Microbiologically influenced corrosion of orthodontic metallic appliances

Microbial corrosion of metallic biomaterials in the oral environment

A wide variety of microorganisms have been closely linked to metal corrosion in the form of adherent surface biofilms. Biofilms allow the development and maintenance of locally corrosive environments and/or permit direct corrosion including pitting corrosion. The presence of numerous genetically distinct microorganisms in the oral environment poses a threat to the integrity and durability of the surface of metallic prostheses and implants used in routine dentistry. However, the association between oral microorganisms and specific corrosion mechanisms is not clear. It is of practical importance to understand how microbial corrosion occurs and the associated risks to metallic materials in the oral environment. This knowledge is also important for researchers and clinicians who are increasingly concerned about the biological activity of the released corrosion products. Accordingly, the main goal was to comprehensively review the current literature regarding oral microbiologically influenced corrosion (MIC) including characteristics of biofilms and of the oral environment, MIC mechanisms, corrosion behavior in the presence of oral microorganisms and potentially mitigating technologies. Findings included that oral MIC has been ascribed mostly to aggressive metabolites secreted during microbial metabolism (metabolite-mediated MIC). However, from a thermodynamic point of view, extracellular electron transfer mechanisms (EET-MIC) through pili or electron transfer compounds cannot be ruled out. Various MIC mitigating methods have been demonstrated to be effective in short term, but long term evaluations are necessary before clinical applications can be considered. Currently most in-vitro studies fail to simulate the complexity of intraoral physiological conditions which may either reduce or exacerbate corrosion risk, which must be addressed in future studies. STATEMENT OF SIGNIFICANCE: A thorough analysis on literature regarding oral MIC (microbiologically influenced corrosion) of biomedical metallic materials has been carried out, including characteristics of oral environment, MIC mechanisms, corrosion behaviors in the presence of typical oral microorganisms and potential mitigating methods (materials design and surface design). There is currently a lack of mechanistic understanding of oral MIC which is very important not only to corrosion researchers but also to dentists and clinicians. This paper discusses the significance of biofilms from a biocorrosion perspective and summarizes several aspects of MIC mechanisms which could be caused by oral microorganisms. Oral MIC has been closely associated with not only the materials research but also the dental/clinical research fields in this work.

Sulfate-Reducing Bacteria in Patients Undergoing Fixed Orthodontic Treatment

OBJECTIVES

The oral microbiological environment may be implicated in the corrosion of orthodontic metals. This study aimed to examine the prevalence of sulfate-reducing bacteria (SRB) in orthodontic patients undergoing fixed appliance treatment.

METHODS

Sixty-nine orthodontic and 69 healthy non-orthodontic participants were enrolled in the study. Supragingival and subgingivaloral biofilm were collected and tested for the presence of SRB. The DNA extraction, polymerase chain reaction (PCR), and 16sRNA Sanger sequencing method was performed from the SRB-positive samples. The sequenced PCR products were analysed and compared with databases to identify the bacterial genus.

RESULTS

Amongst 69 orthodontic patients, characteristic black precipitates developed in 14, indicating the presence of iron sulfides which demonstrates the likelihood of SRB. Alternatively, 2 out of 69 showed the presence of SRB in healthy non-orthodontic participants (controls). Desulfovibrio spp was confirmed by analyses of 16sRNA sequencing, which revealed that the SRB prevalence was 20% in the examined participants with orthodontic appliances.

CONCLUSIONS

The prevalence of SRB was found to be significantly higher amongst orthodontic patients compared to non-orthodontic participants. Presence of stainless steel in the oral environment may have facilitated the colonisation of SRB.

Allergenicity and Bioavailability of Nickel Nanoparticles Compared to Nickel Microparticles in Mice

