Oral epithelial cell reaction after exposure to Invisalign plastic material

Effect of post-printing curing time on cytotoxicity of direct printed aligners: A pilot study

INTRODUCTION

The aim of this in vitro study was to examine the potential impact of different curing times of 3D-printed orthodontic aligners on their cytotoxicity.

METHOD

Some 60 samples of aligner material were directly 3D printed using Tera Harz TC-85 DAC resin and randomly allocated to three different curing time groups (14, 24 and 50 min). Zendura FLX samples were used as control. The samples were incubated in saliva for 14 days, and then the supernatant was collected. Human gingival fibroblasts (HGF-1)-CRL2014 were used to evaluate potential cytotoxicity. Furthermore, HGF-1 cells were plated on the samples as well as on a glass control sample. After 72 h of growth, their viability was tested.

RESULTS

Compared with the glass, only the 50-min curing time markedly reduced fibroblast cell growth. Additionally, a negative linear trend was observed between curing time and fibroblast growth. In comparison with the aligner control group, all samples, including the aligner control samples, exhibited a significant reduction in the viability of human fibroblasts when exposed to saliva.

CONCLUSIONS

3D directly printed aligners showed a cytotoxic effect similar to that of thermoformed conventional aligners in terms of fibroblasts growth. A linear trend was found between curing time and cells growth, indicating that directly printed aligners could exhibit higher cytotoxicity if exposed to a longer curing time. This dependence on curing time underscores the importance of following a strict manufacturing process.

Bisphenol-A release from thermoplastic clear aligner materials: A systematic review

AIM

The aim of the present study was to undertake a systematic review of the available evidence on the release of bisphenol-A (BPA) from thermoplastic materials used in the fabrication of clear aligners (CA).

METHODS

Electronic databases, such as MEDLINE (via PubMed), Google Scholar, Scopus, Cochrane Library, Web of Science, OpenGrey, and the U.S. National Institute of Heath-Clinical Trials, were searched up to 27 October 2022. In vivo/in vitro studies that assessed the release of BPA from different thermoplastic CA materials, with or without a control group, were selected. The risk of bias (RoB) in the randomised controlled trials (RCT) and in vitro studies was assessed using the Cochrane RoB tool and the guidelines for the reporting of pre-clinical studies, respectively. The quality of evidence was determined using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) Pro tool.

RESULTS

Six studies were considered for review from a total of 1926 records. This included one RCT and five in vitro studies. Only two studies found leaching of BPA, while four did not report any traces. The RoB was found to be moderate to high. The GRADE evidence level ranged from low to very low. Five of the included studies were conducted in vitro. Significant heterogeneity among the included studies prevented a quantitative synthesis.

CONCLUSION

In light of the available conflicting evidence, BPA release from CAs can neither be confirmed nor denied. Safety remains questionable until high-quality in vivo trials prove otherwise.

REGISTRATION

PROSPERO CRD42022310434.

Optimized High-Content Imaging Screening Quantifying Micronuclei Formation in Polymer-Treated HaCaT Keratinocytes

Research on nano- and micro-plastic particles (NMPPs) suggests their potential threat to human health. Some studies have even suggested genotoxic effects of NMPP exposure, such as micronuclei (MN) formation, while others found the opposite. To clarify the ability of NMPP to induce MN formation, we used non-malignant HaCaT keratinocytes and exposed these to a variety of polystyrene (PS) and poly methyl methacrylate (PMMA) particle types at different concentrations and three different sizes. Investigations were performed following acute (one day) and chronic exposure (five weeks) against cytotoxic (amino-modified NMPPs) and genotoxic (methyl methanesulfonate, MMS) positive controls. An optimized high-content imaging workflow was established strictly according to OECD guidelines for analysis. Algorithm-based object segmentation and MN identification led to computer-driven, unsupervised quantitative image analysis results on MN frequencies among the different conditions and thousands of cells per condition. This could only be realized using accutase, allowing for partial cell detachment for optimal identification of bi-nucleated cells. Cytotoxic amino-modified particles were not genotoxic; MMS was both. During acute and long-term studies, PS and PMMA particles were neither toxic nor increased MN formation, except for 1000 nm PS particles at the highest concentration of unphysiological 100 µg/mL. Interestingly, ROS formation was significantly decreased in this condition. Hence, most non-charged polymer particles were neither toxic nor genotoxic, while aminated particles were toxic but not genotoxic. Altogether, we present an optimized quantitative imaging workflow applied to a timely research question in environmental toxicity.

