Assessment of the Susceptibility of Aesthetic Orthodontic Brackets to Discoloration

Abstract

The high aesthetic value of orthodontic appliance elements during treatment constitutes an important factor for an increasing number of adult patients. The aim of this study was to determine whether food dyes could significantly affect the color of both plastic and ceramic orthodontic brackets. Four brands of orthodontic brackets were investigated in the present study. Twenty-five samples of each kind were prepared. Five brackets of each series were stored in pure water, coffee, black tea and red wine for 1 h, 24 h, 7 days and 14 days. Total color change ΔE* of the samples was analyzed after storage with the use of the SpectroShade dental spectrophotometer (MHT, Verona, Italy) according to the CIE L*a*b* color scale. Correlations between bracket brand, kind of food dye and intensity of color change at the significance level p = 0.05 were investigated. After 1 and 24 h of incubation, water had the least influence on the color change of aesthetic orthodontic brackets, and tea had the greatest effect (p = 0.05). After 7 and 14 days of incubation of the samples, water still remained the environment influencing ΔE* change to the smallest extent, whereas storage in red wine changed the color of brackets to the significantly (p = 0.05) highest degree. The degree of discoloration of the assessed brackets depended on the type of material and the storage time in the environment of the individual food dyes (p = 0.05). The results of the present study show that, in the event of contact with food dyes, aesthetic orthodontic brackets discolor, the intensity of which can be influenced by the materials they are made of, the kind of food dye and the time of samples’ storage.

1. Introduction

Orthodontic treatment includes both developmental and adult patients. Its aim is not only to obtain correct occlusal conditions and proper function of the masticatory system but also to improve the aesthetics of teeth and patient’s facial features. In the case of children, the medical aspect in relation to the decision to start therapy is critical. In the case of adolescents and adults, the decisive factor is obtaining desired visual effects [1].

Fixed thin-arch orthodontic appliances commonly used to treat malocclusion in permanent dentition consist of brackets glued to the labial surfaces of incisors, canines and premolars and buccal tubes or bands attached to molars. The time of malocclusion therapy significantly affects the aesthetics of teeth during treatment. The solution that does not interfere with the appearance of dental arches is the use of braces mounted on the lingual surfaces of teeth or the use of aligners, which are becoming more and more popular. Unfortunately, both of the above-mentioned methods have limitations that do not allow optimal results to be achieved in the treatment of severe malocclusion [2,3,4,5]. In addition, the use of lingual techniques significantly increases the costs of therapy and often extends the period of follow-up visits. It is also associated with less comfortable working conditions for the doctor due to hindered access to elements of the appliance [6,7].

The solution that allows improved visual properties of a fixed orthodontic appliance glued on labial surfaces of teeth is the use of elements made of materials that harmonize in color with enamel and dentine. On the market, there are orthodontic arches coated with polymers similar in color to tooth enamel or produced in the process of ion deposition, e.g., rhodium ions [8], as well as flexible ligatures, the colors of which allow for changing of the aesthetic properties of the appliance. Unfortunately, these products are not free from disadvantages, such as the susceptibility of elastomers to discoloration and frequent damage to arc coatings. Due to the price and good performance characteristics, the most commonly used brackets are made of various types of iron alloys, but their opacity and metallic color make the appliance clearly visible and do not harmonize with the colors of the patient’s dental arches.

Manufacturers of orthodontic materials, following the expectations of patients and orthodontists, have introduced brackets made of polymeric materials and ceramics similar in color to enamel and dentin.

Plastics were used for the production of orthodontic brackets as early as the 1970s. The initially used acrylics and polycarbonates were replaced with polyurethane and polycarbonates reinforced with inorganic fillers or glass fiber. The advantages of polymer-based brackets also include their relatively low price, which made them very popular. However, these products turned out to be susceptible to discoloration caused by exposure to UV radiation, products of metabolism of bacterial flora present in the oral environment and dyes delivered with food or drinks.

Highly aesthetic, ceramic orthodontic brackets were introduced to the market in the 1980s. Even though they are not ideal products, they have been widely used until now. The disadvantages of this type of product include:

-

Lower precision of manufacture, compared to metal brackets.

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High brittleness.

-

Higher hardness than in the case of enamel, causing its abrasion and, compared to brackets based on iron alloys, higher friction between the slot and the arch of the appliance, which adversely affects treatment time.