Metal allergy is a common disease that afflicts many people. Nevertheless, the mechanism underlying metal allergy development has not been completely elucidated. Metal nanoparticles might be involved in the development of a metal allergy, but the associated details are unknown. In this study, we evaluated the pharmacokinetics and allergenicity of nickel nanoparticles (Ni-NPs) compared with those of nickel microparticles (Ni-MPs) and nickel ions. After characterizing each particle, the particles were suspended in phosphate-buffered saline and sonicated to prepare a dispersion. We assumed the presence of nickel ions for each particle dispersion and positive control and orally administered nickel chloride to BALB/c mice repeatedly for 28 days. Results showed that compared with those in the Ni-MP administration group (MP group), the Ni-NP administration group (NP group) showed intestinal epithelial tissue damage, elevated serum interleukin (IL)-17 and IL-1β levels, and higher nickel accumulation in the liver and kidney. Additionally, transmission electron microscopy confirmed the accumulation of Ni-NPs in the livers of both the NP and nickel ion administration groups. Furthermore, we intraperitoneally administered a mixed solution of each particle dispersion and lipopolysaccharide to mice and then intradermally administered nickel chloride solution to the auricle after 7 days. Swelling of the auricle was observed in both the NP and MP groups, and an allergic reaction to nickel was induced. Particularly in the NP group, significant lymphocytic infiltration into the auricular tissue was observed, and serum IL-6 and IL-17 levels were increased. The results of this study showed that in mice, Ni-NP accumulation in each tissue was increased after oral administration and toxicity was enhanced, as compared to those with Ni-MPs. Orally administered nickel ions transformed into nanoparticles with a crystalline structure and accumulated in tissues. Furthermore, Ni-NPs and Ni-MPs induced sensitization and nickel allergy reactions in the same manner as that with nickel ions, but Ni-NPs induced stronger sensitization. Additionally, the involvement of Th17 cells was suspected in Ni-NP-induced toxicity and allergic reactions. In conclusion, oral exposure to Ni-NPs results in more serious biotoxicity and accumulation in tissues than Ni-MPs, suggesting that the probability of developing an allergy might increase.

Corrosion behavior of high nitrogen nickel-free austenitic stainless steel in the presence of artificial saliva and Streptococcus mutans

High nitrogen nickel-free austenitic stainless steels (HNSs) have great potentials to be used in dentistry owing to its exceptional mechanical properties, high corrosion resistance, and biocompatibility. In this study, HNSs with nitrogen of 0.88 wt% and 1.08 wt% displayed much lower maximum pit depths than 316L stainless steel (SS) after 21 d of immersion in abiotic artificial saliva (2.2 μm and 1.7 μm vs. 4.5 μm). Microbiologically influenced corrosion (MIC) evaluations revealed that Streptococcus mutans biofilms led to much severer corrosion of 316L SS than HNSs. Corrosion current densities of HNSs were two orders of magnitude lower than that of 316L SS after incubation of 7 d (37.5 nA/cm² and 29.9 nA/cm² vs. 5.63 μA/cm²). The pitting potentials of HNSs were at least 550 mV higher than that of 316L SS in the presence of S. mutans, confirming the better MIC resistance of HNSs. Cytotoxicity assay confirmed that HNSs were not toxic to MC3T3-E1 cells and allowed better sessile cell growth on them than on 316L SS. It can be concluded that HNSs are more suitable dental materials than the conventional 316L SS.

Influence of Conventional or Invisalign Orthodontic Treatment on Mineral and Trace Element Salivary Levels: Longitudinal Study with Total Reflection X-ray Fluorescence

This work aimed to evaluate the salivary concentration of chemical elements in patients undergoing orthodontic treatment with fixed orthodontic appliances and removable aligners. Twelve Angle Class I and II orthodontic patients undergoing treatment with conventional fixed appliances and 15 patients treated with removable aligners provided unstimulated whole saliva samples before treatment (pre) and after 3 months of treatment (post). The concentration and secretion rate of chemical elements in saliva were determined by total reflection X-ray fluorescence. Differences from pre to post and between groups were determined with the paired T test or Wilcoxon test, and two-way ANOVA, considering P < 0.05. The concentrations of S, Cl, and K decreased, while Zn increased significantly (P < 0.05) between pre and post treatment with the fixed appliance treatment. The salivary secretion rate of S was decreased from pre to post in the fixed appliance group. No differences in the concentration and secretion rate of chemical elements were detected from pre to post in the Invisalign group. Fe secretion rate presented an interaction between time and treatment, with lower secretion at post (P = 0.02) in the Invisalign group. Increased Br secretion rate and decreased Rb, Fe, P, and K in Invisalign patients suggested a better salivary electrolyte profile regarding periodontal bone remodeling. No significant alterations in ions associated with metal corrosion and inflammatory reactions were detected in orthodontic patients under dental plaque control.

In Vitro evaluation of immediate cytotoxicity of resterilised orthodontic bands on HGF-1 cell line

OBJECTIVES

The aim of this study was to evaluate the immediate cytotoxic effects of orthodontic molar bands, on HGF-1 cell line, after multiple times of sterilization following size selection procedure.

