Polymerization Shrinkage Of Dental Composites Research Articles

Measuring polymerization shrinkage of dental composite by using PhS-OCT

A method based on phase-sensitive optical coherence tomography (PhS-OCT) was developed to investigate the depth-resolved polymerization shrinkage of dental composites. The method uses optical coherence tomography (OCT) system to acquire the interference spectra during dental composite polymerization and then estimate the refractive index variation and shrinkage strain from the phase variations of the acquired spectrum. Since OCT is a tomographic imaging technique, PhS-OCT is also available for imaging full-field polymerization behavior inside dental composites. For validation, the polymerization processes of three dental composites, FiltekTM Z350, FiltekTM Z250 and FiltekTM P60, were monitored with PhS-OCT. The strain results agree with the reference values and the relative errors are less than 10%. In addition, the vitrification-point delayed curves were measured, revealing the vitrification-point retardation phenomenon. Finally, the flowable dental composite that polymerized inside the human artificial tooth was also measured, successfully demonstrating the depth-resolved full-field polymerization behavior inside the dental composite.

Improved performance of Bis-GMA dental composites reinforced with surface-modified PAN nanofibers

In the present work, polyacrylonitrile (PAN) nanofibers reinforced dental composites were investigated to achieve the improved interfacial adhesion between the PAN nanofiber and resin matrix using surface modification of nanofibers. PAN nanofibers mat were prepared by electrospinning and then, surface treated with the activated bisphenol A glycidyl methacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) (50/50 mass ratio) dental resin followed by photo-curing. Also, the treated nanofibers mat was milled into a powder to achieve the uniform distribution of nanofibers in the matrix resin. The reinforced dental composite were prepared by mixing the various mass fraction of the powder (0.5–15 wt%) with the Bis-GMA/TEGDMA dental monomers. The effect of weight ratio of surface-modified nanofibers to blend resin on the chemical structure, morphology, compression and flexural properties, color and polymerization shrinkage of dental composites was evaluated. The results showed that using surface-treated nanofibers with content of 5 wt% enhanced the compression strength, flexural strength, flexural modulus and work of rupture of the resultant dental composite by factors of 23%, 7%, 80%, and 145%, respectively, comparing to the unreinforced neat resin. Also, the polymerization shrinkage reduces by 37%. These significant improved properties of the dental composite could be due to the semi-interpenetration network formation between surface-modified nanofibers and resin matrix and well distribution of nanofibers in the dental resin. Further increasing the nanofiber content led to poor mechanical properties of obtained dental composites. The results also, revealed that the color of resin composite could be whiter using modified PAN nanofibers as the filler.

Evaluation of polymerization shrinkage of dental composites by microcomputed tomography

Objective: The aim of this study was to evaluate the polymerization shrinkage of different commercially available dental composites with micro-computed tomography (μ-CT). Method: The study group included eight different flowable composites; Surefil SDR Flow (SDR), Charisma Flow (CHF), Clearfil Majesty Flow (CMF), Vertise Flow (VF), Grandio Flow (GF), Filtek Supreme Ultimate Flow (3MEFU), Filtek Bulk Flow (3MBF), X-Tra Base Flow (XTB). Composite materials were placed in standard cylindrical teflon molds and these were scanned for 1 h using μ-CT Skyscan 1172. Polymerization was not observed during scanning since the internal chamber of the equipment was completely dark. When the scanning was complete, materials were polymerized with LED light as per producer recommendations and rescanned with Skyscan 1172 using the same parameters. After the scanning process was over, test materials were analyzed using μ-CT-CTAn software programme. Results: Data were analyzed using Kruskal Wallis, Mann Whitney U and Wilcoxon test at a significance level of α=0.05. Polymerization shrinkage ranged between 1.44% (CMF) and 2.73% (CHF). Bulk-fill and Self-adhesive composites were significantly lower (p<0.05) than those of the others. Conclusion: All tested materials were able to achieve acceptable shrinkage at 2-4 mm depth. The higher shrinkage of hybrid composites over that of other groups may indicate a potential for higher interfacial stresses. However, the bulk fill composite showed low shrinkage that may prove less damaging to the interface.

Evaluation of Difunctional Vinylcyclopropanes as Reactive Diluents for the Development of Low‐Shrinkage Composites

Polymerization shrinkage of dental composites remains a major concern. Free‐radically polymerizable cyclic monomers can be a conceivable alternative to methacrylates for the development of low‐shrinkage composites. In this study, the one‐step synthesis of the novel low viscosity difunctional vinylcyclopropanes 1–4 is described. Photopolymerization kinetics of these monomers are investigated by photo‐differential scanning calorimeter, using bis(4‐methoxybenzoyl)diethylgermane as photoinitiator. Real‐time near‐infrared photorheology measurements are performed to evaluate rheological behavior (i.e., time of gelation, polymerization‐induced shrinkage force) and chemical conversion (i.e., double bond conversion at the gel point, final double bond conversion) of the vinylcyclopropanes in situ. The potential of these monomers as reactive diluents in dental restorative materials is evaluated. Composites based on vinycyclopropanes 1–4 show good mechanical properties and exhibit significantly lower volumetric shrinkage and shrinkage stress than corresponding dimethacrylate‐based materials. The results indicate that such monomers are promising candidates for the replacement of commonly used low viscosity dimethacrylates such as triethylene glycol dimethacrylate in dental composites. image

