Colour Stability between In-House 3D-Printed Resin Brackets and Conventionally Aesthetic Brackets: An In Vitro Study

Abstract

Background: To evaluate colour stability in artificial saliva by mechanically simulating brushing between in-house 3D-printed resin brackets (IH3DBs) and commercially available brackets. Methods: The samples consist of four sets of ten aesthetic brackets each supplied by four different manufacturers: clear Damon, Synovate C, Crystal and in-house 3D printed brackets (IH3DBs). The brackets were immersed in a plastic tank containing artificial saliva maintained at a constant temperature of 37 °C with a 65-minute brushing system. Staining changes at T0 (pre-brushing) and T1 (post-brushing) were measured with a spectrophotometer according to the VITA and Master scale and brightness values. Statistical analysis compared the colour changes with the Wilcoxon test and in case of significance, severity was investigated. The significance level considered was p < 0.05. Results: The IH3DBs shows statistically significant differences for both scales and brightness values. The Damon and the Crystal brackets report a statistically significant difference only for brightness. The Synovate C bracket shows no statistically significant differences. Conclusions: The IH3DB produced chromaticity differences for the VITA and Master scale possibly due to the surface roughness created during the printing process. However, the IH3DBs together with Damon Clear improved brightness, due to the mechanical action of brushing.

1. Introduction

Aesthetics represents an essential aspect of labial orthodontic treatment.

The desire to improve dental aesthetics is the principal motivation that guides patients to undergo treatment [1]. Patients’ growing demand for the most invisible appliance [2] has led to an increasing use of alternative solutions to traditional labial metal brackets, such as aesthetic brackets [3]. Stainless steel brackets are widely employed as labial orthodontic treatment [4] even if they have poor aesthetic properties.

Plastic brackets took over in the early 1970s. Initially, they were constructed from acrylic and later polycarbonate, but inherent problems were soon noticed, including staining and odours, but, more importantly, their lack of strength and stiffness resulted in bonding problems, tie wing fractures and permanent deformation.

To compensate for these deficits, polymer brackets were also developed with different filler materials, e.g., ceramic or fibreglass, or reinforced with metal slots [5,6].

Ceramic brackets were introduced in the 1980s, offering many advantages such as higher strength, more resistance to wear and deformation, better colour stability and, most importantly, superior aesthetics.

On the other hand, ceramic brackets, despite their superior optical properties, have several mechanical disadvantages compared to metal ones: an increasing enamel damage risk during debonding [5,7,8], the inability of plastic deformation, a major risk of wing fracture [5,7,8] and a higher frictional resistance [9,10,11]. The colour stability of aesthetic brackets has always been a topic of study because of their great aesthetic need. However, studies report colouration changes related to the brackets’ material themselves and to extrinsic factors such as smoking, dyes in food and frequent consumption of colour-intensive beverages [12,13].

In order to satisfy both patient demands and biomechanical properties it would be desirable to use a bracket material that combines both the biomechanical and aesthetic requirements. In addition, it would be a further advantage of future bracket systems to turn away from conventional, standardised bracket designs towards a patient-specific, customised bracket system [14].

The current increasing rate of three-dimensional printing applications in dentistry disciplines leads to the development of in-house 3D-printed resin brackets (IH3DBs) which can overcome the limits of the latter [15]. Thanks to the relatively long history of computer-aided design/computer-aided manufacturing (CAD-CAM) technology in dentistry and the advances in 3D imaging and modelling technologies, such as intraoral scanning and Cone Beam Computed Tomography (CBCT), the 3D printing technology has found wide applications. This has led to an increase in 3D printing techniques but research has demonstrated that stereolithography (SLA) and digital light processing (DLP) printers have achieved the best accuracy [16]. A digital workflow could enable clinicians to design and fabricate brackets of desirable colour and translucency in customised shapes and prescriptions to meet the individual needs of their patients′ tooth anatomy. The customisation of bracket shape and prescription, and the optimal bracket placement, could lead to greater efficiency and minimisation of finishing bends at the end of the treatment [17].

CAD-CAM bracket production is practical, economical and customised; additionally, IH3DBs have better biomechanical properties such as less force loss and better sliding mechanics than ceramic ones, in some cases equivalent to metal brackets [14,18,19,20].

