Analysis of behavior of the wear coefficient in different layers of acrylic resin teeth.

STATEMENT OF PROBLEM: Analysis of the wear coefficient (k) of the superficial and deep layers of acrylic resin teeth can help predict denture durability, but little has been published on the wear coefficient of denture teeth. PURPOSE: The purpose of this in vitro study was to determine the k value for the superficial and deep layers of the acrylic resin teeth of 6 different brands by using the fixed-ball microabrasive wear method. MATERIAL AND METHODS: Six artificial tooth specimens of 4 commercial brands were tested: Artiplus IPN (Ar), Biotone IPN (Bi), Magister (Ma), Premium (Pr), Trilux (Tr), and SR Vivodent (Vi). Two specimens from each brand were created, one for the superficial layer and the other for the deep layer. The test was performed on fixed-ball microabrasive wear equipment set to operate at a constant normal force of 0.5 N and a rotation speed of 100 rpm. The test time periods were 5.00, 8.33, and 11.66 minutes. The characteristics of the wear craters were measured by using an optical microscope at a magnification of ×50 and Leica Microsystems software. Wear coefficient (k) values were deduced by using the Archard equation for abrasive wear, Q=k·N, and were analyzed by using 1-way analysis of variance, complemented by the Tukey HSD test (α=.05). A different analysis was used for each layer. RESULTS: The analysis of variance of the wear coefficient revealed significant differences among the groups regarding the superficial layers (P=.009). The Tukey HSD test showed that the k values for the superficial layers of Artiplus specimens were significantly lower than those of the Vivodent and Magister specimens. CONCLUSIONS: One brand (Ar) presented significantly lower wear coefficient value for the surface layer. No difference in wear coefficient values was found among the tooth brands for the deep layer.

Long-term bisphosphonate treatment coupled with ovariectomy in mice provokes deleterious effects on femoral neck fracture pattern and modifies tibial shape.

AIMS: The processes linking long-term bisphosphonate treatment to atypical fracture remain elusive. To establish a means of exploring this link, we have examined how long-term bisphosphonate treatment with prior ovariectomy modifies femur fracture behaviour and tibia mass and shape in murine bones. METHODS: Three groups (seven per group) of 12-week-old mice were: 1) ovariectomized and 20 weeks thereafter treated weekly for 24 weeks with 100 µm/kg subcutaneous ibandronate (OVX+IBN); 2) ovariectomized (OVX); or 3) sham-operated (SHAM). Quantitative fracture analysis generated biomechanical properties for the femoral neck. Tibiae were microCT scanned and trabecular (proximal metaphysis) and cortical parameters along almost its whole length measured. RESULTS: Fracture analyses revealed that OVX+IBN significantly reduced yield displacement (vs SHAM/OVX) and resilience, and increased stiffness (vs SHAM). OVX+IBN elevated tibial trabecular parameters and also increased cortical cross-sectional area and second moment of area around minor axis, and diminished ellipticity proximally. CONCLUSION: These data indicate that combined ovariectomy and bisphosphonate generates cortical changes linked with greater bone brittleness and modified fracture characteristics, which may provide a basis in mice for interrogating the mechanisms and genetics of atypical fracture aetiology.Cite this article: Bone Joint Open 2020;1-9:512-519.

Elastic modulus of posts and the risk of root fracture.

The definition of an optimal elastic modulus for a post is controversial. This work hypothesized that the influence of the posts' elastic modulus on dentin stress concentration is dependent on the load direction. The objective was to evaluate, using finite element analysis, the maximum principal stress (sigma(max)) on the root, using posts with different elastic modulus submitted to different loading directions. Nine 3D models were built, representing the dentin root, gutta-percha, a conical post and the cortical bone. The softwares used were: MSC.PATRAN2005r2 (preprocessing) and MSC.Marc2005r2 (processing). Load of 100 N was applied, varying the directions (0 degrees, 45 degrees and 90 degrees) in relation to the post's long axis. The magnitude and direction of the sigma(max) were recorded. At the 45 degrees and 90 degrees loading, the highest values of sigma(max) were recorded for the lowest modulus posts, on the cervical region, with a direction that suggests debonding of the post. For the 0 degrees loading, the highest values of sigma(max) were recorded for higher modulus posts, on the apical region, and the circumferential direction suggests vertical root fracture. The hypothesis was accepted: the effect of the elastic modulus on the magnitude and direction of the sigma(max) generated on the root was dependent on the loading direction.

