Objectives The present study investigated the effect of cements on fracture resistance of monolithic zirconia crowns in relation to their compressive strength. Materials and methods Four different cements were tested: zinc phosphate cement (ZPC), glass-ionomer cement (GIC), self-adhesive resin-based cement (SRC) and resin-based cement (RC). RC was used in both dual cure mode (RC-D) and chemical cure mode (RC-C). First, the compressive strength of each cement was tested according to a standard (ISO 9917-1:2004). Second, load-to-failure test was performed to analyze the crown fracture resistance. CAD/CAM-produced monolithic zirconia crowns with a minimal thickness of 0.5 mm were prepared and cemented to dies with each cement. The crown–die samples were loaded until fracture. Results The compressive strength of SRC, RC-D and RC-C was significantly higher than those of ZPC and GIC (p < 0.05). However, there was no significant difference in the fracture load of the crown between the groups. Conclusion The values achieved in the load-to-failure test suggest that monolithic zirconia crowns with a minimal thickness of 0.5 mm may have good resistance against fracture regardless of types of cements.

ObjectivesTo investigate how heat development in the pulp chamber and coronal surface of natural teeth with and without cusps subjected to irradiance using light-emitting diode (LED)–light-curing units (LCUs) is associated with (i) irradiance, (ii) time, (iii) distance, and (iv) radiant exposure.Materials and methodsThree different LED-LCUs were used. Their irradiance was measured with a calibrated spectrometer (BlueLight Analytics Inc., Halifax, Canada). An experimental rig was constructed to control the thermal environment of the teeth. The LED-LCU tip position was accurately controlled by a gantry system. Tooth surface temperature was measured by thermography (ThermaCAM S65 HS, FLIR Systems, Wilsonville, USA) and pulp chamber temperature with a thermocouple. LED-LCU tip distance and irradiation times tested were 0, 2, and 4 mm and 10, 20, and 30 s, respectively. Ethical permission was not required for the use of extracted teeth.ResultsMaximum surface and pulp chamber temperatures were recorded in tooth without cusps (58.1 °C  ± 0.9 °C and 43.1 °C ± 0.9 °C, respectively). Radiant exposure explained the largest amount of variance in temperature, being more affected by time than irradiance.ConclusionsAt all combinations of variables tested, repeated measurements produced consistent results indicating the reliability of the method used. Increased exposure time seems to be the factor most likely to cause tissue damage.Clinical relevanceRisk of superficial tissue damage at irradiances >1200 mW/cm2 is evident. There is a risk of pulp damage when only thin dentin is left at higher irradiances (>1200 mW/cm2). Clinicians should be aware of LED-LCU settings and possible high temperature generated.

The study aimed to investigate factors contributing to heat development during light curing of a flowable bulk-fill resin-based composite (SDR TM , Lot # 602000876, Dentsply Sirona, Konstanz, Germany) (RBC). Materials and Methods: Temperatures were measured with calibrated thermocouples. A multi-wave light-emitting diode (LED) light curing unit (LCU) was used (Ivoclar Vivadent, Schaan, Lichtenstein). In all experiments, the RBC was first cured (cured) for 30 s and, after 5 min of recovery time, received a second LCU irradiation (post-cured) for 30 s. The exothermic reaction was measured by calculating the Δ temperature between cured and post-cured RBC. In a cylinder-shaped polymer mold, temperature was recorded inside of RBC during curing (part 1) and light transmission through RBC during curing was investigated (part 2). Pulpal temperatures were assessed in an extracted third molar during light curing (part 3). Data were statistically analyzed using one-way ANOVA (α=0.05). Results: Increased thickness of RBC led to decreased pulp chamber temperatures. Inside RBC, there was a large variation in heat development between the cured and post-cured groups (p<0.05). The cured group absorbed more LCU irradiation than the post-cured group. Conclusion: The irradiance of the LCU seemed to be a more important factor than exothermic reaction of RBCs for pulp chamber heat development. Flowable bulk-fill RBCs can act as a pulpal insulator against LCU irradiation, despite their exothermic curing reaction.

This study aimed to investigate pulp chamber and surface temperature development using different LED light curing units (LCUs). Eight brands of LED-LCUs were tested in a laboratory bench model. The pulp chamber and surface temperature were recorded with a type T thermocouple and infrared cameras, respectively. The highest pulp chamber and surface temperature increase was 6.1±0.3°C and 20.1±1.7°C, respectively. Wide-spectrum LED-LCUs produced higher pulp chamber temperature increase at 0 mm and 2 mm but lower at 4 mm. Narrow-spectrum LED-LCUs produced higher surface temperature increase. LED-LCU featuring modulated output mode resulted in lower increase in pulp chamber temperature but higher on surface temperature. LED-LCU with light guide tip delivering an inhomogeneous beam caused higher increase in temperature on the surface and in the pulp chamber. LED-LCUs with different spectral emission, output mode and light guide tip design contributed to different temperature development in the pulp chamber and at the surface of teeth.

Objective: To assess whether composite polymer resin delivered in compules include pores and the possible effect on the amount of porosity in dental restorations. Method and materials: Original compules containing unpolymerised composite polymer resin (CPR) were scanned in a micro-CT. Four products were examined, which comprised universal composites (Herculite XRV Ultra, Ceram.X Universal, Tetric Evo Ceram) and a flowable bulk-fill composite (SDR) (n ¼ 10 per group). The pore size distribution and amount of porosity (vol.%) were estimated for the unpolymerized and polymerized material used to restore a standardised cavity in a typodont tooth. Manufacturers' instructions were followed regarding material handling, and polymerisation by use of a calibrated light-curing unit. The pore characteristics and their size distribution, and the amount of porosity in the dental restoration were contrasted with the values measured in the compule. Non-parametric tests were used to analyse differences between the four products. Results: All the composite polymer resin compules contained unpolymerised material that included pores. The universal composite compules included pores predominantly in the sub-100 mm sizes. In contrast, the flowable bulk-fill compules included a few pores with a diameter >100 mm, which were assumed to be air-bubbles. The unpolymerised material within the compule included consistently more pores compared to the extruded portion from the compule tip, and in the final restoration (p < .001). The amount of porosity in the restorations differed amongst the tested materials, with the flowable bulk-fill composite showing the lowest amount of porosity (p < .01).