Effect of different modes of light curing and resin

• O.S.T. - T.D.J • Juin/June 2009, Vol..32, N°126•

Effect of different modes of light curing and resin composites on microleakage of Class II restorations - Part II L.S. Hardan¹, E.W. Amm², A. Ghayad³, C. Ghosn¹, A. Khraisat 4

Abstract Introduction: To reduce the polymerization shrinkage of the composite resins and after the introduction of the “slow and gradual polymerization” by GORACCI et al. in 1992, many light curing units (LCU) presented the “soft-start polymerization” in addition to the classical high light intensity mode. This study investigated whether this slow and gradual polymerization has the ability of reducing the marginal debonding, by minimizing the internal stress of the composite, knowing that rare are the studies which compared this mode of polymerization. Material and methods: One hundred twenty Class II cavities were prepared at the mesial and the distal side of human extracted premolars. The teeth were randomly divided into four groups (n=15). In each tooth the mesial Class II cavities were restored with the micro-hybrid resin composite Filtek Z250

1. Dpt of Restorative Dentistry, School of Dental Medicine, SaintJoseph University, Beirut, Lebanon. 2. Dpt of Orthodontics, School of Dental Medicine, Saint-Joseph University, Beirut, Lebanon. 3. Private practice, Beirut, Lebanon. 4. Dpt of Conservative Dentistry and Prosthodontics, Faculty of Dentistry, University of Jordan, Amman, Jordan.

(3M ESPE) and the distal Class II cavities were restored with the nano-filled resin composite Filtek Supreme (3M ESPE) The light curing unit used in this study was a conventional quartz tungsten halogen (QTH) curing light, the QHL 75 (Dentsply) modified by adding a regulating electronic device, controlled by a special software that gives 4 different modes of polymerization as follows: Group A: Standard polymerization: Exposure for 20 seconds at 700 mW/cm², this group was used as control. Group B: Modified pulsedelay polymerization: Exposure for 2 seconds at 700 mW/cm², 5 seconds at 0 mW/cm², then 20 seconds at 700mW/cm². Group C: Modified ramp polymerization: Exposure for 20 seconds with a slow rise of the intensity to reach the 700 mW/cm² then 20 seconds at 700 mW/cm2. Group D: Slow and Gradual polymerization: Exposure for 4 minutes with a slow rise of the intensity to reach 700 mW/cm². The teeth were stored in water at 37°C for 48 hours, then finished and subjected to thermocycling (3000 cycles between 5°C and 55°C). All the teeth were immersed in 2% methylene blue solution for 12 hours at 37°C, sectioned, and evaluated at the gingival margins. Data were statistically analyzed by two-way analysis of variance followed by Tukey HSD multiple comparisons. Results: The main effect for the type of composites (p= 0.682), and the interaction effect (p=0.678) did not reach statistical significance. There was a statistically significant main effect for the type of polymerization used (p=0.014). Post-hoc comparisons using the Tukey HSD test indicated that the mean score for the Group A (M=2.40, SD= 1.102) was significantly different from the group D (M= 1.30, SD= 1.393, p= 0.010). The group D did not differ significantly from group B (M= 1.63, SD= 1.351, p=0.767), and group C (M= 1.60, SD= 1.404, p=0.819). There was a difference between the group A (M=2.40, SD= 1.102), group B (M= 1.63, SD= 1.351) and group C (M= 1.60, SD= 1.404). However, when the Tukey HSD was used, no statistically significant differences between Group A, B and C were found (p=0.122 and p =0.098). Conclusions: There was a statistically significant difference in microleakage between the “slow and gradual polymerization” and the standard polymerization, however no significant differences were found when using the nano-filled or the micro-hybrid composite materials cured with different polymerization regimens.

