Evaluation of internal adaptation on resin composite using micro-CT and SS-OCT

Abstract

I. Introduction

Internal adaptation, which means how well a restoration adapts to tooth material inside, can involve the evaluation of a microgap at the pulpal floor of a restoration. As a non-destructive method for evaluation of internal adaptation, micro-CT was introduced to evaluate the internal adaptation of restorations. For another method, optical coherence tomography (OCT) came to be used as a non-invasive cross-sectional imaging method for biological systems. A specific type of OCT is a swept-source optical coherence tomography (SS-OCT).

The theme of this thesis is the evaluation of the internal adaptation on resin composite in different cavity configuration. This paper includes three different experiments on internal adaptation of resin composite using micro-CT and/or SS-OCT.

II. Materials, methods and results

1. The first experiment of evaluation on internal adaptation

Materials and Methods: Two cylindrical cavities were created on the labial surface of twelve bovine incisors. The 24 cavities were randomly assigned to four groups of dentine adhesives; 1) three-step etch-and-rinse adhesive, 2) two-step etch-and-rinse adhesive, 3) two-step self-etch adhesive, and 4) one-step self-etch adhesive. After application, the cavities were filled with resin composite. All restorations underwent a thermo-cycling challenge, and then, eight SS-OCT images were taken using a Santec OCT-2000TM (Santec Co., Komaki, Japan). The internal adaptation was also evaluated using micro-CT (Skyscan, Aartselaar, Belgium). The image analysis was used to calculate the percentage of defective spot (%DS) and compare the results. The groups were compared using one-way ANOVA with Duncan analysis at the 95% significance level. The SS-OCT and micro-CT measurements were compared with a paired t-test, and the relationship was analyzed using a Pearson correlation test at the 95% significance level.

Results: After thermo-cycling, the %DS results showed that Group 3 ≤ Group 4 < Group 1 ≤ Group 2 on both SS-OCT and micro-CT images. The %DSs on micro-CT were lower than those of SS-OCT (p<0.05) and the Pearson correlation coefficient between SS-OCT and micro-CT was r=0.787 (p<0.05).

2. The second experiment of evaluation on internal adaptation

Materials and Methods: Standardized MOD cavities were prepared in 40 extracted human third molars. They were randomly divided into five groups (n=8). After being applied by dentin adhesive, the teeth were restored with the following resin composites: Group 1- Filtek Z350 (3M); Group 2- SDR (Dentsply) + Z350; Group 3- Venus Bulk Fill (Heraeus Kulzer) + Z350; Group 4- Tetric N-ceram bulkfill (Ivoclar Vivadent); and Group 5- SonicFill (Kerr). After thermo-mechanical load cycling, micro-CT images were taken cross-sectionally. Internal adaptation was measured as imperfect margin percentage (IM%). IM% is the percentage of defective margin length to the whole margin length. On the micro-CT images, IM% was measured at five interfaces to compare the differences. Linear polymerization shrinkage and polymerization stress were measured on each composite. To verify the correlation of polymerization stress and IM%, regression analysis was used.

Results: IM%s on the cavity floors were higher than those of the cavity walls. IM% showed as Groups 4, 5 ≤ Groups 1, 2 ≤ Group 3 (p<0.05). The relationship between polymerization stress and IM% was found to be R2=0.636. The relationship between linear polymerization shrinkage and IM% turned out to be R2=0.618.

3. The third experiment of evaluation on internal adaptation

Materials and Methods: Cylindrical cavities 3 mm in diameter were prepared in 100 human third molars in two depths; 4 mm (high C-factor: H-CF) or 1 mm (low C-factor: L-CF). After adhesive application (Clearfil SE One, Kuraray Noritake), the composite was placed in two increments in 3 subgroups: Filtek Supreme (FS, 3M ESPE); Charisma Diamond (CD, Heraeus Kulzer); Amelogen Plus (AP, Ultradent); and as a single increment in 2 subgroups; Tetric EvoCeram Bulk Fill (TB, Ivoclar Vivadent) and Venus Bulk Fill (VB, Heraeus Kulzer). After thermo-mechanical load-cycles, imperfect margin percentage (%IM) was calculated using optical coherence tomography (SS-OCT) imaging. The relationships between %IM and linear shrinkage (LS) and shrinkage stress, measured under either zero-compliance (PS0) or compliance-allowed (PS) conditions were evaluated.

Results: %IM was significantly different between H-CF and L-CF groups. The %IM in H-CF turned out to be as: groups 2, 1 ≤ group 4 < groups 3, 5. The %IM in L-CF showed as the following: groups 2, 4 ≤ groups 1, 3 < group 5. There were significant correlations between shrinkage parameters and %IM, except between PS0 and %IM in L-CF.

III. Conclusion

Micro-CT and SS-OCT could be used as non-destructive methods for evaluation on the internal adaptation of composite restoration. Measured imperfect margin percentage (%IM) in micro-CT showed different values to those of SS-OCT, however, these two methods were relatively highly correlated. Self-etching adhesive systems showed fewer defective spots than etch-and-rinse adhesive systems in class I cavity.

At the gingival floor of the proximal box and pulpal floor of the cavity, flowable bulk-fill resin showed an inferior internal adaptation when compared with non-flowable ones. For Class II resin restorations, bulk-filling material of the non-flowable type could be preferable to flowable type ones. Polymerization shrinkage and stress, which was measured under the compliance-allowed setup, showed some relation to the internal adaptation.

Within the limitations of the present study, it was shown that a higher imperfect margin percentage was found in the cavities of the High C-factor group. Internal adaptation was different depending on the composite material. Internal adaptations both in the High and Low C-factor cavities are correlated with polymerization stress measured under the compliance-allowed condition. In the Low C-factor cavity group, the polymerization shrinkage stress measured under the zero-compliance condition did not show a significant correlation to internal adaptation.