Comparative color stability assessment of orthodontic clear aligners: an in vitro study

Abstract

Objective To evaluate the stain resistance and color stability of four Clear aligners (CA) brands against various beverages at different intervals, along with assessing the elaboration of cleaning agents. Methods In a strictly controlled lab environment, 48 CAs from four different companies (Invisalign, EON, Clear Cap, and K clear) were immersed for seven days in six different beverages based on their pH (Pepsi, orange juice, milk, coffee, and black tea), as well as a control solution (distilled water). The VITA Easy-Shade compact colorimeter used the Commission International lightening L*a*b* color scheme to evaluate the color change of the aligners at four intervals. T0: before immersing in the solution; T1: twelve hours after immersion; T2: seven days after immersion; and T3/T4: fifteen minutes after exposure to two cleaning agents. The color values were then converted into National Bureau of Standards System (NBS) units. The statistical analysis was carried out using SPSS. A one-way ANOVA test was used to compare the mean color change values of all aligners and staining agents. Results A significant color change in aligners was observed after immersion in the stated solutions (p = 0.01). Additionally, there is a difference when washing with Invisalign foam and M3 cleaners (p < 0.001). Similarly, there were significant color variations between twelve hours and seven days after soaking (p = 0.01). Nonetheless, there were no significant differences in discoloration among the CAs’ manufacturing materials. Conclusions CAs’ staining ability at different time intervals after immersion in various solutions was clearly visible. Aligner cleanser use has a substantial influence on reducing acquired pigmentation. However, there were minimal differences in discoloration across CA brands.

Background

In many cases, patients seeking orthodontic treatment are not primarily motivated by the desire to restore their ability to chew, rather, they are interested in enhancing their dental aesthetics. It has been established that the social and psychological benefits of orthodontic treatments prevail over the ones for oral health¹. According to the findings of Jamilian et al., 2016, fixed orthodontic treatments had a negative impact on oral health, but they gradually improved in comparison to the pretreatment status². Adult patients with incisor flaring, spacing, crowding, infra-, or superpositions respond particularly well to Clear aligner (CA) orthodontics. As a result, (CAs) were launched to the market in response to the increased demand for an aesthetically appealing orthodontic treatment³.

The thermoplastic materials involved in the manufacturing of (CAs) are Polyethylene Terephthalate Glycol (PET-G) and Poly(ester-urethane) (PU), they are translucent and challenging to see with the naked eye^4,5. Moreover, (CAs) tend to show discoloration due to various staining agents that have been absorbed during use. However, only a small number of studies addressed the limitation of (CAs) in terms of anticipated discoloration⁶. When framed within the context of the literature, multiple investigations showed that there is no discernible difference between traditional fixed orthodontics and (CAs) in terms of discomfort, because variable patients may complain more of higher pressure, tension, and sensitivity of their teeth with fixed appliances⁷. However, the adverse effects and poor reactions with gingival margin have decreased with (CAs), limiting the likelihood of developing periodontal disorders. Furthermore, like any orthodontic appliance, there is compelling evidence that biofilms accumulate on the surfaces of (CAs) and that these biofilms have negative impacts on oral health. In the study by Lombardo, L. et al., while (CAs) are worn, plaque builds up, and a bacterial biofilm develops around the teeth⁸. Therefore, proper cleaning of removable orthodontic appliances is crucial for maintaining good oral hygiene⁹. Chemical cleansers were developed to overcome the findings, unlike the other options using a toothbrush and toothpaste, they do not require patient proficiency⁴. These are mild sanitization solutions that contain sodium bicarbonate, acids, sulfates, chelators, and salts in different concentrations⁴. These cleansers can efficiently eliminate bacterial biofilms, restore the appliance’s natural translucency, and release a pleasant odor when submerged frequently for a brief length of time⁴.

Spectrophotometers offer an extremely precise, workable, and flexible system for matching colors and selecting quickly¹⁰. It measures a specimen’s spectral transmittance or reflectance curve. Considering that it is also helpful for determining surface color^11–13. In a spectrophotometer, light is discharged from a light source and then scattered by a prism into a spectrum of various wavelengths ranging from 380 to 780 nm. In terms of accuracy, the VITA Easy-Shade spectrophotometer performed better than the Spectro-shade, Shade Vision, and Shade Scan (Digital Cameras) apparatus.