The advantages of brackets made of mono- or polycrystalline aluminum oxide include:

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Good color matching with tooth tissues.

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High translucency in the case of monocrystalline brackets and very good biocompatibility of ceramics, which allows use of these medical devices in patients who are allergic to nickel.

Although ceramic orthodontic brackets are considered resistant to absorption of colored substances from the oral cavity, there are reports in the available literature that they are susceptible to discoloration under the influence of prolonged contact with various food dyes [9,10,11].

At the same time, clinical experience shows that in the case of hygiene negligence combined with specific dietary habits, some of the aforementioned materials become discolored in the course of orthodontic treatment.

The preliminary hypothesis of the present study was that aesthetic orthodontic brackets discolor to a different degree under the influence of contact with dyes contained in popular foodstuffs.

2. Aim

During orthodontic treatment of both adolescents and adults, the high aesthetics of the components of orthodontic appliances mounted on visible surfaces of teeth are extremely important. Patients expect dental materials to maintain their clinical and visual properties throughout the treatment period. Orthodontic brackets available on the market today, both ceramic and polymer-based, should, according to the manufacturers’ declarations, meet the requirements set for them.

The aim of this study was to compare the susceptibility to discoloration of the assessed orthodontic brackets in contact with individual food dyes in subsequent observation periods.

3. Material and Methods

The study evaluated brackets intended for use on maxillary central incisors (Hype (Ortho Classic, Mcminnville, OR, USA)) made of copolymer and ceramic brackets based on aluminum oxide (Pure Lucent (Ortho Technology, Tampa, FL, USA), Symetri Clear (Ormco, Brea, CA, USA), 20/40 Brackets (American Orthodontics, Sheboygan, WI, USA) and Discovery Pearl (Dentaurum, Ispringen, Germany)).

The materials assessed in the study belong to the group of generally available orthodontic bracket systems from leading manufacturers offered on European markets.

The list of assessed materials is presented in

Table 1. Materials assessed in the study.

Commercial Name	Producer	Declared Composition Based on Producers’ Information
Hype *	Ortho Classic, Mcminnville, OR, USA	Medical-grade compound copolymer *
Pure Lucent	Ortho Technology, Tampa, FL, USA	Aluminum oxide 99.998% *
Symetri Clear	Ormco Corp., Brea, CA, USA	“ceramic” *
20/40	American Orthodontic, Sheboygan, WI, USA	Aluminum oxide 100% *
Discovery Pearl	Dentaurum, Ispringen, Germany	Aluminum oxide 99.99% *

As stated in the safety data sheet/SDS on the manufacturer’s website.

.

The brackets were subjected to initial colorimetric analysis using the SpectroShade dental spectrophotometer (MHT, Verona, Italy).

The brackets were placed on a white, non-transparent background. During the colorimetric evaluation of the samples, measurements were taken in the central part of the brackets. The area of the analyzed surface had constant dimensions of 3 mm (horizontal) by 2 mm (vertical), regardless of the sample size.

The color of each sample was described with numerical values in the CIE L*a*b* system (Commission Internationale de l’Eclairage L*a*b* color scale). This system allows assignment of numerical values to colors of the tested objects, determining their brightness (L*), saturation of green/red (+/−a *) and blue/yellow (+/−b *) colors.

Five pieces of each type of brackets were placed, respectively, in distilled water, black tea and coffee solutions and red wine at 36 °C. After one hour, 24 h, 7 days and 14 days of incubation, the brackets were removed from water and food dye solutions, cleaned with a prophylactic brush (5 s each) under running water and then subjected to another colorimetric evaluation.

Discolored ceramic brackets are shown in Figure 1.

The obtained results were entered into an Excel spreadsheet (Microsoft, Redmond, WA, USA) and then statistically analyzed using the IBM SPSS Statistics ver. 27.0 software (IBM Corp., Armonk, NY, USA). Among others, mean, minimum, maximum and standard deviation values for L*, a* and b* values were calculated. Changes in the mean values describing the color, i.e., ΔL*, Δa*, Δb*, were used to calculate the value of the ΔE* parameter describing the total color change of the tested material sample, according to the following formula [12]:

ΔE* = (ΔL*² + Δa*² + Δb*²)^1/2

In order to investigate relationships between the total color change and the type of assessed brackets, time of incubation in the environment of food dyes and type of dye, the one-way analysis of variance was calculated. To test ANOVA assumptions, Shapiro–Wilk’s tests were used to test normality of dependent variables, and Levene’s tests were implemented to verify the homogeneity of variances. In few cases, significant results of Levene’s tests were observed; therefore, Welch’s variance-weighted ANOVA with Games–Howell post hoc tests was used. All statistical hypotheses were verified at the significance level of p = 0.05.

