Electrogalvanism in Oral Implantology: A Systematic Review

Purpose The objective of this work is to study galvanic corrosion of different couples of prosthetic and implant alloys through the realization of a systematic review. Materials and Methods An electronic search was performed on Pubmed, Google Scholar, Scopus, ScienceDirect, EbscoHost, and Web of Science for published studies related to electrogalvanism in oral implantology. The keywords used were “dental implants” and “galvanic corrosion.” Two independent readers read the scientific articles. Results From 65 articles initially identified, only 19 articles met the eligibility criteria. The evaluation of the selected articles allowed us to determine the parameters compared, such as the resistance to galvanic corrosion, the influence of fluorine and pH on the electrochemical behavior, and the release of metal ions and their cytotoxicity. Indeed, Ti6Al4V and precious alloys coupled to titanium were found to be the most resistant to galvanic corrosion, followed by cobalt-chromium alloys and nickel-chromium alloys which were least resistant. This resistance decreases with increasing fluorine concentration and with decreasing pH of the environment. Discussion. The implant-prosthetic system's galvanic resistance is influenced by many intrinsic factors: alloy composition and surface condition, as well as extrinsic factors such as pH variations and amount of fluorine. The effects of oral electrogalvanism are essentially the result of two main criteria: effects due to electric currents generated by corrosion and effects due to the release of metal ions by corrosion. Conclusion To avoid this phenomenon, it is wise to follow the proposed recommendations such as the use of the minimum of distinct metals as much as possible, favoring the commercially pure titanium implant of Ti6Al4V, opting for the choice of couples, titanium/titanium, favoring daily mouthwashes of 227 ppm of fluoride, and avoiding fluorinated acid solutions.


Introduction
Electro galvanism is the result of the coupling of different metals or alloys with different corrosive potentials in an aqueous conducting environment (� electrolytic) [1,2]. e sustainability of the implant-prosthetic complex depends on the osseointegration of the implant and the stability of the surrounding soft tissue. Morphology and surface roughness have a great influence on osseointegration. e presence of microgaps within the system because of ionic release caused by galvanic corrosion can lead to the accumulation of bacterial biofilm on these surfaces. e dispersion in the tissue of particles of titanium oxide or other derivatives triggers an inflammatory reaction of the nonspecific immune system which certainly activates the resorption of the bone, causing long-term damage to the implant. e level of peri-implant inflammation affects the survival of the implants in the long term [3,4]. e galvanic corrosion of biomaterials, used in oral implantology, in direct contact with the oral environment depends not only on their own properties but also on their interactions with their environment [5,6].
In the light of the abovementioned facts, the objective of our work is to study the galvanic corrosion of different pairs of prosthetic and implant alloys, through the realization of a systematic review.

Materials and Methods
A literature search was conducted using the databases: Pubmed, Google Scholar, Cochrane Library, Science Direct, EbscoHost, Web of Science, Embase, and Clinical Trials. e keywords used were "dental implants" and "galvanic corrosion." e search languages used were English and French. Any study dealing with the galvanic behavior of prosthetic alloys when coupled with implant alloys was selected. Studies dealing with galvanic couples in orthodontics and orthopedics were excluded. Studies dealing with other types of corrosion other than galvanic corrosion were also excluded. Two independent readers read the scientific articles.

Results
e search of the scientific literature yielded 65 articles. We identified 19 studies that met our inclusion and exclusion criteria, of which only one was in vivo and the rest were in vitro ( Figure 1). e evaluation of the selected articles allowed to determine the parameters compared, namely the resistance to galvanic corrosion of the different couples (Table 1), the influence of fluorine and pH on the electrochemical behavior (Table 2), the oxidation surface state on titanium (Table 3), and the release of metal ions (Table 4) and their cytotoxicity.

Cytotoxicity.
e decrease in the cell growth rate allowed Lee et al. to report that the cytotoxicity of nickelchromium alloy with beryllium was greater than that of nickel-chromium alloy without beryllium. e addition of beryllium is therefore detrimental to the cellular activity of the tissues surrounding the implant. On the other hand, increasing the chromium content in the composition of the nonprecious nickel-chromium alloy has a beneficial effect on cytotoxicity [22].

Factors Influencing Corrosion Phenomena.
e resistance of a metal or an alloy to corrosion depends not only on its own properties but also on its interactions with its environment.
ere are different factors influencing the corrosion of an alloy ( Figure 2).

Intrinsic Factors
1. Alloy Composition. Cp Ti implants have excellent biocompatibility and good galvanic corrosion resistance but low mechanical strength, whereas Ti6Al4V has high mechanical properties but low galvanic corrosion resistance [10,11,25].
Excellent galvanic corrosion resistance due to the high thermodynamic stability of gold characterizes high gold alloys. e addition of palladium greatly improves the corrosion resistance of silver alloys. Alloys based on gold and palladium have a lower dissolution rate and therefore a higher corrosion resistance than those made of nonnoble base metals such as NiCr or CoCr [12-15, 18-20, 26].  2. Surface Condition. Oxide film thickness, energy, roughness, and grain size on the titanium surface influence corrosion resistance, biomaterial interaction with cells, and osseointegration mechanisms [7,27,28]. e acid-etch surface treatment of Ti cp directly affects the formation of galvanic couples, improves osseointegration, and increases corrosion resistance in the oral environment [7].