Immune System Acts on Orthodontic Tooth Movement: Cellular and Molecular Mechanisms

Orthodontic tooth movement (OTM) is a tissue remodeling process based on orthodontic force loading. Compressed periodontal tissues have a complicated aseptic inflammatory cascade, which are considered the initial factor of alveolar bone remodeling. Since skeletal and immune systems shared a wide variety of molecules, osteoimmunology has been generally accepted as an interdisciplinary field to investigate their interactions. Unsurprisingly, OTM is considered a good mirror of osteoimmunology since it involves immune reaction and bone remolding. In fact, besides bone remodeling, OTM involves cementum resorption, soft tissue remodeling, orthodontic pain, and relapse, all correlated with immune cells and/or immunologically active substance. The aim of this paper is to review the interaction of immune system with orthodontic tooth movement, which helps gain insights into mechanisms of OTM and search novel method to short treatment period and control complications.

Advances in orthodontic clear aligner materials

Rapid technological improvements in biomaterials, computer-aided design (CAD) and manufacturing (CAM) have endorsed clear aligner therapy (CAT) as a mainstay of orthodontic treatment, and the materials employed for aligner fabrication play an all-important role in determining the clinical performance of clear aligners. This narrative review has attempted to comprehensively encompass the entire gamut of materials currently used for the fabrication of clear aligners and elucidate their characteristics that are crucial in determining their performance in an oral environment. Historical developments and current protocols in aligner fabrication, features of contemporary bioactive materials, and emerging trends related to CAT are discussed. Advances in aligner material chemistry and engineering possess the potential to bring about radical transformations in the therapeutic applications of CAT; in the absence of which, clear aligners would continue to underperform clinically, due to their inherent biomechanical constraints. Finally, while innovations in aligner materials such as shape memory polymers, direct three-dimensional (3D) printed clear aligners and bioactive materials combined with clear aligner materials are essential to further advance the applications of CAT; increased awareness of environmental responsibilities among aligner manufacturers, aligner prescribing clinicians and aligner users is essential for better alignment of our climate change goals towards a sustainable planet.

Leaching from a 3D-printed aligner resin

AIM

To quantitatively assess the degree of conversion and the water-leaching targeted compound from 3D-printed aligners.

MATERIALS AND METHODS

3D-printed aligners were made of photopolymerized resin (Tera Harz TC85A). The molecular structure and degree of conversion of the set resin were investigated by ATR-FTIR spectroscopy (n = 5). The aligners (n = 10) were immersed in double distilled water for 1 week at 37°C and the eluents were analysed using liquid chromatography/mass spectrometry methods (LC-ESI-MS/MS for urethane dimethacrylate [UDMA] and LC-APCI-MS/MS for bispenol-A [BPA]).

RESULTS

The resin was composed of aliphatic vinyl ester-urethane monomers, with acrylate and/or methacrylate functionalization. The degree of conversion was estimated as to 83%. There was no detection of BPA in any of the assessed samples (0.25 µg/l). Quantifiable amounts of UDMA were detected in all the exposed samples, ranging from 29 to 96 µg/l.

CONCLUSIONS

Although efficiently polymerized and BPA free, the great variability in the amount of UDMA monomer leached from the examined samples may raise concerns on potential health hazards after repeated intraoral exposure, which is indicated for this class of materials.

Cytotoxicity assessment of different clear aligner systems: An in vitro study

OBJECTIVES

To evaluate and compare the cytotoxicity of multiple clear aligner systems (Invisalign, Eon, SureSmile, and Clarity).

