In vitro evaluation of gap formation and fracture resistance of indirect inlay restorations cemented with preheated composite resins

The aim of this study is to investigate the gap formation of indirect inlay restorations produced with CAD/CAM, cemented with preheated composites and conventional resin cement in comparison with teeth restored with direct composites and to examine and evaluate the fracture strength values and fracture types of all produced restorations in comparison with natural teeth. For the preparation of the specimens, 50 human molar teeth of similar dimensions, without any caries, cracks or hypoplastic defects and with an indication for extraction due to orthodontic or periodontal reasons were selected and randomly divided into 5 groups with 10 specimens in each group. After standard inlay preparation, dual-cure resin cement Bifix Hybrid Abutment (VOCO GmbH, Cuxhaven, Germany) was used for cementation of indirect restorations in Group 1. VisCalor® preheated composite (VOCO GmbH, Cuxhaven, Germany) was used for cementation in Group 2 after being heated for 30 seconds according to the manufacturer's instructions. Filtek One Bulk-Fill Composite (3M/ESPE, St. Paul, USA) was applied by heating using the VisCalor® Dispenser (VOCO GmbH, Cuxhaven, Germany) for cementation in Group 3. Cavities in Group 4 were restored with direct resin composite Filtek Z250 (3M/ESPE, St. Paul, USA) using the incremental method. Specimens in Group 5 were kept in their natural state without opening any cavity. After the extracted teeth were cleaned, they were first stored in chloramine-T trihydrate solution and then in distilled water until the experimental stage. Standardized Class II MOD inlay cavities were prepared with one proximal box in the enamel (1-1.5 mm above the cementoenamel junction) and the other in the dentin (1-1.5 mm below the cementoenamel junction). The dimensions of the cavities were prepared as 4.0 mm wide and 3-3.5 mm deep in the occlusal isthmus and 5.0 mm wide in the proximal boxes. The depth of the proximal box to the axial pulp walls was 1.5 mm. A periodontal probe was used to measure the cavity dimensions. The cavity walls were prepared using diamond burs under water cooling to obtain a 10 to 12 degree angle taper. Internal point and line angles were rounded and prepared in a butt-joint configuration without beveling the enamel edges. Digital impressions were taken on the teeth and indirect restorations were fabricated and cemented using resin nano-ceramic blocks (Grandio Blocs, VOCO GmbH, Cuxhaven, Germany) via CAD-CAM. All teeth were thermally cycled using SD Mechatronik Thermocycler THE-110 (Feldkirchen-Westerham, Germany) for 10,000 cycles between 5°C and 55°C with a dwell time of 30 seconds and a transfer time of 10 seconds. The samples were then placed in a chewing simulator (CS-4.2; SD Mechatronik, Feldkirchen-Westerham, Germany) and subjected to 50,000 cycles (100 N and 1.7 Hz) at room temperature (23 ± 1°C) and 100% humidity. Micro-CT gap formation analysis was performed using the microtomography system SkyScan 1174 (Skyscan, Kartuizersweg, Kontich, Antwerp, Belgium). Since there would be no deterioration on the samples with micro-CT analysis, the samples were placed in the universal testing machine (instron) and fractured with a 5-mm-diameter steel ball applied to the central fossa of the teeth at a speed of 0.5 mm/sec. Fracture strength values were measured in Newtons (N). The fractured samples were examined and photographed using a stereo microscope (SMZ1000, Nikon, Japan) using x0.8 magnification and classified according to the modes of fracture. For gap formation and fracture strength analysis, normality was checked with Shapiro Wilks and variance homogeneity was checked with Levene test. Differences between groups were determined with one way analysis of variance (One way ANOVA). The group creating the difference was determined with Tukey test. In the gap formation analysis, the highest value (mm3) among the M (mesial) values was observed in Group 3 (0.572±0.326), and the lowest value (mm3) was observed in Group 1 (0.162±0.044). In the D (distal) values, all groups had statistically similar gap formation values (p>0.05). M and D values in Group 2 and Group 3 are statistically similar (p<0.05). In the fracture strength analysis, the highest value (Newton) was observed in Group 5 (2905.15±492.69). Type 4 fractures were mostly observed in Group 1 with a rate of 40%. Mild type 1 fractures, which only included the restoration, were mostly observed in Group 3 with a rate of 60%. The preheated composite VisCalor (VOCO GmbH, Cuxhaven, Germany) showed similar performance to conventional resin cement when used for cementation of indirect restorations and can be recommended as an alternative cementation agent.