Do Low-shrink Composites Reduce Polymerization Shrinkage Effects?

Progress in polymer science has led to continuous reduction of polymerization shrinkage, exemplified by a new generation of ""low-shrink composites"". The common inference that shrinkage stress effects will be reduced in teeth restored with such restoratives with lower shrinkage was tested in extracted human premolars. Mesio-occluso-distal slot-shaped cavities were cut and restored with a conventional (SupremePlus) or low-shrink (RefleXions, Premise, Kalore, and LS) composite (N = 5). We digitized the coronal surfaces before and 10 min after restoration to determine cuspal deflection from the buccal and lingual volume change/area. We also determined the main properties involved (total shrinkage, post-gel shrinkage, degree of conversion, and elastic modulus), as well as microleakage, to verify adequate bonding. It was shown that, due to shrinkage stresses, buccal and lingual surfaces pulled inward after restoration (9-14 microns). Only Kalore and LS resulted in significantly lower tooth deformation (ANOVA/Student-Newman-Keuls post hoc, p = 0.05). The other two low-shrink composites, despite having the lowest and highest total shrinkage values, did not cause significant differences in cuspal deflection. Deflection seemed most related to the combination of post-gel shrinkage and elastic modulus. Therefore, even for significantly lower total shrinkage values, shrinkage stress is not necessarily reduced.

Understanding Contradictory Data in Contraction Stress Tests

The literature shows contradictory results regarding the role of composite shrinkage and elastic modulus as determinants of polymerization stress. The present study aimed at a better understanding of the test mechanics that could explain such divergences among studies. The hypothesis was that the effects of composite shrinkage and elastic modulus on stress depend upon the compliance of the testing system. A commonly used test apparatus was simulated by finite element analysis, with different compliance levels defined by the bonding substrate (steel, glass, composite, or acrylic). Composites with moduli between 1 and 12 GPa and shrinkage values between 0.5% and 6% were modeled. Shrinkage was simulated by thermal analogy. The hypothesis was confirmed. When shrinkage and modulus increased simultaneously, stress increased regardless of the substrate. However, if shrinkage and modulus were inversely related, their magnitudes and interaction with rod material determined the stress response.

Mechanical characterization of proanthocyanidindentin matrix interaction

Objectives To characterize the properties of dentin matrix treated with two proanthocyanidin rich cross-linking agents and their effect on dentin bonded interfaces. Methods Sound human molars were cut into 0.5mm thick dentin slabs, demineralized and either treated with one of two cross-linking agents (grape seedGSE and cocoa seedCOE extracts) or left untreated. The modulus of elasticity of demineralized dentin was assessed after 10 or 60min and the swelling ratio after 60min treatment. Bacterial collagenase was also used to assess resistance to enzymatic degradation of samples subjected to ultimate tensile strength. The effect of GSE or COE on the resindentin bond strength was evaluated after 10 or 60min of exposure time. Data were statistically analyzed at a 95% confidence interval. Results Both cross-linkers increased the elastic modulus of demineralized dentin as exposure time increased. Swelling ratio was lower for treated samples when compared to control groups. No statistically significant changes to the UTS indicate that collagenase had no effect on dentin matrix treated with either GSE or COE. Resindentin bonds significantly increased following treatment with GSE regardless of the application time or adhesive system used. Significance Increased mechanical properties and stability of dentin matrix can be achieved by the use of PA-rich collagen cross-linkers most likely due to the formation of a PAcollagen complex. The short term resindentin bonds can be improved after 10min dentin treatment.(C) 2010 Academy of Denta lMaterials. Published by Elsevier Ltd. All rights reserved.