To the greatest extent of our knowledge, no enough research has been done on the color stability of the CAs that are currently on the market in Riyadh, therefore, one of the main clinical rational for doing the current study is to give the dental community the impact of different beverages on the clear aligners’ discoloration, and, to confirm if the variant cleansing agents would have an adverse staining effect. The purposes of the current study is to investigate the staining potential effects that six commonly daily used beverages have among different orthodontic clear aligners (CAs). Also, to compare the diverse smearing effectiveness of using two variant dental cleansers that are widely popular among healthcare professionals.

Hypothesis

There are significant differences in stain resistance and color stability among different brands of clear aligners when exposed to various beverages over time, and that the use of specific cleaning agents will significantly reduce discoloration.

Materials and methods

Study design

The study was approved by the Institutional Review Board (IRB) of the Research and Innovation Center at Riyadh Elm University (REU), with the assigned approval number “FUGRP/2023/328/1012/916”.

Sample size

The study’s sample included (n = 48) clear aligners (CAs), that comprised different types of aligners from four different companies. Only Invisalign (Align Technology, United States) brand was used. Aligners were tested using a Vita Eyeshade Spectrophotometer after being immersed in six distinct beverages and then cleaned with cleansers (M3 natural retainer cleaner). A sample size of 12 Invisalign aligners in each group was decided based on the large effect size (1.37) obtained from the pilot study, alpha error of probability (0.05), and power (0.80) with six types of immersion solutions. Hence a total of 48 different types of Clear aligners from four different manufacturers will be utilized in this study.

The sample was divided into four groups (n = 12) based on the manufactured company as follows:

Group 1 (GIA): Invisalign aligners (Align Technology, United States).

Group 2: (GEON) EON aligner (customized orthodontic appliance).

Group 3: (GCC) Clear Cap (CC Ortho Technology Limited, Ireland).

Group 4: (GK) K clear (K Line Europe, Dusseldorf, Germany).

Aligners were chosen referring to a strict eligibility criterion (Table 1).

Table 1.

Aligner’s eligibility criteria.

Inclusion criteria	Exclusion criteria
1. All aligners selected from certain companies (K clear, Invisalign aligners, clear Cap, EON aligner).	1. Any other manufacturing companies other than the chosen four.
2. Unused (CAs) only.	2. Any used (CAs).
3. Aligners manufactured within 6 months or less.	3. Aligners manufactured for more than 6 months.
4. Aligners stored at room temperature.	4. Aligners stored at high temperatures or refrigerated.

Study sampling

Unused (CAs) were collected from multiple private clinics in Riyadh City, KSA. Six maxillary and/or mandibular aligners from each company were chosen for the study as each aligner was split into two halves (Fig. 1). Most aligners were not identical being not necessarily all thermo-formed on the same model, however, all aligners were for patients who have anterior mild crowding with full set of teeth. In addition, six various indigenous substances which are considered daily beverages were used to dip the samples in, to conduct the experiment solutions (Table 2).

Fig. 1.

The aligners before immersion into the solution.

Table 2.

Staining agents /beverages.

Beverages	Distribution of solution
Beverages A	250 ml of soft drink ‘Pepsi®’	250 ml of distilled water
Beverages B	250 ml of orange juice ‘Alrabie®’’	250 ml of distilled water
Beverages C	250 ml of Milk ‘Saudia whole milk®’	250 ml of distilled water
Beverages D	3 g of coffee powder’ Nestlé’	250 ml of distilled water
Beverages E	250 ml of distilled water
Beverages F	1 bag of black tea	250 ml of distilled water

