2022, Number 1
Photoelastic shrinkage analysis of dental resins using different filling techniques
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ABSTRACT
Introduction: composite resins are widely used materials in which research continues in pursuit of decreasing shrinkage which is the main cause for their failure. Objective: to compare the contraction by polymerization of three different dental resin placement techniques, using photoelasticity. Material and methods: 33 photo elastic resin plates were made with a circular cavity in the center of 7 mm in diameter. These cavities were filled with resin by the block technique, wall by wall, in oblique increments, and polymerized. One hour later, the samples were placed in the polariscope, photographed, and measured. Statistical analysis was performed using the Kolmogorov-Smirnov, ANOVA, and post hoc Tukey tests, in the SPSS V.22 IBM program. Results: 100% of the samples showed polymerization shrinkage. They all expressed isochromes in the photoelastic resin. The block technique showed the lowest shrinkage results, 40% of the samples presented only the first isochromatic band corresponding to the lowest stress value. Conclusion: the use of resins for the block technique reduces stress on the cavity walls.INTRODUCTION
One of the most widely used materials in dentistry are composite resins; they are also one of the materials that have undergone the most modifications in order to achieve physical properties similar to enamel and dentin. Resins are constituted by a polymeric base; for their chemical reaction of hardening, they resort to the process of polymerization, in which the union of each monomer supposes its approach. Therefore they present, in different proportions, shrinkage inherent to the formation of polymeric chains, called polymerization shrinkage.1 Due to the ceramic nature of enamel and dentin, the formation of micro-cavities in the dentinal tubules is necessary to generate the bond. Another polymeric-based material is housed in these cavities: the dental adhesive, which functions as the bonding link between the dental structure and the resin.2 When there is bonding to the tooth structure and shrinkage of the polymers, the bonded wall undergoes stress, which is the cause of restoration failure and postoperative pain. For this reason, many researchers and clinicians focus their attention on placement techniques, polymerization, and different materials, as it has been shown that variations regarding these factors may result in shrinkage reduction and thus offer a better prognosis of treatments involving the use of composite resins.
Techniques for the placement of dental composite resinsIncremental technique. It consists of creating the restoration by adding layers of resin less than 2 mm thick, photo-activating one layer before the next. This technique proposes that better polymerization is achieved by performing it for each layer, thus reducing the effect of polymerization shrinkage, under the principle that "polymerization shrinkage is directly proportional to the volume of the resin".3
Oblique incremental technique. In this technique, small portions of less than 2 mm are added, but from one wall to the opposite one. Each portion is set up in triangles formed by the floor and the wall in contact; when performing the photopolymerization, this can be irradiated directly or through the cavity walls.4
Wall-by-wall technique. It is another variation of the incremental technique with the same condition regarding thickness, but the layers are placed in a specific order: the first increment is placed on one of the walls without touching the others and the photopolymerization process is performed; then another portion of resin is taken to the opposite wall, in the same way without touching the previous one. And so on until all the walls and the floor are covered; each layer is polymerized for 20 seconds with an intensity greater than 1,000 mW/cm2.5
Block techniques. The application techniques in single increments are also known as "in block or Bulk-Fill", it is said that they are simpler, faster, and more practical because the number of clinical steps is reduced. The composite resin of the bulk system was created to achieve increments of up to 4 mm, decreasing the shrinkage effect and reducing the number of voids within the increments. In order to achieve increments of 4 mm in depth, the polymerization light must be used with an intensity greater than 1,000 mW/cm2 for a minimum time of 20 seconds.6
The research and development of this type of technology have been increasing, according to Furness,7 the use of an increment of 4 mm with the resin for the Bulk-Fill system did not present significant differences in comparison with the conventional composite resins regarding the percentage of polymerization. On the other hand, Czasch8 investigated the degree of conversion of resins placed in 4 and 2-mm increments without finding significant differences. Although many studies on bulk resins have been reported, there is no conclusive evidence of their advantage or disadvantage.