4. Results

Mean changes in the color ΔE* of Pure Lucent (Ortho Technology, Tampa, FL, USA) brackets with respect to incubation time and type of dye are presented in

Table 2. Mean color changes of Pure Lucent brackets with respect to incubation time and type of dye.

Material	Dye	Period	n	Mean ΔE*	SD	Minimum	Median	Maximum
Pure Lucent Ortho Technology	Coffee	1 h	5	2.55	0.76	1.69	2.28	3.63
		24 h	5	4.96	1.58	2.93	4.53	7.00
		7 days	5	10.81	2.84	7.91	10.75	14.97
		14 days	5	14.73	4.04	10.41	14.48	20.57
	Tea	1 h	5	5.79	1.37	4.81	5.35	8.17
		24 h	5	12.02	2.35	10.23	11.23	15.98
		7 days	5	20.41	4.37	15.67	21.28	26.00
		14 days	5	23.94	0.67	22.99	23.99	24.67
	Wine	1 h	5	5.12	2.26	1.77	5.60	8.01
		24 h	5	10.04	2.46	7.15	9.99	13.71
		7 days	5	34.35	5.93	25.90	37.23	39.34
		14 days	5	42.24	5.73	33.50	43.68	49.12
	Water	1 h	5	1.08	0.31	0.75	1.12	1.43
		24 h	5	0.96	0.94	0.45	0.59	2.63
		7 days	5	4.17	1.24	2.82	3.84	6.16
		14 days	5	1.35	0.90	0.50	0.88	2.63

. The greatest change in sample color was caused by storage of the tested materials in red wine for 14 days.

Mean changes in the color ΔE* of Hype (Ortho Classic) brackets with respect to incubation time and type of dye are presented in

Table 3. Mean color changes of Hype brackets with respect to incubation time and type of dye.

Material	Dye	Period	n	Mean ΔE*	SD	Minimum	Median	Maximum
Hype Ortho Classic	Coffee	1 h	5	1.98	0.51	1.16	2.15	2.44
		24 h	5	3.23	0.52	2.65	3.16	3.98
		7 days	5	5.21	0.54	4.56	5.09	5.99
		14 days	5	7.12	0.79	6.17	7.40	7.83
	Tea	1 h	5	1.24	0.42	0.83	1.12	1.91
		24 h	5	2.92	0.43	2.34	2.94	3.54
		7 days	5	3.62	0.64	2.54	3.99	4.02
		14 days	5	9.14	1.89	7.50	8.80	12.09
	Wine	1 h	5	1.49	0.73	0.32	1.72	2.14
		24 h	5	2.74	1.15	1.91	2.42	4.70
		7 days	5	9.90	1.86	7.97	10.03	12.64
		14 days	5	14.81	6.16	8.10	13.21	24.60
	Water	1 h	5	0.85	0.29	0.42	0.88	1.22
		24 h	5	0.82	0.16	0.57	0.81	0.95
		7 days	5	4.32	1.29	2.78	4.15	5.69
		14 days	5	1.95	0.92	0.95	2.40	2.85

. Storing the brackets in red wine for 14 days resulted in the greatest color change.

As in the case of the materials described above, the 20/40 brackets (American Orthodontic, Sheboygan, WI, USA) also discolored the most after 14 days of contact with the coloring substances contained in red wine. The mean values of ΔE* are presented in

Table 4. Mean color changes of 20/40 brackets with respect to incubation time and type of dye.

Material	Dye	Period	n	Mean ΔE*	SD	Minimum	Median	Maximum
20/40 American Orthodontics	Coffee	1 h	5	3.72	1.03	1.91	4.22	4.31
		24 h	5	6.57	1.70	3.68	6.96	7.82
		7 days	5	7.26	1.73	4.18	7.94	8.23
		14 days	5	9.25	1.57	6.86	9.40	10.89
	Tea	1 h	5	5.91	2.83	2.12	5.59	9.34
		24 h	5	9.95	4.29	5.26	9.09	16.16
		7 days	5	11.27	4.64	6.27	10.37	18.44
		14 days	5	15.15	4.61	10.67	13.43	22.66
	Wine	1 h	5	3.95	0.54	3.41	3.73	4.71
		24 h	5	5.94	1.16	4.15	6.08	7.14
		7 days	5	16.49	1.94	14.21	16.84	18.46
		14 days	5	31.13	3.91	27.09	30.17	36.29
	Water	1 h	5	0.97	0.40	0.60	0.88	1.52
		24 h	5	0.79	0.59	0.41	0.51	1.83
		7 days	5	3.77	1.63	1.96	4.57	5.47
		14 days	5	0.95	0.68	0.00	1.10	1.58

.

