Assessing color stability of orthodontic esthetic wires in different staining solutions

Abstract

BACKGROUND:

Esthetic orthodontic wires are preferred for their ease to fit in with natural tooth color, but their susceptibility to staining in the oral environment poses a concern. Various Coatings such as Teflon and Epoxy aim to enhance appearance and biocompatibility but may still result in discoloration. Understanding the color stability of these wires under different staining conditions is crucial for a better and enhanced treatment plan.

OBJECTIVE:

This study intended to assess the color stability of esthetic orthodontic wires under various staining solutions that are often used in daily life.

METHOD:

Color changes of Teflon and Epoxy-coated esthetic orthodontic wires were meticulously measured at baseline, 7, 14, and 21-day intervals utilizing the precise CIE Lab* color measurement system. A total of thirty-two samples of wires from each brand were prepared ( $n=$ 8/group) and immersed in staining solutions (coffee, tea, cola, and saffron). The color change within and between the groups was statistically evaluated ( $p<$ 0.05).

RESULTS:

Significant variations in color stability were observed across different staining solutions. Saffron emerged as the most potent agent, inducing the most pronounced color changes, whereas cola demonstrated the least impact. Furthermore, Epoxy-coated wires consistently exhibited superior color stability compared to their Teflon-coated counterparts across all staining solutions and time intervals.

CONCLUSION:

This study underlines the significance for orthodontists to consider staining agents’ possible effects on orthodontic wires into account when selecting the orthodontic wires. The findings suggest that Epoxy-coated wires hold promise in mitigating discoloration issues during orthodontic therapy.

Keywords

Esthetic orthodontic wire color change tea coffee cola saffron

1. Introduction

The orthodontic wire has been an integral part of orthodontic appliances. The increasing emphasis on esthetics in orthodontic treatment, driven by patient preferences and advancements in materials like composite and ceramic brackets, has spurred research into the development of esthetically compatible orthodontic wires [1].

In pursuit of esthetically pleasing orthodontic wires, composite materials, incorporating ceramic fibers within a polymeric matrix, show promise [2]. However, solid polymeric wires, prized for their transparency or translucency [3], lack essential mechanical properties such as rigidity and strength required for orthodontic applications [4].

The predominant materials used for orthodontic wires remain metals, including stainless steel (SS) and nickel-titanium (NiTi) [5, 6]. The current solution to address this esthetic challenge involves coating stainless steel and NiTi orthodontic wires with tooth-colored resin materials like Teflon or Epoxy. However, certain authors have suggested that over time, the color of coated archwires may change, and there is a risk of the coating splitting during intraoral use, thus revealing the underlying metal [7, 8].

Color stability is a crucial clinical characteristic and an alteration in color might suggest deterioration or aging of a material [9]. Similarly, the color stability of esthetic orthodontic wires during orthodontic treatment is clinically important, any staining or discoloration will not only affect the cooperation and acceptance from the patient [7, 10] but also affect the mechanical properties. The intake of food and drinks together with the absorption of water cause alterations in color and esthetic attributes [11]. Additionally, smoking can impact the color stability of dental appliances and exacerbates discoloration and deterioration to both the esthetic and functional integrity of the appliances [12]. Quite a few factors, such as oral hygiene, water sorption, and food dyes, can influence the extent of color change [13]. Our daily intake of tea, coffee, and cola drinks and the use of spices, e.g., saffron in food promotes the discoloration of these wires. All these are considered the most chromogenic agents.

The CIE L*a*b* color space is a contemporary and highly recommended system for color evaluation, particularly proficient in discerning even minute color discrepancies. Numerous investigators have employed $\Delta$ E values to gauge the “perceptibility” of color differences. According to clinical threshold limit, if 50% of observers notice color difference, it would correspond to 50:50% perceptibility threshold while the difference in color that is acceptable for 50% of observers would corresponds to 50:50% acceptability threshold [14]. Additionally, to counter such differences and disagreements in the criteria used, the NBS rating system is frequently used to determine the degree of color difference, since it offers absolute criteria by which $\Delta$ E* values can be converted to definitions with clinical significance [5].

Therefore, this study intended to evaluate the color stability of two widely used brands of esthetic orthodontic wires using a CIE L*a*b* color measurement system after staining in above-named agents. It was hypothesized that there will be no significant variation in color over time at each time point when subjected to four distinct staining solutions in both esthetic orthodontic wires.