The aim of the study is to evaluate colour stability in artificial saliva by mechanically simulating brushing between in-house 3D-printed resin brackets (IH3DBs) and commercially available brackets.

2. Materials and Methods

The present in vitro study compared IH3DBs with three different commercial 0.022″ × 0.028″ brackets: ceramic Clear Damon bracket (Ormco Corporation, Glendora, CA, USA), ceramic Synovate C bracket (Genesis Orthodontics, Castle Rock, CO, USA) and composite Crystal bracket (Genesis Orthodontics, Castle Rock, CO, USA).

The IH3DBs, with a slot size of 0.022 × 0.028 inches and an MBT prescription, were printed using the desktop SprintRay PRO 95 printer (SprintRay Inc., Los Angeles, CA, USA), which relies on DLP (Digital Light Projection) technology. [21] The resin for 3D-printing of temporary crowns BEGO Verseosmile Temp (BEGO GmbH & Co. KG, 28359 Bremen, Germany) certified IIa with a flexural strength of ±80 Mpa and colour A2 on the Vita scale calibrated for the SprintRay Pro 95 printer was used and a print-layer thickness of 50 μm on the Z axis was adopted.

There were ten brackets for each type to a resin bar with cyanoacrylate adhesive, for a total of four bars, and a first colour measurement (T0) was taken by a spectrophotometer (Vita Easyshade Compact, Bad Säckingen, Germany), which was calibrated by the white light spectrum before each measurement. The digital spectrophotometer provides emission of all tooth colours in the two VITA Classical A1-D4 scale and VITA System 3D-master colour standards scale, and in tooth brightness according to the American Dental Association scale. Colour values were recorded for each of the ten brackets for each bar and subsequently averaged.

In order to test the repeatability of the analysis, two measurements were taken under the same initial conditions.

Subsequently, the four bars were immersed individually in a plastic box with artificial saliva, Oral Balance (Biotène Oral Balance, Biopharm Sas, Peschiera Borromeo, Italia) and brown food colouring [18]. The latter solution was prepared by mixing 1.5 g of Brooke Bond tea with 150 mL of boiling water. The solution was cooled to 37 °C (room temperature) and subsequently was filtered out using a filter paper to eliminate any residues, thus obtaining a homogenous solution [22].

A constant temperature was maintained by simulating the condition of the oral cavity at 37 ± 1 °C. The brushing cycle of about 65 min corresponding to two years of orthodontic treatment was calculated to be equivalent to 8.5 years of brushing, based on a brushing time of 120 s twice daily for all teeth. For the brushing simulation, a manual toothbrush connected to an electric motor (24vdc 60w 250 rpm brush type electric machinery reducer motor) and 16,250 cycles were performed setting the cleaning force to 2.5 N [23] (Figure 1).

At the end of the brushing procedure, the colour values of all brackets were re-measured with the same spectrophotometer by the same operator (T1) (L.L.).

In order to perform the statistical analysis, it was necessary to convert the results of the VITA scale and Master scale into absolute values. Firstly, the Master scale’s values were transformed into the VITA scale’s colours [24] so that there were two VITA scale values for each bracket analysed. Then, both values were transformed into absolute values by means of the conversion table (Figure 2) [25].

3. Statistical Analysis

A descriptive analysis was performed.

Three Wilcoxon tests were performed in order to verify any statistical differences between the values. In case of significance, the effect size was calculated in absolute values, which was considered as follows: 0.10 ≤ 0.3 (small effect), 0.30 ≤ 0.5 (moderate effect) and ≥ 0.5 (large effect).

For all tests, the significance level considered was p < 0.05.

Analyses were conducted with the IBM SPSS v28 software.

