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ORIGINAL RESEARCH Table of Contents   
Year : 2009  |  Volume : 20  |  Issue : 4  |  Page : 394-399
SEM evaluation of marginal sealing on composite restorations using different photoactivation and composite insertion methods


1 Department of Restorative Dentistry, University of North Parana - UNOPAR, Londrina, Brazil
2 Department of Restorative Dentistry, Piracicaba Dental School, State University of Campinas, Piracicaba, Brazil

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Date of Submission25-Aug-2008
Date of Decision05-Oct-2008
Date of Acceptance26-Jan-2009
Date of Web Publication29-Jan-2010
 

   Abstract 

Aim: This in vitro study evaluates the influence of marginal sealing methods in composite restorations with different adhesive systems submitted to mechanical load.
Materials and Methods: Eighty bovine incisor crowns were embedded in Polyvinyl chloride (PVC) molds with the buccal surface exposed, where cavities (4mm x 4mm x 3mm) were made. Samples had the adhesive systems, Single Bond or Clearfil SE Bond, applied according to the manufacturer's recommendations. The cavities were filled with a Z-250 composite according to the restoration technique (bulk filling or three increments) and photoactivation (conventional, soft start, pulsatile light or light-emitting diode [LED]). The samples were duplicated with epoxy resin for scanning electron microscopy observations. Samples were also submitted to mechanical load (200,000 cycles; 2 Hz) and new replicas were made.
Results: The results, in percentages, were submitted to ANOVA followed by Tukey's test (P < 0.05). There was statistical difference between the cycle group (23.84%) and the non cycle group (18.63%). Comparing the restoration technique, there was no statistical difference between bulk filling (19.62%) and three increments (22.84%). There was no statistical difference among the groups: Pulsatile light (24.38%), soft start (22.75%), LED (21.47%) or conventional (16.34%). Furthermore, there were no statistical differences between the adhesive systems: Clearfil SE Bond (21.32%) and Single Bond (20.83%).
Conclusions: The photoactivation methods, the restorative techniques and the adhesive systems did not influence gap formation.

Keywords: Adhesive systems, light-curing mode, marginal gap, mechanical cycling, resin composite, scanning electron microscopy

How to cite this article:
Lopes MB, Costa LA, Consani S, Gonini AJ, Sinhoreti MA. SEM evaluation of marginal sealing on composite restorations using different photoactivation and composite insertion methods. Indian J Dent Res 2009;20:394-9

How to cite this URL:
Lopes MB, Costa LA, Consani S, Gonini AJ, Sinhoreti MA. SEM evaluation of marginal sealing on composite restorations using different photoactivation and composite insertion methods. Indian J Dent Res [serial online] 2009 [cited 2014 Nov 27];20:394-9. Available from: http://www.ijdr.in/text.asp?2009/20/4/394/59428
The adhesion process of composites to dental structures has proved to be a challenge for dentists and researchers. Also, the development of agents that will provide strong and stable adhesive bonds both to the dentine and to the enamel in the oral environment is a major challenge. [1],[2] An adhesion failure results in microleakage on the tooth restoration interface, with possible consequences of post operative sensitivity, marginal discoloration, occurrence of decay and pulp alterations. [3]

Control of rate of polymerization reaction may increase the resin flow index, allowing release of tensions, which were caused by polymerization contraction, [4],[5],[6] resulting in a more efficient adhesion. According to Koran and Kurschner, [7] greater light intensity will induce a faster composite viscosity increase, resulting in a reduced flow of material during the polymerization procedure. Some authors [4],[7],[8] have demonstrated a direct correlation between contraction and intensity of photoactivation and, furthermore, this correlation could be influenced by light intensity. Although low light intensity decreases the conversion degree and, consequently, the physical properties of the material, [9] this problem may be solved by increasing the time of photoactivation. According to some authors, [4],[10],[11] the density of irradiated energy (intensity vs. time) determines the polymerization degree of the material. Lower intensity of light and more exposure time could improve the marginal integrity of the restorations without decreasing the physical properties. [11]