Influence of matrix composition on polymerization stress development of experimental composites

Objective. Stress development at the tooth/restoration interface is one of the most important reasons for failure of adhesive restorations. The aim of this study was to evaluate the influence of BisGMA/TEGDMA (B/T) and UDMA/TEGDMA (U/T) ratios on polymerization stress (PS) and on the variables related to its development: degree of conversion (DC), polymerization maximum rate (Rp(max)), volumetric shrinkage (VS), elastic modulus (E), stress relaxation (SR) and viscosity of experimental composites. Method. Composites were formulated containing B/T or U/T in mol% ratios of 2: 8, 3: 7, 4: 6, 5: 5, 6: 4, 7: 3 and 8: 2, and 15 wt% of fumed silica. PS was determined with a universal testing machine. VS was measured with a linometer. E and SR were obtained in three-point bending. DC and Rp(max) were determined by real time NIR spectroscopy and viscosity was measured in viscometer. Data were submitted to one-way ANOVA, Tukey test (alpha = 0.05%) and regression analyses. Results. PS, VS, E and DC decreased and viscosity and Rp(max) increased with base monomer content in both series. PS showed strong correlation with VS, DC and viscosity. PS, VS and DC were higher and viscosity was lower for UDMA-based materials. Significance. Reduced viscosity, kinetics parameters and molecular characteristics led UDMA-based composites to elevated conversion and relatively lower PS at lower TEGDMA contents, compared to B/T composites. (C) 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Contraction stress related to composite inorganic content

Objectives. The role of inorganic content on physical properties of resin composites is well known. However, its influence on polymerization stress development has not been established. The aim of this investigation was to evaluate the influence of inorganic fraction on polymerization stress and its determinants, namely, volumetric shrinkage, elastic modulus and degree of conversion. Methods. Eight experimental composites containing 1:1 BisGMA (bisphenylglycidyl dimethacrylate): TEGDMA (triethylene glycol dimethacrylate) (in mol) and barium glass at increasing concentrations from 25 to 60 vol.% (5% increments) were tested. Stress was determined in a universal test machine using acrylic as bonding substrate. Nominal polymerization stress was obtained diving the maximum load by the cross-surface area. Shrinkage was measured using a water picnometer. Elastic modulus was obtained by three-point flexural test. Degree of conversion was determined by FT-Raman spectroscopy. Results. Polymerization stress and shrinkage showed inverse relationships with filler content (R(2) = 0.965 and R(2) = 0.966, respectively). Elastic modulus presented a direct correlation with inorganic content (R(2) = 0.984). Degree of conversion did not vary significantly. Polymerization stress showed a strong direct correlation with shrinkage (R(2) = 0.982) and inverse with elastic modulus (R(2) = 0.966). Significance. High inorganic contents were associated with low polymerization stress values, which can be explained by the reduced volumetric shrinkage presented by heavily filled composites. (C) 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Influence of the bonding substrate in dental composite polymerization stress testing

Our objective was to compare the polymerization stress (sigma(pol)) of a series of composites obtained using poly(methyl methacrylate) (PMMA) or glass as bonding substrates, and to compare the results with those from in vitro microleakage of composite restorations. The tested hypothesis was that stress values obtained in a less rigid testing system (i.e. using PMMA) would show a better relationship with microleakage data. Five dental composites were tested: Filtek Z250 (FZ), Z100 (Z1), Concept (CO), Durafill (DU) and Heliomolar (HM). sigma(pol) was determined in 1 mm high specimens inserted between two rods (empty set = 5 mm) of either PMMA or glass. The composite elastic modulus (E) was obtained by three-point bending. sigma(pol) and E data were submitted to a one-way analysis of variance/Tukey test (alpha = 0.05). For the microleakage test (MI), bovine incisors received cylindrical cavities (empty set = 5 mm, h = 2 mm), which were restored in bulk. After storage for 24 h in water, specimens were subjected to dye penetration using AgNO(3) as tracer. Specimens were sectioned twice, perpendicularly, and microleakage was measured (in millimeters) under 20x magnification. Data from MI were submitted to the Kruskal-Wallis test. Means (SD) of sigma(pol) (MPa) using glass/PMMA were FZ: 7.5(1.8)(A)/2.5(0.2)(bc); Z1: 7.3(0.5)(A)/2.8(0.3)(ab); CO: 6.8(1.1)(A)/3.2(0.5)(a); DU: 4.5(0.7)(B)/2.0(0.2)(bc); HM: 3.5(0.2)(B)/2.3(0.3)(c). sigma(pol) obtained using PMMA rods were 34-67% lower than with glass. Means (SD) for tooth average/tooth maximum microleakage were FZ: 0.92(0.19)(B)/1.53(0.30)(a); Z1: 1.19(0.21)(A)/1.75(0.20)(a); CO: 1.26(0.25)(A)/1.78(0.24)(a); DU: 0.83(0.30)(B)/1.68(0.46)(a): HM: 0.81(0.27)(B)/1.64(0.54)(a). The tested hypothesis was confirmed, as the composites showed the same ordering both in the polymerization stress test using PMMA rods and in the microleakage test. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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.

