In vitro colour stability of aesthetic brackets
* Departments of Orthodontics
** Prosthetic Dentistry, University Medical Centre, Regensburg, Germany
Address for correspondence Dr Andreas Faltermeier, Department of Orthodontics, University Medical Centre, Franz-Josef-Strauss-Allee 11, D-93042 Regensburg, Germany. E-mail: andreas.faltermeier{at}klinik.uni-regensburg.de
 |
Summary |
|---|
 Top Summary IntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
Contrary to their popularity in satisfying aesthetic demands, plastic brackets still present some problems because of their decreased hardness and wear resistance. A problem of plastic brackets is discolouration, due to ultraviolet (UV) light and food dyes.
The aim of this study was to investigate the colour stability of aesthetic brackets during UV irradiation and exposure to food dyes. Four different polymer brackets were exposed in a Suntest CPS+ ageing device to a xenon lamp to simulate natural day light. Because most tooth-coloured bracket systems are used in adult treatment, red wine, coffee, and tea were chosen as food colourants. After 24 and 72 hours of exposure, colour measurements were performed by means of a spectrophotometer according to the CIE L*a*b* system and colour changes (
E*) were computed. Statistical differences were investigated using three-way analysis of variance (ANOVA).
With the exception of the Aesthetic-Line bracket, almost all investigated polymer brackets showed clinically unacceptable discolouration during in vitro exposure to colourants. Most of the brackets became yellower after UV light treatment. In spite of the short exposure period of 72 hours, almost all polymer brackets showed undesirable discolouration.
These current in vitro findings indicate that even newly developed plastic brackets, consisting of composite materials or modern polymers (polyoxymethylene) may have clinically unacceptable colour stability in the long-term.
|
Introduction |
|---|
|
TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
Although plastic and ceramic brackets improve the appearance of fixed appliances, they are still far from ideal in fulfilling the requirements of orthodontic brackets. The advantages of ceramic brackets are colour stability and strength. Nevertheless, the use of ceramic brackets may result in problems with excessive bond strength, damage to the enamel during removal, and bracket breakage because of brittleness (Arici and Regan, 1997
).
Initially plastic brackets were manufactured from unfilled polycarbonate. Nowadays, alternative polymers such as polyoxymethylene and composite are used. Composite brackets are reinforced with special fillers, or consist of a fibreglass reinforcement. In addition, plastic brackets with a metal slot are available. Safe debonding is uncomplicated because of the low modulus of polymer appliances and a peel-off effect similar to that found for metal brackets (Zinelis et al., 2005). Contrary to their popularity in satisfying aesthetic demands, plastic brackets still present some problems because of their decreased hardness, wear resistance, and an inability to withstand the torquing forces generated by rectangular wires (Arici and Regan, 1997). The main problem of plastic brackets is discolouration, because of ultraviolet (UV) light and food dyes.
There are internal and external causes for the discolouration of aesthetic brackets. External discolouration can be caused by food dyes and coloured mouth rinses (Khokar et al., 1991; Seher and Viohl, 1992; Dietschi et al., 1994; Leibrock et al., 1997). The material, e.g. the polymeric structure or filler content, and surface roughness play a decisive role in the extent of discolouration caused by diverse substances (Dietschi et al., 1994; Leibrock et al., 1997). The amount of colour change can be influenced by a number of factors including oral hygiene, water sorption, and incomplete polymerization (Arthur et al., 2004). The reason for internal discolouration can be found in UV irradiation and thermal energy. UV light is able to induce physico-chemical reactions in the polymer, which cause irreversible colour changes of the brackets.
The purpose of the present in vitro study was to investigate the influence of food dyes and UV irradiation on plastic brackets. Four different plastic brackets, two composite, one polymer (polyoxymethylene), and one experimental composite bracket were exposed to UV irradiation and food dyes for 72 hours.
|
Materials and methods |
|---|
|
TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
A total of 160 right upper central incisor brackets were investigated. The groups (40 per bracket group) consisted of the composite bracket, Aesthetic-Line (Forestadent, Pforzheim, Germany), the plastic bracket Brillant (Forestadent), the composite bracket Envision (Ortho Organizers, San Marcos, California, USA), and one experimental bracket, ‘Exper’. The Exper bracket was a composite bracket, consisting of urethane dimethacrylate as a monomer matrix and a functional silane-treated SiO2 filler. The filler content was 40 vol%.