Until the first half of the 90 s, most photoactivator equipments emitted a constant intensity of light. Thus, due to the necessity for decreasing the contraction stress, photoactivator equipment with additional resources appeared on the market. Soft start and pulsatile light may increase the pre gel phase to promote better accommodation of the composite on the cavity walls, decreasing the stress on the interface restoration tooth when high intensities of light are used. [6]

Recently, equipments for composite activation were developed utilizing technology based on diode lamps (light-emitting diode [LED]), allowing more biological advantages. [12],[13] The LED photoactivator emits lower intensities of light, although the spectrum of light is the same as the one responsible for camphorquinone activation. Studies have verified that the LED photoactivator cures resins with a similar degree of conversion to that of quartz-tungsten-halogen (QTH) light-curing units. [12],[14] However, composite resins that present another initiator type are not appropriately cured.

Another factor that influences the adhesion of the resin to the dental structures is the technique of material insertion. A method used to reduce contraction is the resin placement in small layers and its polymerization, decreasing the stress on the cavity walls generated during the polymerization contraction and also increasing the depth of cure. [10],[15] The insertion of the composite resin in one increment, filling the whole cavity, can promote more stress on the cavity, which could cause the rupture of the adhesive interface. [10]

Because of the polymerization contraction and the stress generated by the polymerization process, the influence of the different photoactivation methods on the marginal seal in composite restorations is of interest. This study proposed to evaluate the influence of gap formation and fractures on the interface of restorations, before and after a mechanical load was applied, according to the following variables: 1, mechanical cycling: With or without; 2, photoactivation: Conventional, soft start, LED and pulsatile light; 3, adhesive agents: Conventional and conditioning primer; 4, insertion of material: One or three increments.


   Materials and Methods Top


Eighty incisive bovine teeth were used in the study. The teeth were submitted to prophylaxis with pumice stone and water using a Robinson brush with a hand piece at low rotation speed. The roots were sectioned and the crowns were stored at 0°C in a refrigerator for the maximum period of 6 months until the beginning of the experiment.

The crowns were embedded in acrylic resin inside polyvinyl chloride (PVC) plastic tubes (20 mm diameter per 20 mm height), with the buccal surface exposed and projected 1mm from the superior border. An adapted optical microscope was used to standardize the cavities. The micrometer of the microscope was used to control the movement of the hand piece, which was fixed to the microscope base. The plastic tubes were adapted on the metallic base of the microscope platform and class V cavities were prepared with dimensions of 4 mm x 4 mm x 3 mm at the center of the buccal surface of the crown, with cylindrical diamond burs 4102 (KG Sorensen, São Paulo, SP, Brazil) with a high-rotation hand piece under copious air-water spray.

After cavity preparation, the 80 samples were divided into two groups (n = 40) according to the adhesive system used: Single Bond (3M Dental Products Division, St. Paul, MN, USA) and Clearfil SE Bond (Kuraray Co. Ltd., Osaka, Japan), applied according to the manufacturers' instructions.

The two groups were subdivided in two subgroups, each one with 20 samples each, according to the material insertion method: 1, one increment; 2, multiple increments. For the one increment technique, the Z-250 composite (3M) was inserted in one layer and cured. For the multiple increments technique, the Z-250 composite was applied in three layers of the same amount, the first and second layers were seated in the axial-pulpal walls on opposite sides and the third was used to fill the remaining of the cavity. After each application, curing was performed.

The four groups were subdivided again into four new subgroups of five samples each, according to the curing method: 1, continuous light (XL-2500, 3M); 2, soft start light (XL-2500, 3 M); 3, pulsatile light (Degulux modified to pulse; Degussa, Hanau, Germany); 4, LED (Ultrablue Is; DMC Equipamentos, São Carlos, Brazil). For the continuous light method, the light was applied for 20 s with an intensity of 520 mW/cm 2 . For the soft start method, a hollow cylindrical extension of the light tip made with heavy silicon and calibrated to decrease the light intensity to 150 mW/cm 2 was used for the initial 10 s and then a further 20 s, with an intensity of 520 mW/cm 2 without the apparatus. For the pulsatile method, the light was applied at intervals of 1 s (1 s with and 1 s without light) for 40 s. For the LED method, the application was for 20 s. The energy density for each method was calculated [Table 1].

