As Ti itself is not able to promote HA precipitation, attempts to alter its surface to promote HA nucléation have been numerous. These have largely involved pre-treatment (in solutions or through thermal oxidation) or the provision of a ceramic or glass coating rich in calcium and phosphate ions to enhance the biological response by achieving true bone bonding at the interface. Other strategies for chemical surface modification have focused on controlling the cellular response to the surface rather than HA formation. The following lists and highlights the most popular concepts of Ti modification, as the amount of work conducted in this area is beyond the limitations and the objectives of this section. A limited review of some of the clinical and/or laboratory findings is also given for the most commonly applied techniques:
a) Pre-treatment by
• Nitric acid passivation
• Calcium and phosphate containing solutions • Alkali solutions
• H2O2 treatment
Passivation in nitric acid was repeatedly found to result in thinning of the oxide layer on Ti surfaces, unless coupled with thermal aging of the surface in boiling water or through heating, which was found to elicit a more favourable cellular response (cell proliferation and ALP activity) at long exposure times of
72 h - 4 weeks (Ku et al. 2002). A detailed discussion of this treatment is presented in chapter 6.
Treatment of CP Ti with NaOH to form sodium hydrogels at the surface layer was reported to induce apatite deposition in simulated body fluid (SBF) environments (Kim et al. 1999). Others reported on the order of Ca and phosphate deposition on CP Ti surfaces following pre-treatment in alkali solutions (Yang et al. 1999). In a surface science report by Jones, several methods of Ti surface modification were listed, among which was the inclusion of Ca ions into Ti plates through immersion in aqueous Ca solutions. This was reported to result in the formation of calcium hydroxides and/or calcium titanate, which, upon immersion in Hanks balanced salt solution (HBSS), was found to induce apatite formation which was not detectable on untreated CP Ti plates. This report also included the results of Ti pre-treatment in solutions (containing both Ca and phosphate ions) that was found to enhance calcium phosphate precipitation in HBSS depending on the pH and ion concentration of the original solution.
Exposure to H2O2 solutions containing various metal chlorides was also found to create bioactive Ti surfaces (Jones 2001). H2O2 treatment results in oxidation and hydroxylation of the surface, as well as the production of Ti- peroxy gels, which are thought to enhance nucléation (Brunette et al. 2001). However, treatment of Ti with H2O2 containing tantalum chloride, followed by heating, provided titanium with bioactivity and enhanced interfacial in vitro bond strength between bone and the Ti implant, which was thought to be due to the thinner titania layer on the rougher Ti surface (Wu et al. 2002).
• Protein coatings
Hydroxyapatite coatings are commonly used to enhance osseointegration and short-term exposure revealed that such surfaces provide more osteophilic substrates than CP Ti resulting in more rapid bone-implant integration. The force required to detach hydroxyapatite coated Ti from bone was found to be higher than that for Ti at periods of around 12 weeks (Tengvall
et al. 1992). However, the popular hydroxyapatite coating suffers from a number
of drawbacks among which are delamination and fracture of the coating and in some cases reports of bacterial colonization. Long-term experiments have revealed problems with the adhesion of the coating and the occurrence of coating-substrate interfacial fracture (Kim at al. 1991, Li at al. 1997). Long-term perspective work over a period of 1 - 10 years found that the hydroxyapatite coated Ti was integrated in a similar manner to the CP Ti implants (Tengvall at al. 1992). The reported increase of bacterial susceptibility of HA coatings compared with Ti implants indicates the need for an alternative technique in surface modification to render the Ti itself bioactive (Ong at al. 2000).
Applying ion beam technology to modify Ti results in improved adhesion of the coating to the surface. Ca-rich coatings can be developed through several processes including ion beam mixing, ion beam deposition and plasma spraying. Ohtsuka at al (Ohtsuka at al. 1994) obtained films with improved adhesion by evaporating HA in combination with dynamic ion beam mixing using 50 keV Ca ions. Alternatively, sputtering of an HA coating with additional oxygen flow and simultaneous irradiation with 3 keV oxygen ions was reported to enhance adhesion of the coating (Ektessabi 1997).
Thin film hydroxyapatite coatings on Ti surfaces at a thickness of 5 - 50
A
were developed by an Italian group of researchers, through either ionsputtering of an hydroxyapatite matrix or alternatively, by pulsed laser deposition. This was at around the same time as the development of ion beam dynamic mixing to produce thin hydroxyapatite films by a Japanese group (Jones 2001).
Laboratory studies have shown that thin calcium phosphate coatings produced by ion beam mixing instead of plasma spraying easily dissolve in simulated body fluid (SBF) within 1 day. Treatment of these films by rapid, low temperature heating resulted in minimal dissolution and ensured the adherence of the coating to the substrate (Yoshinari et al. 1997). Hydroxyapatite coated Ti implants were found to fail at 36 months due to peri-implantitis as compared to plasma sprayed Ti which had an even earlier failure pattern associated with overloading (Ellingsen 1998).
Surface analysis of two failed plasma sprayed Ti coated dental implant systems (IMZ and ITI) showed a significant decrease in the coating thickness compared to the unused controls. This could possibly be a result of ion release into the surrounding tissue but whether such ion release initiated failure or is a common event in such implants is unknown (Esposito et al. 1998b).
Generally the hydroxyapatite coatings showed wide variation in terms of chemical composition, porosity, and crystalline and amorphous phases. Such differences are thought to influence the physical properties such as dissolution rates. Furthermore, these modified Ti implants were considered by some to be at an experimental stage in need of long-term follow up studies, as it is still unclear whether these coatings are advantageous or detrimental over the years (Ellingsen 1998).
• Reaction with antibiotics
Most of the above concepts displayed significant results in vitro but the in vivo findings are less significant and require further studies (Brunette et al.
2001).
There seems to be a general agreement that the early stages of healing are the most critical in determining the biological response, while long-term studies have shown no significant difference in the host tissues surrounding integrated Ti implants prepared by different methods. Thus it would seem logical to attempt to modify Ti surfaces to accelerate short-term bone formation on Ti through direct incorporation of Ca and/or phosphate ions. This can be achieved by a number of methods, one of which is ion implantation, which forms the basis of this investigation.
Ion implantation is a technique which enables the direct incorporation of the ions intended for implantation into the substrate. This is achieved by the acceleration of highly energised ions through an electrostatic field and onto the surface to be doped. This technique offers a number of advantages including high dopant purity and homogeneity of the dopant layer as well as control over the dose and distribution of the implanted species. It overcomes some of the common problems associated with coatings such as delamination and fracture. Ion implantation has been widely used in the semiconductor industry to alter the physical, chemical and optical surface properties of the substrate without weakening or adversely affecting the bulk material (Ziegler 1988).