Tailoring of physical properties in highly filled experimental nanohybrid resin composites

Objectives. To assess the elastic modulus (EM), volumetric shrinkage (VS), and polymerization shrinkage stress (PSS) of experimental highly filled nanohybrid composites as a function of matrix composition, filler distribution, and density. Methods. One regular viscosity nanohybrid composite (Grandio, VOCO, Germany) and one flowable nanohybrid composite (Grandio Flow, VOCO) were tested as references along with six highly filled experimental nanohybrid composites (four Bis-GMA-based, one UDMA-based, and one Ormocer (R) -based). The experimental composites varied in filler size and density. EM values were obtained from the ""three-point bending"" load-displacement curve. VS was calculated with Archimedes` buoyancy principle. PSS was determined in 1-mm thick specimens placed between two (poly) methyl methacrylate rods (empty set = 6 mm) attached to an universal testing machine. Data were analyzed using oneway ANOVA, Tukey`s test (alpha = 0.05), and linear regression analyses. Results. The flowable composite exhibited the highest VS and PSS but lowest EM. The PSS was significantly lower with Ormocer. The EM was significantly higher among experimental composites with highest filler levels. No significant differences were found between all other experimental composites regarding VS and PSS. Filler density and size did not influence EM, VS, or PSS. Significance. Neither the filler configuration nor matrix composition in the investigated materials significantly influenced composite shrinkage and mechanical properties. The highest filled experimental composite seemed to increase EM by keeping VS and PSS low; however, matrix composition seemed to be the determinant factor for shrinkage and stress development. The Ormocer, with reduced PSS, deserves further investigation. Filler size and density did not influence the tested parameters. (C) 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Finite Element Analysis of Shear Versus Torsion Adhesive Strength Tests for Dental Resin Composites

Stress distributions in torsion and wire-loop shear tests were compared using three-dimensional (3-D) linear-elastic finite element method, in an attempt to predict the ideal conditions for testing adhesive strength of dental resin composites to dentin. The torsion test presented lower variability in stress concentration at the adhesive interface with changes in the proportion adhesive thickness/resin composite diameter, as well as lower variability with changes in the resin composite elastic modulus. Moreover, the torsion test eliminated variability from changes in loading distance, and reduced the cohesive fracture tendency in the dentin. The torsion test seems to be more appropriate than wire-loop shear test for testing the resin composite-tooth interface strength. (c) Koninklijke Brill NV, Leiden, 2009

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 (sigma(exp)) 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 (sigma(y)) and transverse (sigma(x)) axes of models representing the composites with the highest and lowest E (FZ and HM, respectively) were evaluated by finite element analysis (FEA). Results. sigma(exp) 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 or sigma(exp) 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 (r(2) = 0.946). FEA revealed that system compliance was influenced by the composite (FZ led to higher compliance than HM). sigma(x) distribution was similar in both substrates, while cry distribution showed larger areas of compressive stresses in specimens built on PMMA. Significance. sigma(exp) determined in PMMA was 53-68% lower than in glass. Composite ranking varied slightly due to differences in substrates` longitudinal and transverse deformation. (c) 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

