The use of a liner under different bulk-fill resin composites: 3D GAP formation analysis by x-ray microcomputed tomography

Abstract Gap formation of composite resin restorations is a serious shortcoming in clinical practice. Polymerization shrinkage stress exceeds the tooth-restoration bond strength, and it causes bacterial infiltration within gaps between cavity walls and the restorative material. Thus, an intermediate liner application with a low elastic modulus has been advised to minimize polymerization shrinkage as well as gap formation. Objective: The purpose of this in vitro study was to assess gap formation volume in premolars restored with different bulk-fill composites, with and without a resin-modified glass-ionomer cement (RMGIC) liner, using x-ray micro-computed tomography (micro-CT). Methodology: Sixty extracted human maxillary premolars were divided into six groups according to bucco-palatal dimensions (n=10). Standardized Class II mesio-occluso-distal cavities were prepared. G-Premio Bond (GC Corp., Japan) was applied in the selective-etch mode. Teeth were restored with high-viscosity (Filtek Bulk Fill, 3M ESPE, USA)-FB, sonic-activated (SonicFill 2, Kerr, USA)-SF and low viscosity (Estelite Bulk Fill Flow, Tokuyama, Japan)-EB bulk-fill composites, with and without a liner (Ionoseal, Voco GmbH, Germany)-L. The specimens were subjected to 10,000 thermocycles (5-55°C) and 50,000 simulated chewing cycles (100 N). Gap formation based on the volume of black spaces at the tooth-restoration interface was quantified in mm3 using micro-computed tomography (SkyScan, Belgium), and analyses were performed. Data were analyzed using repeated-measures ANOVA and the Bonferroni correction test (p < 0.05). Results: The gap volume of all tested bulk-fill composites demonstrated that Group SF (1.581±0.773) had significantly higher values than Group EB (0.717±0.679). Regarding the use of a liner, a significant reduction in gap formation volume was observed only in Group SFL (0.927±0.630) compared with Group SF (1.581±0.773). Conclusion: It can be concluded that different types of bulk-fill composite resins affected gap formation volume. Low-viscosity bulk-fill composites exhibited better adaptation to cavity walls and less gap formation than did sonic-activated bulk-fill composites. The use of an RMGIC liner produced a significant reduction in gap formation volume for sonic-activated bulk-fill composites.


Introduction
Increasing demand for esthetics and improvements in adhesive system technology has made resin composite restorations a popular choice for clinicians. 1 However, shrinkage associated with the polymerization of materials is a serious shortcoming in clinical practice. 2 Polymerization shrinkage stress exceeds the tooth-restoration bond strength, and it causes fluid passage and bacterial infiltration within gaps between cavity walls and the restorative material. 3 Microleakage, which is described as clinically undetectable penetration, could lead to post-operative hypersensitivity, marginal staining, secondary caries, pulpal inflammation and necrosis. 4 Several procedures have been developed to decrease polymerization shrinkage stress, such as modifying the chemical composition in the resin formulation, control of light irradiance, incremental layering techniques and intermediate liner application. 5 However, no definitive method to eliminate polymerization shrinkage has been described in the literature. 6 The incremental layering is a standard protocol used to place restorative materials in the cavity, but this technique has many disadvantages, such as placement difficulty in small cavities, increased chair time, voids and contamination risk between composite layers. 7 Therefore, novel composite resin materials with the use of bulk-filling techniques have been placed on the market. 8 Bulk-fill composite resins can be applied in 4-5-mm thicknesses with relative ease of use and a claim of low polymerization shrinkage compared with conventional composites. 9 These materials have a short curing time due to new initiation systems and increased translucency based on reduced filler amounts and increased filler size. 10 Furthermore, polymerization shrinkage stresses are reduced through the incorporation of stress-relievers; thus, they have a decreased risk of gap formation at the tooth-restoration interface. 11 Gaps on the margins of the restorations may cause material deterioration and marginal infiltration. 12 Although bulk-fill composite resins are claimed to exhibit low polymerization shrinkage, there is not enough information with respect to the effects of gap formation of bulk-fill composites using an intermediate liner in the literature. The use of a liner (flowable composites, resin-modified glass-ionomers, filled adhesives) with a low elastic modulus/low viscosity could provide better cavity adaptation with less gap formation as a stress-absorbing layer and lessen the polymerization shrinkage at the tooth-restoration interface. 13 Currently, different types of bulk-fill composite resins that are classified according to their rheological properties are commercially available. 14 For this in vitro study, high-viscosity, sonic-activated and low-viscosity bulk-fill composite resins were used. The purpose of this in vitro study was to assess the gap formation volume of maxillary premolars restored with three different types of bulk-fill resin composites, with and without a resin-modified glass-ionomer cement liner (RMGIC) as an intermediate material using microcomputed tomography.
The research null hypotheses were: There would be no difference in the gap formation volume between different types of bulk-fill composite resins.
The RMGIC liner would not reduce gap formation volume and enhance the cavity adaptation of teeth restored with bulk-fill composite resins.