Why a zero CTE mismatch may be better for veneered Y-TZP structures.

OBJECTIVE: Compare residual stress distribution of bilayered structures with a mismatch between the coefficient of thermal expansion (CTE) of framework and veneering ceramic. A positive mismatch, which is recommended for metal-ceramic dental crowns, was hypothesized to contribute to a greater chipping frequency in veneered Y-TZP structures. In addition, the multidirectional nature of residual stresses in bars and crowns is presented to explore some apparent contradictions among different studies. METHODS: Planar bar and crown-shaped bilayered specimens with 0.7 mm framework thickness and 1.5 mm porcelain veneer thickness were investigated using finite element elastic analysis. Eight CTE mismatch conditions were simulated, representing two framework materials (zirconia and metal) and six veneering porcelains (distinguished by CTE values). Besides metal-ceramic and zirconia-ceramic combinations indicated by the manufacturer, models presenting similar mismatch values (1 ppm/°C) with different framework materials (metal or zirconia) and zirconia-based models with metal-compatible porcelain veneers were also tested. A slow cooling protocol from 600 °C to room temperature was simulated. The distributions of residual maximum and minimum principal stresses, as well as stress components parallel to the long axis of the specimens, were analysed. RESULTS: Planar and crown specimens generated different residual stress distributions. When manufacturer recommended combinations were analysed, residual stresses obtained for zirconia models were significantly higher than those for metal-based models. When zirconia frameworks were combined with metal-compatible porcelains, the residual stress values were even higher. Residual stresses were not different between metal-based and zirconia-based models if the CTE mismatch was similar. SIGNIFICANCE: Some conclusions obtained with planar specimens cannot be extrapolated to clinical situations because specimen shape strongly influences residual stress patterns. Since positive mismatch generates compressive hoop stresses and tensile radial stresses and since zirconia-based crowns tend to be more vulnerable to chipping, a tensile stress-free state generated with a zero CTE mismatch could be advantageous.

Influence of residual thermal stresses on the edge chipping resistance of PFM and veneered zirconia structures: Experimental and FEA study.

OBJECTIVE: Chipping fractures of the veneering porcelain are frequently reported for veneered all-ceramic crowns. In the present study, the edge chipping test is used to measure the toughness and the edge chipping resistance of veneered zirconia and porcelain-fused-to-metal (PFM). The aim is to describe an edge chipping method developed with the use of a universal testing machine and to verify the accuracy of this method to determine the influence of residual thermal stresses on the chipping fracture resistance of veneering porcelain. A finite element analysis (FEA) was used to study the residual stress profiles within the veneering porcelain. METHODS: Veneered zirconia and PFM bar specimens were subjected to either a fast or a slow cooling protocol. The chipping resistances were measured using the edge chipping method. The load was applied in two different directions, in which the Vickers indenter was placed in the veneering porcelain either parallel or perpendicular to the veneer/framework interface. The mean edge chipping resistance (ReA) and fracture toughness (KC) values were analysed. ReA was calculated by dividing the critical force to cause the chip by the edge distance. KC was given by a fracture analysis that correlates the critical chipping load (FC) regarding edge distance (d) and material toughness via KC=FC/(ßd1.5). RESULTS: The ReA revealed similar values (p>0.005) of chipping resistance for loads applied in the parallel direction regardless of framework material and cooling protocol. For loads applied in the perpendicular direction to the veneer/framework interface, the most chip resistant materials were slow cooled veneered zirconia (251.0N/mm) and the PFM fast cooled (190.1N/mm). KC values are similar to that for monolithic porcelain (0.9MPa.√m), with slightly higher values (1.2MPa.√m) for thermally stressed PFM fast cooled and veneered zirconia slow cooled groups. SIGNIFICANCE: The developed and reported edge chipping method allows for the precise alignment of the indenter in any predetermined distance from the edge. The edge chipping method could be useful in determining the different states of residual thermal stresses on the veneering porcelain.

