Resin Composite Properties and Energy Density of Light Cure

According to the ‘total energy concept’, properties of light-cured resin composites are determined only by energy density because of reciprocity between power density and exposure duration. The kinetics of polymerization is complex, and it was hypothesized that degree of cure, flexural strength, and flexural modulus were influenced not only by energy density, but also by power density per se. A conventional resin composite was cured at 3 energy densities (4, 8, and 16 J/cm2) by 6 combinations of power density (50, 100, 200, 400, 800, and 1000 mW/cm2) and exposure durations. Degree of cure, flexural strength, and flexural modulus increased with increasing energy density. For each energy density, degree of cure decreased with increasing power density. Flexural strength and modulus showed a maximum at intermediate power density. Within clinically relevant power densities, not only energy density but also power density per se had significant influence on resin composite properties.

A survey of failed post-retained restorations

Survival of endodontically treated, post-restored teeth depends on a multitude of factors, all of which are practically impossible to include in a randomized, controlled clinical study. The purpose of this survey was to characterize and analyze reported failures of post-retained restorations to identify factors critical to failure and to type of failure. A questionnaire was mailed to private practitioners in Denmark with a request to complete the questionnaire whenever a patient presented with a failed post-retained restoration. Information was gathered on factors related to the patient, the tooth, the restorative materials, and the techniques. Two-hundred and sixty questionnaires were collected from 171 practitioners over a 3-year period. Functioning time until failure varied between 3 months and 38 years. Mean survival time until failure was 11 years. Of the failed restorations, 61% had functioned for 10 years or less. Fracture of the tooth was the most common type of failure reported, followed by loosening of the post and fracture of the post. Tapered posts implied an increased risk of tooth fracture compared to loosening or fracture of the post, and the relative risk of tooth fracture increased with the functioning time until failure. Fracture of the post was more common among male than female patients. On the basis of this survey of failed post-retained restorations, it was concluded that tapered posts were associated with a higher risk of tooth fracture than were parallel-sided posts.

Influence of Curing Rate of Resin Composite on the Bond Strength to Dentin

When deciding on a specific curing protocol, dental professionals should be aware of the advantages and limitations of each curing mode.

Resistance to cyclic loading of teeth restored with posts

This in vitro study evaluated the effect of presence of post, presence of core, and of shape, type, and surface treatment of posts on resistance to cyclic loading of crowned human teeth. For all teeth, crowns designed without ferrule were cast in sterling silver and luted with resin cement (Panavia F). Each tooth underwent cyclic loading of 600 N at two loads per second until failure. Teeth that had only been crowned showed significantly higher resistance to cyclic loading than teeth with cores or with post and cores. No significant differences were found between teeth restored with cores only or with post and cores, irrespective of surface-treatment of the posts. Teeth restored with parallel-sided cast post (ParaPost XP) and cores showed significantly higher resistance to cyclic loading than teeth with either tapered cast posts or untreated prefabricated posts of titanium alloy (ParaPost XH) or glass fiber composite (ParaPost Fiber White). No significant difference was found between teeth restored with parallel-sided cast post and cores and teeth restored with untreated prefabricated posts of zirconia (Cerapost). Surface treatment of posts significantly increased the resistance to cyclic loading compared with untreated posts. When posts are used, surface treatment is recommended.

The influence of relative humidity on the effect of dentin bonding systems

PURPOSE

The aim of the study was to measure the influence of relative humidity of the ambient air on bond strength to enamel and dentin of six different bonding systems. In this manner, the role of the primer solvent was assessed.

MATERIALS AND METHODS

The bonding systems selected were Prime & Bond 2.1, One Step, Scotchbond MP, Syntac Single Component, Optibond Solo, and Perma Quick. The primers of these systems are based on either acetone, water or ethanol. Plane enamel and dentin surfaces were placed at relative humidities of 18%, 45%, 80% or 99% RH and then treated in accordance with the recommendations of the respective manufacturers. A resin composite was then bonded to the treated surfaces. After storage in water at 37 degrees C for 1 day, the bonded specimens were broken in shear.

RESULTS

The bonds to enamel were either unaffected or in one case moderately increased by increasing ambient humidity. The bonds to dentin were either unaffected or in four cases strongly reduced by increasing humidity.

CONCLUSIONS

The response of the bonding systems to differences in ambient humidity were not related to the solvent of the primer in a simple way. To exclude the damaging effect of high humidity on dentin bonding, the use of rubber-dam is recommended.

Effect of pulse-delay curing on in vitro wall-to-wall contraction of composite in dentin cavity preparations

PURPOSE

To determine the effect of pulse-delay curing of resin composite on marginal gap formation in dentin cavity preparations.