Simple optical method for measuring free shrinkage

ObjectivesA simple optical method for measuring polymerization shrinkage of dental composites is compared with an established dilatometer. MethodsFive restorative composites were used to test the methods: Filtek Supreme Ultra (3M ESPE), Filtek LS (3M ESPE), Premise (Kerr), Gradia Direct (GC), and GC Kalore (GC). Uncured composites were attached to sandblasted silane-treated glass slides. The slides were placed sample side inside a mercury-filled dilatometer (ADAF). The mercury levels were recorded as the materials were light-cured through the glass-slides (40s). Mercury levels, which correlated with volumetric shrinkage, were recorded for 60min (N=6). For the optical method, uncured composite was placed on a smooth silicone platform. A pre-polymerization image was captured under a stereomicroscope, and the specimen was light-cured (40s). Post-polymerization images were captured at 2, 10, 60, and 90min (N=10). Composite outlines were traced to obtain projected surface areas (ImageJ) and volumetric shrinkage was calculated. Results were analyzed using two-way ANOVA (α=0.05) and Pearson Correlation tests. Shrinkage deformation for both methods was modeled using finite element analysis. ResultsVolumetric shrinkage at 60min ranged between 1.24% and 2.24% for dilatometer and 1.35–2.68% for optical methods. Optical method shrinkage was consistently higher than the dilatometer (P=.0001), but the ranking of the composites was the same (Pearson Correlation Coefficient 0.9997). Finite element analysis showed that lower shrinkage values of the dilatometer method could be attributed to bonding of its samples. SignificanceThe optical method using a general-purpose stereomicroscope and public-domain software is a simple and accurate alternative to measure free shrinkage.

Evaluation of polymerization shrinkage of dental composites by an optical method.

This study proposes an alternative methodology for evaluating polymerization shrinkage of dental composites using an advanced video extensometer (AVE) system. This equipment measures the displacement between two points drawn on a tooth's wall without requiring physical contact with the tooth. By doing so, the polymerization process was monitored by the cusp deflection. This technique was used in human and bovine teeth, where the cavities were prepared under controlled conditions so that the volume of the composite used was the same in both types of teeth. After the cavity preparation, the specimens were acid etched, washed and dried, and then the adhesive was applied and polymerized. The composite was then inserted into the cavity. Polymerization was performed with two different light polymerizing units (LD Max and Optilight Max - Gnatus do Brasil), and the displacement curve of the tooth cusp was recorded for a period of 400 s. After a statistical analysis, it was concluded that the technique was capable of evaluating shrinkage by the deflection from the cusps and that the human and bovine teeth do not react in a similar manner towards the polymerization shrinkage of composites.

Analysis of Composite Shrinkage Stresses on 3D Premolar Models with Different Cavity Design Using Finite Element Method

Local polymerization stress occurs due to polymerization shrinkage of resin based composites adhesively bonded to tooth tissues. Shrinkage causes local displacements of cavity walls, with possible occurrence of micro-cracks in the enamel, dentin and/or material itself. In order to design a cavity for experimental testing of polymerization shrinkage of dental composites using 3D optical analysis, in this paper finite element method (FEM) was used to analyze numerical models with different cavity radiuses. 3D optical strain and displacement analysis of composite materials and cavity walls is limited by equipment sensitivity i.e. 0.01% for strain and 1 micron for displacement. This paper presents the development of 3D computer premolar models with varying cavity radiuses, and local stress, strain and displacement analysis using FEM. Model verification was performed by comparing obtained results with data from the scientific literature. Using the FEM analysis of local strains, displacements and stresses exerted on cavity walls, it was concluded that the model with 1 mm radius was optimal for experimental optical 3D displacement analysis.

Application of digital image correlation to full-field measurement of shrinkage strain of dental composites