Moreover, in the case of the Symetri Clear (Ormco, Brea, CA, USA) and Discovery Pearl (Dentaurum, Ispringen, Germany) brackets, the prolonged incubation of samples in red wine caused the most color change of these orthodontic materials. Mean color changes of the above-mentioned brackets depending on incubation time and type of storage environment are presented in

Table 5. Mean color changes of Symetri Clear brackets with respect to incubation time and type of dye.

Material	Dye	Period	n	Mean ΔE*	SD	Minimum	Median	Maximum
Symetri Clear Ormco	Coffee	1 h	5	4.71	1.80	3.48	4.08	7.89
		24 h	5	7.94	2.21	6.40	7.34	11.80
		7 days	5	8.27	2.30	6.97	7.06	12.30
		14 days	5	11.08	2.56	9.46	9.66	15.50
	Tea	1 h	5	7.41	1.81	4.58	8.49	8.79
		24 h	5	10.47	1.62	9.03	9.79	12.77
		7 days	5	10.98	1.42	9.59	10.64	13.29
		14 days	5	15.74	2.26	13.32	14.66	18.95
	Wine	1 h	5	3.99	1.13	2.31	4.62	5.05
		24 h	5	7.04	2.01	3.71	7.92	8.65
		7 days	5	23.06	4.55	18.40	21.82	29.32
		14 days	5	34.15	5.55	29.14	31.92	40.86
	Water	1 h	5	0.54	0.27	0.22	0.62	0.83
		24 h	5	0.58	0.17	0.37	0.51	0.79
		7 days	5	1.39	0.34	0.94	1.36	1.91
		14 days	5	0.62	0.28	0.22	0.73	0.93

and

Table 6. Mean color changes of Discovery Pearl brackets with respect to incubation time and type of dye.

Material	Dye	Period	n	Mean ΔE*	SD	Minimum	Median	Maximum
Discovery Pearl Dentaurum	Coffee	1 h	5	5.27	2.15	2.45	6.58	7.12
		24 h	5	7.63	2.17	4.88	8.75	9.55
		7 days	5	10.99	2.39	7.09	11.10	13.41
		14 days	5	10.25	2.79	6.91	11.40	12.89
	Tea	1 h	5	6.30	1.65	3.39	6.94	7.34
		24 h	5	10.21	2.79	5.58	11.28	12.44
		7 days	5	11.63	1.91	8.51	12.16	13.33
		14 days	5	16.26	2.03	14.79	15.76	19.80
	Wine	1 h	5	2.27	0.64	1.44	2.42	3.11
		24 h	5	6.57	0.48	5.97	6.39	7.12
		7 days	5	32.39	0.94	31.30	32.76	33.47
		14 days	5	38.93	1.02	37.34	39.56	39.70
	Water	1 h	5	0.45	0.18	0.24	0.41	0.68
		24 h	5	2.25	2.42	0.45	1.35	6.50
		7 days	5	1.11	0.35	0.58	1.10	1.57
		14 days	5	1.59	0.76	0.85	1.39	2.44

, respectively.

Figure 2, Figure 3, Figure 4 and Figure 5 present diagrams showing mean changes in the color of the brackets in individual incubation environments.

The results of statistical tests illustrating differences in discoloration of the assessed orthodontic brackets in individual incubation environments and subsequent observation periods are presented in

Table 7. Comparisons of ΔE* between materials within dye (water, coffee) and period subgroups.