MATERIALS AND METHODS

A cytotoxicity assessment was carried out by immersing three sets of aligners from the included four systems in normal saline for 1 month at 37°C. The solutions were then diluted to three different concentrations (5%, 10%, and 20% volume/volume). Gingival fibroblasts were exposed to the solution after being seeded to 96-well microplates for 48 hours, and the medium was substituted with an MTT solution (MTT: 3-[4,5-dime- thylthiazol-2-yl]-2,5-diphenyltetrazolium bromide). Optical density was then measured to determine cell viability and evaluate cytotoxicity subsequently.

RESULTS

Cytotoxicity comparison showed no statistically significant difference among the four included systems. However, when cell viability of each system was compared with the control, a significant difference was reported at the 10% and 20% solution concentrations. The Clarity system had the lowest toxicity across all solution concentrations.

CONCLUSIONS

The thermoplastic materials used by all tested systems (Invisalign, Eon, SureSmile, and Clarity) presented some degree of toxicity (slight to moderate), with statistically significant mean differences compared with the control.

Cytotoxicity evaluation of different clear aligner materials using MTT analysis

Backround: The in vitro cytotoxic effects of six different clear aligner materials were evaluated using the MTT analysis.

Methods: The clear aligner material samples [Duran (ScheuDental GmbH, Iserlohn, Germany), Zendura-Flx (Bay Materials LLC, Fremont, CA, USA), Taglus (Laxmi Dental Export Pvt. Ltd, Mumbai, India), Smart Track (Align Technology, San Jose, CA, USA), Zendura (Bay Materials LLC, Fremont, CA, USA), Essix C + (Essix® (Raintree Essix, Inc., 4001 Division St, Metairie, LA-USA)] were initially kept in a saline solution in airtight test tubes for 8 weeks at 37°C. According to the recommended ISO standards, the weights of the samples were divided by the volumes of the dilutions in the ratio of 0.1 g/ml. To evaluate the cytotoxicity of the samples, an MTT analysis was performed using a human gingival fibroblast cell line (HGF). To analyse the data, the Kruskal– Wallis test was applied (a=0.05).

Results: Zendura was the most cytotoxic material resulting in 67.3 ± 16.20% cell viability, followed by Smart Track with 87.6 ± 5.53% cell viability. While Duran, Essix C + had 92.6 ± 26.34% and 94.9 ± 8.54% cell viability, Zendura-Flx, Taglus had 106.9 ± 12.76% and 113.183 ± 7.45% cell viability, respectively.

Conclusion: While Zendura and Smart Track showed mild cytotoxicity, other materials showed greater cell viabilities. According to the ISO standards, the clinical use of each brand of aligners, except Zendura, may be considered reliable. Taking into account standard deviation, Zendura and Duran should be used with caution. The suppliers of aligners should adhere to the manufacturer’s recommendations since an increase in ion release might arise from material wear.

Behaviour of Human Oral Epithelial Cells Grown on Invisalign® SmartTrack® Material

Invisalign aligners have been widely used to correct malocclusions, but their effect on oral cells is poorly known. Previous research evaluated the impact of aligners’ eluates on various cells, but the cell behavior in direct contact with aligners is not yet studied. In the present study, we seeded oral epithelial cells (cell line Ca9-22) directly on Invisalign SmartTrack material. This material is composed of polyurethane and co-polyester and exhibit better mechanical characteristics compared to the predecessor. Cell morphology and behavior were investigated by scanning electron microscopy and an optical cell moves analyzer. The effect of aligners on cell proliferation/viability was assessed by cell-counting kit (CCK)-8 and 3,4,5-dimethylthiazol-2-yl-2,5-diphenyl tetrazolium bromide (MTT) assay and live/dead staining. The expression of inflammatory markers and proteins involved in epithelial barrier function was measured by qPCR. Cells formed cluster-like structures on aligners. The proliferation/viability of cells growing on aligners was significantly lower (p < 0.05) compared to those growing on tissue culture plastic (TCP). Live/dead staining revealed a rare occurrence of dead cells on aligners. The gene expression level of all inflammatory markers in cells grown on aligners’ surfaces was significantly increased (p < 0.05) compared to cells grown on TCP after two days. Gene expression levels of the proteins involved in barrier function significantly increased (p < 0.05) on aligners’ surfaces after two and seven days of culture. Aligners’ material exhibits no cytotoxic effect on oral epithelial cells, but alters their behavior and the expression of proteins involved in the inflammatory response, and barrier function. The clinical relevance of these effects has still to be established.