The suitability of different FEA models for studying root fractures caused by wedge effect.

Finite element analysis (FEA) utilizing models with different levels of complexity are found in the literature to study the tendency to vertical root fracture caused by post intrusion ("wedge effect"). The objective of this investigation was to verify if some simplifications used in bi-dimensional FEA models are acceptable regarding the analysis of stresses caused by wedge effect. Three plane strain (PS) and two axisymmetric (Axi) models were studied. One PS model represented the apical third of the root entirely in dentin (PS-nG). The other models included gutta-percha in the apical third, and differed regarding dentin-post relationship: bonded (PS-B and Axi-B) or nonbonded (PS-nB and Axi-nB). Mesh discretization and material properties were similar for all cases. Maximum principal stress (sigma max) was analyzed as a response to a 165 N longitudinal load. Stress magnitude and orientation varied widely (PS-nG: 10.3 MPa; PS-B: 0.8 MPa; PS-nB: 10.4 MPa; Axi-B: 0.2 MPa; Axi-nB: 10.8 MPa). Axi-nB was the only model where all sigma max vectors at the apical third were perpendicular to the model plane. Therefore, it is adequate to demonstrate the tendency to vertical root fractures caused by wedge effect. Axi-B showed only part of the sigma max perpendicular to the model plane while PS models showed sigma max on the model plane. In these models, sigma max)orientation did not represent a situation where vertical root fracture would occur due to wedge effect. Adhesion between post and dentin significantly reduced sigma max.

Polymerization stress of resin composites as a function of system compliance.

OBJECTIVES: Evaluate the effect of testing system compliance on polymerization stress and stress distribution of composites. METHODS: Composites tested were Filtek Z250 (FZ), Herculite (HL), Tetric Ceram (TC), Helio Fill-AP (HF) and Heliomolar (HM). Stress was determined in 1-mm thick specimens, inserted between two rods of either poly(methyl methacrylate), PMMA, or glass. Experimental nominal stress (sigmaexp) was calculated by dividing the maximum force recorded 5 min after photoactivation by the cross-sectional area of the rod. Composites' elastic modulus (E) was obtained by three-point bending. Data were submitted to one-way ANOVA/Tukey's test (alpha=0.05). Stress distribution on longitudinal (sigmay) and transverse (sigmax) axes of models representing the composites with the highest and lowest E (FZ and HM, respectively) were evaluated by finite element analysis (FEA). RESULTS: sigmaexp ranged from 5.5 to 8.8 MPa in glass and from 2.6 to 3.4 MPa in PMMA. Composite ranking was not identical in both substrates, since FZ showed sigmaexp statistically higher than HM in glass, while in PMMA FZ showed values similar to the other composites. A strong correlation was found between stress reduction (%) from glass to PMMA and composite's E (r2=0.946). FEA revealed that system compliance was influenced by the composite (FZ led to higher compliance than HM). sigmax distribution was similar in both substrates, while sigmay distribution showed larger areas of compressive stresses in specimens built on PMMA. SIGNIFICANCE: sigmaexp determined in PMMA was 53-68% lower than in glass. Composite ranking varied slightly due to differences in substrates' longitudinal and transverse deformation.

Composite shrinkage stress as a function of specimen dimensions and compliance of the testing system.

