Cytotoxicity of esthetic, metallic, and nickel-free orthodontic brackets: cellular behavior and viability

Nanomaterials Application in Orthodontics

Nanotechnology has gained importance in recent years due to its ability to enhance material properties, including antimicrobial characteristics. Nanotechnology is applicable in various aspects of orthodontics. This scientific work focuses on the concept of nanotechnology and its applications in the field of orthodontics, including, among others, enhancement of antimicrobial characteristics of orthodontic resins, leading to reduction of enamel demineralization or control of friction force during orthodontic movement. The latter one enables effective orthodontic treatment while using less force. Emphasis is put on antimicrobial and mechanical characteristics of nanomaterials during orthodontic treatment. The manuscript sums up the current knowledge about nanomaterials' influence on orthodontic appliances.

Release and toxicity of bisphenol-A (BPA) contained in orthodontic adhesives: A systematic review

OBJECTIVE

The objectives of this systematic review of the literature are to search for BPA release from biomaterials used in orthodontics and to highlight their possible impact on human health.

MATERIALS AND METHODS

Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) model, digital and manual searches were conducted in February 2020 in the database of CENTRAL, MEDLINE and EMBASE. Thirteen parameters relating to the release and biological effects of BPA were identified and analysed.

RESULTS

Thirty-one non-randomized clinical trials were included. In general, the level of evidence was low, and the risk of bias ranged from moderate to high. Furthermore, although at different concentrations, most of in vitro and in vivo studies found BPA release from orthodontic adhesives. The lack of standardized protocols and the clinical and methodological heterogeneity of the studies prevented a valid interpretation of the actual results. Concerning the possible toxicity of BPA, no conclusive scientific evidence could be drawn, but it seems that orthodontic biomaterials containing BPA have potential adverse biological effects in humans.

DISCUSSION AND CONCLUSION

Until we have solid evidence from clinical trials, clinicians should consider that orthodontic adhesives containing BPA have potential short- and long-term adverse biological effects in humans. Careful consideration should therefore be given to bonding, polymerization and debonding protocols in orthodontics.

In Vitro Determination of Genotoxicity Induced by Brackets Alloys in Cultures of Human Gingival Fibroblasts

Orthodontic brackets release ions that can be reabsorbed in the oral mucosa, potentially causing complications, including cytotoxic effects and mutagenic alterations. The aim was to evaluate the genotoxicity induced by orthodontic appliance alloys in cultures of human gingival fibroblasts by comet assay. Eluates were obtained from the following brackets alloys: EconoLine (SS: stainless steel), MiniMirage (Ni-Ti: nickel-titanium), Nu-Edge (Co-Cr: cobalt-chromium), In-Vu (PC-polycrystals (PC) aluminum oxide), and Monocrystal IZE (monocrystalline (MC) aluminum oxide). Each bracket was sterilized and exposed to a corrosive process for 35 days. The obtained eluates were tested for genotoxicity of human gingival fibroblasts (HGFA) by the alkaline comet assay. All study groups showed genotoxic effects; there was a significant difference (p < 0.0001) among groups. The eluates obtained from Ni-Ti showed a 16-times greater genotoxic effect. There were differences in genotoxicity after comparing the Ni-Ti with SS (p < 0.01) and Co-Cr brackets (p < 0.001). The ceramic was more genotoxic than metallic brackets (SS and Co-Cr), but less than the Ni-Ti. This in vitro model will be useful for further study of early DNA damage caused by brackets and other biomaterials used in the oral cavity before their introduction into the clinical setting.

Evaluation of toxicity and response to oxidative stress generated by orthodontic bands in human gingival fibroblasts

OBJECTIVE

To evaluate the cytotoxicity of stainless-steel orthodontic bands and their influence on the expression of the antioxidant genes in human gingival fibroblasts.

MATERIALS AND METHODS

Ten bands of each brand (Dentsply-Sirona, Dentaurum, TP Orthodontics, and Morelli) were conditioned in 0.2 g/mL culture medium at 37°C for 14 days, and the corresponding conditioned media were applied over the fibroblasts. Cell viability was assessed after 24, 48, and 72 hours of exposure to the conditioned media by trypan blue exclusion assay. Expression of the antioxidant defense genes peroxiredoxin 1 (PRDX1), superoxide dismutase 1 (SOD1), and glutathione peroxidase 1 (GPX1) were evaluated by quantitative polymerase chain reaction after 24 hours of exposure. These parameters were compared to those of the cells not exposed to the conditioned media of the bands (control).