The Exper brackets were first produced by hand mixing the monomer and filler in appropriate portions. To obtain a homogenous mixture, the composite matrix was additionally mixed in a mixing device (Speed Mixer DAC 150FVZ, Hauschild Engineering, Hamm, Germany) for 60 seconds (1800 rpm). After preparation, the composite was stored in opaque receptacles to prevent premature polymerization. To avoid the formation of bubbles, the composite was placed carefully in a mould made of a silicone impression of a Brillant bracket and polymerization was carried out using the polymerization device Targis-Power-Lichtofen (Ivoclar-Vivadent AG, Schaan, Liechtenstein) for 25 minutes. After polymerization, the Exper brackets were taken out of the silicone mould and the surplus was removed with a scalpel.
As food dyes, red wine (Chateau Romefort 2001, Bordeaux, France), coffee (Krönung, Jacobs, Bremen, Germany), and tea (Darjeeling, Lord Nelson, Lidl, Neckersulm, Germany) were chosen and placed in three small receptacles. In each receptacle, eight samples per bracket group were stored for 72 hours. After 24 and 72 hours, colour measurements were performed. Eight brackets per group, which served as the controls, were stored in distilled water under light exclusion for the same period.
To investigate the influence of UV irradiation on the brackets, eight samples of each group were subjected to artificial ageing following DIN EN 27491 (1991) (International Organization for Standardization, 1985) in a Suntest CPS+ ageing device (Figure 1, Heraeus Instruments, Hanau, Germany). The brackets were exposed to a filtered xenon lamp with an irradiation value of 765 W/m2 for 24 and 72 hours. This technique is able to simulate a light strength of approximately 160 kLux and corresponds to intensive natural sunlight in equivalent exposure time. To simulate the moist environment of saliva, each 20-minute cycle consisted of a 3-minute rinse with deionised water at 37°C followed by a 17-minute dry phase. After 24 and 72 hours, the colour values of the brackets were measured and compared with the control group.
|
The colour measurements were carried out using the Minolta spectrophotometer CM-C3500 (Minolta Co. Ltd, Tokyo, Japan) with a pinhole diaphragm diameter of 3-mm according to the CIE L*a*b* system (Commission Internationale de l'Eclairage, 1976). A colour graph consisting of L*, a*, and b* co-ordinates can be produced by means of mathematical transformations. The L* parameter corresponds to the degree of lightness and darkness and the a* and b* values to the chroma, where +a* is red, –a* is green, +b* is yellow, and –b* is blue (Eldiwany et al., 1995
). Ruyter et al. (1987) reported that a colour change (E*) of 3.3 is visually perceptible. Therefore, in this investigation, colour changes of E* 
3.3 were considered to be clinically unacceptable. The calculation E* between two colour positions in the three-dimensional L*a*b* colour space (Figure 2) is as follows:
 |
|
Statistics
Statistical differences were investigated using three-way ANOVA. The level of significance was set at 
= 0.05. The median, 25th and 75th percentiles were calculated.
|
Results |
|---|
|
TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
All the examined brackets showed a significant discolouration after 24 hours of storage in red wine (Figure 3a, Table 1). Significant colour changes during exposure to red wine (
E* > 5) were observed for Envision and Brillant brackets. After 72 hours storage, all brackets showed a significant enhancement of discolouration compared with 24 hours (P = 0.012).
|
|
Comparing different colouring agents and UV light treatment, red wine caused the greatest colour changes followed by coffee. Tea and UV light resulted in less colour change (Table 1). The Envision bracket showed significantly greater discolouration in comparison with the other bracket groups (P = 0.012, Table 2).
|
With the exception of the Aesthetic-Line bracket, unacceptable colour changes of
E* > 3.3 were determined for all brackets after storage in coffee (Figure 3b). A significant increase (P = 0.012) in discolouration after 72 hours compared with 24 hours of exposure time was observed for the Brillant and Aesthetic-Line brackets (Table 1).
