Polymerization Contraction Research Articles

Contraction stresses in dental composites adjacent to and at the bonded interface as measured by crack analysis

The objective of this study was to calculate stresses produced by polymerization contraction in regions surrounding a dental resin composite restoration. Initial cracks were made with a Vickers indenter at various distances from the edge of a cylindrical hole in a soda-lime glass disk. Indentation crack lengths were measured parallel to tangents to the hole edge. Resin composites (three brands) were placed in the hole and polymerized (two light irradiation protocols) at equal radiation exposures. The crack lengths were re-measured at 2 and 10 min after irradiation. Radial tensile stresses due to polymerization contraction at the location of the cracks ( σ crack) were calculated from the incremental crack lengths and the fracture toughness K c of the glass. Contraction stresses at the composite–glass bonded interface ( σ interface) were calculated from σ crack on the basis of the simple mechanics of an internally pressurized thick-walled cylinder. The greater the distance or the shorter the time following polymerization, the smaller was σ crack. Distance, material, irradiation protocol and time significantly affected σ crack. Two-step irradiation resulted in a significant reduction in the magnitude of σ interface for all resin composites. The contraction stress in soda-lime glass propagated indentation cracks at various distances from the cavity, enabling calculation of the contraction stresses.

Formation of gaps at the filler–resin interface induced by polymerization contraction stress: Gaps at the interface

Objectives To investigate the movement of resin matrix with respect to the filler particles of filled composites during their photo cure without or with polymerization contraction stress (PCS). Methods Two types of composites were prepared. Glass beads as macroscopic fillers were placed into the center of a bis-GMA/TEGDMA resin to make single bead-embedding “composites” and a variety of fillers of different compositions, sizes, and shapes were mixed with another bis-GMA/TEGDMA resin to make lightly filled composites. They were photo cured in a cavity constructed with an acrylic or aluminum ring sitting on a polyester strip. Bonding to the ring constrained the polymerization shrinkage and thus produced a PCS. The formation of gaps between the filler and the resin was detected by optical microscopy for the glass bead–resin systems, and by light attenuation and scanning electron microscopy (SEM) for the filler–resin composites. Results In general, for composites with untreated fillers, the optical microscopy and SEM revealed gaps at the filler–resin interface only when they were cured under constrained shrinkage conditions. These composites attenuated more light when cured under constrained shrinkage conditions than when under non-constrained conditions. For the composites with silane-treated fillers, no gaps were observed. Some did not show any significant difference in light attenuation when cured under either constrained or non-constrained conditions. Conclusions The resin tends to move away from the filler particles under the influence of PCS. Strengthening the filler–resin interaction, such as by the use of silane-treated filler, may help prevent the resin departure and thus the formation of gaps.

Hyperbranched polymers: Their use in dental resins to reduce polymerization contraction

Hyperbranched polymers: Their use in dental resins to reduce polymerization contraction

Surface texture and roughness of polished nanofill and nanohybrid resin composites

Nanofiller-containing resin composites have gained appreciable market share in dentistry due to their claims of high mechanical strength and low polymerization contraction. In this study, the polishability of one nanofill (Filtek Supreme XT/FIL) and three nanohybrid materials (Grandio/GRA, Tetric EvoCeram/TET, Venus Diamond/VED) was investigated using surface profilometry and SEM. After the specimens were cured under a Mylar strip or pre-ground with 600-grit SiC paper, three polishing systems were applied and their polishing effects compared: diamond polishing points, a diamond paste, and urethane-backed aluminum oxide disks. Except for the profilometry results obtained by glass filler-containing GRA and VED with one polishing system that comprised the consecutive application of diamond particles and a diamond polishing paste, the final roughness (Ra) of all other specimens were lower than the clinically acceptable 0.2 microm threshold. The surface textures of the polished nanofill FIL and nanohybrid TET were uniformly smooth, whereas relief polishing effects and filler extrusion of varying extents were seen on the nanohybrid composites GRA and VED.