Methodology
This in vitro study was approved by the local ethics committee (process no. 06/06/2018-9063).

Sample Size Calculation
The sample size was calculated based on the estimated effect size between groups according to the literature. 15,16 It was determined that 10 samples were needed for each group to achieve a medium effect size (d=0.50), with 80% power and a 5% type 1 error rate in this study.

Specimen Preparation
A total of 60 intact human maxillary premolar teeth, freshly extracted for orthodontic and periodontal purposes, were selected. To standardize the dimensions of the teeth before the study, the maximum buccopalatal width (BPW) of each tooth was measured using a digital micrometer. 17 Then, the teeth were allocated into six groups according to the BPW (n=10). The mean bucco-palatal dimensions of the teeth between groups differed no more than 5% (p=0.061) according to one-way ANOVA using the Statistical Package for The teeth were embedded in acrylic resin blocks with the crown extended to 2 mm from the cementoenamel junction (CEJ) along the vertical axis.
A standardized Class II mesio-occluso-distal (MOD) cavity was opened in each tooth ( Figure 1) using a coarse diamond fissure bur (FC Diamond, GZ Instrumente, Austria) in a high-speed handpiece under water cooling. A new bur was employed for each of the five specimens. The dimensions of the approximal box of each cavity were arranged such that they were two-thirds of the BPW of the tooth (A), and the occlusal isthmus was arranged to half of the BPW (B). The total depth of the cavity was adjusted to 4 mm, with an axial wall height of 2 mm. Approximal boxes had 1.5 mm mesiodistal width on the gingival floors 1 mm above the CEJ. 18 The dimensions of cavity preparation were confirmed with a digital caliper. The specimens were then stored in distilled water at room temperature (23±1°C) before and after preparation.

Restorative Procedure
After cavity preparations, a metal auto matrix

Group Filtek Bulk Fill (FB)
High-viscosity bulk-fill resin composite (Filtek TM Bulk Fill, A2 Shade, 3M ESPE, USA) was used to restore the cavity and was polymerized with an LED LCU from the occlusal surface for 10 s. After removing the metal matrix, the restorations were polymerized from the mesial and distal surfaces for 10 s on each side.

Group Filtek Bulk Fill with liner (FBL)
The cavities were lined with one-component RMGIC  The cavities were lined with RMGIC liner, polymerized as described in group FBL. G-Premio Bond was applied as previously described and then restored with SonicFill TM 2 resin composite as described in group SF.

Group Estelite Bulk Fill Flow (EB)
Low-viscosity bulk-fill resin composite (Estelite Bulk Fill Flow, A2 Shade, Tokuyama Dental Corp., Japan) was used to restore the cavity, and it was polymerized with an LED LCU for 10 s. After removing the metal matrix, the restorations were polymerized from the mesial and distal surfaces for 10 s on each side.

Group Estelite Bulk Fill Flow with liner (EBL)
The cavities were lined with a RMGIC liner and polymerized as described for group FBL. G-Premio Bond was applied as previously described, followed by restoration with Estelite Bulk Fill Flow as described for group EB. and 100% humidity. A vertical load was applied with a 3.2-mm stainless-steel ball-shaped stylus at the center of the restorations. 19 During the aging procedure, the specimens remained immersed in distilled water.

Results
The obtained data were assessed based on the recorded volume (mm 3 ). An analysis of the gap formation between bulk-fill composites and/or the RMGIC liner and cavity walls was performed for all tested groups (n=10). Micro-CT-based gap formation volumes with standard deviations are shown in *Different letters indicate that there was statistically significant differences in mean gap formation volumes with standard deviations for bulk-fill composites without liner/with liner (p<0.05). **Horizontal bars indicate that there was statistically significant differences in mean gap formation volume with standard deviations for same bulk-fill composites according to the use of liner (p<0.05).   In this study, micro-CT imaging was used to quantify the gap formation between the cavity walls and restorative materials as the volume (mm 3 ) after a thermo-mechanical aging procedure. Thermocycling and mechanical aging are the most effective and frequently used methods for imitating clinical situations. 26 Thermocycling is a water storage protocol that subjects specimens to the extreme temperature differences present in the oral cavity due to hot or cold drinks, inducing the composite resin to contract and expand several times for hydrolytic degradation. 27 Mechanical aging is performed to simulate the exposure of the tooth-restoration interface to cyclic subcritical loadings produced during chewing. 28 In the current study, all restored teeth were subjected to 10,000 thermocycles (5-55°C), which represents 1 year of clinical functions, 29 and 50,000 simulated chewing cycles (100 N loading).
Significantly higher gap formation volumes were found for Group SF compared with Group EB among all tested bulk-fill composites. In contrast to this finding, Han, et al. 16  The use of an RMGIC liner yielded a significant reduction in gap formation volume for only sonicactivated bulk-fill composites.