The dynamics of the oral microbiome and oral health among patients receiving clear aligner orthodontic treatment

OBJECTIVES

This 6-month prospective clinical study assessed the impacts of Invisalign appliances on the oral bacterial community and oral health of patients.

METHODS

Salivary samples were obtained from twenty-five adult patients receiving Invisalign aligner treatment before the treatment (Group B) and at a 6-month follow-up (Group P). The bacterial composition of each sample was determined using Illumina MiSeq sequencing of the bacterial 16S rRNA. Intra- and intergroup biodiversity was analyzed. Clinical periodontal parameters and daily oral hygiene habits were recorded.

RESULTS

Reduction in plaque, increased daily brushing frequency, and decreased dessert intake were observed in Group P compared with that in Group B. A total of 1,853,952 valid reads were obtained from the 50 salivary samples, with 37,904 sequences per sample. No significant differences were detected in the intra- and intergroup biodiversity comparisons between the two groups. By clustering, 8,885 OTUs were identified and categorized into six major phyla: Firmicutes, Proteobacteria, Bacteroidetes, Fusobacteria, Actinobacteria, and Candidate_division_TM7_norank. At the genus level, compared with Group B, Group P demonstrated significantly increased Bacillus abundance and decreased Prevotella abundance.

CONCLUSIONS

Our results suggested that the general biodiversity and salivary microbial community structure did not change significantly and that patients had increased beneficial oral hygiene habits and awareness during the first six months of Invisalign treatment. Hence, on the basis of this study, it appears that Invisalign aligner treatment did not induce deterioration of oral health nor significant biodiversity changes in oral bacterial communities, assuming that detailed oral hygiene instructions for both teeth and aligners were provided.

In vitro cytotoxicity of different thermoplastic materials for clear aligners

OBJECTIVES

To investigate the in vitro cytotoxicity of different thermoplastic materials for clear aligners on human primary gingival fibroblasts (HGFs).

MATERIALS AND METHODS

Four materials for clear aligners were considered in this study: Duran (Scheu-Dental GmbH, Iserlohn, Germany), Biolon (Dreve Dentamid GmbH, Unna, Germany), Zendura (Bay Materials LLC, Fremont, CA, USA), and SmartTrack (Align Technology, San Jose, CA, USA). Three out of four materials (Duran, Biolon, Zendura) were assessed as thermoformed and nonthermoformed, whereas the SmartTrack was assessed only as thermoformed. The samples were placed at 37°C in airtight test tubes containing Dulbecco's Modified Eagle's Medium (DMEM; 0.1 mg/mL) for 14 days. The cell viability of 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 means of one-way and two-way analysis of variance and post hoc tests (α = 0.05).

RESULTS

Each material exhibited a slight cytotoxic effect after 14 days. The highest cytotoxicity level on HGFs was achieved by Biolon (64.6% ± 3.3 of cell viability), followed by Zendura (74.4% ± 2.3 of cell viability), SmartTrack (78.8% ± 6.3 of cell viability), and finally Duran (84.6% ± 4 of cell viability), which was the least cytotoxic. In the comparison between nonthermoformed and thermoformed materials for Duran, Biolon, and Zendura, the thermoformed materials showed the highest level of cytotoxicity (P < .001).

CONCLUSIONS

Under the experimental conditions of this study, all the materials for clear aligners presented a slight cytotoxicity. Biolon was the most cytotoxic and the thermoforming process increased the cytotoxicity of the materials.