Polymerization Contraction Stress of Resin Composite

Polymerization Contraction Stress of Resin Composite

Polymerization composite shrinkage evaluation with 3D deformation analysis from μCT images

Objectives The aim of this study was to develop a method to experimentally determine and visualize the direction and amount of polymerization shrinkage. Methods We modified a composite to include 1.5 wt% traceable glass beads. A cylindrical cavity (6 mm diameter, 3 mm height) was restored with this traceable composite, with and without dentin adhesive, and digitized with high-resolution micro-computed tomography (μCT). Image segmentation was performed to extract the glass beads from the acquired 3D μCT images (uncured and cured). Afterwards, each glass bead was subjected to local rigid registration. The resulting displacement vectors were used to examine and calculate the changes. Results In unbonded restorations, the displacement vectors were oriented inwards to the center of mass, although not perfectly. Bonded restorations exhibited two contraction patterns: either toward one side of the cavity or toward the top-surface of the restoration. The displacement vector length values (mean/SD) for the bonded group (46.8 μm/10.0 μm) was significantly higher ( p < 0.01) than unbonded group (31.3 μm/8.5 μm), and the histogram curve was flatter (skew/kurtosis: 0.10/−0.56) as compared to the unbonded group (skew/kurtosis: 0.03/−0.26). Significance The proposed method can visualize real 3D displacement vectors generated by polymerization shrinkage. The bonding quality and cavity geometry are critical for the direction of polymerization contraction. This method has the potential to validate current models concerning the amount and orientation of shrinkage vectors.

Influence of different light sources and photo-activation methods on degree of conversion and polymerization shrinkage of a nanocomposite resin

The purpose of this study was to evaluate the influence of different light sources and photo-activation methods on degree of conversion (DC%) and polymerization shrinkage (PS) of a nanocomposite resin (Filtek™ Supreme XT, 3M/ESPE). Two light-curing units (LCUs), one halogen-lamp (QTH) and one light-emitting-diode (LED), and two different photo-activation methods (continuous and gradual) were investigated in this study. The specimens were divided in four groups: group 1—power density (PD) of 570 mW/cm2 for 20 s (QTH); group 2—PD 0 at 570 mW/cm2 for 10 s + 10 s at 570 mW/cm2 (QTH); group 3—PD 860 mW/cm2 for 20 s (LED), and group 4-PD 125 mW/cm2 for 10 s + 10 s at 860 mW/cm2 (LED). A testing machine EMIC with rectangular steel bases (6 × 1 × 2 mm) was used to record the polymerization shrinkage forces (MPa) for a period that started with the photo-activation and ended after two minutes of measurement. For each group, ten repetitions (n = 40) were performed. For DC% measurements, five specimens (n = 20) for each group were made in a metallic mold (2 mm thickness and 4 mm diameter, ISO 4049) and them pulverized, pressed with bromide potassium (KBr) and analyzed with FT-IR spectroscopy. The data of PS were analyzed by Analysis of Variance (ANOVA) with Welch’s correction and Tamhane’s test. The PS means (MPa) were: 0.60 (G1); 0.47 (G2); 0.52 (G3) and 0.45 (G4), showing significant differences between two photo-activation methods, regardless of the light source used. The continuous method provided the highest values for PS. The data of DC% were analyzed by Analysis of Variance (ANOVA) and shows significant differences for QTH LCUs, regardless of the photo-activation method used. The QTH provided the lowest values for DC%. The gradual method provides lower polymerization contraction, either with halogen lamp or LED. Degree of conversion (%) for continuous or gradual photo-activation method was influenced by the LCUs. Thus, the presented results suggest that gradual method photo-activation with LED LCU would suffice to ensure adequate degree of conversion and minimum polymerization shrinkage.