The Exper bracket showed small changes in colour (E* < 2.2) after storage in tea for 24 and 72 hours (Figure 3c, Table 1). In contrast, median values of 6 E* units (24 hours) and 7 E* units (72 hours) were measured for the Envision bracket after exposure to tea (Table 3).
|
After 24 hours of exposure to UV light, only the Envision bracket demonstrated undesirable colour changes (
E* > 3.3) compared with the control group (Figure 3d). Clinically acceptable colour stability was observed for the Brillant, Aesthetic-Line, and Exper brackets after UV light exposure (Table 3). All brackets showed a significant increase of E* values by enhancing the UV light exposure time from 24 to 72 hours (Table 1). After 72 hours of storage, red wine caused the greatest colour changes of all investigated brackets in comparison with the other food dyes (Figure 4, Table 3).
|
|
Discussion |
|---|
|
TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
Colour changes can be distinguished by a colourimeter or visually. However, the sensitivity of the human eye in observing small colour differences is limited and the interpretation of visual colour comparisons is subjective. Therefore, colourimetric measurements are necessary to allow reproducible results of colour determination (Buyukyilmaz and Ruyter, 1994
; Rinke et al., 1996).
Numerous tests have been used for artificial ageing of dental materials to investigate colour stability in vivo and in vitro (Rosentritt et al., 1998; Stober et al., 2001; Arthur et al., 2004). As clinical testing is difficult to carry out and results may not be comparable due to a combination of miscellaneous factors, an in vitro investigation was undertaken. In this study, the exposure time to red wine, coffee, tea, and UV light was set to 72 hours, as Stober et al. (2001) reported that a period of 24-hour artificial treatment is too short to investigate discolouration of dental composites.
Rosentritt et al. (1999) examined the in vitro colour stability of veneer composites after exposure to UV light for 72 hours and storage in red wine or coffee for 10 days. They described discernible but acceptable colour changes (E* < 3.3) after UV irradiation (72 hours). They reported no synergetic effects on colour behaviour after UV ageing and storage in food dyes.
Compared with the results of other similar in vitro studies of colour stability of dental composites (Um and Ruyter, 1991; Seher and Viohl, 1992; Fruits et al., 1997), colour stability of the polymer brackets investigated in this research was unsatisfactory. With the exception of the Aesthetic-Line bracket, almost all polymer brackets showed clinically unacceptable discolouration during in vitro exposure to colourants, and seemed to become yellower after UV light treatment. The cause of this yellow discolouration was investigated by Ferracane et al. (1985) who found that yellowing of the polymer was accompanied by a reduction in the quantity of residual unreacted double bonds in the resins. They stated that a possible explanation for the yellowing could be an oxidation of the unreacted C=C to produce coloured peroxide compounds. Thus, the polymeric structure and filler content, as well as the polymerization conversion, seem to be the most important factors, which influence the colour stability of dental polymers.
In the present study, red wine caused the greatest colour changes in comparison with other food dyes after 72 hours of exposure. Only the Aesthetic-Line bracket showed, after 72 hours of UV irradiation and food dye exposure, a clinically acceptable E*. Undesirable discolouration after 72 hours of storage in red wine, coffee, and tea was observed for the Envision and Brillant brackets, consisting of polyoxymethylene. Satisfactory colour stability was measured for those brackets only after UV irradiation. This polymer bracket seems to be more susceptible to external discolouration, caused by food dyes, than to UV light leading to internal discolouration. The Envision bracket is manufactured from a special thermoplastic polyurethane, which is post-cured by heat treatment. Despite this, the bracket showed colour changes after 72 hours of artificial ageing.
Arthur et al. (2004) suggested that changes in the optical properties within the polymer could be responsible for colour changes seen clinically. They stated that chemical discolouration was due to the oxidation of unreacted double bonds in the matrix of the polymer and the subsequent formation of degradation products from water diffusion or the oxidation of the polymer.