MATERIAL AND METHODS

48 stainless steel orthodontic molar bands were divided into 4 groups according to times of sterilization (1, 2, 4 and 8 times). A liquid extract containing the ions released from each band was prepared and the HGF-1 cell line was exposed to the extracts. 2 control groups (positive and negative) were designated. An MTT assay was performed, and the absorbance was read at 492nm in a microplate reader (Antos 2020, Austria).

RESULTS

There was no significant difference in pure optical density (OD) among the 4 groups (P=0.749) however a statistically significant difference was seen between the positive control group and other 4 groups (P<0.001).

CONCLUSION

The stainless-steel orthodontic bands used in this study were inert as manufactured and even multiple times of sterilization did not decrease the biocompatibility of these bands for clinical use. The present study shows that clinicians can sterilize the tried-in molar bands for at least 8 times without any risk of cytotoxicity for patients.

Assessing the Potential Association Between Microbes and Corrosion of Intra-Oral Metallic Alloy-Based Dental Appliances Through a Systematic Review of the Literature

Objective: Systematic review assessing the association between oral microorganisms and corrosion of intra-oral metallic alloy-based dental appliances. Design: PubMed, Scopus, and Web of Science were searched using keyword combinations such as microbes and oral and corrosion; microbes and dental and corrosion; microorganisms and oral and corrosion; microorganisms and dental and corrosion. Results: Out of 141 articles, only 25 satisfied the selection criteria. Lactobacillus reuteri, Streptococcus mutans, Streptococcus sanguis, Streptococcus mitis, Streptococcus sobrinus, Streptococcus salivarius, sulfate-reducing bacteria, sulfate oxidizing bacteria, Veilonella, Actinomyces, Candida albicans were found to have a potential association with corrosion of intraoral metallic alloys such as stainless steel, titanium, nickel, cobalt-chromium, neodymium-iron-boron magnets, zirconia, amalgam, copper aluminum, and precious metal alloys. Conclusion: The included studies inferred an association between oral microorganisms and intra-oral metallic alloys-based dental appliances, although, it is vital to acknowledge that most studies in the review employed an in-vitro simulation of the intra-oral condition.

Effects of magnetic fields from electric toothbrushes on fluoride- and oral bacteria-induced corrosion of orthodontic metallic wires

Corrosion of metallic materials in the oral cavity could trigger metal allergy in patients. To clarify the risk elevation of magnetic fields (MFs) exposure on metallic corrosion when combined with fluoride-containing dental care products and indigenous oral bacteria, we investigated electric toothbrush-derived MF-induced corrosion of orthodontic stainless steel (SUS) and nickel titanium (Ni-Ti) wires in the presence of fluoride and oral bacteria, i.e. Streptococcus (S) mutans and S. sanguinis. MFs induced an electric current in the wires under both environments. Oral bacteria corroded SUS wires, and fluoride corroded SUS and Ni-Ti wires as previously reported; however, no additive or synergistic effects of MF exposure on fluoride- and microbiologically-induced metallic corrosion were observed. These results suggest that the MFs from electric toothbrushes do not increase the risk of corrosion of metallic appliances, given that the oral environment of patients is exposed to oral bacteria and fluoride-containing products.

Orthodontic appliances did not increase risk of dental caries and periodontal disease under preventive protocol

OBJECTIVES

To assess periodontal parameters and microbial species levels after orthodontic appliance placement in patients who received oral hygiene instructions and who were monitored and motivated throughout the study.

MATERIALS AND METHODS

The Periodontal Index was recorded and saliva collection was performed before (T0) and 30 (T1), 60 (T2), and 90 (T3) days after orthodontic appliance placement in 15 patients (mean age 17.53 ± 8.0 years). Analysis was carried out using checkerboard DNA-DNA hybridization. Nonparametric statistical analysis was performed.

RESULTS

The Periodontal Index did not change. The total amount of the purple and red complexes and Candida species showed a significant decrease from T2. The green, yellow, and orange complex showed a significant decrease at T3. The specific species analysis showed that Prevotella nigrescens, Pseudomonas putida, Fusobacterium periodonticum, Pseudomonas aeruginosa, Peptostreptococcus anaerobius, and Tanerella forsythia showed high incidence before bonding, and their levels decreased at T2 and T3. Only Porphyromonas gingivalis showed increased levels at T2 and displayed the highest level at T3. The Streptococcus group decreased their levels from T2 onward.

CONCLUSIONS

A dynamic change in microbial levels was identified. The decrease in the levels of complexes present was only possible due to the mechanical method of oral hygiene implemented in this sample.

