In vivo and in vitro color evaluation of clear thermoplastic retainers

Abstract

Introduction: Several orthodontic retention protocols and devices have been proposed in order to minimize post-treatment changes. Although removable retainers are preferable when compared with fixed retainers regarding oral hygiene, they require patient compliance and prolonged use. In order to increase patient acceptance of removable devices, clear thermoplastic retainers have been introduced as an esthetically acceptable, comfortable and inexpensive alternative to conventional removable retainers. While fulltime wear of clear retainers immediately after the removal of fixed orthodontic appliances is recommended, there are different opinions concerning the length of time and the wear regimen of these devices. Various materials are currently used for the fabrication of thermoplastic retainers, such as polypropylene, polyurethane or co-polyester polymers. While thermoplastics have the potential to provide significant esthetic advantages and greater patient comfort and compliance during orthodontic retention, their chemical and physical properties may be compromised when used in the oral cavity. Poor wear resistance and durability have been demonstrated along incisal and occlusal surfaces after only a few months of intra-oral use. Moreover, polyurethane is not an inert material and is affected by heat, moisture, and prolonged contact with oral enzymes. From an esthetic point of view, the color stability and transparency of thermoplastic material during the full-time wear of clear retainers remains a critical factor for clinicians and patients. Aims: To investigate the in vitro effect of different staining solutions on the color stability of Vivera® retainers and to assess in vivo the color alterations of two different clear thermoplastic retainers, Vivera® and Essix® C+. Material and methods In vitro study Thirty flat specimens fabricated using the polyurethane-based Vivera® material (Align Technology Inc., CA, USA) were assigned into five groups (A, B, C, D and E) and immersed at 37°C in solutions of distilled water (control), coffee, tea, red wine, and Coca- Cola®, respectively. The CIE color parameters (L*, a*, b*) of each specimen were measured with a UV2401PC UV-VIS Recording Spectrophotometer (Shimadju®, Tokyo, Japan; UVPROBE software version 2.21) before immersion (T0) and after 12 hours (T1), three days (T2) and seven days (T3) of solution exposure. Color differences (ΔΕ) between the interval groups were calculated. A two-way ANOVA was applied for the analysis of the data and pairwise comparisons were done with the Bonferroni method. Statistical significance was set at p<0.05.In vivo study Thirty patients were randomly allocated into two groups after the completion of active orthodontic treatment. Group 1 (n=15) received Vivera® retainers (Align Technology Inc., CA, USA) and Group 2 (n=15) received Essix® C+ retainers (Raintree Essix, New Orleans, LA, USA). For each patient, two retainers were fabricated, one for oral use and the other to serve as control. The CIE color parameters (L*, a*, b*) of each patient’s upper central incisors were measured with a SpectroShade™ Micro spectrophotometer (MHT Optic Research AG, Zurich, Switzerland; software version 2.20) immediately after retainer insertion in the mouth (T0) and after 15 days (T1), 1 month (T2) and 3 months (T3) of intraoral use. At each time point the measurements were also performed both with and without the control retainer on (tooth-only). Color differences (ΔΕ) between the interval groups were calculated. A 4-way ANOVA was applied for the statistical analysis of the data. In the case of rejection of the sphericity assumption, the Greenhouse-Geisser method for the epsilon correction was used. The study of pairwise comparisons was performed using the Bonferroni method. The statistical significance was set at p<0.05.ResultsIn vitro study Significant differences were observed concerning (i) L* between T3 and T0, T1 and T2 for Group B; (ii) a* between T0 and T1, T2 and T3 for Groups B, C, D and E, as well as between T1 and T2 for Group C; and (iii) b* between T0 and T1, T2 and T3 for Group B and between T0 and T2 and T3 for Group C. The differences between ΔΕ(T1-T0), ΔΕ(T2-T0) and ΔΕ(T3-T0) were statistically significant for Groups B and C, as well as the difference between ΔΕ(T1-T0) and ΔΕ(T2-T0) for Group D.In vivo studyΔΕ changes from T0 to T1, T0 to T2 and T0 to T3 showed no statistically significant differences between upper central incisors. No significant differences in ΔΕ changes were observed between Vivera® and Essix® C+ groups within any combination of used retainers, control or teeth-only measurements or right or left central incisors. ΔΕ changes between T3 and T0 for used retainers were 1.55 times higher than those of control retainers (p=0.002) and 1.44 times higher than those of teeth-only (p=0.004). In addition, for used retainers, color changes between T3 and T0 were 1.56 times higher than those between T1 and T0 and 1.47 higher than those between T2 and T0 (p<0.001). Conclusions: The in vitro part of this study indicates that the color of Vivera® retainers exhibited visible changes induced by several staining solutions, especially coffee, tea and, to a lesser extent, red wine. The in vivo part of this study indicates that the used retainers exhibited greater color changes than control retainers or teeth without retainers and these changes became more pronounced with the duration of use. Vivera® and Essix® C+ retainers exhibited similar color stability over a 3 month period of oral use, as no significant differences in color change were observed between the two retainer types. There were no differences in color changes between right and left incisor measurements during a 3 month post-treatment period. All retainer color differences observed in vivo during a 3 month post-treatment period were considered clinically acceptable (ΔE<3.7), although prolonged use of the retainers could possibly cause clinically significant changes in coloration. Knowledge of the effects of the staining susceptibility of the surface of the retainer could guide clinicians regarding the choice of the thermoplastic retainer material used for their patients or the instructions to patients about retainer use, so that better long-term retainer maintenance and color stability are preserved. The present findings suggest that polyurethane and polypropylene retainers have similar optical behavior during a three month oral wear and that it is important for patients to avoid drinking staining beverages, such as coffee and tea, while the retainers are in the mouth. Further studies would be necessary to safeguard the physical and optical properties of thermoplastic material when retainers are subjected to the hostile conditions of the oral cavity for longer periods of use.

