(Padros, et al., 2020). The area with the higher oxygen level is cathodic to the less well-oxygenated region, which is anodic and experiences higher rates of corrosion. A common example of this is the enhanced corrosion of amalgam restorations beneath plaque or within crevices and scratches of an amalgam restoration (Kapoor and Ahmed, 2021). To combat against this, all metallic dental restorations should be polished (Panov
and Markova, 2018). Corrosion is associated with a number of potentially serious consequences, such as mechanical or metallurgical failure of componentsadverse tissue reactions to metal debris, cytotoxicity, and the potential to develop a sensitivity to the restorative metals involved. (Padros, et al., 2020; Sukumaran, et al., 2020).
CORROSION OF METALS IN DENTISTRY
In dentistry, it is important to understand the corrosion behavior of the alloys because the degradation of alloy-containing dental materials could lead to the release of metal ions into the body, causing numerous adverse side effects such as a localized inflammatory reaction, bone loss and even an allergic reaction (Vrbova, et al, 2021; Moslehifard, et al., 2019). Corrosion susceptibility of dental materials is based on a number of factors, including salivary pH, food composition, medications utilized, bacterial plaque, temperature and changes in the oral health status (Haugli, et al., 2020; Narayanan, et al., 2018). One study compared the shapes and finishes of various palladium-based dental materials (Milheiro et al.,2015). In this study, the researchers reported significantly higher rates of corrosion in disc-shaped specimens than in crown-shaped specimens. Both types of specimens had also undergone the same casting procedure (Milheiro et al.,2015). Another study compared possible corrosion effects of tungsten inert gas (TIG) welding with those obtained with conventional welding on nickel-chromium-based dental alloy specimens (Matos et al.,2015). In this study, specimens that underwent conventional brazing exhibited poorer corrosion resistance than those in the TIG method group (Matos et al.,2015). Saliva composition and the pH of saliva have also been shown to influence corrosion susceptibility. A study of titanium implants in varying pH and glucose level environments suggests that titanium alloy is more susceptible to corrosion in low-pH, high-glucose environments (Berbel, et al.,2019). A study of the effects of different mouthwashes on dental implants confirmed these findings (Al Subari et al., 2012).The composition, electrode potential and surface roughness of a given alloy used in restorative dentistry are also factors which influence their potential for corrosion (Narayanan, et al., 2018). A dense biofilm layer on the surface layer of a metallic restoration provides some protection against corrosion until it is compromised at which point the metallic restoration is subject to the elements of the corrosive process (Mystkowska, et al., 2018). The types of metals used in dental materials also affect corrosion and the risk that metallic substances will leach into surrounding oral mucosa. Noble alloys, such as gold, platinum, and palladium, have a higher corrosion resistance when compared with base metal alloys, such as nickel (Haugli, et al, 2020; American Dental Association, 2021). Noble alloys release lower levels of metal ions into the oral cavity, thus reducing the local and/or systemic adverse effects associated with an imbalance of metals (Arregui, et al. 2021; Nierlich et al.,2016). Even though these restorations exhibit little gross corrosion galvanic coupling and galvanic shock can occur when they contact metal such as amalgam which is used in restorative dentistry (James, 2020). Although noble metals are highly resistant to corrosion and more thermodynamically stable, base metal alloys, that is, those based on or containing nickel, cobalt, copper, iron, and titanium, grew in popularity because they are more affordable and generally offer higher rates of surface hardness and tensile strength than noble metal alloys (American Dental Association, 2021; Alnazzawi, 2018). Unlike the noble metals, the corrosion resistance of base metal alloys relies on the formation of a thin,
protective oxide film on the surface of the metal (Moslehifard, et al., 2019). American Dental Association, 2021). Many metals used for restorative dentistry undergo passivation, or an enhancement of this oxide layer, prior to implantation in the oral environment to make them more resistant to corrosion and thus less likely to release metal ions into the oral environment (Gopalakrishnan, et al., 2021). This protective surface oxide layer can be disrupted by insertion of the implant and movement of the implant within the bone from masticatory forces and cause difficulty in re-passivating making them susceptible to pitting and/or crevice corrosion (Yarram, et al, 2019). The amount of metal ions released by base metal alloys is generally below toxic limits. There is concern surrounding the use of certain alloys, especially nickel-based alloys, as it is estimated that 12-19% of females and 3-6% of males are allergic to nickel (American Dental Association, 2021; Buxton, et. al., 2019). The release of any amount of metal localizes in tissue near the implant and various organs and is thought to result in allergic reactions (Sukumaran, et al., 2020; Padros, et al, 2020; Haugli, et al, 2020). Corrosion testing of nickel-titanium alloys revealed pitting and/or crevice corrosion (Narayanan, et al., 2018; Barbeiri, et al., 2017). Pitting corrosion is a localized form of corrosion which involves breakdown of the passive oxide film and the development of holes on the surface of a metal used in restorative dentistry. Crevice corrosion features the development of stagnant solutions in crevices which development on the surface of the restorative metal and initiate the corrosive process (Yarram et al., 2019; Narayanan, et al., 2018). Copper- based alloys have also been shown to yield significantly higher corrosion rates, and subsequently higher rates of metal ion release, when compared to other alloy-based restorative materials (Holm, Morisbak, Kalfoss, & Dahl, 2015). Metallic ions which are released from the corrosion of implants can cause a cascade of inflammatory events which can lead to the activation of osteoclasts and resorption of the bone which supports the implant. (Yarram, et al., 2019). Titanium and titanium-based alloys are among the most corrosion-resistant materials used for biomedical applications (Berbel, et al., 2019; Stanec et al., 2016).However, it should be noted that pH significantly influences titanium’s corrosion resistance (Berbel, et al., 2019; Stanec et al., 2016). Studies show that titanium’s corrosion resistance is significantly lower in basic and neutral solutions (Sukumaran, et al., 2020; Stanec et al., 2016). Corrosion resulting from intraoral mixed metal and other galvanic cells may also be associated with lichen planus, leukoplakia, and oral cancer (Mortazavi, et.al, 2019; Neville, et. al., 2016; Alnazzawi, 2018). Patients with galvanic couples also have complaints of metallic or salty taste, electric sensation, or chronic inflammation of the mucosa, paresthesia, burning mouth syndrome and tooth sensitivity (Panov and Markova, 2018; James, 2020). Biodegradation products resulting from galvanic coupling due to the contact of a dental implant with a nickel- based alloy restoration influence the tissue condition, especially bone resorption (Sukumaran, et al., 2020).
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