Copyright © 2009 Tous droits réservés

Keywords: Polymerization, Light intensity, Composites

• Effect of different modes... •

Résumé Introduction : Pour réduire la rétraction de polymérisation des résines composites et après l’introduction de la « polymérisation lente et progressive » par GORACCI et al. en 1992, plusieurs appareils à photopolymériser ont présenté la polymérisation «soft-start», en plus du mode de polymérisation classique à intensité lumineuse maximale. Cette étude a testé si cette polymérisation lente et progressive pourra réduire le décollage marginal en diminuant le stress interne des composites, sachant que les études qui ont comparé ce mode de polymérisation sont rares. Matériel et Méthodes : Cent vingt cavités classe II ont été préparées au niveau des faces mésiales et distales de prémolaires humaines extraites. Ces dents ont été réparties aléatoirement en 4 groupes (n = 15). Au niveau de chaque dent, les cavités mésiales ont été restaurées avec le composite micro-hybride Filtek Z250 (3M ESPE) et les cavités distales ont été restaurées avec le composite nano-chargé Filtek Suprême (3M ESPE). Dans cette étude, l’appareil à photopolymériser utilisé était un appareil halogène conventionnel, le QHL 75 (Dentsply), modifié en ajoutant un dispositif de réglage électronique, contrôlé par un logiciel spécial qui nous permet d’avoir 4 modes de polymérisation différents comme suit : Groupe A : Polymérisation standard : Exposition de 20 secondes à 700 mW/cm². Groupe B : Polymérisation intermittente modifiée : Exposition de 2 secondes à 700 mW/cm², suivie par 5 secondes à 0 mW/cm² puis 20 secondes à 700 mW/cm². Groupe C : Polymérisation progressive modifiée : Exposition de 20 secondes avec une augmentation progressive de l’intensité pour atteindre 700 mW/cm² puis de 20 secondes à 700 mW/cm². Groupe D : Polymérisation lente et progressive : Exposition de 4 minutes avec une augmentation progressive de l’intensité pour atteindre 700 mW/cm². Les dents ont été stockées dans de l’eau à 37°C pendant 48 heures, polies, puis soumises à un thermocyclage (3000 cycles entre 5°C et 55°C). Toutes les dents ont été immergées dans une solution de bleu de méthylène à 2% pour une durée de 12 heures à 37°C, sectionnées et évaluées au niveau de l’interface marginale. Les données ont été analysées statistiquement par le test de l’analyse de variance à deux facteurs suivie par des comparaisons multiples de Tukey HSD. Résultats : La différence statistique entre les deux types de composites n’a pas été significative (p = 0,682), de même que pour l’effet de l’interaction (p = 0,678). Concernant les techniques de photopolymérisation testées, la différence était significative (p = 0,014). Le test de Tukey HSD a indiqué que la moyenne des scores du groupe A (M = 2.40, SD=1.102) était significativement différente de celle du groupe D (M=1.30, SD=1.393, p=0.010). Pas de différence significative entre le groupe D et le groupe B (M=1.63, SD=1.351, p=0.767), ainsi que le groupe C (M=1.60, SD=1.404, p=0.819). Il y avait une différence entre le groupe A (M=2.40, SD=1.102) et le groupe B (M=1.63, SD=1.351) ainsi que le groupe C (M=1.60, SD=1.404). Cependant, lorsque le test de Tukey HSD a été utilisé cette différence n’était plus statistiquement significative entre le groupe A et les groupes B et C (p=0,122 et p=0.098). Conclusions : Il y avait une différence statistiquement significative, dans la formation de microhiatus, entre la « polymérisation lente et progressive » et la polymérisation standard, cependant il n’y avait pas de différence statistiquement significative entre les composites nano-chargés et micro-hybrides polymérisés avec les différents modes de polymérisation.