The Role of Oral Cavity Biofilm on Metallic Biomaterial Surface Destruction-Corrosion and Friction Aspects

Metallic biomaterials in the oral cavity are exposed to many factors such as saliva, bacterial microflora, food, temperature fluctuations, and mechanical forces. Extreme conditions present in the oral cavity affect biomaterial exploitation and significantly reduce its biofunctionality, limiting the time of exploitation stability. We mainly refer to friction, corrosion, and biocorrosion processes. Saliva plays an important role and is responsible for lubrication and biofilm formation as a transporter of nutrients for microorganisms. The presence of metallic elements in the oral cavity may lead to the formation of electro-galvanic cells and, as a result, may induce corrosion. Transitional microorganisms such as sulfate-reducing bacteria may also be present among the metabolic microflora in the oral cavity, which can induce biological corrosion. Microorganisms that form a biofilm locally change the conditions on the surface of biomaterials and contribute to the intensification of the biocorrosion processes. These processes may enhance allergy to metals, inflammation, or cancer development. On the other hand, the presence of saliva and biofilm may significantly reduce friction and wear on enamel as well as on biomaterials. This work summarizes data on the influence of saliva and oral biofilms on the destruction of metallic biomaterials.

Intraoral electric potential via oral bacterial power generation -A novel mechanism of biofilm formation

In the early stages of biofilm accumulation, the electric charge of the dental enamel and pellicle surfaces is known to be involved. We therefore investigated the relationship between oral hygiene and intraoral electric potential (IoP) in 45 male participants using a double-blind study. IoP, but not body surface electric potential, was loosely correlated with oral hygiene condition (Oral Hygiene Index; OHI). IoP was also loosely correlated with smartphone use; however, there was no significant correlation between smartphone use and OHI. IoP elevation might be caused by OHI elevation resulting from biofilm formation as an internal factor, with smartphone use as an external factor. This in vitro study revealed the generating capacity of Streptococcus mutans accompanied by biofilm accumulation using a microbial fuel cell. These results suggest that IoP elevation is caused by biofilm accumulation induced by power generation of oral bacteria, resulting in elevation of OHI.

Corrosion of dental alloys in artificial saliva with Streptococcus mutans

A comparative study of the corrosion resistance of CoCr and NiCr alloys in artificial saliva (AS) containing tryptic soy broth (Solution 1) and Streptococcus mutans (S. mutans) species (Solution 2) was performed by electrochemical methods, including open circuit potential measurements, impedance spectroscopy, and potentiodynamic polarization. The adherence of S. mutans to the NiCr and CoCr alloy surfaces immersed in Solution 2 for 24 h was verified by scanning electron microscopy, while the results of electrochemical impedance spectroscopy confirmed the importance of biofilm formation for the corrosion process. The R(QR) equivalent circuit was successfully used to fit the data obtained for the AS mixture without S. mutans, while the R(Q(R(QR))) circuit was found to be more suitable for describing the biofilm properties after treatment with the AS containing S. mutans species. In addition, a negative shift of the open circuit potential with immersion time was observed for all samples regardless of the solution type. Both alloys exhibited higher charge transfer resistance after treatment with Solution 2, and lower corrosion current densities were detected for all samples in the presence of S. mutans. The obtained results suggest that the biofilm formation observed after 24 h of exposure to S. mutans bacteria might enhance the corrosion resistance of the studied samples by creating physical barriers that prevented oxygen interactions with the metal surfaces.

Biocorrosion of 316LV steel used in oral cavity due to Desulfotomaculum nigrificans bacteria

Corrosion processes of metallic biomaterials in the oral cavity pose a significant limitation to the life and reliable functioning of dental materials. In this article, the influence of environment bacteria Desulfotomaculum nigrificans sulfate reducing bacteria on the corrosion processes of 316LV steel was assessed. After 14 and 28 days of contact of the material with the bacterial environment, the surfaces of the tested biomaterial were observed by means of confocal scanning laser microscopy, and their chemical composition was studied using X-Ray Photoelectron Spectrometry and a scanning transmission electron microscopy. Corrosive changes, the presence of sulfur (with atomic concentration of 0.5%) on the surface of the biomaterial and the presence of a thin oxide layer (thickness of ∼20 nm) under the surface of the steel were observed. This corrosion layer with significant size reduction of grains was characterized by an increased amount of oxygen (18% mas., p < 0.001) in comparison to untreated 316LV steel (where oxygen concentration - 10% mas.). Image analysis conducted using APHELION software indicated that corrosion pits took up ∼2.8% of the total tested surface. The greatest number of corrosion pits had a surface area within the range of 100-200 μm² . © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 222-229, 2017.