RESULTS

All bands promoted a reduction in the number of viable cells in the periods of 48 and 72 hours (P < .01). Analysis of gene expression showed a significant increase in the levels of PRDX1 transcripts caused by the conditioned media of the Dentsply-Sirona, TP Orthodontics, and Morelli bands (P < .01) as well as induction of SOD1 by the conditioned media of the Dentaurum and Morelli (P < .01). Expression of GPX1 was not influenced by the conditioned media.

CONCLUSIONS

The orthodontic bands showed toxicity to fibroblasts and increased the expression of PRDX1 and SOD1 antioxidant genes, indicating induction of oxidative stress in the cells.

Cytotoxicity of bracket identification dyes

OBJECTIVES

To investigate the potential cytotoxicity of the bracket identification dyes commonly used in orthodontic fixed appliances.

MATERIALS AND METHODS

Six bracket brands representing the market in various aspects were selected. Ten sets of each bracket brand were acquired, and the identification dyes on them were scraped. They were tested for cytotoxicity at three different levels of concentration (2.5 mg/mL, 5 mg/mL, and 10 mg/mL), with the aid of a real-time cell analysis system. The results were compared within and between the groups. One-way analysis of variance and Tukey's post hoc test were used for statistical analysis.

RESULTS

None of the six investigated dyes displayed cytotoxicity at the 2.5 mg/mL concentration. Of the investigated brands, three at 5 mg/mL and four at 10 mg/mL displayed cytotoxicity.

CONCLUSIONS

Some of the identification dyes in this study did display cytotoxicity at the higher concentrations tested. Alternative methods for bracket identification should be considered.

Inactivation of Planktonic Escherichia coli by Focused 1-MHz Ultrasound Pulses with Shocks: Efficacy and Kinetics Upon Volume Scale-Up

This study addresses inactivation of E. coli in either 5- or 10-mL volumes, which were 50- to 100-fold greater than used in an earlier study (Brayman et al. 2017). Cells were treated with 1-MHz pulsed high-intensity focused ultrasound (10 cycles, 2-kHz repetition frequency, +65/-12.8 MPa focal pressures). The surviving fraction was assessed by coliform assay, and inactivation demonstrated curvilinear kinetics. The reduction of surviving fraction to 50% required 2.5 or 6 min in 5- or 10-mL samples, respectively. Exposure of 5 mL for 20 min reduced the surviving fraction to ∼1%; a similar exposure of 10-mL samples reduced the surviving fraction to ∼10%. Surviving cells from 5-min exposures appeared normal under light microscopy, with minimal debris; after 20 min, debris dominated. Transmission electron microscopy images of insonated samples showed some undamaged cells, a few damaged but largely intact cells and comminuted debris. Cellular damage associated with substantive but incomplete levels of inactivation can be variable, ranging from membrane holes tens of nanometers in diameter to nearly complete comminution.

Effect of heat treatment on cytotoxicity of self-adhesive resin cements: Cell viability analysis

Objective

The aim of the study was to assess, in vitro, the influence on cytotoxicity of heat treatment applied before photopolymerization, while mixing three self-adhesive resin cements, in an NIH/3T3 fibroblast cell culture, based on cell viability measures.

Methods

Samples were divided into three groups: (1) no heat treatment while mixing (control), (2) 37°C, and (3) 60°C heat treatment while mixing. Cements were light-cured immediately after mixing and immersed in Dulbecco's Modified Eagle Media for the extraction of possibly uncured products after 24 h and 7 days. Cultures contained 0.5 mL of NIH/3T3 fibroblasts per well at a concentration of 0.4 × 10⁵ cells/mL and specific extracts for each sample.

Statistical Analysis Used

Data were statistically analyzed with ANOVA and post hoc Student-Newman-Keuls (significance of 5%).

