Effect of Layering Techniques on Polymerization Shrinkage Stress of High- and Low-viscosity Bulk-fill Resins

Restorative procedures using composite resin have become an essential part of daily practice because of material enhancements¹  and improvements in adhesive systems.²  Material improvements over the years have resulted in an increase in patient demands for esthetics³  and more emphasis on preserving tooth structure.⁴  The wear resistance of composite resins has continued to improve, since their introduction as restorative materials, enabling expanded use in posterior restorations of large cavities with long-term durability.⁵  A systematic review⁶  comparing direct and indirect composite resin restorations in posterior teeth showed that there was no difference in longevity between direct and indirect restorations, regardless of the type of composite resin. In addition, a recent 20-year clinical study⁷  of direct composite resin restorations in posterior teeth showed a high success rate and increased mean survival time.

However, polymerization shrinkage and its associated stress have remained a major concern of direct resin restorations, especially in large cavities in posterior teeth.⁸  Numerous studies have been performed to assess polymerization shrinkage and stress development in composite resins.^9,10  Polymerization shrinkage causes stress at the interface between a tooth and a restoration as the elastic modulus of the resin increases during polymerization.¹¹  This stress may result in marginal gap formation, microleakage, and enamel micro-cracks and can give rise to pulpal irritation, secondary caries due to bacterial infiltration, and postoperative sensitivity, which in turn can lead to restoration failure requiring restoration replacement (Figure 1).¹²  Efforts have been made to reduce polymerization shrinkage and stress by increasing inorganic filler loading,¹³  modifying filler particle size and shape,¹⁴  and developing new types of resin matrices.¹⁵ 

However, increases in inorganic filler loading and modification of filler size and shape are limited by the need for a physicochemical combination within the resin matrix. Polymerization shrinkage and stress of composite resins can also be reduced by decreasing the reactive sites per unit volume through increasing the molecular weight per reactive group.¹⁶  Nevertheless, this strategy also has its limitations because the use of monomers with a high molecular weight may produce increased viscosity, resulting in poorer handling characteristics, and potentially reducing mechanical strength. In addition, attempts to develop composite resins with completely new resin matrices that shrink considerably less, have been ongoing. Recently, a composite resin with a silorane resin matrix containing siloxane and oxirane was introduced, but this resin needs special adhesive systems for bonding.¹⁷ 

Clinical strategies to minimize the shrinkage stress of composite resins, such as horizontal and oblique layering techniques to decrease the C-factor, are still widely used.¹⁸  In addition, recent developments have led to the introduction of high- and low-viscosity bulk-fill resins that can be used for large cavities with the bulk-fill technique.¹⁹  These materials are designed to be used for placing the resin in increments of up to 4 mm, and one of their central characteristics is a much greater depth of cure than that found in conventional resins.²⁰  However, they have also been designed to reduce polymerization shrinkage stress to support their use in bulk-fill techniques.⁵  No independent research has yet investigated whether the filling techniques used to reduce polymerization shrinkage stress are still necessary or valuable with these newly developed bulk-fill resins.

Although measurement of cuspal deflection is a useful way to evaluate the polymerization shrinkage stress of composite resins, the use of extracted teeth to measure cuspal deflection can produce significant discrepancies among specimens due to the lack of standardization of the anatomical and histochemical characteristics of the individual teeth.²¹  Recently, Tsujimoto and others⁵  fabricated aluminum blocks with identical shapes and dimensions instead of using extracted human teeth to examine cuspal deflection. The cuspal deflection of the aluminum blocks was measured using a digimatic micrometer or a confocal laser scanning microscope.

This laboratory study examined cuspal deflection of bulk-fill resins using standardized uniform aluminum blocks with mesial-occlusal-distal (MOD) cavity preparations. The purpose of this study was to investigate the influence of filling techniques on the polymerization shrinkage stresses of high- and low-viscosity bulk-fill resins. The null hypothesis was that the layering techniques would have no influence on the polymerization shrinkage stresses of high- and low-viscosity bulk-fill resins.

Six high-viscosity bulk-fill resins were used: 1) Beautifil Bulk (BB, Shofu, Kyoto, Japan); 2) EverX Posterior (EP, GC, Tokyo, Japan); 3) Filtek Bulk Fill (FB, 3M OralCare, St Paul, MN, USA); 4) Quixx Fill Posterior Restorative (QF, Dentsply Sirona, York, PA, USA); 5) Tetric Evo Ceram Bulk Fill (TEB, Ivoclar Vivadent, Schaan, Liechtenstein); and 6) Tetric N Ceram Bulk Fill (TNB, Ivoclar Vivadent). Six low-viscosity bulk-fill resins were used: 1) Beautifil Bulk Flow (BF, Shofu); 2) EverX Flow (EF, GC); 3) Filtek Fill and Core Flowable Restorative (FF, 3M Oral Care); 4) SDR Flow+ (SD, Dentsply Sirona); 5) Tetric Evo Flow Bulk Fill (TF, Ivoclar Vivadent), and 6) X-tra Base (XB, Voco GmbH, Cuxhaven, Germany).

