Influence of specimens’ geometry and materials on the thermal stresses in dental restorative materials during thermal cycling

Abstract

Objectives

Thermal cycling is widely used to simulate the aging of restorative materials corresponding to the changes of temperature in the oral cavity. However, test parameters present in literature vary considerably, which prevents comparison between different reports. The aim of this work is to assess the influence of the specimens’ geometry and materials on the thermal stresses developed during thermal cycling tests.

Materials and methods

Finite elements method was used to simulate the conditions of thermal cycling tests for three different sample geometries: a three-points bending test sample, a cylinder rod and more complex shape of a restoration crown. Two different restorative systems were considered: all-ceramic (zirconia coupled with porcelain) and metal-ceramic (CoCrMo alloy coupled with porcelain). The stress state of each sample was evaluated throughout the test cycle.

Results

The results show that the sample geometry has great influence on the stress state, with difference of up to 230% in the maximum stress between samples of the same composition. The location of maximum stress also changed from the interface between materials to the external wall.

Conclusions

Maximum absolute stress values were found to vary between 2 and 4 MPa, which might not be critical even for ceramics. During multi-cycle testing these stresses would cause different fatigue in various locations. The zirconia-based specimens and zirconia-based restoration (crown) exhibited the most similar stress states. Thus it might be recommended to use these geometries for fast screening of the materials for this type of restorations.

Clinical significance

The selection of specimens’ geometry and materials should be carefully considered when aging conditions close to clinical ones want to be simulated.

Introduction

The oral cavity faces temperature changes during to eating and drinking of hot or chilled foods and liquids, besides mild changes due to breathing. These constant changes create residual stresses in bilayered dental restorations due to their continuous contraction and expansion. The stresses are created mainly at the interface between the restorations’ materials, which can induce crack propagation through these surfaces, debonding and microleakage of pathogenic fluids, known as percolation . The need of dental restorations to be made of two different materials derives from their two main requirements: good mechanical resistance and aesthetics. Usually porcelain is used as veneering material in all-ceramic and metal-ceramic restorations in order to match the restoration’s colour to the remaining natural teeth; however, porcelain is brittle and subjected to premature failure . All-ceramic restorations have been developed over the last few years due to its better aesthetic and biocompability compared with the traditional metal-ceramic restorations. Zirconia is nowadays the material of choice to be used as framework in all-ceramic restorations due to its high mechanical resistance, good aesthetic and biocompability .

In vivo studies are the best methods to evaluate the performance of new restorative materials. However, they present several limitations, such as operator variability, substrate differences, patient compliance and recall failure, which prevents these studies to be standardized and invariably employed . Thus, in vitro tests are used to simulate, to some extent, the oral environment and predict the lifetime of dental restorations. Thermal cycling in such tests is supposed to predict the behaviour of restorations in the oral cavity through the aging process of restorations and natural teeth. Thermal cycling tests attempt to simulate the oral cavity conditions, considering its temperature changes and chemical environment. The test consists in immersing the samples into a high temperature liquid for a certain time (called dwell time), and then transfer them to a low temperature bath, where it will stay for another period of time. The liquid usually is distilled water, however studies also have used artificial saliva or alcohol . The cycle is then repeated several times. This procedure generates stresses in the bonding between the materials, which affects the bonding strength and the marginal integrity of the restoration, and it could cause microleakage, marginal breakdown, hypersensitivity, staining and development of pulpal pathology .

Although thermal cycling test is widely used as an in vitro test of dental materials, its conditions vary considerably. Protocol standardization is necessary to allow comparison between different tests and reports. Different temperatures, dwell and transfer times and bath liquids were used in several studies. Although most authors use the temperatures proposed by ISO 11405 (between 5 °C and 55 °C) , other temperatures have been used, such as 4–60 °C and 10–50 °C . The dwell time appears arbitrary, usually ranging from 15 s up to 2 min . Gale and Darvell and Morresi et al. reported this lack of standardization through reviews showing the different temperatures, dwell times and number of cycles studied by previous reports published. However, there are no comparative studies reporting the influence of the specimens’ geometry and materials on the thermal stresses developed within each type of specimen during a complete cycle. Usually, cylinder rods or three-points bending test samples are used in thermal cycling tests, since they will be afterwards subjected to an adhesion strength assessment through a shear or bending tests. The aim of this work is to analyse through simulations using finite elements method the stress state in these geometries during one cycle of the test, and compare them with an actual dental restoration geometry. The stress is critical when it is located at the interface between the materials. This study will evaluate the location and magnitude of maximum stress on each sample. Besides the geometry, this study will also assess the influence of the sample materials on both stress and temperature distribution throughout the thermal cycle.

Materials and methods

Three different geometries were used in the simulations ( Fig. 1 ). The first one is a cylindrical rod made of two materials: zirconia or a cobalt alloy (CoCrMo) on the bottom half and porcelain on the top half. As this geometry is a revolution solid, it was modelled as 2D axi-symmetric to simplify calculations. The second geometry is a rectangular specimen, used for 3-points-bending test, made of a metallic/ceramic strip veneered in the centre part by porcelain. The bottom plate is made of zirconia or CoCrMo, and the upper plate is made of porcelain. The edges of the plates were rounded (r = 0.2 mm) to avoid stress raisers points. A 3D axi-symmetric model was used to simulate this geometry. The third geometry is a modelled dental restoration crown. The restoration has an inferior layer made of zirconia or CoCrMo and a top porcelain layer. The thickness is measured at the restoration centre, and it changes along its external outline. This geometry was simplified to a 2D axi-symmetric model.