Introduction

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The clinical failure of light-cured composites, widely used in restoring anterior and posterior teeth, is related to the changes during the polymerization process (21) and that is the chemical reaction that transforms small

Polymérisation, Intensité lumineuse, Composites

molecules into large polymer chains or networks. The distance between monomer molecules is 3-4 Å, but after polymerization this distance is reduced to 1.5 Å leading to the shrinkage during the polymerization process

• O.S.T. - T.D.J • Juin/June 2009, Vol..32, N°126 •


Mots clés :


(26). The volumetric shrinkage during the composite polymerization creates internal stress which may disrupt the bonding between the composite and the cavity walls or may even cause cohesive failure of the restorative material or the surrounding tooth tissue (7). The polymerization of resin composites with high power density will lead to a fast monomer conversion, the stress generation will be high and so, gap formation and microleakage may result as a consequence (10, 12, 18, 22, 23). Studies of polymerization dynamics show that post-gelation polymerization shrinkage generates stress (6, 9). Thus the harmful effects of polymerization shrinkage can be avoided by delaying gelation which can be achieved using a low light intensity for curing (15) to increase the ‘plastic flow’ period during the pre-gel polymerization status (13), knowing that high light intensities are essential to get complete polymerization and optimal mechanical properties (26). To increase the flow capabilities of the composites, the time span before getting to the gel point should be delayed. Hence, GORACCI et al. (13) introduced the ‘slow and gradual polymerization’ method in 1992, which consists in the polymerization of the first composite layer with an increasing gradual light intensity during four minutes. For that purpose many light curing units (LCU), incorporating different soft-start polymerization modes, have been placed on the market, to lengthen the pre-gel phase and to minimize the internal stress during the polymerization of the composite. None of those modes respects the time of cure and the way of increasing the light intensity as proposed by GORACCI et al. (13). It was demonstrated that the modes of cure commonly found in most of the commercial LCUs didn’t show statistically significant difference in microleakage formation at cementum/dentin interface (14). Multiple

studies compared different modes of cure to see what mode best reduces the composite internal stress (10, 12, 20, 23, 28), but rare are those who tried to study the ‘slow and gradual polymerization’ proposed by GORACCI et al. The question is whether this technique is able to minimize the internal stress of the composite and to reduce the marginal debonding by increasing the time available for the flow of the material, slowly, with a gradual intensity. The aim of this in vitro study was to evaluate the marginal seal, measured as microleakage on cementum/dentin margins of class II composite restorations, using four different modes of polymerization including the ‘slow and gradual polymerization’ method described by GORACCI et al. and two different resin composites: a micro-hybrid Filtek Z250 (mean particle size of filler= 0.6 μm) (28) and a nano-filled Filtek Supreme (mean particle size of filler: Nanocluster = 0.6–1.4 μm, Nanofiller= 20nm) (28) using a quartz tungsten halogen (QTH) curing light which allows to have different modes of cure by adding a regulating device controlled by a special software.

I - Material and methods Sixty freshly human premolars, extracted for orthodontic reasons, were collected and stored in a 0.1% thymol solution at room temperature. The criteria for tooth selection included intact enamel without cracks caused by extraction forceps, absence of caries and previous restorations. The teeth were then cut 3 mm coronal to the cementum/enamel junction, with a doublefaced diamond disk (KG Sorensen Ind. Com. Ltda, Barueri, SP, Brazil). In each tooth a Class II cavity was prepared at the mesial and the distal side with a #SG 835.014 diamond bur (Edenta AG, Hauptstrasse 7, AU/SG,