Holographic Measurement of a Tooth Model and Dental Composite Contraction

We have developed a real-time holographic technique to observe deformation induced by dental composite contraction. The standard split beam method was used, in conjunction with in situ holographic plate processing. Experiments were performed on a mechanical model of a human tooth with cavity. A silicone mold was used to manufacture a number of identical casts, using photoactivated composite. A LED lamp was used to induce photo-polymerization reaction in a composite. We have shown that the proposed method is ideal to analyze various polymerization strategies, with the purpose of recommending one which minimizes the polymerization contraction.

The Current Status of Materials for Posterior Composite Restorations: The Advent of Low Shrink

Polymerization contraction, and the stresses associated with this, have presented problems with resin composite materials, particularly when used to restore cavities in posterior teeth. This paper summarizes the problems associated with polymerization contraction and examines methods used to overcome this, in particular, by the use of materials which have reduced percentage contraction when compared with traditional materials. Use of a material with reduced polymerization contraction should lead to simpler restoration placement.

Effects of polymerization contraction on interface’s µTBS of luting material and dentin

Polymerization contraction of composite resin luting materials is known to produce high stresses in the interfaces being cemented that are described as perpendicular to them. This study describes the effect of shearing strains of curing luting materials on microtensile bond strength (microTBS) of interfaces. A flat surface of labial dentin of bovine incisors was exposed and teeth randomly assigned to A (n = 12) or B (n = 6) groups. Adoro rectangular (2 x 3 x 11 mm) restorative composite resin blocks were cemented (Excite DSC + Variolink II) completely (group A) or partially (group B, only on extremes and center) occupying luting space. After visible light curing, stick compound bars were sectioned perpendicular to interface and submitted to tension until detachment. microTBS decreased from the center to the extremes in group A (Spearman tests p < 0.0008) and not in group B, where microTBS was higher in extremes than in correspondent locations in group A and equivalent to that in group A in the central location. Weibull's analysis showed that m modulus and characteristic stresses also decreased from the center to periphery of restorations in group A. Mechanical resistance of bonded interface of a luting material and dentin decreases peripherally, and this reduction is caused by polymerization contraction.

Polymerization contraction stress in resin–tooth bonds under hydrated and dehydrated conditions

Objective This study hypothesizes that, with enamel or dentin as a bonding substrate, intrinsic water affects the development of polymerization contraction stress in the bonds of self-etching adhesives during bonding. Materials and methods The influence of the water content in dentin and enamel (wetness with water as control and acetone-dried specimens) on the stress development in self-etching adhesives was determined with a tensilometer. Thin layers of self-etching primer and/or adhesive resins were created between a glass plate and a flat enamel or dentin surface. Results After an initial maximum shortly after light curing for 30 min, the contraction stress was decreased in the dentin (30–70%) and enamel (approximately 20%). In the acetone-dried specimens, the stress was continuously increased for 20–50%. Significance The intrinsic water content of tooth tissue influences the initial polymerization of polymers. This effect is favorable for stress relief in resin restoration but causes unwanted nanoleakage channel formation in resin–tooth bonds.

Flowability of composites is no guarantee for contraction stress reduction

Objectives The purpose of this study was to measure the contraction stress development of three flowable resin-composite materials (Grandio Flow, VOCO GmbH, Cuxhaven, Germany; Tetric Flow, Ivoclar Vivadent, Schaan, Liechtenstein; Filtek Supreme XT Flowable Restorative, 3 M ESPE, ST. Paul, MN, USA) and an universal micro-hybrid composite resin (Filtek Z250, 3 M ESPE, St. Paul, MN, USA) during photopolymerization with a halogen curing light, using a novel stress-measuring gauge. Methods Curing shrinkage stress was measured using a stress-analyzer. Composites were polymerized with a halogen curing unit (VIP, Bisco Inc., Schaumburg, IL, USA) for 40 s. The contraction force (N) generated during polymerization was continuously recorded for 180 s after photo-initiation. Contraction stress (MPa) was calculated at 20 s, 40 s, 60 s, 120 s and 180 s. Data were statistically analyzed. Results Filtek Supreme XT Flowable Restorative exhibited the highest stress values compared to other materials ( p < 0.05), while the lowest values were recorded with Tetric Flow ( p < 0.05). Tetric Flow was also the only flowable composite showing stress values lower than the conventional composite Filtek Z250 ( p < 0.05). Significance Flowable composites investigated with this experimental setup showed shrinkage stress comparable to conventional resin restorative materials, thus supporting the hypothesis that the use of flowable materials do not lead to marked stress reduction and the risk of debonding at the adhesive interface as a result of polymerization contraction is similar for both type of materials.

