Biomedical Engineering Reference
In-Depth Information
is differed according to aimed application circumstances of the implants, and many types
of SBF have been reported, such as 0.05 mol Tris(hydroxy)methylaminomethane-HCl (pH =
7.3) (Ducheyne et al. 1993), Hank's balanced salt solution (HBSS) (Reis et al. 1996), isotonic
saline solution (0.15M NaCl) (Reis et al. 1996), 37°C sterile Hank's physiologic (pH = 7.2)
balanced salt solution (Maxian et al. 1993a), Ringer's solution (Gross et al. 1994), Gomori's
buffer (Klein et al. 1994). Nevertheless, the most direct and effect way of validating the
biocompatibility of the thermal-sprayed HA coatings is to conduct cell culture experi-
ment. The SBF study is able to give helpful information about the coatings for further
investigation.
Different phases in the HA family exhibit different structures and could thus lead to
different biological responses in vitro. It was reported that the rate of cell proliferation was
highly dependent on the composition of HA powders and sintering characteristics (Best
et al. 1997). Because the coating phase composition is significantly dependent on the start-
ing powders, this indicates the effect of final coating composition. Generally, the in vitro
behavior of HA coatings mostly involves dissolution/precipitation processes. The varia-
tion of the crystal structure and stoichiometry of calcium phosphate ceramics produces
dissolution behavior that varies over a wide range. The dissolution rate of monophasic cal-
cium phosphates increases in the following order: HA < CDHA < OHA < β-TCP < α-TCP <
TTCP (Ducheyne et al. 1993), even though Maxian et al. reported that crystalline coatings
showed significantly greater Ca dissolution than amorphous coatings under some condi-
tions (Maxian et al. 1993a). It was also found that the dissolution of α-TCP phase in TCP
coating could favor the precipitation of the nonwell-crystallized apatite phase on the coat-
ing surface while for the HA coating the absence of dissolution would delay this precipita-
tion (Cleries et al. 2000). Results indicate the possible mutual effect among different phases
in vitro. It was revealed that the precipitation rate of bonelike apatite on their surface was
in the order of: β-, α-TCP (β-TCP contains some α-TCP) > HA > Ti > ACP (Cleries et al. 2000).
Another study also showed that poorly crystallized coatings resorbed faster and showed
greater surface film precipitation and greater chemical changes than amorphous coatings
(Maxian et al. 1993b). It suggests that the crystalline structure is indeed important in dic-
tating the differences in the rate of nucleation.
HA coating surface usually plays the most important role in overall dissolution/pre-
cipitation behavior. The underlying surface was not a significant factor in Ca dissolution,
but was significant for surface chemistry and morphological changes (Maxian et al. 1993).
Results already showed that that the precipitation rate was directly influenced by local
Ca
2+
concentration near to the coating surface and dissolution of certain phases accelerated
the precipitation of the bonelike apatite (Khor et al. 2003a).
It was revealed that the in vivo behavior of the prosthesis was significantly dependent
on its composition (Suominen et al. 1996) regarding degradability and bioactivity of dif-
ferent phases. Despite the content of ACP in HA coatings, it was revealed that once early
osteointegration was achieved, biodegradation of a bioactive coating should not be detri-
mental to the bone/coating/implant fixation (Maxian et al. 1993b). As well, the remodeling
process is fundamental for implant fixation and stability for a long term (Pazzaglia et al.
1998). Investigations of the dissolution-reprecipitation phenomena involved in the forma-
tion of HA bone bond have identified two joining modes (Mattioli Belmonte et al. 1998):
(1) bone tissue components bonded to HA through a recrystallization zone similar in struc-
ture to the reprecipitation layer, and (2) bone tissue components bonded directly to HA
crystals with no morphologically discernible signs of dissolution. And the reabsorption-
reconstruction cycles do not reduce the mechanical fixation of the prosthesis, and do not
attain the threshold of loosening (Hardy et al. 1999). Concerning the bonding of bone with
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