Fig. 1
Modelled geometries for the simulations. A) Cylindrical specimen; B) rectangular specimen; C) dental crown restoration.

The materials properties are considered temperature independent in this work. The range of temperature is relatively small and it should not change considerably these properties. The values for the mechanical and thermal properties used in this work are either based in experimental values by and or informed by the manufacturer. The values are shown in Table 1 . Two different porcelains compatible with the zirconia and CoCrMo alloy used as framework materials were used, VITA VM9 (VITA Zahnfabrik, Germany) and CERAMCO3 (Dentsply, York, USA), respectively. Properties from conventional zirconia (Y-TZP) and Keramit NP CoCrMo alloy (Nobil Metal, Italy) were used in this study.

Table 1
Materials properties used in this model.
Young’s modulus (GPa) Density (kg/m 3 ) Poisson’s ratio Thermal conductivity (W/(m K)) Heat capacity (J/(kg K)) CTE (10 −6 1/K)
Zirconia 210 6095 0.31 2.92 466 10.17
Porcelain ZrO2 70 2431 0.26 1.37 734 9.05
CoCrMo Alloy 220 8600 0.3 13 466 14.4
Porcelain C ° CrMo 70 2431 0.26 1.37 734 13.4

The stress state of thermal cycling test samples was simulated after one full cycle. The cycle starts with the samples at a temperature of 310 K (37 °C). All stresses are zero at t = 0. The samples are then exposed to 328 K (55 °C) for 15 s, which simulates the hot bath. The samples are then transferred to a 278 K (5 °C) environment, which is the cold bath ( Fig. 2 ). The bath temperatures chosen for this work are recommended by ISO 11405 . The transfer time, which is when the samples are in contact with air, takes 5 s. Each change of temperature generates a new stress profile on the samples, which has been analysed on this work.

Fig. 2
Temperature change during the thermal cycle for the bending test and cylinder rod samples. In real tests temperature drop between phases is not sharp due to thermal inertia.

A heat flux was considered on all external walls of the samples. The heat transfer coefficient has been taken as 66 W/(m 2 K) during the cold bath, 1.5 W/(m 2 K) during the transfer, and 110 W/(m 2 K) during the hot bath. The external temperature is chosen 328 K (55 °C) in the hot bath, 278 K (5 °C) in the cold bath and 298 K (25 °C) during the transfer time. The temperature profile was slightly changed for the restoration crown: the transfer time was removed simulating an extreme condition in the oral environment, where cold liquid is swallowed just after a hot liquid, resulting in a cycle of 30 s.

The simulations were carried out by commercial software COMSOL Multiphysics. The boundary conditions used on the models were point constraints, to avoid rotation and translation of the body. On the cylinder rod, a predefined free triangular mesh was used, which resulted in 1644 elements. On the three-points-bending test sample, a predefined free tetrahedral mesh was used resulting in 10467 elements. The mesh of the 2D axi-symmetric restoration model resulted in 9716 free triangular elements. Since these models are simple, the mesh generated by COMSOL showed good results and a greater refinement was not necessary.

Materials and methods

Three different geometries were used in the simulations ( Fig. 1 ). The first one is a cylindrical rod made of two materials: zirconia or a cobalt alloy (CoCrMo) on the bottom half and porcelain on the top half. As this geometry is a revolution solid, it was modelled as 2D axi-symmetric to simplify calculations. The second geometry is a rectangular specimen, used for 3-points-bending test, made of a metallic/ceramic strip veneered in the centre part by porcelain. The bottom plate is made of zirconia or CoCrMo, and the upper plate is made of porcelain. The edges of the plates were rounded (r = 0.2 mm) to avoid stress raisers points. A 3D axi-symmetric model was used to simulate this geometry. The third geometry is a modelled dental restoration crown. The restoration has an inferior layer made of zirconia or CoCrMo and a top porcelain layer. The thickness is measured at the restoration centre, and it changes along its external outline. This geometry was simplified to a 2D axi-symmetric model.

Fig. 1
Modelled geometries for the simulations. A) Cylindrical specimen; B) rectangular specimen; C) dental crown restoration.

The materials properties are considered temperature independent in this work. The range of temperature is relatively small and it should not change considerably these properties. The values for the mechanical and thermal properties used in this work are either based in experimental values by and or informed by the manufacturer. The values are shown in Table 1 . Two different porcelains compatible with the zirconia and CoCrMo alloy used as framework materials were used, VITA VM9 (VITA Zahnfabrik, Germany) and CERAMCO3 (Dentsply, York, USA), respectively. Properties from conventional zirconia (Y-TZP) and Keramit NP CoCrMo alloy (Nobil Metal, Italy) were used in this study.

Table 1
Materials properties used in this model.
Young’s modulus (GPa) Density (kg/m 3 ) Poisson’s ratio Thermal conductivity (W/(m K)) Heat capacity (J/(kg K)) CTE (10 −6 1/K)
Zirconia 210 6095 0.31 2.92 466 10.17
Porcelain ZrO2 70 2431 0.26 1.37 734 9.05
CoCrMo Alloy 220 8600 0.3 13 466 14.4
Porcelain C ° CrMo 70 2431 0.26 1.37 734 13.4
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Jun 17, 2018 | Posted by in General Dentistry | Comments Off on Influence of specimens’ geometry and materials on the thermal stresses in dental restorative materials during thermal cycling
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