MATERIALS AND METHODS

Cylindrical cavities were prepared in extracted human molar dentin and were distributed into 8 groups (A-H). The cavity preparations were treated with a dentin-bonding agent (Prime & Bond NT) and filled with a light-cured resin composite (Z100). The restorations were irradiated initially for 1, 2 or 3 seconds at a power density of 425 mW/cm2. Following water storage for varying time intervals, the restorations were re-exposed for 10 seconds at 750 mW/cm2. Marginal gap formation was measured by light microscopy after 20 minutes of water-storage, and the wall-to-wall contraction was calculated as the widest gap in percent of cavity diameter.

RESULTS

Pulse-delay curing in combination with a waiting time of at least 1 minute between exposures reduced wall-to-wall contraction in dentin cavities compared to continuous-cure control.

Influence of selected components on crosslink density in polymer structures

Bifunctional methacrylates polymerize to form crosslinked polymer structures which may be characterized by the quantity of remaining double bonds and by the crosslink density. This study investigated the influence of composition variation on the crosslink density of model methacrylate polymers. It was hypothesized that addition of a monofunctional monomer would reduce crosslink density and that compositions giving rise to many centers of polymer growth would result in increased crosslink density. Unfilled resins with varying content of BisGMA, TEGDMA, a monofunctional monomer, camphoroquinone, and amine were polymerized by visible light irradiation. After polymerization, the quantity of remaining double bonds and the Wallace hardness were determined. Polymer softening upon storage in ethanol was taken as a measure of the crosslink density. After ethanol storage, the relationship between hardness number and amount of monofunctional monomer showed a minimum. At same quantity of remaining double bonds, a high content of camphoroquinone compared to amine resulted in reduced softening in ethanol. Composition influenced not only remaining double bonds but also the crosslink density of the resulting polymer structures.

The effect of secondary curing of resin composite on the adherence of resin cement

PURPOSE

The aim of this study was to measure the adherence between a resin cement and a resin composite polymerized with and without an additional secondary cure of the resin composite.

MATERIALS AND METHODS

The resin cement was 3M Opal Luting Composite and the resin composite was Z-100. The resin composite was either polymerized by light only, or given an additional secondary cure at 110 degrees C for 10 min. For each curing mode, the composite was either ground on carborundum paper #1000 or sandblasted with alumina powder. The adherence was determined as bond strength and as bond energy.

RESULTS

Without sandblasting of the resin composite, the heat treatment resulted in reduced adherence both in the bond strength and the bond energy mode of measurement. With sandblasting, only the bond energy was reduced as a consequence of the heat treatment.

CONCLUSION

On the basis of the experiments conducted, it may be concluded that secondary cure of resin composites takes place at the expense of the adherence to the resin cement.

Influence of pulse-delay curing on softening of polymer structures

Resin composites may be polymerized according to one of several light-curing modes. These modes include variations in intensity of the curing lights and time delay before final cure. The so-called pulse-delay method has earlier been found to reduce the formation of gaps due to polymerization contraction, without mechanical properties of the resin composite being compromised. It was hypothesized that the slow pre-cure of this method would give rise to a different polymer structure than results when the polymer is cured in one step at high intensity. It was found that although the quantity of remaining double bonds was unaffected, the pulse-delay technique led to polymers of increased susceptibility to softening in ethanol. The softness increased with the intensity of the pre-cure and with the waiting time before final cure. The increased softening may be interpreted as the manifestation of a polymer structure having fewer crosslinks.

Soft-start polymerization and marginal gap formation in vitro

PURPOSE

To determine the effect of soft-start polymerization of resin-based composites on the marginal gap formation in dentin cavities.

MATERIALS AND METHODS

Cylindrical cavities were prepared in dentin in extracted human molars. The cavities were distributed into 8 groups. The teeth in groups 1-4 were treated with Scotchbond Multi-Purpose and restored with Z100, and the teeth in groups 5-8 were treated with OptiBond FL and restored with Herculite XRV. The restorations were light-cured for 40 s by four different curing modes. The marginal gap formation was measured in a light microscope after a 20 min water-storage, and the wall-to-wall contraction was calculated as the widest gap in percent of the cavity diameter.

RESULTS

Soft-start light curing did not influence wall-to-wall contraction in dentin cavities. OptiBond FL and Herculite XRV showed less wall-to-wall contraction than did Scotchbond Multi-Purpose and Z100.

CLINICAL SIGNIFICANCE

The soft-start polymerization procedures studied did not improve the marginal adaptation of two resin composites bonded to dentin cavities compared with conventional polymerization.

