Light-cured Composites Research Articles

Effect of filler content of light-cured composites on bond strength to bovine dentine

The bonding of light-cured composites to tooth tissues is known to be disturbed by polymerization shrinkage, and polymerization shrinkage is affected by the filler content of composites. This in vitro research examined the relationship between the filler content and bond strength to dentine of light-cured composites. Experimental light-cured composite systems with filler contents of 45, 55, 65 and 75 per cent by volume were used in both bond strength to dentine and shrinkage tests. The surfaces selected as substrates were a flat surface and a box-shaped cavity prepared in bovine dentine. The lowest bond strength was obtained with the 45 per cent filler centent composite in the box-shaped cavity. Bond strength increased with increasing filler content. Volumetric polymerization shrinkage decreased with increasing filler content. The polymerization shrinkage at 120 s after light curing was 5.24 per cent for the 45 per cent filler content system, 4.77 per cent for the 55 per cent, 2.14 per cent for the 65 per cent and 1.68 per cent for the 75 per cent. The correlation between bond strength and shrinkage was greater for the cavity than it was for the flat surface. This implies that bond strength in the box-shaped cavity may have been affected more by polymerization shrinkage than with the flat surface. The findings lend support to the view that filler content is one of the important factors influencing the physical properties of composites.

Bond strengths of five different ceramic brackets: an in vitro study

The purpose of this investigation was to compare the bond strengths to enamel obtained in shear/peel and tensile/peel modes of testing using five different ceramic brackets and two different light-cured composites. The site of failure was also examined. Shear/peel bond strength was significantly affected by adhesive type, with Heliosit having higher mean bond strengths than Prismafil. Bond strength was significantly affected by bracket type; the highest mean bond strength in both tests being attained using Transcend with Heliosit. All brackets resisted shear/peel forces better than tensile/peel forces. High bond strengths are associated with an increased incidence of bracket and enamel fractures with Transcend and Starfire.

The Relationship between Polymerization Shrinkage Measured by a Modified Dilatometer and the Inorganic Filler Content of Light-Cured Composites

The Relationship between Polymerization Shrinkage Measured by a Modified Dilatometer and the Inorganic Filler Content of Light-Cured Composites

Light-cured dental composites characterization by Electron Microscopy

The service performance of current dental composite materials, such as anterior and posterior restoratives and/or veneer cements, needs to be improved. As part of a comprehensive effort to find ways to improve such materials, we have launched a broad spectrum study of the physicochemical and mechanical properties of photopolymerizable or visible light cured (VLC) dental composites. The commercially available VLC materials being studied are shown in Table 1. A generic or neat resin VLC system is also being characterized by SEM and TEM, to more fully understand formulation variables and their effects on properties.At a recent dental research meeting, we reported on the differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) characterization of the materials in Table 1. It was shown by DSC and DMA that the materials are substantially undercured by commonly used VLC techniques. Post curing in an oral cavity or a dry environment at 37 to 50°C for 7 or more hours substantially enhances the cure of the materials.

Textbook of dental pharmacology and therapeutics

A reciprocating abrasion testing machine was used to measure the mutual abrasion of light-cured composites and clasps in vitro. Composite specimens were produced by packing the material into hemispherical wells in a polytetrafluoroethylene block, light-curing for 20 seconds and conditioning in distilled water for one week. The composites tested included ultrafine, hybrid and conventional types. Nine clasp types were studied including cast and wrought, round and half-round, cobalt-chromium, stainless steel and gold. The tests were carried out at 37 °C. The vertical overlap (undercut) of composite and clasp was set at 0.25mm. Ten specimens were subjected to 14,500 traverses in each test Abrasion of composite was measured as weight loss and reduction in thickness of the specimens, while abrasion of clasps was measured as weight loss. Abrasion of the composites was greatest for the conventional type and least for the ultrafines, but in all cases the changes were small and unlikely to impair the clinical performance of clasps during several years' usage. The abrasion of the clasps by conventional composite was considerable and would be expected to cause a significant reduction in retention clinically.