Period	Material	Dye
		Water						Coffee
		p *	p **					p *	p **
			PL	H	20/40	SC	DP		PL	H	20/40	SC	DP
1 h	PL	0.023	-	0.736	0.984	0.097	0.037	0.013	-	0.654	0.331	0.226	0.193
	H		0.736	-	0.980	0.442	0.169		0.654	-	0.076	0.111	0.110
	20/40		0.984	0.980	-	0.349	0.184		0.331	0.076	-	0.817	0.625
	SC		0.097	0.442	0.349	-	0.974		0.226	0.111	0.817	-	0.991
	DP		0.037	0.169	0.184	0.974	-		0.193	0.110	0.625	0.991	-
24 h	PL	0.247	-	0.996	0.996	0.887	0.797	0.002	-	0.277	0.564	0.203	0.269
	H		0.996	-	1.000	0.226	0.699		0.277	-	0.046	0.036	0.043
	20/40		0.996	1.000	-	0.929	0.701		0.564	0.046	-	0.802	0.902
	SC		0.887	0.226	0.929	-	0.593		0.203	0.036	0.802	-	0.999
	DP		0.797	0.699	0.701	0.593	-		0.269	0.043	0.902	0.999	-
7 days	PL	0.001	-	1.000	0.991	0.028	0.019	0.002	-	0.048	0.227	0.561	1.000
	H		1.000	-	0.972	0.028	0.019		0.048	-	0.225	0.163	0.022
	20/40		0.991	0.972	-	0.126	0.089		0.227	0.225	-	0.926	0.125
	SC		0.028	0.028	0.126	-	0.710		0.561	0.163	0.926	-	0.418
	DP		0.019	0.019	0.089	0.710	-		1.000	0.022	0.125	0.418	-
14 days	PL	0.055	-	0.828	0.926	0.494	0.989	0.008	-	0.057	0.158	0.488	0.337
	H		0.828	-	0.371	0.129	0.957		0.057	-	0.166	0.106	0.259
	20/40		0.926	0.371	-	0.843	0.641		0.158	0.166	-	0.669	0.950
	SC		0.494	0.129	0.843	-	0.185		0.488	0.106	0.669	-	0.986
	DP		0.989	0.957	0.641	0.185			0.337	0.259	0.950	0.986	-

* Welch’s variance-weighted ANOVA p-value; ** pairwise Games–Howell post hoc test p-value.

(incubation/storage in water and coffee) and

Table 8. Comparisons of ΔE* between materials within dye (tea, wine) and period subgroups.

Period	Material	Dye
		Tea						Wine
		p *	p **					p *	p **
			PL	H	20/40	SC	DP		PL	H	20/40	SC	DP
1 h	PL	0.000	-	0.006	1.000	0.538	0.981	0.001	-	0.092	0.788	0.846	0.191
	H		0.006	-	0.088	0.006	0.009		0.092	-	0.003	0.026	0.425
	20/40		1.000	0.088	-	0.849	0.999		0.788	0.003	-	1.000	0.013
	SC		0.538	0.006	0.849	-	0.840		0.846	0.026	1.000	-	0.119
	DP		0.981	0.009	0.999	0.840	-		0.191	0.425	0.013	0.119	-
24 h	PL	0.000	-	0.004	0.869	0.744	0.797	0.001	-	0.007	0.081	0.304	0.140
	H		0.004	-	0.091	0.001	0.018		0.007	-	0.015	0.029	0.004
	20/40		0.869	0.091	-	0.999	1.000		0.081	0.015	-	0.821	0.790
	SC		0.744	0.001	0.999	-	1.000		0.304	0.029	0.821	-	0.982
	DP		0.797	0.018	1.000	1.000	-		0.140	0.004	0.790	0.982
7 days	PL	0.000	-	0.004	0.071	0.032	0.039	0.000	-	0.002	0.008	0.060	0.939
	H		0.004	-	0.089	0.000	0.002		0.002	-	0.004	0.008	0.000
	20/40		0.071	0.089	-	1.000	1.000		0.008	0.004	-	0.132	0.000
	SC		0.032	0.000	1.000	-	0.969		0.060	0.008	0.132	-	0.042
	DP		0.039	0.002	1.000	0.969	-		0.939	0.000	0.000	0.042	-
14 days	PL	0.000	-	0.000	0.056	0.004	0.003	0.000	-	0.001	0.049	0.248	0.719
	H		0.000	-	0.179	0.007	0.003		0.001	-	0.010	0.005	0.004
	20/40		0.056	0.179	-	0.999	0.985		0.049	0.010	-	0.850	0.045
	SC		0.004	0.007	0.999	-	0.995		0.248	0.005	0.850	-	0.434
	DP		0.003	0.003	0.985	0.995	-		0.719	0.004	0.045	0.434	-

* Welch’s variance-weighted ANOVA p-value; ** pairwise Games–Howell post hoc test p-value.