2. Materials and methods

2.1 Study sample

The study was conducted on a total of 84 coated esthetic orthodontic wires. The parameters were recorded at 4 different intervals i.e., baseline (0 days; T0), 7 days (T1), 14 days (T2) and 21 days (T3). The study samples included Epoxy-coated NiTi and Teflon-coated NiTi orthodontic wire of dimensions 0.019 $\times$ 0.025” (Table 1, Fig. 1). A total of thirty-two samples of each brand were prepared ( $n=$ 8/group). Each sample were made by placing 10-mm-long wire segments together and uniting their juxtaposed ends with ethyl cyanoacrylate (Superglue). The esthetic coating surface of each wire segment was oriented in the same direction, ensuring uniformity. Additionally, each sample was constructed to have a minimum width of 7 mm to enable accurate color measurement.

Table 1
NiTi wire with coating type and cross-sectional size used in study groups and subgroups

composition	Cross section size	coating type	Manufacturer
NiTi	0.019* 0.025”	Teflon	EverWhite, American Orthodontics, USA)
NiTi	0.019* 0.025”	Epoxy	Ultraesthetic, G&H Orthodontics, USA

2.2 Preparation of staining solution

Four types of staining solutions were prepared for the study. The details are present in Table 2.

Table 2
Preparation of staining solution

Solutions	Preparation	Replenishment
Coffee	15 g of coffee powder was infused with 500 ml of boiled distilled water and stirred every 30 min for 10 s. Once cooled to 37^∘C, the mixture was filtered through filter paper.	The solution was changed every 7 days and stirred for 1 min every day.
Tea	Five tea bags were boiled in 500ml of distilled water for 4 min. Next, the solution was allowed to cool at room temperature.	The solution was changed every 6 hours, that was 4 times a day.
Cola	500 ml of Cola was used at room temperature (24^∘C).	It underwent replacement twice daily.
Saffron	The solution was prepared by dissolving 0.2 g of Saffron leaves in 500 mL of boiled distilled water. It was filtered through a filter paper.	The solution was replaced every 7 days and stirred for 1 min every day.

Figure 1.

NiTi orthodontic wires with 0.019 $\times$ 0.025" cross-section: (A) Epoxy-coated and (B) Teflon-coated.

The findings were transformed to National Bureau of Standards (NBS) units in the manner described below to correlate the magnitude of colour change ( $\Delta$ E) with a clinical setting: $\Delta$ E $\times$ 3 $=$ 0.92 for NBS units. The NBS unit-quantified definitions of colour changes were applied. The concept of color change was recommended by Koksal and Dikbas [15], as given in Table 3.

Table 3

Critical marks of color change according to the national bureau standards

NBS unit	Amount	Definitions of color differences
0.0–0.5	Trace	Extremely slight change
0.5–1.5	Slight	Slight change
1.5–3.0	Noticeable	Perceivable change
3.0–6.0	Appreciable	Marked change
6.0–12.0	Much	Extremely marked change
12.0+	Very much	Change to other color

2.3 Color measurements

Color measurements were conducted using the Vita Easyshade V spectrophotometer (Vita Zahnfabrik, Bad Sackingen, Germany), adhering to the CIELAB system. Readings were taken for each wire band and solution. The color differences were presented as $\Delta$ E*, and were computed using the formula: $\Delta$ E* $=$ [( $\Delta$ L1*)^ 2 $+$ ( $\Delta$ a1*)^ 2 $+$ ( $\Delta$ b1*)^ 2^ (1/2). This formula considers the differences in lightness ( $\Delta$ L1*), green to red axis ( $\Delta$ a1*), and blue to yellow axis ( $\Delta$ b1*) between the specified time points.

2.4 Statistical analysis

The Statistical Package for Social Sciences (SPSS) version 28.0 of the statistical analysis software was used to examine the data. For every research group, the mean and standard deviation of the descriptive statistics were computed. The color change within the groups at different time intervals was evaluated using a paired sample $t$ -test. However, the inter-group comparisons were calculated with a two-way analysis of variance and a post hoc Tukey’s multiple comparison tests. The statistical significance was set at $p<$ 0.05.

Figure 2.

Mean $\Delta$ E values of the study groups at different time intervals using a Teflon-coated orthodontic wire.

3. Results

Figure 2 shows the $\Delta$ E values of the study groups at different time intervals using a Teflon-coated orthodontic wire. Saffron solution consistently produced the highest mean $\Delta$ E, ranging from approximately 4.98 at 7 days to 8.78 at 14 days and 14.13 at 21 days, indicating significant color alteration in Teflon materials. Conversely, Cola solution consistently yields lower mean Delta E values, ranging from approximately 0.16 at 7 days to 0.17 at 14 days and 0.18 at 21 days, suggesting relatively minimal color change compared to other solutions. Additionally, Coffee and Tea solutions exhibited intermediate mean $\Delta$ E values across the time points, with values falling between those of Saffron and Cola solutions.