OBJECTIVES: Verify the influence of specimen dimensions on composite shrinkage stress in testing systems of known compliance and in situations where axial strain of bonding substrates was suppressed. Stress distribution was evaluated using finite element analysis (FEA). METHODS: A chemically activated composite (Bisfill 2B, Bisco) was inserted between the flat surfaces of two glass rods (2, 4 or 6mm diameter, D) attached to a universal testing machine. Specimen height (H) was defined by adjusting the distance between the rods (0.5, 1, 2 or 4mm). An extensometer was used to monitor the distance between them. Maximum force after 30min of polymerization was divided by the cross-sectional area of the rod to obtain nominal stress (sigma(n)). Mathematical equations were employed in order to estimate the 'corrected nominal stress' (sigma(cor)) that would be obtained in ideally rigid systems. Data were analyzed by two-way ANOVA/Tukey test (alpha=0.05) and regression analysis (stress versus 'C factor' and stress versus specimen volume). Axysimetrical 2D models were used to evaluate X-component stress distribution (sigma(x)) in the zero compliance condition. RESULTS: The interactions between D and H were significant for sigma(n) and sigma(cor) (p<0.001). For D=2mm, height influenced only sigma(cor) values. A direct correlation was found between stress and C factor (sigma(n):R(2)=0.959; sigma(cor):R(2)=0.923), but not between stress and volume. FEA evidenced the effect of boundary restraints on sigma(x). SIGNIFICANCE: Specimen dimensions influenced test results, mostly regarding sigma(cor). Stress values strongly increased with the confinement of the specimen probably due to stress concentration adjacent to the bonded interface.

Influence of local factors on composite shrinkage stress development--a finite element analysis.

PURPOSE: Using finite element analysis (FEA), to determine the nominal shrinkage stress of a composite under different restriction conditions defined by the longitudinal compliance (LC) and C-factor (C) of the testing system, and by the elastic modulus of the bonding substrate (E). MATERIALS AND METHODS: Eight axisymmetric models representing an experimental setup used to determine composite shrinkage stress were simulated. Composite thicknesses of 0.5 mm and 4 mm were tested, defining different C and volumes (C = 6 and vol = 14 mm3 or C = 0.8 and vol = 113 mm3, respectively). The E of the substrate was tested in two levels, 12 GPa and 207 GPa. Two LC values (1 x 10(-6) or 28 x 10(-6) mm/N) were defined for each E value by varying the length of the rods used as bonding substrate (0.3 mm and 9.5 mm for E = 12 GPa; 6.0 mm and 163.9 mm for E = 207 GPa). Materials were considered elastic, homogeneous, and isotropic. Shrinkage was simulated by thermal analogy. Nominal stress (nodal force/cross-sectional area) was calculated for each condition. Results were analyzed using Taguchi's method. RESULTS: Nominal stress values varied between 1.7 MPa and 30.3 MPa. The main variables were statistically significant (LC: p = 0.0046; C: p = 0.0153; E: p = 0.0155), as well as the LC x E interaction (p = 0.0354). Stress reduction between low and high LC was more pronounced for E = 207 GPa compared to E = 12 GPa. Stress was lower for the high C conditions for both compliance levels. CONCLUSION: Not only the C-factor of the testing assembly, but also its LC and the E of the bonding substrate influence stresses generated by composite shrinkage.

How mechanical stresses modulate enamel demineralization in non-carious cervical lesions?

OBJECTIVE: To introduce an experimental non-carious cervical lesion (NCCL) model for studying the influence of presence and type of stress (tension or compression) on acid effects involved in NCCL formation on the enamel near the cement-enamel junction (CEJ). METHODS: 108 bovine incisors were cut into 18 × 3×3 mm3 beams, with a notch in the cervical region to generate a standardized area of stress concentration. Half of the specimens were immersed in distilled water and the other half in acetic acid solution (pH 4.5) for 72h. Each group was divided into three subgroups. Two subgroups underwent 800gf static loading, with the specimen positioned in a bending jig with the buccocervical region under either tension or compression. The load was applied simultaneously to immersion (in water or in acid). The third subgroup was not subjected to loading. Transversal and longitudinal 0.05 mm plates of the specimens were analyzed under a light microscope (40, 100 and 200×) to measure the enamel demineralization depth and to assess the presence of cracks, fractures and gaps at the enamel-dentin junction. The demineralization depth data were submitted to ANOVA and Tukey's test at a 5% significance level. RESULTS: Enamel demineralization depth (µm) was higher under tension (158±19 in transversal sections and 229±32 in longitudinal sections) than under compression (transversal: 129±16 and longitudinal: 167±10) or unstressed condition (transversal: 138±21 and longitudinal: 187±21). Specimens immersed in acid and subjected to tensile stress presented enamel micro fractures and wider gaps in the dentin-enamel junction. SIGNIFICANCE: Enamel demineralization was significantly higher in the presence of tensile stress, due to wider gaps between dentin and enamel, stress corrosion cracking and increased enamel permeability to acid.

