Investigation of mechanical performances and polymerization shrinkage of dual-cured resin composites as core build-up material.

This study systematically compared the mechanical performances and polymerization shrinkage of two novel dual-cured resin composites (DCRC) with one conventional packable light-cured resin composite (LCRC) for their application as core build-up material by micro-hardness test, flexural strength test, push-out test, and digital image correlation analysis. The LCRC had a significantly higher micro-hardness (p<0.05) whereas the bond strength demonstrated no difference. The mean values of three materials ranged from 35.16 and 64.82 for the Vickers hardness and from 4.66 MPa to 11.53 MPa for the bond strength. The flexure strength of the three materials was not statistically different from each other. LCRC demonstrated 1.88% of volumetric shrinkage while the two DCRC showed 5.06% and 4.91%, respectively. In general, the DCRC demonstrated a comparable flexural strength and bond strength as the LCRC, however, the significant polymerization shrinkage of DCRC should be emphasized.

Effect of chamfer design on load capacity of reattached incisors.

OBJECTIVE: This study aimed to evaluate the effect of different chamfer preparations on the load capacity of reattached fractured incisors under lingual loading. METHODS: Eighty #8 typodonts were randomly assigned to four groups (n = 20 each). They were sectioned to simulate crown fracture, and reattached with a self-etch adhesive and a resin composite. The preparation for each group was: (1) no chamfer; (2) buccal chamfer; (3) lingual chamfer; and (4) circumferential chamfer. Forty-eight human lower incisors were grouped and prepared similarly (n = 12 each). These teeth were tested for their load capacity under a lingual load on a universal testing machine. Finite element models were used to examine the stresses on the reattached surfaces to help interpret the experimental results. RESULTS: The buccal chamfer did not increase the load capacity when compared with the no-chamfer group. Lingual and circumferential chamfers respectively increased the fracture load by 36.9% and 32.3% in typodonts, and 78.5% and 33.3% in human incisors. The increase was statistically significant (p < 0.05). A higher fracture load tended to be accompanied by a larger area of deflected cohesive fracture. Finite element analysis showed that lingual and circumferential chamfers reduced the fracture-causing tensile stress at the lingual margin of the reattachment interface by approximately 70% and 60%, respectively, in human upper incisors. SIGNIFICANCE: It was the joint design, and not the size of the bond area, that affected the load capacity of reattached incisors. Among the preparations considered, only those with a lingual chamfer could increase the load capacity of reattached incisors under a lingual load.

Shrinkage stress and cuspal deflection in MOD restorations: analytical solutions and design guidelines.

OBJECTIVE: This paper aimed to derive analytical solutions for the shrinkage stress and cuspal deflection in model Class-II mesial-occlusal-distal (MOD) resin-composite restorations to better understand their dependence on geometrical and material parameters. Based on the stress solutions, it was shown how design curves could be obtained to guide the selection of dimensions and materials for the preparation and restoration of this class of cavities. METHODS: The cavity wall was considered as a cantilevered beam while the resin composite was modeled as Winkler's elastic foundation with closely-spaced linear springs. Further, a mathematical model that took into account the combined effect of material properties, sample geometry and compliance of the surrounding constraint was employed to relate the shrinkage stress at the "tooth-composite" interface to the local compliance of the cavity wall. Exact analytical solutions were obtained for cuspal deflection and shrinkage stress along the cavity wall by solving the resulting differential equation, which had the same form as that for a beam on elastic foundation with a distributed load. To quantify the shrinkage stress at the cavity floor, the resin composite was assumed to be a beam, fixed at both ends and loaded with a uniformly distributed load that approximated the shrinkage stress. The analytical solutions thus obtained were compared with results from finite element analysis (FEA). RESULTS: The analytical solution for cuspal deflection contains a dimensionless parameter, γ, which represents the stiffness of the cavity wall relative to that of the cured resin composite. For the same shrinkage strain, cuspal deflection increases with reducing γ, i.e. reducing stiffness of the cavity wall or increasing stiffness of the composite. For the same γ, cuspal deflection increases proportionally with shrinkage strain. Shrinkage stress along the cavity wall is maximum at the cavity corner and reduces towards the occlusal surface; the maximum value depends only on Young's modulus and the shrinkage strain of the resin composite. For low values of γ, the interfacial stress at the occlusal surface can become compressive. The interfacial stress at the cavity floor can be much higher than that along the cavity wall, increasing exponentially with the resin composite's thickness. The analytical solutions agree well with FEA predictions. SIGNIFICANCE: When validated, the analytical solutions and design curves presented in this study can provide useful guidelines for choosing appropriate dimensions of cavity preparations and resin composite materials with suitable mechanical properties for Class-II MOD restorations to help avoid tooth fracture and interfacial debonding caused by polymerization shrinkage.

Mechanical manifestation of the C-factor in relation to photopolymerization of dental resin composites.