Calculation of contraction stresses in dental composites by analysis of crack propagation in the matrix surrounding a cavity

Objectives Polymerization contraction of dental composite produces a stress field in the bonded surrounding substrate that may be capable of propagating cracks from pre-existing flaws. The objectives of this study were to assess the extent of crack propagation from flaws in the surrounding ceramic substrate caused by composite contraction stresses, and to propose a method to calculate the contraction stress in the ceramic using indentation fracture. Methods Initial cracks were introduced with a Vickers indenter near a cylindrical hole drilled into a glass-ceramic simulating enamel. Lengths of the radial indentation cracks were measured. Three composites having different contraction stresses were cured within the hole using one- or two-step light-activation methods and the crack lengths were measured. The contraction stress in the ceramic was calculated from the crack length and the fracture toughness of the glass-ceramic. Interfacial gaps between the composite and the ceramic were expressed as the ratio of the gap length to the hole perimeter, as well as the maximum gap width. Results All groups revealed crack propagation and the formation of contraction gaps. The calculated contraction stresses ranged from 4.2 MPa to 7.0 MPa. There was no correlation between the stress values and the contraction gaps. Significance This method for calculating the stresses produced by composites is a relatively simple technique requiring a conventional hardness tester. The method can investigate two clinical phenomena that may occur during the placement of composite restorations, i.e. simulated enamel cracking near the margins and the formation of contraction gaps.

Influence of thermal expansion on shrinkage during photopolymerization of dental resins based on bis-GMA/TEGDMA

Objective The aim of this study was to assess volume changes that occur during photopolymerization of unfilled dental resins based on bis-GMA-TEGDMA. Methods The resins were activated for visible light polymerization by the addition of camphorquinone (CQ) in combination with dimethylamino ethylmethacrylate (DMAEMA) or ethyl-4-dimethyl aminobenzoate (EDMAB). A fibre-optic sensing method based on a Fizeau-type interferometric scheme was employed for monitoring contraction during photopolymerization. Measurements were carried out on 10 mm diameter specimens of different thicknesses (1 and 2 mm). Results The high exothermic nature of the polymerization resulted in volume expansion during the heating, and this effect was more pronounced when the sample thickness increased. Two approaches to assess volume changes due to thermal effects are presented. Due to the difference in thermal expansion coefficients between the rubbery and glassy resins, the increase of volume due to thermal expansion was greater than the decrease in volume due to thermal contraction. As a result, the volume of the vitrified resins was greater than that calculated from polymerization contraction. The observed trends of shrinkage versus sample thickness are explained in terms of light attenuation across the path length during photopolymerization. Significance Results obtained in this research highlight the inherent interlinking of non-isothermal photopolymerization and volumetric changes in bulk polymerizing systems.