Characterization of resin composites polymerized with plasma arc curing units

OBJECTIVES

Newly developed curing units (plasma arc curing units) operate at relatively high intensity and are claimed to result in optimum properties of resin composites in a short cure time. This study was conducted to determine a number of characteristics of resin composites polymerized by plasma arc curing units.

METHODS

The investigated polymerization characteristics were quantity of remaining double bonds, depth of polymerization, flexural strength and modulus, and wall-to-wall polymerization contraction. The investigated plasma arc curing units were Apollo 95E and 1000 PAC. The conventional curing unit XL 3000 was used as baseline.

RESULTS

Irradiation with Apollo 95E resulted in a higher quantity of remaining double bonds than did XL 3000, whereas the results obtained with 1000 PAC depended on the resin composite. The depth of cure with the plasma arc units was equal to or less than that obtained with the conventional unit, depending on the resin composite. The flexural strength did not depend on the curing unit. The flexural modulus resulting from curing with Apollo 95E was less than that resulting from curing with XL 3000 in 3 out of 4 comparisons. The wall-to-wall polymerization contraction was equal to or less with the plasma arc units than with the conventional unit.

SIGNIFICANCE

Plasma arc curing units make it possible to polymerize resin composite in much shorter times than conventional curing units. However, the polymerization characteristics associated with the units may be less than optimal.

Substitutes for methylene chloride as dental softening agent

Methylene chloride is used as softening agent for guttapercha and as adhesion promotor for polymethylmethacrylate (PMMA). However, methylene chloride has been found to be carcinogenic. It was the aim of the present work to search for harmless substitutes for methylene chloride in these situations. A number of possible candidates with structural similarities to guttapercha or PMMA were selected. The efficacy in softening of guttapercha of these compounds was measured by the depth of penetration of a needle under the load of 2 N. The effect of the compounds on the adhesion between PMMA and an acrylic rebasing material was measured in shear. It was found that a number of alkenes or dialkenes of moderate molecular weight (1,3-pentadiene, cyclopentene, isoprene, 2-methyl-2-butene, and 1,5-hexadiene) were as effective as methylene chloride as softening agent for guttapercha. As adhesion promotor in bonding to PMMA, it was found that low molecular weight methyl esters (methyl formate and methyl acetate) were as effective as methylene chloride. Because of the variation in polarity, the calculated solubility parameter was not a reliable predictor of softening efficacy of the investigated compounds.

Elution of TEGDMA and BisGMA from a resin and a resin composite cured with halogen or plasma light

Plasma arc light units for curing resin composites have been introduced with the claim of relatively short curing times. The purpose of the present study was to measure and compare elution of monomers from an experimental BisGMA-TEGDMA resin and a commercial resin composite when cured with a halogen unit and when cured with a plasma arc unit. Specimens of the materials were immersed in methanol, and the amounts of monomers released with time were analyzed by HPLC. By use of Fick's laws of diffusion, the amount of eluted monomers from the specimen at infinity was estimated. The elution from resin specimens and from resin composite specimens cured with the plasma arc light unit was 7 and 4 times higher, respectively, compared to the elution from specimens cured with the halogen unit. It was concluded that the plasma arc light curing unit did not provide optimal cure when used as recommended by the manufacturer.

The effect of postcuring on quantity of remaining double bonds, mechanical properties, and in vitro wear of two resin composites

OBJECTIVES

To determine the effect of different postcuring methods on degree of conversion, mechanical properties, and in vitro wear of two resin composites (Z100 and Charisma). The postcuring methods involved devices for inlay curing as well as devices present for other purposes in many dental laboratories or dental offices.

METHODS

Specimens of the resin composites were initially light cured and then postcured according to one of the following methods: Translux EC handheld curing unit (10min), Translux EC light box (10min), Triad II (10min), 40 degrees C (10min), 70 or 110 degrees C for 10min, 1, 6, or 24h. The properties were determined following storage of the specimens for 1week in water at 37 degrees C. The degree of conversion was determined using transmission IR. The mechanical properties tested were diametral tensile strength, flexural strength, and flexural modulus. In vitro wear was induced by a three-body wear simulator.

RESULTS

Most postcuring methods increased degree of conversion of both materials. Postcuring increased the mechanical properties and in vitro wear resistance of Charisma, whereas no effect of postcuring was found on these properties of Z100.

CONCLUSION

Postcuring with the use of devices readily available in the dental laboratory and dental office increased the degree of conversion of Z100 and Charisma as well as the mechanical properties and in vitro wear resistance of Charisma. A heat treatment at 110 degrees C for 10-60min was found to be the most promising postcuring method.