Results

Cement cytotoxicity increased with time, as shown by the higher values observed at 7 days. There was a slight difference in intragroup cytotoxicity levels between 24 h and 7 days. Heat treatment at 60°C was associated with a major decrease in cytotoxicity levels in all three groups, both at 24 h and at 7 days, with no differences among the cements.

Conclusions

Heat treatment at 60°C should be considered as a strategy to reduce cytotoxicity of self-adhesive resin cements, as evidenced by the results observed at 24 h and 7 days of analysis.

A CAD/CAM Zirconium Bar as a Bonded Mandibular Fixed Retainer: A Novel Approach with Two-Year Follow-Up

Stainless steel alloys containing 8% to 12% nickel and 17% to 22% chromium are generally used in orthodontic appliances. A major concern has been the performance of alloys in the environment in which they are intended to function in the oral cavity. Biodegradation and metal release increase the risk of hypersensitivity and cytotoxicity. This case report describes for the first time a CAD/CAM zirconium bar as a bonded mandibular fixed retainer with 2-year follow-up in a patient who is subjected to long-term treatment with fixed orthodontic appliance and suspected to have metal hypersensitivity as shown by the considerable increase of nickel and chromium concentrations in a sample of patient's unstimulated saliva. The CAD/CAM design included a 1.8 mm thickness bar on the lingual surface of lower teeth from canine to canine with occlusal rests on mesial side of first premolars. For better retention, a thin layer of feldspathic ceramic was added to the inner surface of the bar and cemented with two dual-cured cement types. The patient's complaint subsided 6 weeks after cementation. Clinical evaluation appeared to give good functional value where the marginal fit of digitized CAD/CAM design and glazed surface offered an enhanced approach of fixed retention.

Salivary levels of nickel, chromium, iron, and copper in patients treated with metal or esthetic fixed orthodontic appliances: A retrospective cohort study

The purpose of this retrospective cohort study was to measure the salivary levels of nickel (Ni), chromium (Cr), iron (Fe) and copper (Cu) released from metal and esthetic fixed orthodontic appliances. Ninety patients were divided into three groups (n=30): control (those who had never undergone orthodontic treatment), metal appliance (stainless steel brackets and bands, and nitinol archwires) and esthetic appliance (polycarbonate brackets and tubes, and rhodium-coated nitinol archwires). Patients undergoing orthodontic treatment had used their appliances for periods between one and six months. Ni, Cr, Fe and Cu salivary concentrations were measured by the Total Reflection X-Ray Fluorescence technique. Kruskal-Wallis and Bonferroni-Dunn test showed that Ni (p=0.027) and Cr (p=0.040) concentrations were significantly higher for patients undergoing metallic orthodontic treatment than for the esthetic group. No significant difference regarding Ni and Cr (p=0.447) concentrations were observed between the metal and the control groups (p=0.464 and p=0.447, respectively) or between the esthetic and the control groups (p=0.698 and p=0.912, respectively). Ni and Cr concentrations were significantly influenced by the type of appliance used. Fe and Cu concentrations were not affected by the type or use of orthodontic appliances.

A systematic study of the release of bisphenol A by orthodontic materials and its biological effects

INTRODUCTION

Bisphenol A (BPA) is a synthetic chemical substance used as a starting ingredient in the manufacturing process of a number or orthodontic materials. It is a well-known endocrine disruptor with low estrogenic properties. The aim of this investigation is to present a systematic review regarding the issue of bisphenol A release by orthodontic materials and its impact in orthodontics.

MATERIALS AND METHODS

A systematic analysis was performed by electronic search (between 1936 and 2015) on several data bases. The search was limited by using several specific key-words in two languages, English and French. Two investigators selected the responses, which met the selection criteria.