Colorimetric changes in composites resulting from visible-light-initiated polymerization

We evaluated three shades of nine light-cured composites to determine the colorimetric changes which occur as a result of the photo-polymerization reaction. A photo-electric tristimulus colorimeter was used to measure the color of a 0.5-mm-thick sample of composite on two different backgrounds before and after the polymerization process had been initiated. The results showed that each of the photo-initiated composites tested produced a visually significant change in color as a result of the polymerization reaction, regardless of the shade of the backing. In general, the light-cured composites produced a characteristic chromatic shift toward the blue region of color space, which resulted in a perceived decrease in yellow chroma. Therefore, direct shade selection of a resin composite which is more yellow or more chromatic than the tooth which is being restored is recommended to compensate for this characteristic immediate color shift.

Radiographic inhomogeneity of posterior composites

A radiographic technique has been devised for determining the structural inhomogeneity between successive specimens fabricated from any given dental composite. This has been applied to nineteen composites currently recommended for the restoration of Class I and Class II cavities. The inhomogeneity data fell within clearly defined bands, with some light-cured posterior composites exhibiting an inhomogeneity parameter of zero. Other hands had average inhomogeneities of 1.5%, 2.5%, 3.8% and 6.8%. The level of inhomogeneity exhibited by a composite system may be an important factor in material selection and in long-term clinical performance.

The use of visible light-curing composites in bonding ceramic brackets

Three hundred and fifty bovine teeth were bonded with a ceramic bracket using nine different materials. Five of the materials were composites used in conservative dentistry, two were light-curing orthodontic adhesive test materials, and two were chemically curing adhesives. Four of the materials were modified by dilution with resin. With the proper consistency, it was found that the use of primer was not necessary. Adequate strength was observed within a 20-second curing time. No statistical difference in bond strength was observed among the materials. It is concluded that light-curing adhesives can offer several advantages in the bonding of ceramic brackets.

A new porcelain repair system with a silane coupler, ferric chloride, and adhesive opaque resin.

A new porcelain repair system was developed which uses a two-liquid primer, self-cured opaque resin, and light-cured composite. The primer consisted of two liquids. One component was 4% 3-trimethoxysilylpropyl methacrylate in methyl methacrylate, and the other component was 0.5% ferric chloride in ethanol. A self-curing opaque resin, 4-META/MMA-TBB opaque resin, was used as an adhesive to bind the porcelain and the composite. Adhesive opaque resin was applied on the roughened and primed porcelain. Light-cured composite was placed over the 4-META opaque layer. A shear test was performed for investigation of the strength and durability of the bonding. The result was that for all specimens after 20,000 thermocycles, fracture or crack propagation during the shear test occurred in the porcelain, rather than in the bond. This system may be used to repair fractured porcelain facings without removal of prostheses.

The strength of layering technique in visible light-cured composites

Despite the extensive use of the layering technique in light-cured restorations there is limited information for the interfacial strength when curing is accomplished in air and an inhibited zone is formed on the first layer. This study analyzed the nature of the inhibited zone to determine the interfacial topography, evaluated the shear interlayer bond strength in the presence of the inhibited layer, and assessed the effect of various interfacial treatments on bonding. Two microfilled, one hybrid, and three small-particle composite resins were included in the study. According to the results, the inhibited zone is primarily composed of catalyst-free monomer. The presence of this zone reduces bond strength, creating interfacial discontinuities. Removal of the inhibited layer by an organic solvent of low boiling point results in the highest strength and better interfacial topography from the tested treatments.

Bond strength of joined posterior light-cured composites: Comparison of surface treatments

Bond strength of joined posterior light-cured composites: Comparison of surface treatments

A technique for preparation of lingual rest seats in light-cured composite

A technique for preparation of lingual rest seats in light-cured composite

Marginal leakage associated with three posterior restorative materials

Seventy sound extracted premolar teeth were prepared with minimal Class II cavities and randomly divided into seven equal groups. One group was restored with posterior light-cured composite. In addition, this material was used to restore three other groups of teeth after ( a) etching the magins, ( b) applying two coats of a dentine bonding agent ( c) etching and bonding. Cermet cement was used to restore two groups of teeth, one being pretreated with polyacrylic acid. The final group was restored with amalgam after the application of two coats of cavity varnish. The specimens were sealed to within 1 mm of the cavity margins and the crowns immersed in buffered eosin for 48 h. After mesio-distal sectioning the specimens were photographed. These were examined on an image analyser to calculate the percentage area of crown dentine exhibiting leakage. It is concluded that when composite is used to restore both etched and bonded cavities, marginal leakage is similar to that of amalgam. If either or both pretreatment stages are omitted leakage is significantly greater ( P < 0·05) than for amalgam. Cermet with polyacrylic acid preconditioning of the cavity exhibits significantly less ( P < 0·001) marginal leakage than with amalgam, composite or the other cermet group.