Initiation of Localized
Corrosion. e initiation of other types of corrosion removes the passivation oxide layer and is likely to aggravate galvanic corrosion by increasing the current [17,19].

Extrinsic Factors
1. pH Variations. e pH of the environment plays a major role in the electrochemical behavior of the different couples; as the pH of the saliva decreases, the values of the galvanic current between the implant and its superstructure increase [2,7,21].
However, the normal pH of saliva secreted by the salivary glands varies between 6 and 7. It can reach acidic levels of about 2 when acidic foods are ingested or when acid regurgitation occurs, as it can vary in the areas around surgical sites and dental implants [29,30].

2.
e Amount of Fluorine. Prophylactic toothpastes, mouthwashes, and gels contain 200 to 20 000 ppm F− and may impair the corrosion resistance of prosthetic and implant dental alloys in the oral cavity [2,5,7].
In fact, increasing the concentration of fluoride ions decreases the corrosion resistance of titanium implants except at 227 ppm F− at pH 5.5, which is the fluoride concentration found in daily mouthwash [21].
It has also been shown that the combination of low pH and the presence of fluoride ions in the solution severely  International Journal of Dentistry  International Journal of Dentistry affects the degradation of the protective passivation layer that normally exists on titanium alloys, resulting in galvanic corrosion [7,21,29,31].

e Coupling between Implant Titanium and Prosthetic
Superstructures. When coupling, compatible metals should be selected for direct contact with each other in the oral cavity to avoid or minimize the formation of undesirable electrochemical couples [32]. e use of titanium alloy prosthetic superstructures on titanium implants avoids the problem of galvanic corrosion. A study by Arismendi et al. suggests that the best restorationimplant pairing can be achieved by using cp titanium and a titanium alloy [8]. Whereas Taher et al. suggest that the best couples are Ti/Ti, Ti/Or, and Ti/CoCr [15].

Cathode/Anode Surface Area
Ratio. e most unfavorable situation is when a small anode is linked to a large cathode.
is ratio can cause more corrosion [17].

Host Response to Electrogalvanism in Oral
Implantology. e effects of oral electrogalvanism are primarily the result of two main factors: (i) Effects due to electrical currents generated by corrosion (ii) Effects due to the release of metal ions by corrosion

Osteolysis Induced by Electrical Currents Generated by
Corrosion. It has been shown that cyclic loads (chewing and  International Journal of Dentistry biting) enhance electrical currents induced by corrosive events. It is suggested that surrounding tissues are chronically exposed to abnormal electrical signals [33]. e bone responds to electrical potentials applied to it, and osteogenesis is proportional to electronegativity [34].

Osteolysis Induced by Corrosion Debris.
Olmedo et al. observed that corrosion-induced ion release may be responsible for periimplantitis and treatment failure [35].
Periimplantitis is characterized by a loss of the supporting bone, both clinically and radiologically proven, and is associated with an inflammatory reaction of the surrounding soft tissue [2,36]. e metal ions released because of the corrosion process are phagocytized by macrophages and release inflammatory mediators in the form of pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNF-α) and interleukin-1 (IL-1), and increased intercellular adhesion molecules (ICAM-1), which inhibit osteoblast production and promote osteolytic activity through the RANKRANK ligand pathway, thus inducing osteolysis of peri-implant tissues ( Figure 3) [2,31,[37][38][39][40][41].

Conclusion
ere is a wide range of materials to be used in implantology, both for the implants and the superstructure, and the most effective treatment of electroplating in International Journal of Dentistry 7 oral implantology remains preventive. e judicious choice of materials is made when establishing the prosthetic treatment plan. e proposed recommendations to practitioners are as follows: (i) e use of biocompatible materials and a minimum of discrete metals whenever possible (ii) e choice of metal couples whose elements are as close as possible in the galvanic scale; the best couples are Ti/Ti, Ti/Or, and Ti/CoCr (iii) e use of supra-implant ceramic restorations (iv) Prefer cp Ti to Ti6Al4V as an implant material for its better resistance to galvanic corrosion (v) Avoid as much as possible, the direct contact between two different metals with cathodic inhibitors, a joint, an insulator, and a coating. (vi) Avoid an unfavorable anode-cathode surface ratio (vii) Avoid acidic fluoride solutions, especially when the implant is made of titanium alloy and the superstructure is made of Co-Cr, and therefore, prefer daily mouthwashes of 227 ppm fluoride

Data Availability
All data used in this review are available on Pubmed, Google Scholar, Scopus, ScienceDirect, EbscoHost, and Web of Science.

Conflicts of Interest
e authors declare that they have no conflicts of interest.