Aluminum blocks (10 [W] × 8[L] × 15 [D] mm) with a simulated MOD cavity (4 [W] × 8 [L] × 4 [D] mm) were milled out using a computer-aided design/computer-aided manufacturing system, creating two remaining cusps. This structure simulated a large Class II cavity preparation in a premolar. The inside of the cavity was submitted to airborne-particle abrasion with 50 μm Al₂O₃ powder for 10 seconds to improve surface characteristics for bonding. Air pressure was set to 0.2 MPa, and the distance between the tip and aluminum surface was approximately 10 mm (Jet Blast II, J. Morita, Tokyo, Japan). A universal adhesive (Scotchbond Universal Adhesive, 3M Oral Care) was applied according to the manufacturer’s instructions before placing the high- and low-viscosity bulk-fill resins. The adhesive was light-cured for 10 seconds at a standardized distance of 1 mm using a quartz–tungsten–halogen curing unit (OptiLux 501, Kerr, Orange, CA, USA). The power density (>700 mW/cm²) of the curing unit was confirmed with a dental radiometer (Model 100, Kerr) before specimen preparation.

The aluminum blocks were randomly divided into three groups for different filling techniques (bulk-fill vs horizontal layering vs oblique layering technique) and were further subdivided according to the type of composite resin (high- vs low-viscosity) (Figure 2).

Group 1 (Bulk-fill Technique)—High- or low-viscosity bulk-fill resin was placed in bulk and light-cured for 40 seconds for each of the three exposed surfaces. Cuspal deflection was measured from the difference in the distance between the center of the two remaining cusps before the placement of composite resin and 10 minutes after light curing using a high-accuracy submicron digimatic micrometer (MDH-25MB, Mitutoyo, Tokyo, Japan).

Group 2 (Horizontal Layering Technique)—High-or low-viscosity bulk-fill resin was placed in two separate horizontal increments (2 mm each). Each increment was light-cured for 40 seconds each to the three exposed surfaces to ensure that an identical curing time was maintained. Cuspal deflection was measured in the same manner as in group 1.

Group 3 (Oblique Layering Technique)—High- or low-viscosity bulk-fill resin was placed in three separate oblique increments. Each increment was light-cured for 40 seconds each to the three exposed surfaces to ensure that an identical curing time was maintained. Cuspal deflection was measured in the same manner as in group 1 and group 2.

SEM (TM 3000, Hitachi-High Technology, Tokyo, Japan) was used to examine the filler size and shape of the tested high- and low-viscosity bulk-fill resins. A thin coating of gold-palladium alloy was applied in a sputter coater (SC7620 Mini Sputter Coater, Emitech, East Sussex, UK) to prevent electrostatic charge from accumulating at the sample surface. The observations were done at an operating voltage of 15 kV.

Statistical analysis was conducted with a commercial statistical software package (SPSS Statistics 25, IBM; Armonk, NY, USA). Two-way analysis of variance with a Tukey post hoc honest significant difference test was used to analyze the gathered data for the influence of 1) layering technique and 2) type of resin with an α level of 0.05.

The effect of layering techniques on polymerization shrinkage stresses is shown in Table 2 for high-viscosity bulk-fill resins and in Table 3 for low-viscosity bulk-fill resins. Cuspal deflection for the high-viscosity resins ranged from 11.2 to 18.2 μm with the bulk-fill technique, 10.7 to 15.5 μm with the horizontal layering technique, and 10.9 to 15.2 μm with the oblique layering technique. The rank order of cuspal deflection using the bulk-filling technique was TEB-FB-TNB-QF-EP-BB and that using the layering technique was TEB-EP-FB-TNB-QF-BB.

Cuspal deflection for the low-viscosity resins ranged from 9.2 to 19.8 μm with the bulk-fill technique, 8.2 to 15.7 μm with the horizontal layering technique and 8.4 to 16.4 with the oblique layering technique. The rank order of cuspal deflection using the bulk-filling technique was SD-FF-TF-EF-XB-BF and that using the layering techniques was SD-FF-EF-TF-XB-BF.

The cuspal deflection of BB, EP, and QF in high-viscosity resins and the cuspal deflection of BF, EF, and XB in low-viscosity resins were decreased when the layering techniques were used, unlike those of other resins.

Representative SEM observations of tested resins are shown in Figure 3. The SEM images clearly show that there are differences in the size and shape of fillers in the high- and low-viscosity bulk-fill resins. BB, TEB, TNB, and QF among high-viscosity resins as well as BF, SD, TF, and XB among low-viscosity resins showed irregular particles with a broad size range (<1 to 20 μm). EP and EF showed short E glass fibers of different diameters and irregular particles of a relatively uniform small size (<1 μm). The concentration of short E glass fibers in EF was higher than that in EP. FB and FF showed quite small (<1to 5 μm) irregular, and spherical particles.

