Biomedical subjects

Rafael Y Ballester

Publications and source records attributed to Rafael Y Ballester.

6 recordsLinked to original sources

PubMed2006

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.

Compliance↗

PubMed2005

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.

Chemical Phenomena↗

PubMed2004

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.

Acrylic Resins↗

PubMed2004

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.

Analysis of Variance↗

PubMed2004

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.

Acrylic Resins↗

PubMed2003

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.

Analysis of Variance↗