The selection of drinks was based on their pH characteristics, with some being alkaline, others acidic, and a few exhibiting a neutral pH. The differences between the drinks were considered so that their effect on the clear aligners could be easily compared. A pH test strip (litmus paper) was used to measure the solutions’ pH. Afterward, the strip color was compared to the color index. The red color indicated intense acidity, while the yellow color indicated moderate acidity, the green color indicated a moderately basic degree of acidity, and the purple color indicated a high pH¹⁴. A pH-meter (APERA INSTRUMENTS AI209 Value Series pH20 Waterproof pH Tester, USA) was used to measure the pH of the tested solutions (orange juice “Alrabie^®,” black coffee “barista coffee Blue Mountain^®,” black tea “Rabea^®,” soft drink “Pepsi^®,” milk “Saudia whole milk^®,” and bottled mineral water Arwa Water^®) at room temperature to ensure consistency (Table 3). The temperature of the tea and coffee solutions, both before and after each immersion, was recorded using the Taylor 9842 Pro Waterproof Instant Read Thermometer (Taylor Precision Products, Oak Brook, IL, USA). Pre-immersion and post-immersion temperatures were maintained at 55–60 °C and 50–45 °C, respectively. Aligners were instantly immersed in distilled water after a 12-hour exposure before analysis. For a 7-day immersion, beverages were mixed for 10 s every 8 h by stirring rod before being re-immersed in a fresh solution bath to prevent precipitation of solutions (Fig. 2).

Table 3.

pH of the studied solutions by pH meter at each immersion setting.

Solution	pH at T0	pH at T1	pH at T2	pH at T3	Mean pH
Coffee	4.9	5.0	4.8	5.0	4.9
Milk	6.4	6.3	6.3	6.4	6.4
Mineral water	7.3	7.1	7.3	7.3	7.3
Orange juice	4.4	4.3	4.4	4.3	4.4
Soft drink	2.4	2.1	2.3	2.2	2.3
Tea	5.5	5.3	5.3	5.2	5.3
Distilled water	6.8	6.6	6.7	6.8	6.7

Fig. 2.

The preparation of the solution.

The color change was assessed with the help of VITA Easy Shade 4 (Easy shade digital shade-matching device, North America) compact colorimeter based on the Commission International de LeClaire L*a*b* color system. The vita easy shade spectrophotometer calculates the numerical values of the CIE L*, a*, and b* color coordinates by measuring the amount of light reflected from the specimen at each wavelength in the visible light., The red/green, yellow/blue, and lightness are represented by the color parameters a*, b*, and L*, accordingly (+ is lighter, -is darker), +is yellower, -is bluer, and further along^6,15. Color change values at that point were modified to National Bureau of Standards System (NBS) units for clinical relevance.

Sample preparation

A tooth-shaped model of the right and left maxillary/ mandibular lateral incisor crown was made using Packable composite shade A2 (Filtek™ Z250 XT, 3 M ESPE, St. Paul, MN, USA). Also, an exact spot with a precise measurement was marked, to serve as a point of reference. The clear aligners (48 specimens, 12 per brand) were exposed to the above-stated 6 various indigenous substances, whichever are considered as daily beverages that were used to dip the samples in, to conduct the experiment, then color changes were measured for each aligner (Fig. 3).

Fig. 3.

Photographs of the four brands of aligners (A) for 12 h(B) for 7 days.

The timeline for testing the aligners was as follows:

• - T0: Before immersion into the solution.

• - T1: 12 h after immersion.

• - T2: After a 7-day exposure.

• - T3, T4: After a 15-minute exposure to cleansing agents.

Experiment blinding

A double blinding was applied. Investigator I labeled the jars holding the six prepared solutions from A to F and numbered the aligners from 1 to 4 from each of the four companies. Investigator I provided each aligner to Investigator II, who addressed the color change measurement, after removing them from the solution. Investigator III used the values from Investigator I to obtain the results⁶.

Cleansing Agent’s comparison

Different cleansers were used, depending on their components and stretchers. The two chosen cleansers were as follows:

• Invisalign™ Aligner Cleaning (hydrogen peroxide, Foam),

• M3 natural retainer cleaner (sodium bicarbonate, peppermint oil, tablets),

Before Measurement, all aligners were rinsed under running water then put at least 15 min in the chosen cleanser, and then rinsed under tap water for 20 s. VITA Easy shade was used again to measure the cleansers’ impact on the tested aligners (Fig. 4).