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Switzerland) in a high-speed water-cooled handpiece (Bien Air Dental SA, Bienne, Switzerland). The burs were replaced every four preparations to maintain uniformity. The cavities had the following dimensions: 1.5 mm of axial depth, 3 mm of bucco-lingual width, and the gingival margin located 1 mm apical to the cementum/enamel junction corresponding to 4 mm cervico-incisal. A 2mm vertical slot was prepared on the buccal surface of the teeth to identify the mesial and the distal cavities. The teeth were randomly divided into four groups (n=15), and each group was the result of the combination of the resin composites and the mode of polymerization used. Within each group, the cavities had gingival margins on cementum/ dentin. In all groups the enamel and dentin were etched with 35% phosphoric acid (3M ESPE, St. Paul, MN, USA) for 15 seconds, rinsed for 15 seconds, and blot dried. Two coats of Adper Single Bond 2 adhesive (3M ESPE, St. Paul, MN, USA) were applied with a microbrush tip, lightly dried for 5 seconds, and polymerized for 10 seconds following the manufacturer’s directions. The mesial class II cavities were restored with the micro-hybrid resin composite Filtek Z250 (3M ESPE, St. Paul, MN, USA) A2 shade and the distal Class II cavities were restored with the nano-filled resin composite Filtek Supreme (3M ESPE, St. Paul, MN, USA) A2 body shade. The light curing unit used in this study was a conventional QTH LCU, the QHL 75 (Dentsply, Milford, DE, USA) modified with a power amplifier unit controlled through a software by a custom application developed using LabVIEW (National Instruments, Austin, TX, USA) which generates a variety of luminosity profiles. The user could control the light profile variables (time, amplitude, ramp) to build linear/non-linear profiles which allow curing

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the restorations with different modes of cure, in reproducible cycles in time and intensity. The light intensity of the light curing unit was assessed with a radiometer (Demetron Research Corporation, Danbury, CT, USA) all along the study. Resin-composites were placed in two horizontal (bucco-lingual) increments, and each increment was cured on the occlusal surface as follows: • Group A: Standard polymerization: Exposure for 20 seconds at 700 mW/cm², this group was used as control. • Group B: Modified pulse-delay polymerization: Exposure for 2 seconds at 700 mW/ cm², 5 seconds at 0 mW/cm², then 20 seconds at 700mW/cm². • Group C: Modified ramp polymerization: Exposure for 20 seconds with a slow rise of the intensity to reach the 700 mW/cm² then 20 seconds at 700 mW/cm². • Group D: Slow and gradual polymerization: Exposure for 4 minutes with a slow rise of the intensity to reach 700 mW/cm². Following the restorative procedure, the teeth were stored in water at 37°C for 48 hours. After that, all restorations were finished with Sof-Lex discs (3M ESPE, St. Paul, MN, USA). Finishing and polishing were done in only one direction with a low-speed handpiece without any water coolant. The restored teeth were subjected to 3000 temperature cycles between 5°C and 55°C. The cycles consisted of 60 seconds in each bath with an exchange time of 15 seconds between baths. The root apices were removed with a diamond disc and sealed with composite resin (Z250, 3M-ESPE, St. Paul, MN, USA) and cyanoacrylate adhesive and the coronal sur-faces were sealed with epoxy resin. The teeth were coated with two applications of nail varnish up to 1 mm from the gingival margins of the




restorations. All the teeth were immersed in 2% methylene blue solution for 12 hours at 37°C. After this time, the samples were rinsed in tap water and dried at room temperature. The specimens were sliced longitudinally in three sections: through the one-quarter, the center and the three-quarters of the restorations with a water-cooled diamond saw (Exact Technologies Inc., Kg of Norderstedt, Germany). Dye penetration at the gingival margin was evaluated by two previously calibrated examiners with a stereomicroscope (Olympus Optical Co. GmbH, Wendenstr., Hamburg, Germany) at 40x magnification and scored using the following scale: (a 0-3 scoring system was used to describe the severity of infiltration): 0 = No dye penetration, 1 = Dye penetration up to one-third of the cavity wall, 2 = Dye penetration more than one third, but less than two–thirds of the cavity wall, 3 = Dye penetration more than two thirds, or to the full extent of the cavity wall.

two times by two evaluators. For statistical analysis, the highest score for each restoration was chosen. The two evaluators differed in scoring 2 sections of 2 restorations; agreement was achieved following discussion. Statistical Analysis Data were statistically analyzed by two-way analysis of variance followed by Tukey HSD multiple comparisons for determination of significant differences in scores based on two independent variables: photopolymerization technique and composite resin. The “restoration” was defined as the statistical unit. The level of significance was set at α = 0.05. The software used was SPSS 15.0 (SPSS inc., Chicago, USA)

II - Results Table I shows the distribution of dye penetration scores in all groups. The main effect for the type of composites (p = 0.682), and the interaction effect (p = 0.678) did not reach statistical significance (Figure I).