Effects of restoration and substrate on polymerization contraction stress of dental composites

The aim of this study was to treat both restoration and substrate as a combined factor (RS-factor) to complement the popular C-factor in prediction of polymerization contraction stress (PCS). A simple model consists of a uniaxial restoration with a curing composite sandwiched between two solid mountings (substrates). By using the equal stress principle and taking into account substrate deformation, a set of equations were developed and solved, resulting in a mathematical relationship between PCS and the size and stiffness of the substrate and the restoration. The strain gage method was used to experimentally assess the PCS of a light-cured composite encircled in an aluminum ring. Differently sized inserts made of cured composites and glass were placed in the center of the ring to control the thickness of the composite to be cured and created different RS factors as well as C-factors. According to the model, a restoration with a small RS-factor will produce a high PCS. The model also predicts that a restoration with a small C-factor will have a high PCS because of the compliance of substrates. These predictions were tentatively confirmed by the strain gage measurements. A higher PCS was detected when a smaller insert was used, which created a smaller RS-factor or C-factor, or when a glass insert was used instead of a less stiff composite insert, in which the former created a smaller RS-factor. The RS-factor may be a dominant factor in determining the PCS outcome in some special cases.

Video-controlled characterization of polymerization shrinkage in light-cured dental composites

This study investigated a procedure to measure the intrinsic polymerization contraction of dental resin-based materials. A CCD-camera interfaced with a computer through video-interface hardware measured the coordinates of the centers of gravity of two dot markers stuck on a resin composite cylinder before and after the irradiation. The distances of the centers of gravity, and hence the linear percentage shrinkage, were calculated. The values obtained were compared with those measured by the deflecting disk technique [D.C. Watts, A.J. Cash, Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development, Dent. Mater. 7 (1991) 281–287.], which assessed polymerization shrinkage through deflection of a glass disk measured by an linear vertical displacement transducer (LVDT). Three resin composites, light-cured for 40 s at 800 mW cm −2, were tested: Solitaire 2 [SOL2], Tetric Ceram [TECE] and Z100 [Z100]. The data were subjected to the Kruskal–Wallis test ( p < 0.05). Whatever the experimental procedure, the three resin composites did not present statistically different polymerization shrinkage, and data obtained with the video-controlled technique were statistically higher than with the deflecting disk method.

Polymerization contraction stress in dentin adhesives bonded to dentin and enamel

Objective In a previous study on of polymerization contraction stress determinations of adhesives bonded to dentin a continuous decline of stress was observed after the adhesives had been light-cured. The decline was ascribed to stress relief caused by diffusion into the adhesive layer of water and/or solvents, left in the impregnated dentin surface after drying and/or evaporation in the application procedure. The purpose of the present study was to test the hypothesis that the contraction stress of adhesives bonded to enamel will not decline after light-curing, based on the assumption that water and/or solvents are more efficiently removed from impregnated enamel surfaces in the drying and/or evaporation step. Materials and methods Contraction stress was determined in a tensilometer for three total-etching adhesives Scotchbond multi-purpose, Single bond and One-step plus and four self-etching adhesives Clearfil SE Bond, Clearfil Protect Bond, AdheSE, and Xeno III. The adhesives were placed in a thin layer between a glass plate and a flat dentin or enamel surface pre-treated with phosphoric acid or self-etching primer and light-cured under constrained conditions. Results All adhesives bonded to enamel showed a stress decline, but significantly less than for dentin with the exception of two self-etching adhesives. The greatest decline was found for the total-etching adhesive systems bonded to dentin. The presence of hydrophobic monomers in the adhesives had a significant influence on the decline. Significance The experiments indicate that fluids are withdrawn from the resin impregnated tooth structures, which may result in small defects in the tooth–resin interfaces.

Improving fiber adhesion by surface oxidation in carbon fiber reinforced bone cement

The mechanical superiority of carbon fiber reinforced PMMA containing additional apatite was shown previously. For further improvement these carbon fibers were now submitted to a superficial oxidation treatment by HNO3. A closer contact between the carbon fibers and PMMA and even trabeculae-like adhesions were detected by Scanning Electron Microscopy. The fatigue strength of the carbon fiber reinforced bone cement could be increased at 17% by this oxidation treatment. This increase, however, is less than that observed in the case of other fiber reinforced composites. Most likely this is caused by the pronounced polymerisation contraction of PMMA. Further improvement of the adhesion of the fibers to cement may be achieved by different oxidation techniques, further extraction of foreign substances or graft polymerization of the carbon fibers by PMMA or other polymers.