Objectives: Polymerization shrinkage of dental composites remains a major concern in restorative dentistry because it can lead to micro-cracking of the tooth and debonding at the tooth-restoration interface. The aim of this study was to measure the full-field polymerization shrinkage of dental composites using the optical digital image correlation (DIC) method and to evaluate how the measurement is influenced by the factors in experiment setup and image analysis. Methods: Four commercial dental composites, Premise Dentine, Z100, Z250 and Tetric EvoCeram, were tested. Composite was first placed into a slot mould to form a bar specimen with rectangular-section of 4 mm×2 mm, followed by the surface painting to create irregular speckles. Curing was then applied at one end of the specimen while the other part were covered against curing light for simulating the clinical curing condition of composite in dental cavity. The painted surface was recorded by a charge-coupled device (CCD) camera before and after curing. Subsequently, the volumetric shrinkage of the specimen was calculated with specialist DIC software based on image cross correlation. In addition, a few factors that may influence the measuring accuracy, including the subset window size, speckle size, illumination light and specimen length, were also evaluated. Results: The volumetric shrinkage of the specimen generally decreases with increasing distance from the irradiated surface with a conspicuous exception being the composite Premise Dentine as its maximum shrinkage occurred at a subsurface distance of about 1 mm instead of the irradiated surface. Z100 had the greatest maximum shrinkage strain, followed by Z250, Tetric EvoCeram and then Premise Dentine. Larger subset window size made the shrinkage strain contour smoother. But the cost was that some details in the heterogeneity of the material were lost. Very small subset window size resulted in a lot of noise in the data, making it difficult to discern the general pattern in the strain distribution. Speckle size did not seem to have obvious effect on the volumetric shrinkage strain along specimen length; however, larger speckles resulted in higher level of noise or heterogeneity in the shrinkage distribution. Compared with bright illumination, dimmer lighting produced larger standard deviations in the measured shrinkage, indicating a higher level of noise. The longer the specimen, the greater was the rate of reduction with distance from the irradiated surface, especially for the longitudinal strain. Significance: The image correlation method is capable of producing full-field polymerization shrinkage of dental composites. The accuracy of the measurements relies on selection of optimal parameters in experimental setup and DIC analysis.

Dimensional change measurements of conventional and flowable composite resins using a laser displacement sensor

The polymerization shrinkage values of commercial flowable and conventional composites were measured with a newly developed laser displacement sensor. Compressive strength and filler content were also measured. Compared to conventional composites, the flowable composites required a longer time - not exceeding 4 minutes - to reach a constant polymerization shrinkage rate. In terms of shrinkage ratio, it was less than 1.5% for conventional composites at 90 minutes, which was lower than that of flowable composites, i.e. more than 2.0%. The compressive strength of conventional composites was higher than that of flowable composites. A highly significant correlation between filler content and polymerization shrinkage at 90 minutes was observed, and polymerization shrinkage tended to decrease as filler content increased. In light of the results obtained in this study, the newly developed displacement meter with a laser displacement sensor proved to be effective for in-depth investigation of the polymerization shrinkage of dental composites.

Measurement of the full-field polymerization shrinkage and depth of cure of dental composites using digital image correlation

Objectives The aim of this study was to measure the full-field polymerization shrinkage of dental composites using optical image correlation method. Methods Bar specimens of cross-section 4 mm × 2 mm and length 10 mm approximately were light cured with two irradiances, 450 mW/cm 2 and 180 mW/cm 2, respectively. The curing light was generated with Optilux 501 (Kerr) and the two different irradiances were achieved by adjusting the distance between the light tip and the specimen. A single-camera 2D measuring system was used to record the deformation of the composite specimen for 30 min at a frequency of 0.1 Hz. The specimen surface under observation was sprayed with paint to produce sufficient contrast to allow tracking of individual points on the surface. The curing light was applied to one end of the specimen for 40 s during which the painted surface was fully covered. After curing, the cover was removed immediately so that deformation of the painted surface could be recorded by the camera. The images were then analyzed with specialist software and the volumetric shrinkage determined along the beam length. Results A typical shrinkage strain field obtained on a specimen surface was highly non-uniform, even at positions of constant distance from the irradiation surface, indicating possible heterogeneity in material composition and shrinkage behavior in the composite. The maximum volumetric shrinkage strain of ∼1.5% occurred at a subsurface distance of about 1 mm, instead of at the irradiation surface. After reaching its peak value, the shrinkage strain then gradually decreased with increasing distance along the beam length, before leveling off to a value of approximately 0.2% at a distance of 4–5 mm. The maximum volumetric shrinkage obtained agreed well with the value of 1.6% reported by the manufacturer for the composite examined in this work. Using irradiance of 180 mW/cm 2 resulted in only slightly less polymerization shrinkage than using irradiance of 450 mW/cm 2. Significance Compared to the other measurement methods, the image correlation method is capable of producing full-field information about the polymerization shrinkage behavior of dental composites.

A simple method for the measurement of polymerization shrinkage in dental composites

Objective: In this study a simple non-contact method was developed to measure the polymerization shrinkage of dental composites. Methods: A gas pycnometer was used to determine the volumes of specimens prior to and after photopolymerization and from which the total volumetric shrinkage could be determined. Results: Four commercial composites were studied and were found to have polymerization shrinkages varying from 1.6 to 2.5%. The method was found to be labour efficient and produced reproducible results with a standard deviation of approximately 10%. Significance: This method is appropriate for shrinkage measurements where only the total amount shrinkage is required and in particular for the measurement of shrinkage of photocured materials which are sensitive to water absorption.