Factors involved in the development of polymerization shrinkage stress in resin-composites: a systematic review.

OBJECTIVES: Polymerization shrinkage stress of resin-composite materials may have a negative impact on the clinical performance of bonded restorations. The purpose of this systematic review is to discuss the primary factors involved with polymerization shrinkage stress development. DATA: According to the current literature, polymerization stress of resin composites is determined by their volumetric shrinkage, viscoelastic behavior and by restrictions imposed to polymerization shrinkage. Therefore, the material's composition, its degree of conversion and reaction kinetics become aspects of interest, together with the confinement and compliance of the cavity preparation. SOURCES: Information provided in this review was based on original scientific research published in Dental, Chemistry and Biomaterials journals. Textbooks on Chemistry and Dental Materials were also referenced for basic concepts. CONCLUSIONS: Shrinkage stress development must be considered a multi-factorial phenomenon. Therefore, accessing the specific contribution of volumetric shrinkage, viscoelastic behavior, reaction kinetics and local conditions on stress magnitude seems impractical. Some of the restorative techniques aiming at stress reduction have limited applicability, because their efficiency varies depending upon the materials employed. Due to an intense research activity over the years, the understanding of this matter has increased remarkably, leading to the development of new restorative techniques and materials that may help minimize this problem.

Polymerization shrinkage: effects of constraint and filling technique in composite restorations.

OBJECTIVES: To evaluate the linear polymerization shrinkage (LPS) and its effect upon mean gap width, bond strength and cohesive strength of a composite placed under different constraints (C-factors--CF) and filling techniques. METHODS: Composite was placed in cavities sized 4 x 4 x 2 mm3 (CF = 3) or on flat dentin surfaces (CF = 0.3) of bovine incisors, after adhesive application. They were inserted in one or three increments, and light cured (600 mW/cm2) for 80 s. The LPS was measured by placing a probe on the top surface of the composite in order to measure its dislodgment in the top-bottom direction. Half of the sample was sectioned to obtain composite resin sticks subjecting them to tensile forces at 0.5 mm/min. The other half of the sample was sectioned and the mean gap width was measured in both sides of the sections. Then the sections were sliced again to obtain composite/dentin sticks. The mean gap width in the sticks was performed before subjecting them to tensile forces at 0.5 mm/min. Data was analyzed by a two-way ANOVA and the correlation between the bond strength and gap width was analyzed by simple linear regression. RESULTS: (1) Linear polymerization shrinkage: significant differences were observed for the interaction (p < 0.05). Under the low constraint, the LPS were similar for both filling techniques. Under higher constraint, polymerization shrinkage was lower for the incremental technique. (2) Gap width and bond strength: no difference was detected either for interaction, or for technique (p > 0.05). Under higher constraint, the gap width was higher and the bond strength lower. (3) The cohesive strength of composite resin was similar for all groups (p > 0.05). No correlation between bond strength and gap width was found (p = 0.17). SIGNIFICANCE: The effects of polymerization shrinkage were not reduced by the filling technique under the different cavity constraints tested.

Relationship between contraction stress and degree of conversion in restorative composites.