RESULTS AND DISCUSSION

Of the 376 studies found, only 21 met our selection criteria: 11 of these dealt with the release of bisphenol by orthodontic materials and 10 in vitro studies described the effects of BPA leaching from orthodontic materials on human and murine cells. The rate of BPA release was well below the daily tolerable intake (DTI) (50mg/kg/day in 2006, then 50μg/kg/day in 2015) according to the European Food Safety Authority (EFSA). Theoretical exposure to BPA was 11,000 times lower than recommendations. However, other studies have shown the presence of BPA and of monomers released in large quantities at very low doses. The effects of observed BPA varied significantly (toxic and carcinogenic potential) while some studies found no effects at all. The relatively small number of studies dealing with the release of Bisphenol A by orthodontic materials, apart from orthodontic materials and their significant biological effects, has led to the absence of standard protocols and has hindered precise determination of released BPA. Moreover, the lack of coherence between the various methodological approaches and variations in the experimental protocols have resulted in a low level of proof regarding the impact of BPA by orthodontic materials.

RECOMMENDATIONS

Through this study, the authors encourage clinicians to observe the following recommendations designed to reduce the amount of BPA released by materials used in orthodontics: keep the tip of the light-curing lamp as close as possible to the composite and perform indirect rather than direct light-curing; Pumice-polish the composite after bonding so as to reduce the potential amount of BPA released; reduce exposure by brushing or rinsing with a mouthwash during the first hour after bonding; follow a standardized, reproducible and expert-validated research protocol aimed at better understanding of BPA release.

Response of Primary Human Bone Marrow Mesenchymal Stromal Cells and Dermal Keratinocytes to Thermal Printer Materials In Vitro

Advancement in thermal three-dimensional printing techniques has greatly increased the possible applications of various materials in medical applications and tissue engineering. Yet, potential toxic effects on primary human cells have been rarely investigated. Therefore, we compared four materials commonly used in thermal printing for bioengineering, namely thermally printed acrylonitrile butadiene styrene, MED610, polycarbonate, and polylactic acid, and investigated their effects on primary human adult skin epidermal keratinocytes and bone marrow mesenchymal stromal cells (BM-MSCs) in vitro. We investigated indirect effects on both cell types caused by potential liberation of soluble substances from the materials, and also analyzed BM-MSCs in direct contact with the materials. We found that even in culture without direct contact with the materials, the culture with MED610 (and to a lesser extent acrylonitrile butadiene styrene) significantly affected keratinocytes, reducing cell numbers and proliferation marker Ki67 expression, and increasing glucose consumption, lactate secretion, and expression of differentiation-associated genes. BM-MSCs had decreased metabolic activity, and exhibited increased cell death in direct culture on the materials. MED610 and acrylonitrile butadiene styrene induced the strongest expression of genes associated to differentiation and estrogen receptor activation. In conclusion, we found strong cell-type-specific effects of the materials, suggesting that materials for applications in regenerative medicine should be carefully selected not only based on their mechanical properties but also based on their cell-type-specific biological effects.

In vitro biocompatibility of nickel-titanium esthetic orthodontic archwires

OBJECTIVE

To investigate the cytotoxicity of nickel-titanium (NiTi) esthetic orthodontic archwires with different surface coatings.

MATERIALS AND METHODS

Three fully coated, tooth-colored NiTi wires (BioCosmetic, Titanol Cosmetic, EverWhite), two ion-implanted wires (TMA Purple, Sentalloy High Aesthetic), five uncoated NiTi wires (BioStarter, BioTorque, Titanol Superelastic, Memory Wire Superelastic, and Sentalloy), one β-titanium wire (TMA), and one stainless steel wire (Stainless Steel) were considered for this study. The wire samples were placed at 37°C in airtight test tubes containing Dulbecco's Modified Eagle's Medium (0.1 mg/mL) for 1, 7, 14, and 30 days. The cell viability of human gingival fibroblasts (HGFs) cultured with this medium was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Data were analyzed by a two-way analysis of variance (α  =  .05).

RESULTS

The highest cytotoxic effect was reached on day 30 for all samples. The archwires exhibited a cytotoxicity on HGFs ranging from "none" to "slight," with the exception of the BioTorque, which resulted in moderate cytotoxicity on day 30. Significant differences were found between esthetic archwires and their uncoated pairs only for BioCosmetic (P  =  .001) and EverWhite (P < .001).

CONCLUSIONS

Under the experimental conditions, all of the NiTi esthetic archwires resulted in slight cytotoxicity, as did the respective uncoated wires. For this reason their clinical use may be considered to have similar risks to the uncoated archwires.