A reverse buildup technique using light-cured composite

A reverse buildup technique using light-cured composite

The retention of self-threading pins embedded invisible light-cured composites

Tensile force was applied to self-threading dentine pins embedded in cylinders of lightcured composite and the failure force determined. Failures were similar to those found in autopolymerized composite, and depended upon both the fracture toughness of the composite and the pin thread geometry. In addition, shadowing of the curing light by the pin reduced the effectiveness of the pins. Most effective retention was obtained when pins with a wide pitch thread were used in combination with the toughest composites.

Surface hardness development in light-cured composites

The surface microhardness of 5 visible light-cured dental composite materials for anterior use was measured at a series of time intervals up to one month, commencing from the end of the light irradiation period. The microhardness steadily increased with time and tended towards a maximum, usually reached after one week. The rate of increase was substantial over the first hour, in all cases, and was greater for samples stored at 37°C as compared with 23°C. These results reflect the progressive cross-linking reaction in the resin phase of the materials which continues after photo-activation. The surfaces of samples stored in water were softened compared to dry samples. Differences between materials may be attributed to compositional differences and extent of cure.

Esthetic modification of removable partial denture teeth with light-cured composites

Esthetic modification of removable partial denture teeth with light-cured composites

Degree of double bond conversion in light-cured composites

The curing pattern of five commercial light-cured composites was investigated. Micro-ATR infrared spectroscopy was used to evaluate the degree of cure as a function of distance from the exposed surface and curing time. KHN measurements were also obtained for each depth to relate the hardness pattern to the degree of conversion. The post-curing efficiency of each system was studied by DSC. The results indicate that microfilled materials present smaller depth of cure and higher amount of unconverted bonds as a function of distance and exposure time. Depth of cure, hardness pattern and curing pattern are extended with increased inorganic loading. The thermal capacity of the filler fraction influences degree of cure. A great loss in the heat induced polymerization is observed 24-h post curing, attributed to the dead end polymerization effect.

The mutual abrasion of light-cured composites and clasps: an in vitro study

A reciprocating abrasion testing machine was used to measure the mutual abrasion of light-cured composites and clasps in vitro. Composite specimens were produced by packing the material into hemispherical wells in a polytetrafluoroethylene block, light-curing for 20 seconds and conditioning in distilled water for one week. The composites tested included ultrafine, hybrid and conventional types. Nine clasp types were studied including cast and wrought, round and half-round, cobalt-chromium, stainless steel and gold. The tests were carried out at 37 °C. The vertical overlap (undercut) of composite and clasp was set at 0.25mm. Ten specimens were subjected to 14,500 traverses in each test Abrasion of composite was measured as weight loss and reduction in thickness of the specimens, while abrasion of clasps was measured as weight loss. Abrasion of the composites was greatest for the conventional type and least for the ultrafines, but in all cases the changes were small and unlikely to impair the clinical performance of clasps during several years' usage. The abrasion of the clasps by conventional composite was considerable and would be expected to cause a significant reduction in retention clinically.

Microleakage between light-cured composites and repairs

M icrofilled composites are subject to fracture, especially at the incisal edge.’ More leakage occurs with microfilled composites than with conventional or combination particle resins.’ Bond strength of microfilled composites to etched enamel is less than conventional composites perhaps because of difference in proportional limit and elastic modulus of the material.3 This finding tends to support the clinical increase in incisal fractures.’ Repair of fractured composites is an acceptable practice. However, after repair it is possible that fracture of the additional material may occur if a different material is added to the original composite or if the fractured surface is inadequately prepared. This study investigated whether microleakage, hence lower bonding capability, occurs when a light-cured microfilled composite and a fine particle resin is added to l-week-old composites. In addition, the study included the feasibility of repairing the fractured composite with a different product. Various methods of treating the fractured surface to reduce the microleakage were investigated.