OBJECTIVES: To verify the relationship between contraction stress and degree of conversion (DC) in different composites (Filtek Z250, Filtek A110, Tetric Ceram and Heliomolar). METHODS: For the contraction stress test, composite (2 mm thick) was applied between two 5-mm diameter glass rods, mounted in a tensilometer. DC was determined by Infrared Photoacoustic spectroscopy in specimens with similar dimensions and geometry, submitted to identical curing conditions. Specimens were exposed to different energy densities (4.5, 13.5, 27.0, 54.0 and 108.0 J/cm2) by varying exposure time. Contraction stress and DC were recorded 10 min after the beginning of photoactivation. Results were analyzed by ANOVA/Tukey's test and regression analysis. RESULTS: For contraction stress, the interaction between composite and energy density was significant. Stress values ranged between 0.6+/-0.2 and 2.0+/-0.3 MPa at 4.5 J/cm2, 2.3+/-0.5 and 4.3+/-0.4 MPa at 13.5 J/cm2, 3.8+/-0.5 and 5.8+/-0.9 MPa at 27.0 J/cm2, 4.2+/-0.8 and 7.9+/-0.9 MPa at 54.0 J/cm2 and 6.6+/-0.8 and 8.1+/-0.9 MPa at 108.0 J/cm2. Tetric Ceram (39+/-5.8%) showed a higher average DC than the other materials. Heliomolar (28+/-5.2%) showed an average DC similar to Filtek Z250 (32+/-6.6%) and to Filtek A110 (24+/-7.5%) regardless of the energy density level. No significant increase in DC was observed above 27 J/cm2. CONCLUSIONS: At high energy levels, DC had a tendency to level off earlier than contraction stress values. SIGNIFICANCE: Using high energy densities may cause a significant increase in stress values, without producing a significant increase in conversion.

Zinc sulfate addition to glass-ionomer-based cements: influence on physical and antibacterial properties, zinc and fluoride release.

OBJECTIVES: The aim of this study is to evaluate the effect of ZnSO(4) addition to a conventional glass ionomer and a resin-modified glass ionomer on solubility, flexural strength, zinc and fluoride (F) release, and Streptococcus mutans growth inhibition. METHODS: 5 or 10% ZnSO(4) was added to Vitremer and Ketac-Fil powders. Solubility test was performed based on ISO 7489. Flexural strength was determined by 3-point bending test based on ISO 4049. Zn release/uptake was determined by atomic emission spectrometry; F release/uptake was measured using a F-specific electrode. Both release measurements were performed for 15 d before and 15 d after recharging. Antibacterial test was conducted according to agar plate methods against S. mutans, by measuring the inhibition halos in 1-h and 15-d specimens. Data were analyzed by ANOVA. RESULTS: Solubility increased with higher ZnSO(4) content, but remained below the ISO 7489 limit. Flexural strength was not affected by ZnSO(4) addition, and Vitremer performed better than Ketac-Fil. The control materials released no zinc. Vitremer with 10% ZnSO(4) released the highest amount of zinc. Fluoride release was similar for Ketac-Fil and Vitremer. In both cases, the highest amounts were released in the first 24 h. The growth inhibition halo of S. mutans was similar for both materials with highest content of ZnSO(4) and occurred only with 1-h specimens. SIGNIFICANCE: Zinc addition decreased microorganisms growth and improved fluoride release, without significantly affecting the materials' flexural strength and solubility.

Stress concentration in microtensile tests using uniform material.

PURPOSE: The objective of this study was to analyze the stress concentration factor (Kt) in specimens of uniform material with the most commonly used geometry (square hourglass) during microtensile tests using finite element analysis. Standardization is emphasized with the aim of obtaining the most representative nominal strength of the material. METHODS: Eighty cases were simulated using three-dimensional models, in which we varied the fixation of specimens in the jig (f = 1 or 2 sides), the height of this fixed region (h = 1 or 2.75 mm), the specimen width (D = 1.5, 2, 3, 4 or 5 mm), and the radius of curvature of the notch (r = 0.2, 0.5, 0.7 or 1 mm). The cross-sectional area (1 mm2) remained constant in all analyses. The stress concentration factor Kt (maximum tensile stress/nominal tensile stress) was calculated. RESULTS: A 150% difference was observed from the lowest Kt value (1.3) to the highest one (3.2). Results indicated that the radius of curvature is a very influential geometric parameter in microtensile strength tests (variation in Kt values up to 47.4%). For two-side fixed specimens, the Kt values varied from 3 to 4%, while the one-side fixed models resulted in variations from 11 to 15%. CONCLUSION: Variations in the specimen geometry and mode of load application can be responsible for part of the different strength values obtained in microtensile tests. The specimen fixation by two sides is a simple and easily performed method to reduce the stress concentration factor and its variations induced by specimen geometry and test assembly.

