16 Implant Dentistry
coupled either metal/ceramic or all-ceramic superstructure, Anwar et al. (2011) examined this scenario by different electrochemical methods in artificial saliva solutions: they concluded that increased fluoride concentration leads to a decrease in the corrosion resistance of all tested couples.25
Discussion From above literature review, it is important to stress that described evidence of fluoride on titanium alloys derives mostly from in-vitro research, which includes oversimplifications in simulating oral environment.13 According to Lopez- Alias et al. (2006), it is difficult to predict the clinical behavior of any alloy from invitro studies, since such factors as changes in the quantity and quality of saliva, diet, oral hygiene, polishing of alloy, the amount and distribution of occlusal forces, or brushing with toothpaste can all influence corrosion to varying degrees.35 On the other hand, choice of materials to be used as suprastructures with titanium implants is crucial. Their galvanic corrosion behaviors should be evaluated in order to avoid any corrosive process.2 The concern of reducing implant corrosion might be addressed by different methods such as noble metal alloying additions, especially gold on the corrosion behavior of titanium; this could be ascribed to an incorporation of noble metal into the passive layer, resulting in increasing its dissolution resistance.8 Nakagawa et al. (2005) concluded that addition of a small amount of Pd or Pt to Ti to create an alloy proved to be very effective in improving the corrosion resistance of titanium in sodium fluoride (NaF) solutions of various concentrations up to 2 %.33 Ag and Au have excellent corrosion resistance in many aqueous solutions, and good in vivo biocompatibility.8 Zhang et al. (2009) clarified that addition of Ag was found to be effective in reducing corrosion current density and increasing the open circuit potential of titanium in artificial saliva environment.36 Shim et al. (2005) reported that Ti-Ag alloys with low Ag content (< 5 %) have better corrosion resistance than pure titanium in artificial saliva.37 Yamazoe et al. (2007) showed that Ti-0,5 Pt, Ti-6 Al- 4N -0,5 Pt, and Ti-6Al7Nb-0,5 Pt alloys had high corrosion resistance in a fluoride containing environment and high mechanical strength. Therefore, use fullness of these alloys as new implants or denture base materials was suggested.38 Design and texture of implant surface was also studied. The presence of pores in implant materials may be a source of corrosion problems. Foit and Joska (2013) showed that titanium implants with porosity of 24 and 33% initiated a local attack of the material.39 Moreover, the abnormal electrical currents produced during corrosion can convert any metallic implant into an electrode, and the negative impact on the surrounding tissues due to these extreme signals could be an additional cause of poor performance and rejection of implants. Electrical protection of the surfaces of the implants was proposed by Gittens et al. (2011) in order to reduce implant corrosion.15 On the other hand, innovative commercial mouthwashes and toothpastes during the last decade have replaced fluoride with biomimetic hydroxyapatite nanocrystals (CHA) as a remineralizing agent to avoid the effects of fluoride on human health.11 Dental News, Volume XXI, Number III, 2014
Conclusion The most favorable suprastructure/implant couple is the one capable of resisting the most extreme conditions that could possibly be encountered in oral cavity. Most studies were conducted in-vitro. Results showed that fluoride– containing products should be controlled and prescribed carefully, since increasing the use of such products can lead to alterations of implants and restoration surfaces, compromising the longevity of the treatment. In fact, fluoride ions seem to exert a negative influence on the corrosion resistance of pure titanium and titanium alloy Ti-6Al-4V, especially in the acidic artificial saliva which contained over 0,1% NaF (fluoride ions = 500 ppm). This may cause the breakdown of protective passivation layer that normally exists on titanium and titanium alloys, leading to severe corrosion.20,29,30,31 However, further in-vivo studies are warranted in order to elucidate corrosion resistance of titanium and titanium alloys exposed to fluoride agents in oral cavity.
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