Biomedical Engineering Reference
In-Depth Information
Clear evidence was shown of significantly different cell proliferation rates on the surface
of the nanostructured HA coatings and the coatings with more crystalline HA exhibit
higher cell proliferation rate (Li et al. 2006). Surprisingly, it was found that since increased
surface area undoubtedly furthers cell proliferation, it can be claimed that the nanostruc-
tures play a less important role than the phases on the attachment/proliferation of the
cells (Li et al. 2006), even though some researchers found that dissolution of the Ca-rich
phases was accompanied by good cell attachment (Wang et al. 2004) and high prolifera-
tion rate (Kim et al. 2004). Although it was reported that there was increased adhesion
with nanophase materials due to the increased surface boundaries (Webster et al. 2004),
the bioactivity of the coatings is more dependent on phases than on the nanostructures (Li
et al. 2006). Nevertheless, the HA coatings with high content of both crystalline HA and
nanostructures are favorably preferred for their biomedical applications.
It has been reviewed that a number of proteins played important roles in mediating the
proliferation/differentiation of the osteoblasts and their content changed with the pro-
liferation/differentiation process (Stein et al. 1993). It was revealed that the shape of the
nanosized grains at the HA coating surface affects the attachment and proliferation of
osteoblast cells and nanosized pores at coating surface promote cell attachment (Li et al.
2007b). Therefore, the nanosized pores instead of the nanosized grains might play the key
roles in enhancing the attachment/proliferation of the osteoblast cells.
Even though the resorption, fracture, fatigue, or nanostructure/microstructure of HA
coatings have been extensively studied, it should be emphasized that the characteristics of
all HA-based coatings are not always the same, and this may affect the chemical, physical,
and mechanical properties of the coating. Therefore, a study on the general factors that
can influence HA coating properties needs to be conducted. Furthermore, other variables
for evaluating the mechanical properties and the intrinsic factors that influence the per-
formance of HA coatings need to be proposed. Generally, HA coatings do not experience
pure tensile stress in vivo, while compound stresses are experienced (Haman et al. 1997).
In clinical applications, the failure of implantation is usually attributed to many factors.
The success or failure of an implant is directly influenced by (Williams et al. 1987):
• The thickness and nature of the implant
• The presence or absence of infection, inflammatory, or foreign body giant cell
reactions
• The reactivity of the implant (bioactive, bioresorbable, etc.)
• The structure of the implant (hardness, porosity, crystallinity, etc.)
The major cause of failure of the implantation (apart from infection) is implant loosening
(Bonfield 1987). Extensive work has been conducted on the properties of HA coatings in
vivo that mostly clarified the effect of implantation duration on the mechanical proper-
ties of the implants. Table 4.3 shows previous results on interfacial mechanical strength
between bone and HA coating after some time of implantation.
It demonstrated that after long-term implantation, such as 24 weeks, the HA coating
would be bonded with bone together firmly, which was indicated by the relatively high
bonding strength and the corresponding failure location, coating/substrate interface. And
it was revealed that HA-coated implants showed significantly higher push-out strength
with bone than dense HA (Ogiso et al. 1998a). Generally, the adhesion of the implants
with bony tissue is realized through either mere close (molecular-scale) approximation of
tissue and implant or a chemical alteration of the implant surface, a true bonding process
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