(incubation/storage in tea and wine).

The comparison of mean ΔE* values calculated (p = 0.05) in total for all the assessed bracket types showed that the fluids used in the study discolored orthodontic materials to significantly different degrees.

After 1 h of incubation, water had the least influence on the color change of aesthetic orthodontic brackets, and tea had the greatest effect.

After 24 h of observation, the above-mentioned relationships remained unchanged.

After 7 days and 14 days of incubation of the samples, water still remained the environment influencing ΔE* change to the smallest extent, whereas storage in red wine changed the color of brackets to the significantly highest degree.

The graphical visualization of the color change ΔE* observed after the 14-day incubation period in relation to the type of brackets and sample storage environment is presented in Figure 6.

5. Discussion

An analysis of the results of the study showed that color change of the assessed aesthetic orthodontic brackets depends on the type of material from which the samples were made, the time of their incubation in external environment conditions and on the type of food dye used.

5.1. Susceptibility of Individual Types of Orthodontic Brackets to Discoloration with Respect to Different Sample Storage Environments and Incubation Periods

An analysis of the results showed that in subsequent time intervals, the orthodontic brackets assessed in the study discolored to a different extent in the environment of individual food dyes.

5.1.1. Water

After 1 h of storing the samples in water, Discovery Pearl brackets were observed to change color to a significantly lesser degree than Pure Lucent brackets. After 24 h of incubation, no significant differences in color change were found between the assessed materials. Observations after 7 days showed that Discovery Pearl and Symetri Clear brackets changed color to a significantly lesser degree than Pure Lucent and Hype brackets. After 14 days of storing the samples in water, changes in ΔE* values were comparable for all the assessed materials.

5.1.2. Coffee

Observations carried out after 1 h of storing the samples in coffee showed no differences in the degree of color change of all tested materials. After 24 h of observation, it was shown that the 20/40, Symetri Clear and Discovery Pearl brackets discolored to a significantly greater degree than the Hype brackets. In the following period of observation, Hype brackets turned out to be much less susceptible to color change compared to Pure Lucent and Discovery Pearl. After 14 days of storage in a coffee solution, the color changes of all the assessed products turned out to be comparable.

5.1.3. Tea

Significantly, after one hour of storage in tea infusion, the smallest color change was observed for Hype brackets compared to the Pure Lucent, Symetri Clear and Discovery Pearl. During the remaining observation periods, the Ortho Classic products also proved to be significantly less susceptible to discoloration with dyes contained in the tea solution compared to the above-mentioned products from Ortho Technology (Tampa, FL, USA), Ormco (Brea, CA, USA) and Dentaurum (Ispringen, Germany).

5.1.4. Red Wine

In the red wine environment, Hype brackets from Ortho Classic proved to be more resistant to discoloration compared to 20/40 and Symetri Clear. After 24 h of storage in red wine, the Ortho Classic products discolored to a significantly lesser extent compared to the other companies’ products. This relationship did not change in the subsequent observation periods.

The authors take into account the limitations of the study due to the fact that it was conducted in laboratory conditions, and it did not acknowledge the complexity of the oral cavity environment, the influence of the type of diet on the supply of dyes and the time of their contact with dental materials, as well as the temperature of consumed drinks. In laboratory conditions, however, it is difficult to recreate all environmental variables, such as dietary and hygienic habits for each patient (including individual tooth brushing time and type of agents used) or saliva flow and the influence of bacterial flora. It should be realized that in clinical conditions, contact of colored substances with dental materials is much shorter than in conditions of a study, and the color change of dental materials treated with food dyes may not be as intense. It should be noted that the scheme of the study, including incubation times of samples in food dye solutions, is similar to methodology described by other authors of similar studies [9,10,11,13].

However, in the studies described by Seyidaliyev et al. [14], in which susceptibility to discoloration of reconstructive materials was assessed, it was assumed that 1 day of sample storage corresponds to 4 weeks of consumption of solutions containing food dyes. Of course, as mentioned earlier, such assumptions do not exactly match the clinical scenario.

On the other hand, conducting similar studies in vivo does not ensure their standardization, due to different conditions in the oral cavity of individual patients.