PhotoelasticityPhotoelasticity is an optical technique that consists of the fact that, when observing a birefringent material body subjected to stress through polarized light, it presents stress concentration lines called isochromes. These are expressed as a result of differences in the refractive index within the body of the material according to the stresses received. Each of the lines is expressed in different colors and thicknesses, which have been associated with a stress value, indicating the presence of different stress magnitudes. They have also been revealed in different areas of the material pointed in different directions.9
In dentistry, this technique has been applied to analyze the stresses generated within the walls of dental cavities associated with the polymerization shrinkage generated in composite resins.10 The objective of the present investigation is to compare the polymerization stress of three different dental resin filling techniques, utilizing photoelasticity, to determine which of the techniques generates less stress on the preparation walls.
MATERIAL AND METHODS
An experimental study was performed with a sample that included a total of 33 epoxy resin molds, which were randomly numbered and divided into 3 groups (n = 11). They were filled with resin samples using 3 different techniques: Group 1. Block technique with Ivoclar Tetric N-Ceram Bulk-Fill IV resin. Group 2. Wall-by-wall incremental technique with 3M Z250 XT Nano Hybrid resin. Group 3. Oblique increment techniques with 3M Z250 XT Nano Hybrid resin.
Preparation of the photoelastic resin molds consisted of silicone matrices; square molds of 5 × 5 × 4mm were fabricated with a circular cavity of 7 mm in diameter and 2.5 mm deep in the center of the mold. They were poured into epoxy resin Cristal Líquido AWS 520 from Epolyglas México. The hole on one side of the mold was closed with the same resin and the other side was completely free. Before the test, it was verified that there were no residual stress concentration lines around the hole, those samples that showed them were eliminated (Figure 1A). Each mold was numbered and divided into 3 groups. Subsequently, with the help of a Teflon-coated spatula, the cavities were filled up to the superficial edge of the cavity by different techniques; when the edge was reached, a celluloid tape was placed and light curing was performed. The filling was performed according to the corresponding technique for each group:
1. Block filling technique; the cavity was filled with the resin in its entirety and light cured for 40 seconds, group. 2. Incremental wall-by-wall technique; increments were placed covering only one wall with an approximate thickness of 1.5 mm, light curing each one before placing the other for 20 seconds, until the cavity was filled, group 3. Oblique increment techniques; oblique increments were placed starting from one of the walls extending and reducing the height of the increment from a maximum thickness of 1.5 mm, one on top of the other until the entire cavity was filled; light curing was done between each layer for 20 seconds (Table 1). The 3M Elipar lamp (wavelength 430-480nm and intensity of 1,200mW/cm2) was used. The light curing lamp was calibrated with a radiometer to verify the intensity every 5 samples and the light curing distance was constant at 2mm from the edge of the resin sample.
To implement the photoelasticity test, the sample was placed in the polariscope in which the mold was observed and photographed before and after the test pointing out the isochromes. For stress quantification, the sample was compared with a scale ruler considering a constant diameter of 7 mm. The isochromes present were measured up to the end of the darkest band, both on the right and left side (Figure 1B).
PHOTOELASTICITY SCALE
The isochromatic bands show the stress in different colors and thicknesses: the wider they are, the lower the stress; the thinner and more continuous they become, the higher the stress. Figure 1C shows the color scale exhibited by the resin used; it was coded according to what has been reported in the literature from 0 to 5; where 0 and 1 correspond to low stress, 2 and 3 medium, and 4 and 5 high (Figure 1D).
In this study, the distance from the origin to band 0 was measured, because the higher the stress, the farther this band moves away and gives rise to the appearance of new color bands.
For the statistical analysis, the SPSS V.22 IBM program was used. The Kolmogorov-Smirnov normality test was performed followed by the comparison of means through the one-way ANOVA test and post hoc Tukey, all with a level α = 0.05.
RESULTS
All the samples exhibited a stress level in the low classification (between 0 and 1) by showing isochromes horizontally on both the right and left sides of black, dark blue, white, and yellow; none reached level 2 (from the pink band) (Figure 2). The results were compared based on the aforementioned color bands, considering the highest according to the distance exhibited from the origin to the end of the black band.
Group 1 filled with the block technique showed the lowest wall stress results: 40% of the samples presented only the first isochromatic band (black) corresponding to the lowest stress value (Figure 3A-B). Group 2. Incremental wall-by-wall technique (Figure 3C-D), as well as group 3- Oblique incremental technique (Figure 3E-F), showed similar stress results; in group 2 about 10% of samples exhibited only the black band and group 3 was considered the one with the highest stress value: all samples exhibited from level 1, i.e. blue band. When comparing the groups, groups 2 and 3 showed statistically significant differences when relating each technique with group 1 (p = 0.002 and p = 0.000, respectively). No statistically significant differences were found between groups 2 and 3.