Figure 3 presents the mean $\Delta$ E values using Epoxy-coated orthodontic wire across different solutions and time points. Notably, while Saffron solution induced the most significant color alteration, with mean $\Delta$ E values ranging from approximately 10.13 at 7 days to 8.93 at 14 days and 9.26 at 21 days, Cola solution consistently produced the lowest mean $\Delta$ E values, ranging from approximately 2.69 at 7 days to 3.21 at 14 days and 2.89 at 21 days. Conversely, Tea solution falls between these extremes, with mean $\Delta$ E values ranging from approximately 2.36 at 7 days to 2.36 at 14 days and 2.09 at 21 days.

Table 4
Paired differences analysis on $\Delta$ E across time intervals due to immersion of Teflon-coated orthodontic wires in different staining solutions

Staining solution	Time interval	Mean difference	Std. deviation	Std. error mean	95% CI lower	95% CI upper	$t$ -value	df	Sig. (2-tailed)
Coffee	T1	$-$ 0.90169	0.99065	0.3502	$-$ 1.7298	$-$ 0.0734	$-$ 2.574	7	0.037
	T2	$-$ 2.33731	1.35970	0.4807	$-$ 3.4740	$-$ 1.2005	$-$ 4.862	7	0.002
	T3	$-$ 2.56681	1.36286	0.4818	$-$ 3.7062	$-$ 1.4274	$-$ 5.327	7	0.001
Saffron	T1	$-$ 29.04200	5.93214	2.0973	$-$ 34.001	$-$ 24.082	$-$ 13.84	7	0.000
	T2	$-$ 22.33275	2.40572	0.8505	$-$ 24.343	$-$ 20.321	$-$ 26.25	7	0.000
	T3	$-$ 41.2225	3.91450	1.3840	$-$ 44.495	$-$ 37.949	$-$ 29.78	7	0.000
Cola	T1	$-$ 0.14725	0.37425	0.1323	$-$ 0.4601	0.16563	$-$ 1.11	7	0.303
	T2	$-$ 0.09913	0.73482	0.2598	$-$ 0.7134	0.51520	$-$ 0.382	7	0.714
	T3	$-$ 0.22450	0.70013	0.2475	$-$ 0.8098	0.36082	$-$ 0.907	7	0.395
Tea	T1	$-$ 2.4108	1.26610	0.4476	$-$ 3.469	$-$ 1.352	$-$ 5.386	7	0.001
	T2	$-$ 5.2518	0.76350	0.2699	$-$ 5.890	$-$ 4.613	$-$ 19.45	7	0.000
	T3	$-$ 7.0411	0.97170	0.3435	$-$ 7.853	$-$ 6.228	$-$ 20.49	7	0.000

Figure 3.

Mean $\Delta$ E values of the study groups at different time intervals using an Epoxy-coated orthodontic wire.

Table 4 presents the data of Teflon-coated orthodontic wires immersed in coffee solutions. A consistent decline in color stability ( $\Delta$ E) over time intervals of 7, 14, and 21 days was observed. The paired differences analysis revealed significant mean differences from baseline, indicating substantial deterioration at each interval ( $-$ 0.90169 at 7 days, $-$ 2.33731 at 14 days, and $-$ 2.56681 at 21 days) with $p$ -values of 0.037, 0.002, and 0.001, respectively. Similar trends were observed when exposed to saffron, showing significant decreases in color stability at 7, 14, and 21 days ( $-$ 29.04200, $-$ 22.33275, and $-$ 41.22250, respectively), all with $p$ -values of 0.000. However, when exposed to cola and tea staining solutions, Teflon-coated orthodontic wires demonstrated negligible alterations in color stability ( $\Delta$ E) across intervals of 7, 14, and 21 days, with non-significant p-values ranging from 0.303 to 0.714 for cola and 0.001 to 0.395 for tea.