4. Results

Table 1. Descriptive analysis with minimum, maximum, mean, median and standard deviation at T0 and T1 brushing values.

	n	Scale	Minimum		Maximum		Mean		Median		Standard Deviation
			T0	T1	T0	T1	T0	T1	T0	T1	T0	T1
IH3DB	10	VITA	15	15	15	16	15	15.8	15	16	0	0.42163702
		Master	15	15	16	16	15	15.8	15	16	0.4714045	0.42163702
		Brightness	23	23	21	26	20.3	24.3	21	24	1.2516656	0.8232726
Damon	10	VITA	6	6	18	14	5.9	8.7	2	7	6.0635523	3.68329563
		Master	4	4	11	14	4.4	5	1	4	4.7187569	3.16227766
		Brightness	8	8	6	15	4.7	10.7	5	10	1.1595018	2.35937845
Crystal	10	VITA	1	1	1	1	1	1	1	1	0	0
		Master	1	1	2	1	1.1	1	1	1	0.3162278	0
		Brightness	1	1	4	1	3	1	3	1	0.4714045	0
Synovate C	10	VITA	1	1	2	14	1.3	2.7	1	1.5	0.4830459	4.00138865
		Master	1	1	9	9	5	3.4	5	1	4.2163702	3.86436713
		Brightness	1	1	1	2	1	1.1	1	1	0	0.31622777

n: number.

shows the analysis of chromatic change values between T0 and T1 for the four different brackets both in the VITA and Master scales, already converted in absolute values, and in the brightness scale.

The Wilcoxon tests are summarised in

Table 2. Wilcoxon test for different brackets and the VITA, Master and brightness values.

Bracket	n	Scale	Z	p-Value	Effect Size
IH3DB	10	VITA	2.828	0.005 *	0.894
		Master	2.530	0.011 *	0.8
		Brightness	2.823	0.005 *	0.892
Damon Clear (Polycrystalline alumina)	10	VITA	1.278	0.201	-
		Master	0.259	0.796	-
		Brightness	2.812	0.005 *	0.889
Synovate C (Ceramic)	10	VITA	1.141	0.157	-
		Master	1.00	0.317	-
		Brightness	1.00	0.317	-
Crystal (Composite)	10	VITA	0.00	1.00	-
		Master	1.00	0.317	-
		Brightness	2.970	0.003 *	0.939

n: number; * p < 0.05.

.

The IH3DB resulted in being statistically significant with a large effect size. Moreover, the Damon Clear brackets were found to be statistically significant only for the brightness values with a large effect size.

On the other hand, Synovate C brackets were found to be not statistically significant in all scales.

Finally, Crystal brackets were found to be statistically significant only for the brightness values with a small effect size.

However, analysing the differences in values in the VITA and the Master scales between T0 and T1, Table 1 shows that the colour difference in absolute values for the IH3DB brackets ranges from a minimum value of 15 to a maximum value of 16, but, despite this, the p-value was statistically significant (Table 2). On the other hand, the Synovate C brackets’ range from a maximum value at T0 of 2 to a maximum value at T1 of 14 (Table 1) was not statistically significant (Table 2). This difference occurs because, from a statistical point of view, the small variability of measurements may be a limitation that affects the analysis’s significance.

5. Discussion

Aesthetic brackets were introduced in response to patients’ aesthetic demands. According to the study by Russell et al., the better aesthetics of ceramic and polycarbonate brackets compared with conventional stainless-steel brackets is not only well accepted by patients, especially adults, but also positively sought [5].

The majority of commercially available ceramic brackets are based on alumina either of monocrystalline or polycrystalline structure. Despite the superior aesthetic properties, clinicians report often bracket wing fractures during practice, since most ceramic materials are strong but at the same time brittle in nature [26].

Despite offering aesthetic benefits, ceramic brackets present elevated friction resistance (FR) and diminished fracture toughness as compared to stainless-steel brackets.

The differences between ceramic and metallic brackets, in addition to their aesthetic appearance, are the greater residual adhesive after debonding resulting in more difficult removal of resin remnants and greater bracket–wire friction, involving less efficient dental movements.

On the other hand, bracket failure should be considered, as reported by the study of Scribante et al. which compares the failure rates of ceramic and metal brackets in a 12-month clinical study [27].

According to this study, ceramic brackets showed higher failure rates. Patients should be aware that orthodontic treatment with ceramic brackets may involve delays and inconvenience due to the higher failure rate compared to metal brackets [27].

To address these limitations, various innovative approaches have been proposed to develop advanced orthodontic brackets, with zirconia emerging as a promising material for enhancing mechanical properties owing to its remarkable toughness [12].

