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Engineering titanium surfaces for improving osteointegration

Authors Lu, Xiong
Issue Date 2004
Summary Titanium is one of the most important metallic biomedical materials in clinical applications. One of the key issues for successful application of titanium is the interaction at the interface between the titanium and the bone. The present study focuses on improving the surfaces of titanium to achieve better capability to bond with natural bone (i.e. better osteointegration). The objectives of this work include: 1.) Developing microfabrication methods to produce micropatterns on titanium surfaces for promoting osteointegration; 2.) Studying the calcium phosphate (Ca-P) formation on the chemical treated titanium surface and elucidating the mechanism of precipitation theoretically; and 3.) Evaluating osteoconductivity of engineering titanium surfaces in vitro and in vivo. Through mask electrochemical micromachining (TMEMM), jet electrochemical micromachining (Jet-EMM) and the confined etchant layer technique (CELT) were attempted to produce micropatterns on titanium surfaces. TMEMM has a high etching rate and good reproducibility and was used to produce micro-hole arrays on Ti plates for in vivo testing. A Jet-EMM system was successfully designed to fabricate microstructures on the titanium cylinder surfaces with a high aspect ratio. CELT application on Ti etching was successfully developed by carefblly selecting electrolyte (NaF + NaCl + H2O + Na2CO3 + NaHCO3). Formation of Ca-P on alkaline and heat-treated titanium surfaces was investigated using transmission electron microscopy (TEM). Electron diffraction of the precipitates revealed that octacalcium phosphate (OCP), instead of hydroxyapatite (HA), directly nucleates from amorphous Ca-P. The OCP crystals continuously grew on the titanium surfaces rather than transforming into apatite. Calcium titanate was also identified by electron diffraction. Its role in the formation of bioactive Ca-P, however, is not clear from the present study. The driving force and nucleation rate of Ca-P precipitation in simulated body fluid (SBF) were analyzed based on the classical crystallization theory. SBF supersaturation with respect to HA, OCP and DCPD (dicalcium phosphate) was carefilly calculated, considering all the association/dissociation reactions of related ion groups in SBF. The analysis indicates that the nucleation rate of OCP is substantially higher than that of HA, while HA is most thermodynamically stable in SBF. DCPD precipitation is thermodynamically impossible in normal SBF, unless calcium and phosphate ion concentrations of SBF increase. The influences of different SBF recipes, interfacial energies, contact angle and molecular volumes were evaluated and it was found that the parameter variations do not have significant impact on the analysis results. The effects of carbonate incorporation and calcium deficiency in HA were also estimated with available data. Generally, such apatite precipitations are more kinetically favourable than HA. Osteoconduction of Ti6A14V surfaces under various conditions, including micropatterned, alkali-treated, micro-patterned plus alkali-treated, and surfaces without any treatment, was evaluated. TMEMM was used to fabricate micro-hole arrays on the titanium alloy surfaces. In vitro Ca-P formation on titanium surfaces was in static and dynamic SBF conditions. In vivo comparison was conducted in the medullary cavity of a dog femur using the implant cages which could provide the same physiological environment for specimens with different surface conditions. In vitro experiments indicate good conduction of Ca-P on the alkali-treated surfaces, and also better Ca-P deposition on the micro-hole surface than did the flat surfaces in dynamic SBF. In vivo experiments confirm the beneficial effect of alkaline treatment on osteoconduction. The results of in vivo experiments also indicate a synergistic effect of the alkaline treatment and the topographic pattern on osteoconduction.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2004
Language English
Format Thesis
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