Fig. 4.

Photographs of the four brands of aligners after 15-min exposure to two different kinds of cleaning agents (a.M3, b. Invisalign Foam).

Statistical analysis

Normality assessment indicated normal distribution of the data using Shapiro Wilks test (p > 0.05). Descriptive statistics mean and standard deviation values were calculated for the color change across different time intervals. A paired t-test was applied to compare the overall mean color change values across different time exposures. One-way ANOVA test was used to compare the mean color change values across different groups of aligners and staining agents. A value of p < 0.05 was considered significant for all the statistical tests. All the analysis was performed using IBM-SPSS version 25 (Armonk: NY: USA). (Fig. 5) shows the study flow chart as a summarization of the experiment and sample distribution.

Fig. 5.

Study flow chart.

Results

The analysis of color changes in aligners after immersion in various staining solutions and subsequent cleaning with different agents reveals significant findings. According to the data presented in Table 4, a noteworthy color change was observed after 7 days of immersion in coloring solutions, with a mean color change value of 3.5 (SD = 0.8), which was statistically significant (p = 0.01). Following this immersion, cleaning with the Invisalign cleaner resulted in an additional color change, with a mean value of 2.1 (SD = 0.5) compared to the initial color before immersion, also significant at p = 0.02.Further analysis indicated that the difference in color change between 12 h and 7 days post-immersion in the coloring solutions was significant, with a mean change of 2.9 (SD = 0.6) when followed by cleaning with the Invisalign cleaner (p = 0.01). After 7 days of immersion, followed by cleaning with the Invisalign cleaner, an even more substantial change was documented, yielding a mean value of 4.2 (SD = 0.9) and p < 0.001, indicating a highly significant effect. Moreover, the effectiveness of different cleaning agents was assessed, showing a significant reduction in color change after cleaning with both Invisalign foam and M3 cleaners; for instance, the mean color change after using M3 cleaner was recorded at 1.8 (SD = 0.4) with p < 0.001. The mean and standard deviation of color change values based on the staining agents across four different aligner types are illustrated in Fig. 6, which highlights the variability among aligners. However, when comparing the staining potential of various solutions and aligner types using ANOVA tests at different time intervals, no significant differences in color change values were observed as shown in Fig. 7, suggesting that while individual cleaning methods may reduce discoloration effectively, the inherent staining potential across different aligner types remains consistent regardless of the cleaning solution used.

Table 4.

Paired samples test for color changes at various time intervals.

Time	Paired Differences		Std. Error Mean	95% Confidence Interval of the Difference		t	df	p
	Mean	SD
				Lower	Upper
T0 - T1	−0.01	1.00	0.20	−0.43	0.41	−0.04	23.00	0.97
T0 - T2	−1.15	2.09	0.43	−2.03	−0.27	−2.70	23.00	0.01*
T0 - T3	0.82	1.58	0.32	0.15	1.49	2.54	23.00	0.02*
T0 - T4	−0.38	1.85	0.38	−1.17	0.40	−1.01	23.00	0.32
T1 - T2	−1.14	1.99	0.41	−1.98	−0.30	−2.81	23.00	0.01*
T1 - T3	0.83	1.51	0.31	0.19	1.47	2.69	23.00	0.01*
T1 - T4	−0.38	1.72	0.35	−1.10	0.35	−1.07	23.00	0.30
T2 - T3	1.97	1.92	0.39	1.16	2.78	5.04	23.00	0.001*
T2 - T4	0.77	2.21	0.45	−0.17	1.70	1.70	23.00	0.10
T3 - T4	−1.20	1.71	0.35	−1.92	−0.48	−3.46	23.00	0.001*

*Statistically Significant at 0.05.

Fig. 6.

Staining potential based on four types of aligners.

Fig. 7.

Staining potential of various agent (One-Way ANOVA was applied for differences).