While aesthetic bracket options have been limited to commercially fabricated polymeric or ceramic brackets in the past, emerging digital workflows and additive manufacturing technologies present the potential to enable the direct design and fabrication of brackets which combine both aesthetic and biomechanical properties taking into consideration the customization for each patient [17].

Considering the high aesthetic demands, the colour stability of these brackets is often analysed in the literature, but yet there is no study comparing 3D-printed brackets. Although the colour characteristics of ceramic brackets are their main advantage over metal brackets, colour stability is still a cause of disagreement.

The present study compared the colour stability of four commercially available aesthetic brackets with IH3DBs. The present study was conducted by simulating 2-year brushing, with a 16.250 cyclic number of repetitions, in artificial saliva with colouring agents [23,24,25,28], measuring changes in chromaticity with a spectrophotometer. The spectrophotometer is a widely used instrument for measuring surface colour because of its reliability, precision and accuracy [28].

Colour changes in aesthetic brackets have a multifactorial origin.

According to some studies, the colour of ceramic brackets changes over time when exposed to potentially colouring solutions commonly found in people’s diets, such as red wine and products high in caffeine. Moreover, the colouration is cumulative: it increases with increasing exposure time to the colouring elements [29].

On the other hand, discolouration of dental materials can be the result of intrinsic factors such as water absorption, incomplete polymerization of adhesives or resins, material matrix composition, content and size of reinforcement particles and branding or extrinsic factors such as contact with food or beverages containing pigments, use of mouthwashes, saliva, nicotine, lipstick and heat [28,29,30].

An evaluation of brackets using a scanning electron microscope (SEM) showed that discolouration is mainly due to stain absorption and sub-surface stain absorption occurring between the dye solution and ceramic brackets [30]. In addition, the porosity and surface roughness of aesthetic brackets may facilitate the increase in pigment penetration, which contributes to a higher degree of discolouration of the brackets [31].

The in vitro results of the present study indicate that an IH3DB has low colour stability when subjected to continuous staining agents; in fact, the tendency to change colour was found to be greater than other commercially available ceramic brackets. Moreover, 3D-printed brackets, as being resin-based, could experience colour instability to smoke rather than just liquid [32].

By carefully evaluating the descriptive analysis, it is possible to observe little variability in the medians. This result overlaps with the results in the literature on the colour stability of ceramic attachments [5,29,33], but there is no study in the literature comparing it with that of 3D-printed resin attachments. It is hypothesised that the colour change is due to the increased presence of surface roughness due to the layers created during the printing process.

An important aspect to consider in the present study is brightness. Three of the four brackets analysed reported a statistically significant difference in brightness between T0 and T1, with a high effect size. In the literature, the brightness loss is because of water absorption by the dental materials, as is the case with the Crystal brackets of single-crystalline ceramic. In contrast, for the IH3DB and Damon Clear brackets, there is an increase in brightness, probably justified by the fact that the mechanical brushing action has abraded the outer surface of the brackets, making it smoother and brighter [23].

According to De Oliveira et al. [33], all aesthetic ceramic brackets change colour during treatment. However, brackets with the same crystalline formation did not follow the same or similar colour change patterns when exposed to the same staining solutions and under the same conditions. Therefore, the degree of colouration changed considering different brands, and the monocrystalline or polycrystalline structure did not affect the colouration, thus demonstrating that the aesthetic behaviour depends on the bracket manufacturer.

The present study has some limitations. Firstly, it is an in vitro study which fails to faithfully reproduce the condition of the oral environment. Secondly, Easyshade is generally not recommended for in vitro testing [34]. Moreover, the results for VITA and Master scales between IH3DBs and Synovate occur because, from a statistical point of view, the small variability of measurements may be a limitation that affects the analysis’s significance.

In the future, it will be performed an in vivo study in which the condition of the oral environment will be reproduced in order to better understand how each bracket responds to staining agents.

6. Conclusions

Our analyses performed with pigmenting agents and brushing showed that the IH3DB aesthetic brackets have low colour stability when subjected to continuous staining agents reporting significant chromaticity changes in the VITA and Master scale. However, the IH3DBs together with Damon Clear improved brightness, due to the mechanical action of brushing which abraded the outer surface of the brackets, making it smoother and brighter. On the other hand, Synovate C brackets were found to be not statistically significant in all scales.