Each sample was duplicated with an epoxy resin (Epoxicure, Buehler Ltd., Lake Bluff, IL, USA) using a mould made with addition silicone Aquasil Ultra LV regular set (Dentsply Caulk, Milford, DE, USA). The samples were submitted to a mechanical cycling load (100,000 cycles, 30 N) and duplicated again. The replicas in epoxy resin were metalized in gold on a Bal-tec Machine (SCD-050; Bal-Tec AG, Balzers, Liechtenstein) for analysis by scanning electron microscopy (JSM-5600LV; JEOL Ltd., Akishima, Japan) to evaluate the quality of the tooth-material interface. The photomicrographs taken were used to measure the gap perimeter and the total perimeter of the margin. The perimeter of gap was measured on all samples using the microscope software measuring resource [Figure 1] and [Figure 2]. The total perimeter of each restoration, due to small variation among the specimens, was measured using the software UTHSCSA ImageTool (version 3.0, Department of Dental Diagnostic Science at The University of Texas Health Science Center, San Antonio, Texas, USA). The percentage perimeter of the gap was then calculated.


   Results Top


The values for the factors adhesive, restoration technique and photoactivator methods were subject to three-way ANOVA for the cycled [Table 2] and non cycled groups [Table 3] after verifying the homogeneity using Levene's Test of Equality of Error Variances. The averages were compared by Tukey's test with a 5% significance level. The comparison between the cycled and the non cycled groups was subjected to repeated measure ANOVA [Table 4] after verifying the sphericity (Mauchly´s test). The statistics were made on SPSS for Windows (version 15.0, SPSS Inc., Chicago, IL, USA).

[Table 5] shows the results combining all the factors. No statistically significant difference was found for double or triple interactions. [Table 6] shows the gap means before and after mechanical load andthe higher values (P < 0.05) with statistical difference for the cycled group compared to the uncycled one. The means obtained for the different insertion techniques as well as for the two types of adhesive systems and for different photoactivation methods did not demonstrate any statistical difference (P > 0.05).


   Discussion Top


The polymerization contraction of the composite resin restoration materials still represents the main disadvantage of these materials. This contraction is associated with the shortening of the space between the monomers during the organic polymer chain formation, resulting in tension formation in the tooth restoration interface. [16] According to Unterbrink and Liebenberg, [17] the tension intensity is associated with several factors, among which is the technique of restoration. However, according to Ferracane, [18] there is no proven association between the polymerization contraction behavior of dental composite restorations and their clinical outcomes. Whether or not a cause and effect relationship exists, this potential has dictated a fairly precise and careful placement technique by the practitioner in order to optimize clinical outcomes. [19] In this study, when the bulk filling technique was analyzed, no difference compared with the multiple increments technique was found, contradicting McCullock and Smith, [20] who defended the idea that the incremental technique decreases the contraction stress because there is minimum contact between the walls of the cavity and the restorative material during the polymerization, reducing its C-factor [21] as well as the reduction of contraction produced by a small portion of material. However, for Verluis and others, [5] the statement above is true for each increment individually: The total contraction and the stress results on the combination of all increment contraction effects and the dental structure deformation after total filling of the cavity, when the restoration is in full contact with the cavity. The total composite used to fill the cavity using the incremental technique tends to be smaller compared with the bulk filling technique. Polymerization contraction of each increment causes deformation in the cavity, forcing its walls, deflecting them, decreasing the volume of the cavity and, consequently, the amount of material available to fill it. This would result in a cavity filled with less composite than the original volume of the cavity. [12],[22]

Another factor that could influence the result of this study was the period of photoactivation, which, for the one increment technique, may convert less monomer into polymer due to the greater composite volume to be cured in the one increment technique, thus, decreasing the contraction tension. Although the manufacturer's instructions were followed, this became a limitation due to the different dose of energy given to each technique. Multiple increments received a superior energy dose [Table 1] compared with the bulk filling technique. These characteristics of polymerization together may explain the gap formation without a significant difference between bulk and multiple increment fillings.