Direction of shrinkage of light-curing resin composites

It is occasionally stated in the dental literature that light-curing resin composites shrink towards the light source. As light travels at the speed of light, this dictum is not obvious. It was the purpose of the present study to investigate the direction of shrinkage of a light-curing resin composite in relation to the attachment and the thickness of the material. The resin composite was applied in cylindrical brass molds in such a manner that a flash, serving as attachment, was produced at one side of the specimens, while the material was flush with the mold at the other side. The specimens were now irradiated from either the flash or the flush side, and the convexity or the concavity of the specimens was measured. At a material thickness of 3 mm, the shrinkage was towards the light source, irrespective of the position of the flash. At 4 and 5 mm thickness of the molds, the direction of shrinkage could be directed towards or away from the light source, depending on the position of the flash. The number of light quanta emitted from the light source and passing through the material was compared with the number of molecules of camphorquinone present in a resin composite of 3, 4, or 5 mm thickness. It was concluded that under the conditions of the present study, the direction of shrinkage was the result of an interplay between the direction of the light, the attachment of the material, and the thickness of the material.

Stiffness, elastic limit, and strength of newer types of endodontic posts

OBJECTIVES

To determine the stiffness, elastic limit, and strength of a selection of endodontic posts recently introduced onto the market.

METHODS

Endodontic posts of zirconia (Biopost, Cerapost), titanium (PCR), and carbon fiber (Composipost) were cemented in a brass block and loaded at an angle of 45 degrees in an Instron Testing Machine. From the recorded relationship between force and deflection the three mechanical properties were determined (n = 10 in each group).

RESULTS

The ceramic posts were very stiff and strong, with no plastic behavior. The PCR post was as strong as, but less stiff than, the ceramic posts. Composipost had the lowest values for stiffness, elastic limit, and strength of the posts investigated.

CONCLUSION

The posts under investigation differed significantly with respect to mechanical properties.

Influence of eugenol-containing temporary cement on efficacy of dentin-bonding systems

Zinc oxide-eugenol (ZOE) cements are widely used as temporary filling materials. However, eugenol has earlier been shown to have a detrimental effect on both resin composites and dentin-bonding systems. The aim of the present in vitro study was to examine whether ZOE cement would also reduce the efficacy of relatively new dentin-bonding systems. This was done by determination of gap formation around resin composite fillings in dentin cavities and of bond strength of resin composite to enamel and dentin. The tooth surfaces involved were either freshly cut, or had been exposed to a ZOE cement (IRM) or to a non-ZOE cement (Cavit) for 7 d before application of a dentin-bonding system (Gluma CPS or Scotchbond Multi-Purpose Plus) and a resin composite (Z100). Gap formation was assessed in a light microscope on 20-min-old fillings and expressed as wall-to-wall contraction (the width of the maximum marginal gap in % of the cavity diameter). Bond strength was measured in shear on 1-d-old specimens. The mean values of wall-to-wall contraction were 0.06-0.09% with Scotchbond Multi-Purpose Plus and 0.20-0.24% with Gluma CPS. The mean values of bond strength to enamel were 22-25 MPa for Scotchbond Multi-Purpose Plus and 20-23 MPa for Gluma CPS, and to dentin were 20-22 MPa for Scotchbond Multi-Purpose Plus and 13-14 MPa for Gluma CPS. The use of Scotchbond Multi-Purpose Plus resulted in higher bond strength to dentin and less wall-to-wall contraction than did Gluma CPS. No differences were found in either wall-to-wall contraction or in bond strength between the three groups for either dentin-bonding system. Thus, the ZOE cement did not influence the efficacy of two relatively new dentin-bonding systems.

The role of maleic anhydride in adhesive resin cements

The adherence of resin cements depends upon, among other factors, the polar interactions across the interface: resin cement/restorative material. The polar interactions may be augmented by inclusion of polar additives such as maleic anhydride to the cement monomer. However, maleic anhydride is slowly converted to maleic acid when exposed to an aqueous environment. This may affect mechanical properties of such a cement in a negative way. It was the aim of the present investigation to analyze the role of maleic anhydride dissolved in the monomer of resin cements. The resin cement monomers used were common methacrylates, to which maleic anhydride in amounts of up to 30 mol% was added. Polymerization initiators were included to make the materials dual curing. Finally, the preparations were mixed with silanated fillers. The adherence energy of the cements bonded to a chromium-cobalt alloy was assessed by means of the double cantilever beam test. The strength and stiffness of the resin cements were recorded at base line and after two months storage in water. The initial adherence energy increased by a factor of about two as a result of addition of maleic anhydride. However, resin cements containing maleic anhydride suffered significant reductions in long-term adherence, strength and stiffness. These reductions were particularly pronounced in non-irradiated specimens. The use of resin cements containing maleic anhydride is not a viable means of conveying adhesiveness to resin cements.

