Critical considerations on load-to-failure test for monolithic zirconia molar crowns.

Application of monolithic zirconia crowns (MZCs) with reduced thickness to the molar region has been proposed, but potential complications have yet to be fully evaluated in laboratory tests. The present study aimed to develop a clinically relevant load-to-failure test in combination with fatigue treatments involving thermal and mechanical cycling (TC and MC) to evaluate the fracture resistance of molar MZCs. MZCs with a minimal thickness of 0.5 mm were bonded to dies made of resin-based composite (RBC), epoxy resin (EP), or polyoxymethylene-copolymer (POM-C). The samples were either untreated (UT) or subjected to TC (5-55 °C for 1â€¯× 105 cycles) and MC (300 N for 2.4â€¯× 106 cycles). The stress generated by TC and MC was simulated by finite element modeling. The load-to-failure test was performed using an inverse V-shaped two-plane indenter and was followed by fractographic analysis. The median values of fracture load for MZC/RBC and MZC/EP in the TC group were significantly lower than those in the UT group. MC also decreased the median value of fracture load for MZC/RBC significantly, but not that for MZC/EP and MZC/POM-C. Fractography revealed that the fracture started in the cervical area in all groups, which is similar to clinically failed crowns. The simulation confirmed stress concentration at the cervical area in both TC and MC groups. The present study suggests that the load-to-failure test using a two-plane indenter could induce clinically relevant fracture of MZCs, the vulnerability of the MZCs depends largely on the die material employed, and MZCs are more likely to be damaged by thermal fatigue than mechanical fatigue.

Influence of microscale expansion and contraction caused by thermal and mechanical fatigue on retentive strength of CAD/CAM-generated resin-based composite crowns.

CAD/CAM-generated resin-based composite crowns have been proposed as an inexpensive alternative to conventional crowns. However, concerns have been raised about crown loosening in clinical use. Therefore, the present in vitro study aimed to evaluate the influence of thermal and mechanical cycling (TC and MC) on retentive strength of CAD/CAM resin-based crowns in relation to microscale expansion and contraction caused by fatigue. Eighty standardized dies were produced using a resin-based composite material. Crowns were milled from resin-based composite (n = 40) and glass-ceramic blocks (n = 40; control) using a dental CAD/CAM system. The crowns bonded to the dies were subjected to TC (temperature: 5 and 55 °C, cycles: 50,000) and MC (load: 200 N, cycles: 1.2 million). After fatigue treatment, retentive strength of the crowns was evaluated by a crown pull-off test at a crosshead speed of 1 mm/min. Coefficient of thermal expansion (CTE) and modulus of elasticity (E-modulus) of each material were also analyzed to estimate the microscale expansion and contraction during TC and MC. TC and MC significantly reduced the retentive strength of the CAD/CAM resin-based crowns whereas that of the CAD/CAM ceramic crowns was only affected by TC. In addition, the resin-based crowns showed a higher number of crown loosening during TC than the ceramic crowns. Analyses of CTE and E-modulus indicated that the resin-based crowns would be more deformed during TC and MC than the ceramic crowns. The present study demonstrated that the resistance of crowns to microscale expansion and contraction caused by thermal and mechanical fatigue would play an important role in maintaining retentive strength.

Fatigue analysis of computer-aided design/computer-aided manufacturing resin-based composite vs. lithium disilicate glass-ceramic.

Resin-based composite molar crowns made by computer-aided design/computer-aided manufacturing (CAD/CAM) systems have been proposed as an inexpensive alternative to metal-ceramic or all-ceramic crowns. However, there is a lack of scientific information regarding fatigue resistance. This study aimed to analyze the fatigue behavior of CAD/CAM resin-based composite compared with lithium disilicate glass-ceramic. One-hundred and sixty bar-shaped specimens were fabricated using resin-based composite blocks [Lava Ultimate (LU); 3M/ESPE] and lithium disilicate glass-ceramic [IPS e.max press (EMP); Ivoclar/Vivadent]. The specimens were divided into four groups: no treatment (NT); thermal cycling (TC); mechanical cycling (MC); and thermal cycling followed by mechanical cycling (TCMC). Thermal cycling was performed by alternate immersion in water baths of 5°C and 55°C for 5 × 10(4) cycles. Mechanical cycling was performed in a three-point bending test, with a maximum load of 40 N, for 1.2 × 10(6) cycles. In addition, LU and EMP molar crowns were fabricated and subjected to fatigue treatments followed by load-to-failure testing. The flexural strength of LU was not severely reduced by the fatigue treatments. The fatigue treatments did not significantly affect the fracture resistance of LU molar crowns. The results demonstrate the potential of clinical application of CAD/CAM-generated resin-based composite molar crowns in terms of fatigue resistance.

Cutting efficiency of diamond burs operated with electric high-speed dental handpiece on zirconia.

Zirconia-based dental restorations are becoming used more commonly. However, limited attention has been given to the difficulties experienced, concerning cutting, in removing the restorations when needed. The aim of the present study was to compare the cutting efficiency of diamond burs, operated using an electric high-speed dental handpiece, on zirconia (Zir) with those on lithium disilicate glass-ceramic (LD) and leucite glass-ceramic (L). In addition, evaluation of the cutting efficiency of diamond burs on Zir of different thicknesses was performed. Specimens of Zir were prepared with thicknesses of 0.5, 1.0, 2.0, and 4.0 mm, and specimens of LD and L were prepared with a thickness of 1.0 mm. Cutting tests were performed using diamond burs with super coarse (SC) and coarse (C) grains. The handpiece was operated at 150,000 rpm with a cutting force of 0.9 N. The results demonstrated that cutting of Zir took about 1.5- and 7-fold longer than cutting of LD and L, respectively. The SC grains showed significantly higher cutting efficiency on Zir than the C grains. However, when the thickness of Zir increased, the cutting depth was significantly decreased. As it is suggested that cutting of zirconia is time consuming, this should be taken into consideration in advance when working with zirconia restorations.