A method for calculating the compliance of bonded-interfaces under shrinkage: validation for Class I cavities.

OBJECTIVE: The compliance for tooth cavity preparations is not yet fully described in the literature. Thus, the objectives were to present a finite element (FE) method for calculating compliance and to apply this to peak shrinkage stress regions in model cavities restored with resin-composite. METHODS: Three groups of FE-models were created, with all materials considered linear, homogeneous, elastic and isotropic: (a) a pair of butt-joint bonded cubic prisms (dentin/resin-composite), with dentin of known compliance (0.0666 µm/N). Free ends were fixed in the Z-axis direction. A 1% volumetric shrinkage was simulated for the resin-composite. Mean displacements in the Z direction at each node at the dentin-resin interface were calculated and divided by the sum of normal contact forces in Z for each node. (b) A series of more complex restored cavity configurations for which their compliances were calculated. (c) A set of 3D-FE beam models, of 4 mm × 2 mm cross-section with lengths from 2 to 10mm, were also analyzed under both tensile and bending modes. RESULTS: The compliance calculated by FEM for the butt-joint prisms was 0.0652 µm/N and corresponded well to the analytical value (0.0666 µm/N). For more accurate representations of the phenomenon, such as the compliance of a cavity or any other complex structure, the use of the displacement-magnitude was recommended, as loading by isotropic contraction also produces transversal deformations. For the beam models, the compliance was strongly dependent upon the loading direction and was greater under bending than in tension. SIGNIFICANCE: The method was validated for the compliance calculation of complex structures subjected to shrinkage stress such as Class I 'cavities'. The same FEM parameters could be applied to calculate the real compliance of any interface of complex structures. The compliance concept is improved by considering specific load directions.

Influence of specimen dimensions and their derivatives (C-factor and volume) on polymerization stress determined in a high compliance testing system.

OBJECTIVE: To verify the null hypothesis that in a high compliance testing system stress magnitude was not influenced by specimen dimensions and, therefore, by its cavity configuration factor (C-factor) and volume. METHODS: Twelve experimental groups were defined according to the specimen height (0.5, 1, 2 or 4mm) and diameter (4, 6 or 8mm). A selfcure composite (Bisfil 2B, Bisco) was inserted between the flat surfaces of two acrylic rods attached to the opposite ends of a universal testing machine. An extensometer with a gauge length of 10mm was attached to both rods to monitor specimen height. Force development was monitored for 30min and nominal stress was calculated dividing the maximum force value by the crossection of the rod. A second set of data was obtained dividing nominal stress by the corresponding longitudinal compliance (LC) of the system for that particular specimen size. Data was analyzed using two-way ANOVA/Tukey test (nominal stress), Kruskal/Wallis (normalized stress), both at alpha=5%, and regression analysis having either C-factor or volume as independent variable. RESULTS: Regression analysis involving nominal stress did not reveal strong relationships with the independent variables (C-factor: 0.437, volume: 0.662). A strong relationship was found between normalized stress and specimen volume (Radj(2)=0.886). Normalized stress showed no relationship with specimens' C-factor. Pairwise comparisons between groups with similar volumes revealed that normalized stress increased at higher C-factors. SIGNIFICANCE: By eliminating the influence of longitudinal compliance, a strong relationship between polymerization stress and specimen volume was revealed in a high compliance testing system.

Residual stresses in Y-TZP crowns due to changes in the thermal contraction coefficient of veneers.

