying metallic substrates. Furthermore, fatigue [553, 554],
scratch [555] and pullout [555] testing are among the most valuable
techniques to provide additional information on the mechanical
behavior of calcium orthophosphate coatings [194].
Already in the 1980s, de Groot et al. published a paper [556] on
the development of plasma-sprayed HA coatings on metallic implants.
A little bit later, Furlong and Osborn [557], two leading surgeons in
the orthopedics field, began implanting plasma-sprayed HA stems
in patients. Coated implants combine the surface biocompatibility
and bioactivity of calcium orthophosphates with the core strength
of strong substrates (Fig. 4.10). Moreover, calcium orthophosphate
coatings decrease a release of potentially hazardous chemicals from
the core implant and shield the substrate surface from environmental
attack. In the case of porous implants, calcium orthophosphate
coatings enhance bone ingrowth into the pores [233]. The clinical
results for calcium orthophosphate-coated implants reveal that
they have much longer life times after implantation than uncoated
devices and they have been found to be particularly beneficial for
younger patients. Studies concluded that there was significantly less
pin loosening in the HA-coated groups [558]. HA coating as a system
of fixation of hip implants was found to work well in the short to
medium term (8 years [559], 10 to 15.5 years [560], 10 to 17 years
[561], 15 years [562], 16 years [563], 17 years [564], 19 years [565]
and 15 to 21 years [566]). In 2004, a special book summarizing the
studies with HA-coated implants and the “state of the art” of HA
coatings in orthopedics at the close of 2002 was published [567].
Similar data for HA-coated dental implants are also available [568,
569]. Even longer-term clinical results are awaited with a great
interest.
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