OBJECTIVE: This study aims to assess the validity of a recent theory which proposes that (1) the magnitude of the shrinkage stress of resin composites depends on the thickness of the boundary layer under triaxial constraints relative to the total thickness of the specimen and (2) the boundary-layer thickness is proportional to the diameter of the specimen. METHODS: Cylindrical specimens of three commercially available resin composites, three diameters (4, 5 and 6.3mm) and four thicknesses (2, 3, 5 and 6.5mm) were tested. Curing was applied using a LED light for 40s. Microscopic images (32×) of the specimens before and after curing were analyzed to determine the lateral shrinkage profile along the vertical axis. Boundary-layer thickness was determined from the point where lateral shrinkage displacement first reached the maximum value found at mid-thickness. RESULTS: Lateral shrinkage displacement at mid-thickness was close to the theoretical value based on published shrinkage-strain data, with the ratio between experimental and theoretical values being 1.04±0.06. The boundary-layer thickness was found to be proportional to specimen diameter only, independent of material, C-factor, and specimen thickness. The proportionality constant was 0.64±0.07, which was approximately 3 times that of the effective value indicated by shrinkage strain/stress calculations. SIGNIFICANCE: This study validates the assumption made in the shrinkage-stress theory recently proposed and provides a more precise and mechanistic interpretation for the C-factor, i.e. the C-factor, as a measure of a specimen's constraint, is the ratio between the boundary-layer thickness and the total thickness of the specimen.

The two sides of the C-factor.

OBJECTIVE: The aim of this paper is to investigate the effects on shrinkage strain/stress development of the lateral constraints at the bonded surfaces of resin composite specimens used in laboratory measurement. METHODS: Using three-dimensional (3D) Hooke's law, a recently developed shrinkage stress theory is extended to 3D to include the additional out-of-plane strain/stress induced by the lateral constraints at the bonded surfaces through the Poisson's ratio effect. The model contains a parameter that defines the relative thickness of the boundary layers, adjacent to the bonded surfaces, that are under such multiaxial stresses. The resulting differential equation is solved for the shrinkage stress under different boundary conditions. The accuracy of the model is assessed by comparing the numerical solutions with a wide range of experimental data, which include those from both shrinkage strain and shrinkage stress measurements. RESULTS: There is good agreement between theory and experiments. The model correctly predicts the different instrument-dependent effects that a specimen's configuration factor (C-factor) has on shrinkage stress. That is, for noncompliant stress-measuring instruments, shrinkage stress increases with the C-factor of the cylindrical specimen; while the opposite is true for compliant instruments. The model also provides a correction factor, which is a function of the C-factor, Poisson's ratio and boundary layer thickness of the specimen, for shrinkage strain measured using the bonded-disc method. For the resin composite examined, the boundary layers have a combined thickness that is ∼11.5% of the specimen's diameter. SIGNIFICANCE: The theory provides a physical and mechanical basis for the C-factor using principles of engineering mechanics. The correction factor it provides allows the linear shrinkage strain of a resin composite to be obtained more accurately from the bonded-disc method.

1/(2+Rc): A simple predictive formula for laboratory shrinkage-stress measurement.

OBJECTIVE: This paper presents and verifies a simple predictive formula for laboratory shrinkage-stress measurement in dental composites that can account for the combined effect of material properties, sample geometry and instrument compliance. METHODS: A mathematical model for laboratory shrinkage-stress measurement that includes the composite's elastic modulus, shrinkage strain, and their interaction with the sample's dimensions and the instrument's compliance has previously been developed. The model contains a dimensionless parameter, Rc, which represents the compliance of the instrument relative to that of the cured composite sample. A simplified formula, 1/(2+Rc), is proposed here for the normalized shrinkage stress to approximate the original model. The accuracy of the simplified formula is examined by comparing its shrinkage-stress predictions with those given by the exact formula for different cases. These include shrinkage stress measured using instruments with different compliances, samples with different thicknesses and composites with different filler fractions. RESULTS: The simplified formula produces shrinkage-stress predictions that are very similar to those given by the full formula. In addition, it correctly predicts the decrease in shrinkage stress with an increasing configuration factor for compliant instruments. It also correctly predicts the value of the so-called flow factor of composites despite the fact that creep is not considered in the model. SIGNIFICANCE: The new simple formula significantly simplifies the prediction of shrinkage stress for disc specimens used in laboratory experiments without much loss in precision. Its explicit analytical form shows clearly all the important parameters that control the level of shrinkage stress in such measurements. It also helps to resolve much of the confusion caused by the seemingly contradictory results reported in the literature. Further, the formula can be used as a guide for the design of dental composite materials or restorations to minimize their shrinkage stress.

Prediction of water loss and viscoelastic deformation of apple tissue using a multiscale model.

A two-dimensional multiscale water transport and mechanical model was developed to predict the water loss and deformation of apple tissue (Malus × domestica Borkh. cv. 'Jonagold') during dehydration. At the macroscopic level, a continuum approach was used to construct a coupled water transport and mechanical model. Water transport in the tissue was simulated using a phenomenological approach using Fick's second law of diffusion. Mechanical deformation due to shrinkage was based on a structural mechanics model consisting of two parts: Yeoh strain energy functions to account for non-linearity and Maxwell's rheological model of visco-elasticity. Apparent parameters of the macroscale model were computed from a microscale model. The latter accounted for water exchange between different microscopic structures of the tissue (intercellular space, the cell wall network and cytoplasm) using transport laws with the water potential as the driving force for water exchange between different compartments of tissue. The microscale deformation mechanics were computed using a model where the cells were represented as a closed thin walled structure. The predicted apparent water transport properties of apple cortex tissue from the microscale model showed good agreement with the experimentally measured values. Deviations between calculated and measured mechanical properties of apple tissue were observed at strains larger than 3%, and were attributed to differences in water transport behavior between the experimental compression tests and the simulated dehydration-deformation behavior. Tissue dehydration and deformation in the high relative humidity range ( > 97% RH) could, however, be accurately predicted by the multiscale model. The multiscale model helped to understand the dynamics of the dehydration process and the importance of the different microstructural compartments (intercellular space, cell wall, membrane and cytoplasm) for water transport and mechanical deformation.