Table 5

Paired differences analysis on $\Delta$ E across time intervals due to immersion of Epoxy-coated orthodontic wires in different staining solutions

Staining solution	Time interval	Mean difference	Std. deviation	Std. error mean	95% CI lower	95% CI upper	$-$ -value	df	Sig. (2-tailed)
Coffee	T1	$-$ 1.29688	0.56430	0.1995	$-$ 1.7686	$-$ 0.8251	$-$ 6.500	7	0.000
	T2	$-$ 0.35680	0.7108	0.2513	$-$ 0.9511	0.2373	$-$ 1.420	7	0.199
	T3	0.10213	1.48671	0.5256	$-$ 1.1408	1.3450	0.194	7	0.851
Saffron	T1	$-$ 7.87238	1.05045	0.3713	$-$ 8.7505	$-$ 6.9941	$-$ 21.197	7	0.000
	T2	$-$ 9.31088	1.38140	0.4884	$-$ 10.465	$-$ 8.1559	$-$ 19.064	7	0.000
	T3	$-$ 6.35113	1.37647	0.4866	$-$ 7.5018	$-$ 5.2003	$-$ 13.051	7	0.000
Cola	T1	0.03200	0.50659	0.1791	$-$ 0.3915	0.4555	0.179	7	0.863
	T2	0.24538	1.17474	0.4153	$-$ 0.7367	1.2274	0.591	7	0.573
	T3	$-$ 0.03838	1.05920	0.3744	$-$ 0.9238	0.8471	$-$ 0.102	7	0.921
Tea	T1	$-$ 1.2548	0.61239	0.2165	$-$ 1.766	$-$ 0.7420	$-$ 5.796	7	0.001
	T2	$-$ 1.1087	0.71376	0.2523	$-$ 1.705	$-$ 0.5120	$-$ 4.394	7	0.003
	T3	$-$ 0.4795	0.05381	0.0190	$-$ 0.524	$-$ 0.4340	$-$ 25.203	7	0.000

Table 5 presents the data related to Epoxy-coated orthodontic wires when exposed to different staining solutions. In the coffee solution, a significant decrease in color stability at 7 days ( $-$ 1.29688, $p=$ 0.000), suggesting an early impact. At 14 days, an insignificant decrease occurred ( $-$ 0.35688, $p=$ 0.199), implying potential stabilization. Surprisingly, at 21 days, a slight increase was observed (0.10213, $p=$ 0.851), also not significant, revealing a complex pattern in color stability dynamics. When exposed to saffron staining, Epoxy-coated orthodontic wires consistently demonstrated a significant decrease across all periods ( $-$ 7.87238 at 7 days, $-$ 9.31088 at 14 days, and $-$ 6.35113 at 21 days), highlighting the enduring influence of saffron staining on color stability (all $p$ -values $=$ 0.000). However, when immersed in cola solutions, Epoxy-coated orthodontic wires showed negligible alterations in color stability ( $\Delta$ E) over 7, 14, and 21 days, with insignificant p-values ranging from 0.573 to 0.921. For tea staining, a notable decrease was observed at all intervals, with significant mean differences ( $-$ 1.25488 at 7 days, $-$ 1.10875 at 14 days, and $-$ 0.47950 at 21 days) and p-values ranging from 0.001 to 0.000, indicating enduring effects over 21 days.

Table 6

Inter-group comparison of coating types over time on $\Delta$ E

Time interval	Groups	Mean	Std. Deviation	Std. Error Mean	$p$ -value
T0	Teflon-coated	1.1616	0.83765	0.14808	0.001
	Epoxy-coated	2.0474	1.08506	0.19181
T1	Teflon-coated	9.2871	12.84304	2.27035	0.052
	Epoxy-coated	4.6455	3.38121	0.59772
T2	Teflon-coated	8.6669	9.27227	1.63912	0.030
	Epoxy-coated	4.6802	4.19215	0.74107
T3	Teflon-coated	13.9254	17.25620	3.05049	0.002
	Epoxy-coated	3.7392	3.00443	0.53111

Table 6 displays data on two coating types (Teflon-coated and Epoxy-coated) at different time intervals (Baseline, 7 days, 14 days, and 21 days). Notably, the results suggest variations between the coating types over time, with significant differences observed at certain intervals, implying potential influences of coating type on the measured parameter.

4. Discussion

It was observed that there was a color change over time at each time point when both esthetic orthodontic wires were subjected to four distinct staining solutions. Therefore, the hypothesis is rejected.

There is no agreement on the most effective approach to recognize color shifts. Since a spectrophotometer can discriminate between each color scale, it is utilized to lessen subjectivity in the highly subjective visual color comparison approach. Furthermore, color measurements made with a spectrophotometer yield data that are more consistently assessed and more reproducible [16]. The esthetically coated wire should demonstrate ideal biocompatibility, frictional characteristics, color stability, force deflection, and surface coating [8].