Many variables can affect composite colour stability (Eldiwany et al., 1995). On the one hand, chemical differences among the resin components, such as polymeric structure, residual monomers, and the concentration of amines and diketones may influence colour stability. On the other hand, differences in both filler content and composition may explain the fact that composites with a higher content of inorganic filler showed better colour stability than polymers with a low filler content (Eldiwany et al., 1995; Ruyter, 1995).
|
Conclusions |
|---|
|
TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
According to Ruyter et al. (1987)
, a E* of 3.3 is visually perceptible and therefore clinically unacceptable. In this in vitro investigation, an exposure time of 72 hours was chosen. In spite of this short exposure period, almost all investigated polymer brackets showed undesirable discolouration. Nevertheless, it should be remembered that this was an in vitro study, and care should be taken in interpreting the results to those that might occur in the oral cavity. These present in vitro findings indicate that even newly developed plastic brackets, consisting of composite materials or modern polymers, may have clinically unacceptable colour stability in the long term.
|
References |
|---|
|
TopSummaryIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
|---|
-
Arici S, Regan D. Alternatives to ceramic brackets: the tensile bond strengths of two aesthetic brackets compared ex vivo with stainless steel foil-mesh bracket bases. British Journal of Orthodontics (1997) 24:133–137.[Abstract]
Arthur SK, Frederick CS, John C, Tak WC. Color stability of provisional prosthodontic materials. Journal of Prosthetic Dentistry (2004) 91:447–452.[CrossRef][Web of Science][Medline]
Buyukyilmaz S, Ruyter IE. Color stability of denture base polymers. International Journal of Prosthodontics (1994) 7:372–382.[Medline]
Dietschi D, Campanile G, Holz J, Meyer JM. Comparison of the color stability of ten new generation composites: an in vitro study. Dental Materials (1994) 10:353–362.[CrossRef][Web of Science][Medline]
DIN EN 27491. Dentistry; dental materials; determination of colour stability of dental polymeric materials (International Organization for Standardization 7491: 1985) (1991) Beuth, Berlin.
Eldiwany M, Friedl KH, Powers JM. Color stability of light-cured and post-cured composites. American Journal of Dentistry (1995) 8:179–181.[Web of Science][Medline]
Ferracane JL, Moser JB, Greener EH. Ultraviolet light induced yellowing of dental restorative resins. Journal of Prosthetic Dentistry (1985) 54:483–487.[CrossRef][Web of Science][Medline]
Fruits TJ, Duncanson JMG, Miranda FJ. In vitro weathering of selected direct aesthetic restorative materials. Quintessence International (1997) 28:409–414.[Medline]
Khokhar ZA, Razzog ME, Yaman P. Color stability of restorative resins. Quintessence International (1991) 22:733–737.[Medline]
Leibrock A, Rosentritt M, Lang R, Behr M, Handel G. Colour stability of visible light curing hybrid composites. European Journal of Prosthodontics and Restorative Dentistry (1997) 5:125–130.
Rinke S, Hüls A, Kettler MJ. Colorimetric analysis as a means of quality control for dental ceramic materials. European Journal of Prosthodontics and Restorative Dentistry (1996) 4:105–110.
Rosentritt M, Esch J, Behr M, Leibrock A, Handel G. In vivo color stability of resin composite veneers and acrylic resin teeth in removable partial dentures. Quintessence International (1998) 29:517–522.[Web of Science][Medline]
Rosentritt M, Lang R, Behr M, Handel G. In vitro discoloration of restorative materials caused by UV light and food colorants. Acta Odontologica Scandinavica (1999) 4:147–152.
Ruyter IE. Physical and chemical aspects related to substances released from polymer materials in an aqueous environment. Advances in Dental Research (1995) 9:344–347.
Ruyter IE, Nilner K, Moller B. Color stability of dental composite resin materials for crown and bridge veneers. Dental Materials (1987) 3:246–251.[CrossRef][Web of Science][Medline]
Seher J, Viohl J. In vitro discoloration of experimental and dental resins due to the effect of dyes and UV irradiation. Deutsche Zahnärztliche Zeitung (1992) 47:634–636.
Stober T, Gilde H, Lenz P. Color stability of highly filled composite resin materials for facings. Dental Materials (2001) 17:87–94.[CrossRef][Web of Science][Medline]
Um CM, Ruyter IE. Staining of resin-based veneering materials with coffee and tea. Quintessence International (1991) 22:377–386.[Medline]
Zinelis S, Eliades T, Eliades G, Makou M, Silikas N. Comparative assessment of the roughness, hardness and wear resistance of aesthetic bracket materials. Dental Materials (2005) 21:890–894.[CrossRef][Web of Science][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||