ΣΚΟΠΟΣ: Να εκτιμηθεί η in vitro επίδραση διαφορετικών διαλυμάτων χρώσης στο χρώμα των ναρθήκων Vivera® και η in vivo μεταβολή του χρώματος δύο διαφορετικών θερμοπλαστικών ναρθήκων συγκράτησης, Vivera® και Essix® C+. ΥΛΙΚΑ ΚΑΙ ΜΕΘΟΔΟΙ: Τριάντα ασθενείς μετά από ορθοδοντική θεραπεία τυχαιοποιήθηκαν σε δύο ομάδες και έλαβαν συγκρατητικούς νάρθηκες Vivera® και Essix® C+, αντίστοιχα. Τριάντα δείγματα υλικού Vivera® αποδόθηκαν σε πέντε ομάδες και βυθίστηκαν σε διαφορετικά χρωστικά διαλύματα. Για κάθε ένα από τα δείγματα μετρήθηκαν οι παράμετροι χρώματος CIE (L*, a*, b*) in vivo με φασματοφωτόμετρο SpectroShade™ Micro μετά από έως και 3 μήνες ενδοστοματικής χρήσης και in vitro με φασματοφωτόμετρο ορατού-υπεριώδους μετά από έως και μία εβδομάδα εμβάπτισης σε διάλυμα. Υπολογίστηκαν οι διαφορές χρώματος (ΔΕ) μεταξύ των ομάδων. Έγινε ανάλυση ANOVA του στατιστικού δείγματος και συγκρίσεις με τη μέθοδο Bonferroni με επίπεδο στατιστικής σημαντικότητας p < 0,05. ΑΠΟΤΕΛΕΣΜΑΤΑ: Δεν παρατηρήθηκαν in vivo σημαντικές διαφορές στη μεταβολή του χρώματος μεταξύ Vivera® και Essix® C+ στις μεταβολές ΔΕ. Βρέθηκαν σημαντικές διαφορές για τα δείγματα που εμβαπτίστηκαν in vitro σε καφέ, τσάι και κόκκινο κρασί. ΣΥΜΠΕΡΑΣΜΑΤΑ: Οι νάρθηκες Vivera® και Essix® C+ παρουσίασαν παρόμοιες μεταβολές χρώματος μετά από στοματική χρήση τριμήνου. Οι νάρθηκες Vivera® εμφάνισαν ορατές μεταβολές που προκλήθηκαν από καφέ, τσάι και κόκκινο κρασί. Όλες οι διαφορές που παρατηρήθηκαν θεωρήθηκαν κλινικά αποδεκτές (ΔΕ<3,7).