PSI - Issue 28

Vesela Hristova et al. / Procedia Structural Integrity 28 (2020) 1002–1009 Author name / Structural Integrity Procedia 00 (2019) 000–000

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combined - chemical and micro-mechanical. However, the adhesion strength to the dentin is 5-6 times lower than that of the photo-cured composites (Dikova et al. (2020). Due to the relatively high setting shrinkage and low adhesion strength to the HDT, conditions are created for detachment of the GIC from the cavity walls and formation of gaps mainly around the edge of the obturation along the vestibular surface of the tooth (Fig. 2). The result is a microleakage of the dye at 38% of the cavity depth (Fig. 5), confirming the results of other scientists on the use of GIC in the treatment of V-shaped defects (Sahu et al. (2018), Singla et al. (2012). It is known that the shrinkage and mechanical properties of dental composites depend mainly on the amount of the inorganic filler (Rizzante et al. (2019), Van Meerbeek et al. (1993), Dikova T. (2015), Objelean et al. (2016), Anusavice, K.J. (2003). Increasing its content leads to increase in the hardness and strength properties as well as decrease of the shrinkage. The composition of the flowable photo-cured composite Estelite Flow Quick - High Flow consists of 32% organic matrix (BisGMA, TEGDMA and UDMA) and 68% inorganic filler (Table 1), determining relatively high polymerization shrinkage of 3%. In the preparation of the obturation, according to the manufacturer's recommendations, a thin layer of one-component adhesive Adhese Universal is applied over the cavity walls before applying the composite. There is only 4% of highly dispersed silicon dioxide filler in its composition. The rest part consists mainly of 67% methacrylate polymers (BisGMA, HEMA, DMAEMA) and 25% water and ethanol (Table 1). On the other hand, the polymerization process does not run uniformly throughout the whole volume of the composite. It depends on the intensity of the light. Therefore, the most complete polymerization takes place in the surface layer of the obturation, which is closest to the photopolymerization lamp (Singla et al. (2012), Schmidt C. and Ilie N. (2012). The light intensity decreases with the distance to the deeper layers and the polymerization process partially occurs there. This causes inhomogeneous shrinkage in the volume of the composite obturation - it is the largest on the surface of the obturation and decreases in depth. Since the entire layer of the flowable composite is applied in one portion and the entire volume of obturation is irradiated for polymerization, the elastic adhesive layer used can compensate the smaller shrinkage in the cavity depth, but cannot compensate the shrinkage of the surface layer. As a result, the cured composite is detached from the HDT along the border of the obturation on the vestibular surface of the tooth and a 24.49% microleakage is obtained (Fig. 5). The universal Evetric nanohybrid composite consists of 19-20% organic matrix (Bis-GMA, UDMA and Bis-EMA) and 80-81% filler with dimensions of 40 nm - 3 μm, which defines lower polymerization shrinkage of 1.5% (Table 1). In addition, the technique of applying the universal nanohybrid composite differs from that of the flowable one. It is applied incrementally to the cavity - layer by layer, which are photo-cured sequentially. In this way, the volume portions of the composite are smaller, and although the polymerization process is almost complete, the actual shrinkage values are smaller and can be compensated by the deformations of the elastic adhesive layer (Rizzante et al. (2019). In addition, the second layer of composite fills any gaps between the walls of the tooth and the first layer. In this way, the lowest microleakage of 18.2% (Fig. 5) is obtained for the cavities, filled with a universal nanohybrid compositeThe example of the Table and Figure is given below. 5. Conclusion The microleakage degree of wedge-shaped defects, obturated with different materials - GIC, flowable and universal nanohybrid composites was investigated in the present study. It was established that the microleakege depends on the type of material used and the application technique. It was confirmed that the cavities, obturated with GIC, possess the highest relative microleakage of 38.1% compared to the cavities, filled with photo-cured composites. The lowest relative microleakage of 18.2% was found in V-shaped defects, obturated with universal nanohybrid composite. The relative microleakage of the cavities, filled with flowable photo-cured composite, took an intermediate position with 24.5%. As a result of this study, for successful filling of V-shaped defects with minimum microleakege, the universal nanohybrid composites can be recommended in application of incremental technique. References Anastasova, R., Dikova, T., Panov, Vl., 2019. In vitro study of dental composite roughness and microleakage of repaired obturations by various techniques. Journal of IMAB 25 (1), 2419-2425. Anusavice, K.J., 2003. Philips’ Science of Dental Materials. 11th ed., Elsevier, London, 806.