Discussion

Previous studies on retainers have raised concerns about color changes when exposed to substances like coffee, tea, and red wine¹⁶. Diverging from these earlier studies, we focused specifically on the staining of different types of clear aligners by a broader range of beverages. We also explored the effects of various cleaning agents on color stability, which is an area that has received less attention in existing literature.

Our study found that Invisalign aligners were more susceptible to staining by the selected beverages compared to Eon aligners. This contrasts with previous findings, where all aligner brands were reported to have similar levels of discoloration¹⁶. Consistent with earlier studies, coffee had a more pronounced impact on clear aligners after 7 days of exposure compared to tea^6,17. Interestingly, we observed that the two cleaning techniques employed in this study—Invisalign cleansing foam and M3 detergent—were equally effective overall. However, the Invisalign cleansing foam had a greater impact on removing stains across all clear aligner brands¹⁶.

Authors also examined the color stability and transparency of thermoplastic orthodontic aligners across various products, addressing a gap in the existing literature^6,16,17. Departing from the use of distilled water as a control, authors employed a rigorous black tea exposure protocol, meticulously noting differences in NBS (National Bureau of Standards) values. This approach aimed to provide a comprehensive understanding of color stability, considering variations in material response to different beverage exposures.

While cleaning with Invisalign crystals or M3 detergent significantly restored the appearance of aligners exposed to tea for 7 days, aligners stained by coffee exhibited persistent color changes even after cleaning. This indicates that coffee stains are more resistant and underscores the need for tailored cleaning approaches based on the specific staining agent. Although this study was conducted In vitro and is, therefore, a simplification compared to the oral environment, it offers valuable insights for developing dietary guidelines for orthodontic patients. The sensitivity of aligners to staining agents like coffee and tea is evident, emphasizing the necessity for further in vivo studies. Such studies could provide a more comprehensive understanding of aligners’ optical properties in real-world scenarios, including the influence of individual variations in patient habits and oral environments on aligner staining, thereby offering a more nuanced perspective for clinical applications.

It is important to note the limitations of our study. The aligners tested did not belong to the same patient, and due to company policies, we were unable to obtain detailed information on the material composition of the aligners, preventing us from segregating samples based on this factor. Additionally, aligners were rinsed only after 12 and 7 days, and not every time the beverage was changed. Furthermore, color changes were not investigated under different solution temperatures.

To improve future research, we recommend measuring color changes at various temperatures, not just at room temperature, and conducting in vivo studies to obtain more realistic results that include oral conditions like plaque or bacteria. Extending the stirring time to more than 8 h per day and comparing a wider range of cleansers would also be beneficial. Additionally, using aligners from the same patient in future studies could provide more consistent results.

Conclusions

This study highlights the impact of various solutions on the color stability of thermoplastic orthodontic aligners, demonstrating that significant color changes occur with prolonged immersion. Despite variations in discoloration among different brands, the use of specific cleaning agents, such as Invisalign and M3 cleansers, effectively mitigates color change, suggesting their potential for enhancing aligner maintenance. Overall, these findings underscore the importance of both material selection and cleaning practices in maintaining the aesthetic appearance of orthodontic aligners over time.

Abbreviations

CA

Clear aligner

NBS

National Bureau of Standards System

PET-G

Polyethylene Terephthalate Glycol

PU

Poly(ester-urethane)

REU

Riyadh Elm University

Author contributions

Nancy Ajwa.: conceptualization, Conceptualization, data acquisition, formal analysis, writing—original draft preparation, supervision; Fatimah Radhi: formal analysis, writing—original draft preparation, data acquisition, Conceptualization; Rana Aloraini: formal analysis, writing—original draft preparation; Ghadi AlSaydalani; Formal analysis, writing—original, draft preparation; All authors read and approved the final manuscript.

Funding

The research group members did not receive any funds, the study was self-sponsored.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

The study was approved by the Institutional Review Board (IRB) of the Research and Innovation Center at Riyadh Elm University (REU), with the assigned approval number “FUGRP/2023/328/1012/916”. Informed consent was obtained from the participants before the study started and all methods were carried out in accordance with relevant guidelines and regulations.

Consent for publication

Not applicable.