OBJECTIVE: To test the hypothesis that the difference in the coefficient of thermal contraction of the veneering porcelain above (αliquid) and below (αsolid) its Tg plays an important role in stress development during a fast cooling protocol of Y-TZP crowns. METHODS: Three-dimensional finite element models of veneered Y-TZP crowns were developed. Heat transfer analyses were conducted with two cooling protocols: slow (group A) and fast (groups B-F). Calculated temperatures as a function of time were used to determine the thermal stresses. Porcelain αsolid was kept constant while its αliquid was varied, creating different Δα/αsolid conditions: 0, 1, 1.5, 2 and 3 (groups B-F, respectively). Maximum (σ1) and minimum (σ3) residual principal stress distributions in the porcelain layer were compared. RESULTS: For the slowly cooled crown, positive σ1 were observed in the porcelain, orientated perpendicular to the core-veneer interface ("radial" orientation). Simultaneously, negative σ3 were observed within the porcelain, mostly in a hoop orientation ("hoop-arch"). For rapidly cooled crowns, stress patterns varied depending on Δα/αsolid ratios. For groups B and C, the patterns were similar to those found in group A for σ1 ("radial") and σ3 ("hoop-arch"). For groups D-F, stress distribution changed significantly, with σ1 forming a "hoop-arch" pattern while σ3 developed a "radial" pattern. SIGNIFICANCE: Hoop tensile stresses generated in the veneering layer during fast cooling protocols due to porcelain high Δα/αsolid ratio will facilitate flaw propagation from the surface toward the core, which negatively affects the potential clinical longevity of a crown.

Finite element analysis of bonded model Class I 'restorations' after shrinkage.

OBJECTIVES: The C-Factor has been used widely to rationalize the changes in shrinkage stress occurring at the tooth/resin-composite interfaces. Experimentally, such stresses have been measured in a uniaxial direction between opposed parallel walls. The situation of adjoining cavity walls has been neglected. The aim was to investigate the hypothesis that: within stylized model rectangular cavities of constant volume and wall thickness, the interfacial shrinkage-stress at the adjoining cavity walls increases steadily as the C-Factor increases. METHODS: Eight 3D-FEM restored Class I 'rectangular cavity' models were created by MSC.PATRAN/MSC.Marc, r2-2005 and subjected to 1% of shrinkage, while maintaining constant both the volume (20 mm(3)) and the wall thickness (2 mm), but varying the C-Factor (1.9-13.5). An adhesive contact between the composite and the teeth was incorporated. Polymerization shrinkage was simulated by analogy with thermal contraction. Principal stresses and strains were calculated. Peak values of maximum principal (MP) and maximum shear (MS) stresses from the different walls were displayed graphically as a function of C-Factor. The stress-peak association with C-Factor was evaluated by the Pearson correlation between the stress peak and the C-Factor. RESULTS: The hypothesis was rejected: there was no clear increase of stress-peaks with C-Factor. The stress-peaks particularly expressed as MP and MS varied only slightly with increasing C-Factor. Lower stress-peaks were present at the pulpal floor in comparison to the stress at the axial walls. In general, MP and MS were similar when the axial wall dimensions were similar. The Pearson coefficient only expressed associations for the maximum principal stress at the ZX wall and the Z axis. SIGNIFICANCE: Increase of the C-Factor did not lead to increase of the calculated stress-peaks in model rectangular Class I cavity walls.

A method to investigate the shrinkage stress developed by resin-composites bonded to a single flat surface.

OBJECTIVES: To purpose a method for predicting the shrinkage stress development in the adhesive layer of resin-composite cylinders that shrink bonded to a single flat surface, by measuring the deflection of a glass coverslip caused by the shrinkage of the bonded cylinders. The correlation between the volume of the bonded resin-composite and the stress-peak was also investigated. METHODS: A glass coverslip deflection caused by the shrinkage of a bonded resin-composite cylinder (diameter: d=8 mm, 4 mm, or 2 mm, height: h=4 mm, 2 mm, 1 mm, or 0.5 mm) was measured, and the same set-up was simulated by finite element analysis (3D-FEA). Stresses generated in the adhesive layer were plotted versus two geometric variables of the resin-composite cylinder (C-Factor and volume) to verify the existence of correlations between them and stresses. RESULTS: The FEA models were validated. A significant correlation (p<0.01, Pearson's test) between the stress-peak and the coverslip deflection when the resin-composites were grouped by diameter was found for diameters of 2 and 4 mm. The stress-peak of the whole set of data showed a logarithmic correlation with the bonded resin-composite volume (p<0.001, Pearson's test), but did not correlate with the C-Factor. SIGNIFICANCE: The described method should be considered for standardizing the stress generated by the shrinkage of resin-composite blocks bonded to a single flat surface.