The available literature includes reports of studies whose results indicate that dental materials used in orthodontic, conservative and prosthetic treatment are susceptible to discoloration by substances periodically present in the oral cavity.

Color change due to chromogenic substances present in beverages, foods or stimulants applies to both composite materials for filling cavities [15,16,17], polymers used in prosthetics [18,19,20], flexible orthodontic ligatures [21,22,23] and aesthetic brackets.

In the case of restorative materials, the color change of fillings or prosthetic restorations has aesthetic consequences, especially when it comes to composite materials located in the visible area of the dental arch. Changing the color is usually uncomfortable for the patient and may encourage him/her to replace or repair a filling despite an absence of secondary caries symptoms.

It comes as no surprise to both researchers and clinicians that highly transparent or light-colored orthodontic elastomers discolor apparently faster than dark or saturated products. This fact, however, is not a serious problem, as these elements are replaced relatively often during control visits.

A much more serious problem is discoloration of some orthodontic brackets, referred to as aesthetic, that remain in the oral cavity throughout the treatment period. This phenomenon was observed especially often in the case of first generations of aesthetic brackets made of polymeric materials and, apart from unsatisfactory mechanical properties of these products, it contributed to their withdrawal from the market. Maintaining the aesthetics of dental materials throughout their lifespan is consistent with expectations of patients and clinicians and constitutes a technological challenge for manufacturers.

The effect of colored substances on dental materials is usually assessed in laboratory conditions and consists in performing colorimetric analysis of samples before their contact with colored solutions and re-analysis after materials’ contact with dyes. Few of available publications report the results of studies determining the susceptibility of orthodontic brackets to discoloration.

A study by Faltermeier et al. [13] assessed the effect of coffee and red wine on the color change of four types of orthodontic brackets made of composite materials and polymers. The mentioned authors used a Minolta CM-C3500 spectrophotometer (Minolta, Tokyo, Japan) for colorimetric measurements.

After 24 h incubation of the samples in food dye solutions, the cited authors observed changes in the total color ΔE of the samples ranging from 1.93 to 10.41 points for coffee and from 0.33 to 10.15 points for red wine.

The results of the current study describing color changes of orthodontic brackets made of polymers are similar to the values observed by Faltermeier et al. [13] for brackets made of composites. Although the authors assessed mainly ceramic brackets and used a different methodology for color change registration, the results of both studies show that aesthetic orthodontic brackets are, in case of prolonged exposure, susceptible to absorption of food dyes, despite existing differences in material.

An assessment of the effect of food dyes on the color change of orthodontic brackets in in vitro conditions was also described by Wriedt et al. [9]. The authors assessed the effect of UV radiation, orange juice, red wine, coffee and tea on the color stability of six types of orthodontic brackets. Wriedt et al. [9] studied the effect of food dyes on ceramic brackets (Fascination 2 (Dentaurum, Ispringen, Germany), Ceramic 20/40 M (American Orthodontics, Sheboygan, WI, USA), Mystique (GAC, Philadelphia, PA, USA)), as well on polymer materials (Aesthetik-Line (Forestadent, Pforzheim, Germany), Brillant (Forestadent, Pforzheim, Germany), Silkon M (American Orthodontics, Sheboygan, WI, USA)). For colorimetric evaluation of the samples, they used the EasyShade spectrophotometer (VITA GmBH, Baden-Württemberg, Germany). After 24 h of incubation in a black tea solution, the mean total color change ΔE reported by Wriedt et al. [9] was from 7.7 to 11.9 points for brackets made of composite materials and from 6.7 to 9.9 points for ceramic brackets. The values of total color change of aesthetic brackets after 24 h of incubation in an infusion of black tea described by Wriedt et al. [9] are comparable to the values obtained in this study, where the mean color change for the analyzed samples was at the level of ΔE* = 9.11 (in the range from 2.92 to 12.02).

In a study by Yadaw et al. [10], ceramic and composite brackets were assessed in terms of their resistance to dyes contained in coffee and black tea. The authors performed colorimetric evaluation according to the CIE L*a*b* standard using a Spectrolino spectrophotometer (Gretag Macbeth, Grand Rapids, MI, USA). After 24 h contact of the samples with black tea solution, they observed a total color change of 3.08 to 4.31 for porcelain brackets and 5.25 for products made of composite materials. Use of a different research methodology does not allow direct comparison of the results with the current study. It should be noted, however, that the results of the study by Yadaw et al. [10] support the thesis that ceramic brackets are less susceptible to discoloration compared to brackets manufactured on the basis of composite materials.