Surface energy characteristics of adhesive monomers

OBJECTIVES

The aim of the investigation was to determine the surface free-energy components of potentially adhesive monomer mixtures.

METHODS

Four liquids with known components of surface free-energy were used as reference. Small drops of the liquids were placed on the polished surfaces of four types of solid (metal, porcelain, resin composite and hydrocarbon), and the contact angles were measured. By means of the fundamental equations for wetting, the three components of the surface free-energy of the four solids were calculated. Small drops of various monomeric mixtures were then placed on the four solid surfaces, and on the basis of the previously calculated components of surface free-energy of the solids, the surface energy characteristics of the monomeric mixtures were determined. The relationships between contact angles and composition, and between surface tension and composition, were studied by regression analyses. Comparisons between values were carried out by means of Neuman-Keuls' multiple range test at a level of statistical significance of p = 0.05.

RESULTS

Statistically significant differences between the monomeric mixtures as regards the wetting of the four solids were observed. These differences reflected differences in the acid or base component of the surface free-energy of the monomers. In particular, monomeric mixtures containing HEMA, MAN or 4-META exhibited a significant acid component of the surface free-energy.

SIGNIFICANCE

Knowledge of the surface free-energy components of monomers throws light on the mechanisms associated with the adhesion of resin composites, including resin cements. A better understanding of the interfacial interactions may act as guide in a research aimed at developing resin materials of increased adhesion to metal, porcelain or resin composite.

Influence of UEDMA BisGMA and TEGDMA on selected mechanical properties of experimental resin composites

OBJECTIVES

This study was conducted to determine the effect of UEDMA, BisGMA and TEGDMA on selected mechanical properties of experimental resin composites.

METHODS

Thirty monomer mixtures of TEGDMA and BisGMA and/or UEDMA were produced. Five base monomer mixtures had the following molar relationships between TEGDMA and BisGMA: 30:70, 40:60, 50:50, 60:40 and 70:30. Monomer mixtures were then produced in which BisGMA was successively substituted by UEDMA, 10 mol% at a time. The resins were made light-curing and loaded with filler. Diametral tensile strength, flexural strength and modulus of elasticity were determined on 1 week old specimens. The results were analyzed by ANOVA and by response surface methodology.

RESULTS

The diametral tensile strength of the resin composites varied between 52 and 59 MPa, the flexural strength between 137 and 167 MPa, and the modulus of elasticity between 8.0 and 11.1 GPa. The statistical analyses showed that substitution of BisGMA or TEGDMA by UEDMA resulted in an increase in tensile and flexural strength, and that substitution of BisGMA by TEGDMA increased tensile, but reduced flexural strength. Further, it was found that, for a given content of UEDMA, variations in the ratio BisGMA/TEGDMA gave rise to a maximum in modulus of elasticity. The size of this maximum in modulus decreased with increasing content of UEDMA.

SIGNIFICANCE

Varying the relative amounts of UEDMA, BisGMA and TEGDMA has a significant effect on the mechanical properties of the resin composition. Thus, by selecting specific combinations of these components, it may be possible to design composites with properties that are tailor made to specific applications.

Mechanical properties of endodontic posts

Twenty-two commercially available endodontic posts were examined with regard to stiffness, elastic limit and resistance to fracture. Differences in mechanical properties were explained by differences in width, shape and surface structure. In the selection of an endodontic post, its stability rather than its retention should be of primary clinical concern. Accordingly, the authors advocate the use of endodontic posts that have a cylindrical coronal portion and a conical apical portion.

Improved efficacy of dentin-bonding agents

Dentin cavities, prepared in extracted human teeth, were treated with various proprietary dentin-bonding agents and then filled with a light-cured restorative resin for posterior use. All bonding agents were either treated in accordance with the manufacturers' instructions or combined with Gluma, which is an aqueous solution of glutaraldehyde and HEMA, a hydrophilic monomer. 10 min after polymerization, the width and the extent of the marginal contraction gap was measured approximately 0.1 mm below the free surface of the filling, using a light microscope. With nearly all dentin-bonding agents, the marginal contraction gap could be significantly reduced if Gluma was used after conditioning of the dentin. The reason for this improvement may be that glutaraldehyde cross-links the collagen fibers and thereby strengthens the organic part of the hybrid layer, however, other mechanisms might also play a rôle in the improvement found.