Each part of the three sections was scored Table I : Distribution of dye penetration scores and mean in all groups

Dye penetration Scores

Groups

0

1

2

3

Mean ± SD

N

Group A

Distal Mesial

2 2

2 0

0 2

11 11

2.3 ± 1.175 2.47 ± 1.060

15 15

Group B

Distal Mesial

5 4

4 3

0 0

6 8

1.47 ± 1.356 1.8 ± 1.373

15 15

Group C

Distal Mesial

5 6

1 3

1 0

8 6

1.8 ± 1.424 1.4 ± 1.404

15 15

Group D

Distal Mesial

9 5

0 4

1 0

5 6

1.13 ± 1.475 1.47 ± 1.356

15 15

Group A : Standard polymerization - Group B : Modified pulse-delay polymerization Group C : Modified ramp polymerization - Group D : Slow and gradual polymerization Distal : Nano-filled composite - Mesial : Micro-hybrid composite



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III - Discussion

Figure I : Profile plots Filtek Z250

3

Filtek Supreme

Means of scores

2,5 2 1,5 1 0,5 0 A

B

C

D

Photopolymerization technique

Group A : Standard polymerization - Group B : Modified pulse-delay polymerization Group C : Modified ramp polymerization - Group D : Slow and gradual polymerization

There was a statistically significant main effect for the type of polymerization used (p = 0.014). Post-hoc comparisons using the Tukey HSD test indicated that the mean score for the Group A (M = 2.40, SD = 1.102) was significantly different from the group D (M = 1.30, SD= 1.393, p= 0.010). The group D did not differ significantly from group B (M = 1.63, SD= 1.351, p = 0.767), and group C (M= 1.60, SD = 1.404, p=0.819) (Table II).

Table II : Means and Standard deviation (SD) of dye penetration’s scores Photo polymerization technique Technique A Technique B Technique C Technique D

There was a difference between the group A (M = 2.40, SD = 1.102), group B (M = 1.63, SD = 1.351) and group C (M = 1.60, SD = 1.404). However, when the Tukey HSD was used, no statistically significant differences between Group A, B, and C were found (p = 0.122 and p = 0.098) Mean

SD

N

Tukey grouping*

2.40 1.63 1.60 1.30

1.102 1.351 1.404 1.393

30 30 30 30

a a, b a, b b

* Same letters indicate no significant difference Group A : Standard polymerization - Group B : Modified pulse-delay polymerization Group C : Modified ramp polymerization - Group D : Slow and gradual polymerization

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The relation between four modes of polymerization including the “slow and gradual polymerization” and the formation of microleakage between the restoration and the tooth was evaluated in this study using a micro-hybrid and a nano-filled composite. The data presented above proves that those 4 tested modes of soft-start polymerization cannot completely eliminate the marginal leakage of class II composite restorations and no significant differences were observed among them in the composites used in this study. However there was a significant difference between the standard polymerization and the “slow and gradual polymerization” group (p 0.05), was found between the standard polymerization and the modified ramp polymerization. This result is probably due to the fact that the 20 seconds of slow rise polymerization was not enough to allow sufficient flow of the material and to reduce the internal stress of the



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References

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“slow and gradual polymerization” was used. This in vitro study proves that only the “slow and gradual polymerization” shows statistically significant difference in dye penetration at cementum/dentin interface when compared to the standard polymerization, while the modified pulse-delay and ramp polymerizations presented no significant differences. Also, there were no differences observed with regard to the composite resin formulation in this study.

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