Another study assessing the susceptibility of aesthetic orthodontic brackets to discoloration caused by the influence of food dyes is one conducted by Guignone et al. [11]. The cited authors assessed the impact of inter alia, coffee and red wine on the color change of ceramic orthodontic brackets after 24 h and 7 days of observation. After one day of observation, the total color change ΔE* ranged from 44.74 to 109.86 for the samples treated with coffee and from 26.84 to 80.52 for the samples treated with red wine. After 7 days from the beginning of the experiment, the ΔE* parameter assumed the values from 44.12 to 74.13 and from 61.98 to 73.39, respectively. The ΔE* values reported by the quoted authors [11] using a UV-2450 spectrophotometer (Shimadzu, Kyoto, Japan) are much higher than those observed in the present study or described in other publications. This confirms the idea that due to different research methodology, including lack of standardization in terms of color substances and measuring equipment, direct comparison of numerical results of individual experiments is not recommended. However, it is possible to draw conclusions, such as which of the dental materials has the best performance properties under conditions of a specific test.

The susceptibility of aesthetic orthodontic brackets to discoloration was also confirmed in observations by Tangjit et al. [24].

Haynie et al. [25] observed a significant effect of food dyes on orthodontic brackets based on resin composites produced with the use of a 3D printing technique.

An interesting observation is that brackets, which in the conditions of the present experiment were relatively little, susceptible to absorption of dyes in comparison to other products turned out to be Hype copolymer brackets (Ortho Classic). This fact is surprising, as usually ceramic materials are considered less susceptible to discoloration than those based on a polymer network. Polymer-based materials used in conservative dentistry, prosthetics and orthodontics are not completely chemically stable in oral cavity conditions, where they are constantly degraded under the influence of, among other things, chewing forces, temperature fluctuations, presence of water and changes in pH [26,27]. Heterogeneity of the structure and characteristic of polymeric materials [28] may also favor the absorption of colored substances into their interior.

Unfortunately, manufacturers of the mentioned brackets do not provide the chemical composition of the offered product in generally available materials.

According to a commonly prevailing opinion, orthodontic ceramic brackets are resistant to food dyes delivered to the oral cavity with food, drinks and stimulants. The results of the present study and the study by Wriedt et al. [9], however, do not confirm this thesis for the materials assessed in both studies.

On the other hand, the results of studies by Fusco et al. [29] indicate that substances contained in foodstuffs may not only affect aesthetics but also cause a loss of the functional properties of orthodontic materials.

The results of the study and the results described by the quoted authors show that aesthetic orthodontic brackets are susceptible to the absorption of colored substances from food products, in cases of prolonged exposure of dental materials to their action.

Total color change ΔE of more than 1.1 point in the range of red color to more than 2.1 points in the range of yellow is considered aesthetically unacceptable in the case of dental materials used in the oral cavity [30].

According to the information contained in the publication by Mokrzycki i Tatol [12], a standard observer perceives ΔE* color change as follows:

0 < ΔE* < 1—observer does not notice the difference.

1 < ΔE* < 2—only experienced observer can notice the difference.

2 < ΔE* < 3:5—unexperienced observer also notices the difference.

3:5 < ΔE* < 5—clear difference in color is noticed.

5 < ΔE*—observer notices two different colors.

Regardless of the adopted evaluation criteria, the color changes of orthodontic brackets noted in this study significantly exceeded the above-mentioned critical values, especially after longer exposure periods. As mentioned earlier, color changes observed in laboratory conditions should not be directly related to “clinical performance” of dental materials during their use in the oral cavity environment. The trends noted in this type of re-search may, however, facilitate a comparative evaluation of orthodontic brackets with regard to their clinical application.

6. Conclusions

Aesthetic orthodontic brackets, as well as composite-based restorative materials, turned out to be medical materials susceptible to discoloration.

In the event of contact with food dyes, aesthetic orthodontic brackets discolor, the in-tensity of which can be influenced by the materials they are made of, kind of food dye and the time of samples’ storage.

In view of the above observations, clinicians should take into account not only the functional properties of materials used during orthodontic treatment but also their susceptibility to discoloration. At the same time, recommendations for patients treated orthodontically with fixed appliances based on ceramic or polymer-based brackets should include reduced consumption of foods and drinks containing significant amounts of dyes.