BisGMA/TEGDMA ratio and filler content effects on shrinkage stress

Objective. To investigate the contributions of BisGMA:TEGDMA and filler content on polymerization stress, along with the influence of variables associated with stress development, namely, degree of conversion, reaction rate, shrinkage, elastic modulus and loss tangent for a series of experimental dental composites. Methods. Twenty formulations with BisGMA: TEGDMA ratios of 3: 7, 4: 6, 5: 5, 6: 4 and 7: 3 and barium glass filler levels of 40, 50, 60 or 70 wt% were studied. Polymerization stress was determined in a tensilometer, inserting the composite between acrylic rods fixed to clamps of a universal test machine and dividing the maximum load recorded by the rods cross-sectional area. Conversion and reaction rate were determined by infra-red spectroscopy. Shrinkage was measured by mercury dilatometer. Modulus was obtained by three-point bending. Loss tangent was determined by dynamic nanoindentation. Regression analyses were performed to estimate the effect of organic and inorganic contents on each studied variable, while a stepwise forward regression identified significant variables for polymerization stress. Results. All variables showed dependence on inorganic concentration and monomeric content. The resin matrix showed a stronger influence on polymerization stress, conversion and reaction rate, whereas filler fraction showed a stronger influence on shrinkage, modulus and loss tangent. Shrinkage and conversion were significantly related to polymerization stress. Significance. Both the inorganic filler concentration and monomeric content affect polymerization stress, but the stronger influence of the resin matrix suggests that it may be possible to reduce stress by modifying resin composition without sacrificing filler content. The main challenge is to develop formulations with low shrinkage without sacrificing degree of conversion. (C) 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Effect of prolonged erosive pH cycling on different restorative materials

This in vitro study evaluated the effect of a prolonged erosive pH cycling on the superficial microhardness change (SMHC) and the erosive wear of different restorative materials. Eighty enamel specimens with prepared cavities of 1.5 x 1.5 mm were randomly divided into eight groups according to the restorative materials used for the fillings (RMGI - resin-modified glass-ionomer, CGI - conventional glass-ionomer, CR- composite resin, A - amalgam) and immersion media used (ERO - erosive medium or SAL - artificial saliva). During 35 days, half of the specimens were immersed in a cola drink (ERO), for 5 min, three times a day, and they remained in SAL between the erosive cycles. The other half of the specimens was immersed in SAL only, for the entire experimental period (control). Data were tested for significant differences by anova and Tukey`s tests (P < 0.05). Scanning electron microscopy images were made to illustrate the enamel erosive wear and restorative materials alterations. The mean SMHC (%) and mean erosive wear (mu m) of the materials were: RMGI-ERO (30/0.5); CGI-ERO (37/0.5); CR-ERO (-0.3/0.3); A-ERO (-4/0.3); RMGI-SAL (4/0.4); CGI-SAL (-6/0.4); CR-SAL (-3/0.2) and A-SAL (2/0.4). Scanning electron microscopy images showed pronounced enamel erosive wear on groups submitted to erosive pH cycling when compared with groups maintained in saliva. In conclusion, the prolonged pH cycling promoted significantly higher alterations (SMHC and erosive wear) on the glass-ionomer cements than the CR and amalgam.

Effect of erosive pH cycling on different restorative materials and on enamel restored with these materials

This in vitro study evaluated the effect of erosive pH cycling on the percentage of surface micro-hardness change (%SMHC) and wear of different restorative materials and bovine enamel restored with these materials. Eighty enamel specimens were randomly divided into eight groups according to the restorative materials and immersion media used: GI/GV-resin-modifled glass-ionomer, GII/GVI-conventional glass-ionomer, GIII/GVII-resin composite and GIV/GVIII-amalgam. Over a period of seven days, groups GI to GIV were immersed in a cola drink (ERO) for 5 minutes, 3x/day and kept in artificial saliva between erosive cycles. Groups GV to GVIII were immersed in artificial saliva (SAL) throughout the entire experimental period (control). Data were tested for significant differences using ANOVA and Tukey`s tests (p < 0.05). For %SMHC, considering the restorative materials, no significant differences were detected among the materials and immersion media. Mean wear was higher for the resin modified glass ionomer cement when compared to conventional cement, but those materials did not significantly differ from the others. For enamel analyses, erosive pH cycling promoted higher wear and %SMHC compared to saliva. There were no significant differences in wear and %SMHC of enamel around the different restorative materials, regardless of the distance from the restorative material (50, 150 or 300 mu m). In conclusion, there were only subtle differences among the materials, and these differences were not able to protect the surrounding enamel from erosion.