Figure 4 illustrates the averages of the length measurement from the composite resin (taken as the origin of the stress) to the end of the black band.
DISCUSSION
In the present study, photoelasticity was used as a means to demonstrate the polymerization shrinkage of dental composite resins when placed by different techniques in a cavity. Since resinous filling materials have become a fundamental part of daily dental practice, it is important to know and give relevance to their physical characteristics. Authors such as Corral11 and Domínguez3 in 2015 mentioned some disadvantages of incremental techniques, such as the possibility of retention of empty spaces between layers when applying several increments of the material. Additionally, this process is more time-consuming, thus the possibility of error or that the resin suffers sorption despite the use of the rubber dam may increase causing a deficient polymerization; not to mention the fact that the patient spends more time in the dental chair.
In this research, each technique was tested individually to determine which one presented the lowest stress on the cavity walls due to the polymerization shrinkage of each material. This stress evaluation also notes the current polymer technology, which has succeeded in integrating nano-sized filler for shrinkage reduction and thus achieving lower stress. The proposed modifications are those made to resins designed for bulk techniques, which offer advantages in terms of time and steps for clinical work. Some studies with different analysis techniques than the one hereby used, proved to offer advantages in physical properties such as polymerization shrinkage. Al-Harbi 201612 analyzed by SEM microscopy the marginal integrity of resins for both bulk (Tetric N-Ceram Bulk-fill) and incremental techniques without finding significant differences. In 2018 Pereira13 used photoelasticity among other techniques and determined that Bulk-Fill type resins have similar performance to conventional ones, but also demonstrated that concerning polymerization, shrinkage incremental techniques may have advantages. However, in the present study, although all resins show evidence of polymerization shrinkage, the group restored with the bulk-fill technique showed the best results.
Recently, given the improvements and the inclusion of new monomers such as AUDMA or AMF and combinations with nanoclusters among other technologies,14 resins for the bulk technique have a very promising future. There have already been studies with evident results in the reduction of polymerization shrinkage with other analysis techniques, although they will surely continue to evolve.15 Given the importance of polymerization stress in the clinical success of dental resins, different techniques that present results closer to reality have been sought. It is common to use tensiometers to observe the contraction inside the resin structure, however, the results regarding stress in the walls present disadvantages. In 2011 Lopes16 used the same technique and a model similar to the one hereby used to demonstrate that resins based on siloranes do not offer advantages in stress reduction within the cavity walls. Similarly, Oliveira in 201217 analyzed the effect of photoinitiators suggesting that it may be a suitable technique for stress analysis. There are different methods for analyzing stress, however, the one used in the present study is simpler while providing valid and easily observable results.
However, it is still necessary to work with the different adhesive systems that are an important factor for these studies. The molds made were circular, thus avoiding the irregular tension caused by acute angles, which, although they are not recommended in dental cavities, sometimes occur. Therefore, a model more similar to clinical reality might be tested, as Pereira did.
CONCLUSION
Considering the limitations of the present study, it may be concluded that the use of resins for the bulk technique reduces stress in the cavity walls. In the incremental techniques, independent of how these increments are placed, stress values are similar to each other.
AGRADECIMIENTOS/
Al proyecto PAPIME PE206820 por el apoyo al proyecto.
To the PAPIME PE206820 project for support of the project.
REFERENCES
Domínguez Burich R JL, Corral Halal D, Mattar M. Análisis comparativo in vitro del grado de sellado marginal de restauraciones de resina compuesta realizadas con un material mono incremental (Tetric n-ceram Bulk Fill), y uno convencional (Tetric n-ceram). Revista Dental de Chile. 2015; 106 (1): 15-19. Disponible en: https://repositorio.uchile.cl/bitstream/handle/2250/137691/Análisis-comparativo-in-vitro-del-grado-de-sellado-marginal-de-restauraciones-de-resina.pdf?sequence=1
AFFILIATIONS
1 Facultad de Estudios Superiores Zaragoza, Universidad Nacional Autónoma de México.
CORRESPONDENCE
Miriam Marín-Miranda. E-mail: miriam.marin@zaragoza.unam.mxReceived: Octubre 2020. Accepted: Enero 2022.