Transmission of composite polymerization contraction force through a flowable composite and a resin-modified glass ionomer cement

The purpose of this study was to evaluate the individual contraction force during polymerization of a composite resin (Z-250), a flowable composite (Filtek Flow, FF) and a resin-modified glass ionomer cement (Vitrebond, VB), and the transmission of Z-250 composite resin polymerization contraction force through different thicknesses of FF and VB. The experiment setup consisted of two identical parallel steel plates connected to a universal testing machine. One was fixed to a transversal base and the other to the equipment's cross head. The evaluated materials were inserted into a 1-mm space between the steel plates or between the inferior steel plate and a previously polymerized layer of an intermediate material (either FF or VB) adhered to the upper steel plate. The composite resin was light-cured with a halogen lamp with light intensity of 500 mW/cm2 for 60 s. A force/time graph was obtained for each sample for up to 120 s. Seven groups of 10 specimens each were evaluated: G1: Z-250; G2: FF; G3: VB; G4: Z-250 through a 0.5-mm layer of FF; G5: Z-250 through a 1-mm layer of FF; G6: Z-250 through a 0.5-mm of VB; G7: Z-250 through a 1-mm layer of VB. They were averaged and compared using one-way ANOVA and Tukey test at a = 0.05. The obtained contraction forces were: G1: 6.3N ± 0.2N; G2: 9.8 ± 0.2N; G3: 1.8 ± 0.2N; G4: 6.8N ± 0.2N; G5: 6.9N ± 0.3N; G6: 4.0N ± 0.4N and G7: 2.8N ± 0.4N. The use of VB as an intermediate layer promoted a significant decrease in polymerization contraction force values of the restorative system, regardless of material thickness. The use of FF as an intermediate layer promoted an increase in polymerization contraction force values with both material thicknesses.

Die Another Way

Epithelial cells depend for their survival on integrin-mediated signals generated by attachment to the extracellular matrix (ECM). Epithelial cells also form adherens junctions that are dependent on interactions between cadherin proteins on adjacent cells. Detachment of epithelial cells from the ECM triggers their apoptotic cell death in a process known as anoikis. Overholtzer et al . were studying small aggregates of human mammary epithelial cells (MCF10A) in suspension when they noticed that some cells were contained within large vacuoles in other cells. By labeling populations of cells with different fluorescent dyes and mixing the cells, the authors found that cell invasion occurred within a few hours of detachment from the ECM. Whereas cell invasion was not prevented by inhibitors of apoptosis, its occurrence was reduced by inhibitors of actin polymerization or myosin contraction. Inhibition of the guanosine triphosphatase Rho, which regulates actin and myosin, or of its downstream target Rho-associated kinase (ROCK) prevented epithelial cell internalization. Further experiments with differentially labeled and differentially treated cells showed that Rho and ROCK activity in the invading cells were necessary for cell invasion, which the authors refer to as entosis. An antibody against E-cadherin inhibited both the formation of MCF10A aggregates and entosis. Fluorescent microscopy showed that, whereas the invading cells did not contain activated caspase-3, a marker of apoptosis, many of the invading cells colocalized with host cell lysosomal markers and the protease cathepsin B. Finally, immunohistochemical analyses showed evidence of entosis in human primary and metastatic breast tumors. As White points out in commentary, entosis may be a mechanism by which epithelial cancers, characterized by unregulated proliferation of detached epithelial cells, may be suppressed. M. Overholtzer, A. A. Mailleux, G. Mouneimne, G. Normand, S. J. Schnitt, R. W. King, E. S. Cibas, J. S. Brugge, A nonapoptotic cell death process, entosis, that occurs by cell-in-cell invasion. Cell 131 , 966-979 (2007). [PubMed] E. White, Entosis: It’s a cell-eat-cell world. Cell 131 , 840-842 (2007). [PubMed]