Comparison of GPX with or without solvent and hand files in removing filling materials from root canals-An ex vivo study

Objective. The aim of this study was to compare GPX instruments and hand files for gutta-percha removal. Study design. Fifty maxillary central incisors with a single straight canal were instrumented and filled. The teeth were divided into 5 groups of 10 specimens each, according to the gutta-percha removal techniques: group 1: GPX (21-mm-long teeth); group 2: GPX and xylol as solvent (21-mm-long teeth); group 3: GPX (25-mm-long teeth); group 4: GPX and xylol as solvent (25-mm-long teeth); and group 5: hand files and xylol as solvent. The amount of time for gutta-percha removal and the number of fractured instruments were evaluated. Radiographs were taken, and the teeth were grooved longitudinally and split. The area of residual debris was measured using Sigma Scan software. Results. The time for filling material removal was significantly shorter when GPX was used (P < .05). Overall, hand files and solvent produced fewer remnants of filling materials (P < .05). In the GPX 25 mm-long teeth group, the filling material was not removed in the apical third. Conclusions. Under the experimental conditions, the GPX instruments proved to be faster than hand instruments in removing root filling materials; however, hand instruments left a smaller amount of residual filling materials on the canal walls. The GPX instruments did not pull the gutta-percha beyond its tip. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010; 110: 675-680)

Evaluation of the physical and chemical properties of two commercial and three experimental root-end filling materials

Objective. The aim of this study was to evaluate the pH, calcium release, setting time, and solubility of two commercially available mineral trioxide aggregate (MTA) cements (white MTA Angelus and MTA Bio), and of three experimental cements (light-cured MTA, Portland cement with 20% bismuth oxide and 5% calcium sulfate, and an epoxy resin-based cement). Study design. For evaluation of pH and calcium ion release, polyethylene tubes with 1.0 mm internal diameter and 10.0 mm length were filled with the cements and immediately immersed in flasks containing 10 mL deionized water. After 3, 24, 72, and 168 hours, the tubes were removed and the water from the previous container was measured for its pH and calcium content with a pH meter and an atomic absorption spectrophotometer. For analysis of the setting time, Gilmore needles weighing 100 g and 456.5 g were used, in accordance with the American Society for Testing and Materials specification no. C266-03. Solubility of each cement was also tested. Results. All the cements were alkaline and released calcium ions, with a declining trend over time. After 3 hours, Portland cement + bismuth oxide and MTA Bio had the highest pH and light-cured MTA the lowest. After 1 week, MTA Bio had the highest pH and light-cured MTA and epoxy resin-based cement the lowest. Regarding calcium ion release, after 3 hours, Portland cement + bismuth oxide showed the highest release. After 1 week, MTA Bio had the highest. Epoxy resin-based cement and light-cured MTA had the lowest calcium release in all evaluation periods. Regarding setting times, white MTA Angelus and MTA Bio had the shortest, Portland cement + bismuth oxide had an intermediate setting time, and the epoxy resin-based cement had the longest. The materials that showed the lowest solubility values were the epoxy resin-based cement, Portland cement + bismuth oxide, and light-cured MTA. The highest solubility values were presented in white MTA Angelus and MTA Bio. Conclusions. The white MTA Angelus and MTA Bio had the shortest setting times, higher pH and calcium ion release, and the highest solubility. In contrast, the epoxy resin-based cement and light-cured MTA showed lower values of solubility, pH, and calcium ion release. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010; 110: 250-256)