Comparative evaluation of degree of conversion and volumetric shrinkage of two different bulk fill composites, one short fiber reinforced composite and a new nanohybrid composite- an in vitro study

Introduction: The physical and mechanical properties of composite resins have been greatly improved over the past couple of years. However, they still have some shortcomings like an insufficient depth of cure, degree of conversion and volumetric shrinkage. Objective: The purpose of this study was to comparatively evaluate for percentage degree of conversion (DC%) and volumetric shrinkage(VS%) of two bulk fill composite materials, a SFRC and a new Nanohybrid material. Method: A Total of 180 samples were prepared and divided into four groups i.e., Group A (Aura Bulkfill), Group B (Tetric Evocerambulk fill), Group C (Ever X Posterior) and Group D (Harmonize) comprising of 45 samples each. For Group A, samples were further divided into two sub groups. Subgroup A1. For evaluation of DC% (n=30). Subgroup A2. For evaluation of VS(n=15). In Subgroup A1, thirty samples were prepared by cutting teflon tube at two different heights viz; 2mm (n=15) and 4 mm (n=15). These samples were filled with composite in a single increment followed by light curing. Degree of conversion analysis was done with the help of FTIR spectroscope. In Subgroup A2, Fifteen Samples (n=15) were prepared by filling the composite in a stainless-steel mould of 4mm diameter and 4 mm height. These were weighed before and after light curing with the help of an analytical balance. The specific gravity was calculated by using ASTM D-792 method. The volumetric shrinkage was calculated by measuring the difference in specific gravities of uncured and cured samples by mathematical equations. Similar procedures were followed for other Subgroups. However, In Subgroup D1 and D2, 2mm increment of composite material was placed and light cured which was unlike the other tested bulkfills, which were placed in a single increment of 4 mm. Results: EP exhibited highest degree of conversion at both 2mm and 4 mm heights, followed by TEC and AB. Harmonize showed almost similar DC% values at both 2mm and 4mm heights. At 4mm height, DC% of Harmonize was found to be better than AB and TEC. VS% was observed in the order: EP< TEC< AB< Harmonize. Conclusion: BFCs in the present study performed better in terms of DC % at 2 mm height and VS when compared to the newly introduced nanohybrid composite. However, at 4mm height, DC% of Harmonize was observed to be better than AB and TEC, which can be due to its incremental placement and curing of 2mm each.