Colour stabilities of three types of orthodontic clear aligners exposed to staining agents

The aim of this study was to evaluate and compare the colour stabilities of three types of orthodontic clear aligners exposed to staining agents in vitro. Sixty clear orthodontic aligners produced by three manufacturers (Invisalign, Angelalign, and Smartee) were immersed in three staining solutions (coffee, black tea, and red wine) and one control solution (distilled water). After 12-h and 7-day immersions, the aligners were washed in an ultrasonic cleaner and measured with a colourimeter. The colour changes (ΔE*) were calculated on the basis of the Commission Internationale de I'Eclairage L*a*b* colour system (CIE L*a*b*), and the results were then converted into National Bureau of Standards (NBS) units. Fourier transformation infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) were conducted to observe the molecular and morphologic alterations to the aligner surfaces, respectively. The three types of aligners exhibited slight colour changes after 12 h of staining, with the exception of the Invisalign aligners stained with coffee. The Invisalign aligners exhibited significantly higher ΔE* values (ranging from 0.30 to 27.81) than those of the Angelalign and Smartee aligners (ΔE* values ranging from 0.33 to 1.89 and 0.32 to 1.61, respectively, P<0.05). FT-IR analysis confirmed that the polymer-based structure of aligners did not exhibit significant chemical differences before and after the immersions. The SEM results revealed different surface alterations to the three types of aligner materials after the 7-day staining. The three types of aesthetic orthodontic appliances exhibited colour stability after the 12-h immersion, with the exception of the Invisalign aligners stained by coffee. The Invisalign aligners were more prone than the Angelalign and Smartee aligners to pigmentation. Aligner materials may be improved by considering aesthetic colour stability properties.

Patient survey on Invisalign® treatment comparing [corrected] the SmartTrack® material to the previously used [corrected] aligner material

OBJECTIVES

In 2013, a novel material SmartTrack^® was introduced to replace the previous material used for Invisalign^® aligners. We conducted this study to test how this transition was accepted by patients during ongoing Invisalign^® treatment.

METHODS

We surveyed a total of 72 patients (68 % women, 32 % men, mean age: 29.3 ± 9.2 years) who had worn the new material for a mean of 6 months. They completed a questionnaire with 25 items that compared both materials in terms of pain, pressure upon insertion, comfort, mucosal irritation, phonetics, discoloration, and taste.

RESULTS

On a numerical rating scale (NRS) ranging from 0 to 10, the new aligner material was rated by the respondents to cause significantly (p < 0.001) less maximum pain than the old material (2.8 vs. 3.8). Significant reductions were obtained for duration of pain (p < 0.001) and duration of pressure (2.5 vs. 1.9 days; p = 0.001) upon insertion of the aligners. Over 90 % of the respondents indicated less or equal pain during eating, felt that the new material offered a tighter fit, and provided more favorable ratings for impairment, durability, and discoloration. Improved comfort was reported by 50 % of the respondents.

CONCLUSIONS

The new material was favorably rated by the patients and showed significant reductions in pain intensity, pain duration, and pressure upon insertion. Important clinical parameters like overall comfort and impairment also were improved. The clinical effectiveness of the new material remains to be investigated.

INFLUENCE OF FRICTION IN TRANSPARENT TOOTH CORRECTION TREATMENT: FINITE ELEMENT METHOD

Objective: The objective was to evaluate the effect of frictional forces on the canine's translation and rotation in transparent tooth correction treatment. Materials and Methods: Based on computed tomography (CT) images of the teeth and their supporting tissues, solid models were used to build finite-element models. One canine was simulated translation and rotation in the buccolingual direction by aligners with different friction coefficients. Results: The change of the canine's rotation centers and periodontal ligament (PDL) stress with an increase of the friction coefficient in the two movement types were obtained. Conclusions: Friction coefficient has little effect on the canine's translation movement. While in rotation cases, the small friction coefficient seems to be helpful to gain the body movement type.

[Invisible and almost invisible orthodontic appliances]

Patient demand for invisible esthetic orthodontic appliances is steadily increasing. Two types of orthodontic appliances have a high rate of patient acceptance. The lingual technique has continually evolved by offering both prefabricated brackets or custom-made brackets. The various techniques have been improved over time, and the results are better than ever. Thermoplastic aligners are an alternative to lingual devices and their use is rapidly growing. Their specific properties make it easier to assess their indications and limitations. A review of the literature, the development of these systems and a presentation of some clinical examples of treated cases will help us to provide some of the basics for understanding each of these two types of appliances and to demonstrate the advantages and disadvantages of each system.