The need for improved adhesion with dental structures contributed to the development of several adhesive systems, mainly after the establishment of acid conditioning by Buonocore. [8] However, high-concentrate acid solutions accentuate demineralization into dentine to a point that the adhesive cannot fill the total conditioned dentine area. [8],[23],[24] To avoid this problem, a generation of adhesives was developed, called as conditioning primers. The concept of an acidic primer is attractive because, in theory, this system simultaneously infiltrates the collagen fibers as it decalcifies the inorganic component to the same depth in the dentine, which should minimize voids, because there would not be a region of demineralized dentine that was not encapsulated by the resin primer. [25] In this study, when comparing the conditioning primer system, Clearfil SE Bond and the conventional system Single Bond, no statistically significant difference was found, demonstrating that both adhesive systems worked by a similar action. [26],[27] This result is already expected as the adhesive volume could be considered insignificant compared with the total amount of restorative material, having none or little influence on the final stress contraction result.

Another factor that could influence marginal integrity of the restoration is the light intensity used. According to Koran and Kurschner, [7] the higher the light intensity, the faster the composite viscosity increases and, therefore, the lower the material flow during the polymerization. The results of this study, however, did not demonstrate any statistical difference among the light-curing units used. Alonso et al., [13] evaluating marginal and cervical adaptation, found that marginal adaptation was not affected by any photoactivation method corroborating with this study. They also found an inverse relationship between bond strength and internal adaptation.

Contraction stress of a resin paste depends on factors such as filler loading, filler type and filler size. [28] Other factors that affect shrinkage behavior are the monomer and polymerization initiator systems because they determine the polymer structure of the material. [29] Bennett and Watts [30] compared the LED and QTH light-curing unit spectrum of light and found that the LED presented a more energy-efficient emission with peaks of 458 and 468 nm, coincident with the activation zone of camphorquinone. In the present study, because the activator of the restoring composite, Z-250, is camphorquinone, it may explain the similarity between the QTH and the LED light-curing unit.

Stansbury and Bowman [31] found that the predominant portion of the shrinkage stress did not start to develop until a much higher extent of polymerization was reached. Moreover, the latter stage of polymerization, where shrinkage stress was concentrated, a small increase in conversion led to a very significant increase of the shrinkage stress. Not only did this small increase in conversion affect the final magnitude of stress developed in the forming network but it was also critical when the effects of different curing protocols on stress evolution were investigated. [31] It is therefore not surprising to observe that soft start or pulse curing led to decreased shrinkage stress; however, a significantly decreased final conversion was also produced. [31]

Thermal or mechanical cycling is performed to simulate the aging of the specimens. The choice for mechanical or thermal treatment varies according to the geometric configuration of the specimens and the type of environment factor to be evaluated. When restorative materials are submitted to these simulations, there is a tendency for the defects being evidenced. In this study, the mechanical load was chosen to simulate masticatory forces. The marginal gap length of the cycled groups was statistically greater after mechanical cycling; however, no differences were found between materials, curing protocols or insertion technique.

Independent of the factor analyzed, material, insertion technique, aging or photoactivation mode, when they were analyzed together, no statistical difference was found. This may be influenced by the composite resin chosen, Z-250, as well as its shade, A2, and may not be applied to another resin or shade. The Z-250 absorbed a high quantity of light due to its inorganic particle shape and the shade A2 is sufficiently translucent to facilitate the light to cross the resin body, masking the influence of the different curing protocols, materials or insertion technique on the final contraction stress of the resin. Beside this, all the groups showed gap formation, indicating that deficiencies on the restorative materials and procedures still exist. More researches become necessary in order to establish a better tooth-restorative material bond union.


   Conclusions Top


  1. The multiple increments technique presented higher gap means, which were statistically different from those of the one increment technique.
  2. The conditioning adhesive system, Clearfil SE Bond, did not present any statistical difference from the conventional adhesive system, Single Bond.
  3. The different light-curing units and the several light intensities did not influence gap formation at the tooth restoration interface.
  4. The gap formation demonstrates that the composites still present deficiencies due to polymerization contraction.

   Acknowledgment Top


Grateful acknowledgement is made to Cnpq-Pibic for its support in this research.

 
   References Top

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Correspondence Address:
Murilo Baena Lopes
Department of Restorative Dentistry, University of North Parana - UNOPAR, Londrina
Brazil
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DOI: 10.4103/0970-9290.59428

PMID: 20139558

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]

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