The results showed that saffron solution induced the most significant color alteration in both Teflon-coated and Epoxy-coated orthodontic wires, while cola solution consistently produced the lowest mean color change values. In contrast, coffee and tea solutions fell between these extremes, with intermediate color change values. Notably, Teflon-coated orthodontic wires exhibited a decline in color stability over time when exposed to coffee and saffron solutions, with significant decreases observed at each interval. Conversely, minimal color alterations were noted when exposed to cola and tea solutions.

Epoxy-coated orthodontic wires displayed a significant decrease in color stability when immersed in coffee solutions at 7 days, followed by potential stabilization at 14 days and a slight increase at 21 days. Saffron staining consistently led to significant color alterations in Epoxy-coated orthodontic wires across all periods. In contrast, cola solutions induced negligible color changes, while tea staining resulted in notable decreases in color stability over the 21-day period.

Inter-group comparisons highlighted that saffron caused the most significant discoloration in both Teflon-coated and Epoxy-coated orthodontic wires, followed by tea and coffee solutions. Cola solution exhibited the least color change, indicating minimal staining effects. The NBS units further categorized the color changes, with saffron and tea solutions leading to appreciable or marked changes, while coffee and cola solutions resulted in trace or extremely slight alterations.

Intra-group comparisons between Teflon-coated and Epoxy-coated orthodontic wires demonstrated that Epoxy-coated wires were less stained compared to Teflon-coated wires, with significant differences in color stability. The study emphasized the susceptibility of esthetic orthodontic wires to color changes in the oral environment, influenced by the chromogens present in food and beverages. The causes of esthetic archwire’s color differences is due to water absorption, the adsorption or absorption of colorants from solutions [17].

The study by Mundim et al. (2010) reported that when exposed to coffee, cola, tea, and spices for 72 hours, spices and coffee had the most significant effect on color stability, while cola had the least impact [18]. This finding aligns with the notion that different staining solutions can have varying effects on the color stability of esthetic orthodontic wires. The interaction between orthodontic wire materials and staining solutions, as well as the chemical properties of the coatings, likely contribute to these observed differences in color alteration [19].

In contrast, a study by Darabi et al. (2019) suggested a different order of color change when exposed to staining solutions. According to their findings, coffee had the maximum color change, followed by turmeric, tea, and cola [20]. This discrepancy highlights the complexity of color stability in orthodontic materials and the need for further research to understand the underlying mechanisms driving these variations.

The observed variations in color stability among esthetic orthodontic wires might be attributed to a confluence of factors such as the composition of these orthodontic wires, including the chemical properties of Teflon and Epoxy coatings, likely plays a pivotal role, influencing their susceptibility to staining [21, 22]. Interactions between orthodontic wire materials and the diverse chromogens present in staining solutions, each with distinct chemical compositions, could lead to the observed differences in color alteration [4, 23]. Surface properties, microstructural variances, and the chemical stability of coatings over time may contribute to the varying degrees of color stability [6, 24]. Additionally, the absorption and retention of chromogens, influenced by the orthodontic wire’s ability to resist or retain stains, could contribute to the observed outcomes [25]. The chemical properties of staining solutions, such as saffron inducing more significant color changes compared to cola, further underscore the complexity of this interaction [26]. Moreover, the duration of exposure and potential influences of food and beverage components warrant consideration [27]. This research proved that that the type of material, coloring agent, and duration of exposure influence the color stability [11]. Therefore, in order to reduce the staining impact, clinicians should advise patients to rinse their mouths with tap water after intake of coloring food [9].

These findings emphasize the need for orthodontists to carefully select orthodontic wires based on optimal color stability and educate patients on minimizing consumption of staining agents to mitigate discoloration issues [28, 29]. However, caution is advised in interpretation of the results. Achieving reproducibility of oral cavity conditions is intricating owing to myriad factors such as the complex microbial flora and their metabolic by-products, as well as the deposition of biofilms on the tested materials [30]. Therefore, clinical studies are necessary in addition to future research related to nuanced interactions between orthodontic wire materials and staining solutions for advancing the development of esthetically pleasing and durable orthodontic wires [31, 32].

5. Conclusions

Within the limitations we can draw the conclusions that the saffron solution induced the most significant color alterations, while cola solution resulted in the least color change in both orthodontic wires. Although Epoxy-coated wires generally exhibited better color stability compared to Teflon-coated wires. These findings emphasize the importance of selecting orthodontic wires with optimal color stability and educating patients on minimizing consumption of staining agents to mitigate discoloration issues. Further research is warranted to understand the underlying mechanisms driving variations in color stability and to develop more durable orthodontic materials.

Funding

Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024 R331), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Data availability

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

Footnotes

Conflict of interest

The authors declare no conflict of interest.

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