Cuspal deflection is one of the accepted indicators of polymerization shrinkage stress of composite resins and has been evaluated using devices such as linear variable differential transformers, direct current differential transformers, and strain gauges.²²  Recently, Tsujimoto and others⁵  developed a cuspal deflection measurement technique using a digimatic micrometer. Compared with previously reported methods, the digimatic micrometer may be a more accessible and practical approach to measuring cuspal deflection than use of confocal scanning laser microscopy and other devices. In this study, a digimatic micrometer was used to measure cuspal deflection for high- and low-viscosity bulk-fill resins.

Park and others²³  reported results that the cuspal deflection in the layering technique was shown to be considerably lower than that for the bulk-fill technique, and they found no significant difference between horizontal and oblique layering techniques using a conventional resin. However, in that study, the resin used for the bulk-fill technique was a conventional resin with a recommended maximum 2-mm depth of cure. Therefore, the depth of polymerization might not be totally completed in the bulk-fill specimens, making it more difficult to interpret test results. Further, as the bulk-fill technique is not recommended for clinical use with conventional resins, the clinical application of their results may not be applicable.

Recently, high- and low-viscosity bulk-fill resins have been used to expedite the restoration process by enabling increments up to 4-mm thick to be light-cured.^19,20  The present study compared the influence of layering techniques on high- and low-viscosity bulk-fill resins. Bulk-fill resins face no problems with polymerization in smaller increments to reduce polymerization shrinkage stress and can be used in bulk-fill or layering techniques in the clinic, as appropriate to the situation. Thus, it is clinically important to determine whether there are reasons to continue to use layering techniques with bulk-fill resins. While Park and others²³  found a significant difference in conventional resin composites, it is not surprising that the present study showed different behavior in cuspal deflection for bulk-fill resins.

The 12 bulk-fill resins used in this study can be classified into four groups according to the level of shrinkage stress produced: 1) high-viscosity bulk-fill resins with low stress (TEB, FB, QP, and TNB); 2) high-viscosity bulk-fill resins with high stress (BB and EP); 3) low-viscosity bulk-fill resins with low stress (SD, FF, and BB); and 4) low-viscosity bulk-fill resins with high stress (BF, EF, and XB). A high-viscosity bulk-fill resin (BB) and a low-viscosity bulk-fill resin (SF) showed the highest and lowest values of cuspal deflections, respectively. Although, within the high- and low-viscosity bulk-fill resins with low stress, there was no significant difference (p>0.05) between the value of the cuspal deflection measured with bulk-filling and that measured with layering techniques. For high- and low-viscosity bulk-fill resins with high stress, the difference in cuspal deflection was significant (p<0.05). This suggests that, for low shrinkage stress materials, advances in material composition are more important for reducing polymerization shrinkage stress than choice of filling techniques. However, this cannot be said of materials that show high polymerization shrinkage stress.

It is generally accepted that horizontal and oblique layering techniques improve marginal adaptation and bond stability of the adhesive interface between tooth structures and resin.²⁴  Therefore, when filling a large cavity, the use of small increments horizontally and obliquely was still recommended in one of the reviews²⁵  so that the polymerization shrinkage stress could be reduced. Although it is true that all the tested materials showed decreased values of cuspal deflection in this study with horizontal and oblique layering techniques, the values did not show statistically significant differences in high- and low-viscosity bulk-fill resins with low stress. Therefore, it appears that the use of polymerization shrinkage reduction technology in resins is more important than layering techniques. This is consistent with recent systematic reviews.^26,27  In those reviews, the technique protocols of placing material no longer showed a great contribution to polymerization shrinkage stress and, indeed, made less difference than the choice of light-curing method. On the other hand, modifying the resin matrix made the largest contribution to minimizing polymerization shrinkage stress development.

The layering technique for composite resins increases treatment time and increases the risk of creating voids and/or other weaknesses within a restoration.²⁸  Thus, in low polymerization shrinkage bulk-fill resins, the rationale for using layering techniques is diminished because the layering technique has a minimal influence on polymerization shrinkage stress. However, this was not true for all of the bulk-fill resins investigated in the present study; thus, clinicians need to be familiar with the properties of the bulk-fill resin in order to choose the most appropriate procedures. Hayashi and others²⁹  have shown that bulk-fill resins with higher rates of polymerization shrinkage may nevertheless show much lower gap formation than other resins. This suggests that total polymerization shrinkage and stress are not enough information to guide selection of an appropriate technique, and that detailed information about the interaction between resins and adhesives is necessary. As this information is not easily available outside the research literature, the results of this study suggest that at present it would be safer to continue to use the layering technique as a significant number of bulk-fill resins show significantly higher polymerization shrinkage stress without it. Thus, from the overall results of this study, the null hypothesis that the layering method would have no influence on polymerization shrinkage stress, was rejected for some materials but not for all.

The results of this study indicate that the effect of layering techniques on polymerization shrinkage stress for high- and low-viscosity bulk-fill resins was material dependent. A decrease in polymerization shrinkage stress with layering techniques was not observed in bulk-fill resins that showed low cuspal deflection (<15 μm).

The authors thank Dr Masao Irie of Okayama University and Mr Kazuyoshi Shimizu of 3M Oral Care for comments that greatly improved the manuscript.