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.

Visible-light curing units: correlation between depth of cure and distance between exit window and resin surface

Study 1) The depth of proximal cavities was measured on previously taken bitewing radiographs. Study 2) By means of a scrape test, the relationship between depth of cure and irradiation distance was examined with four different curing units and two different restorative resins. Study 3) The microhardness of one of the polymerized resins was measured 0.5 mm below the free surface of the filling and then at 1.0 mm, 1.5 mm, 2.0 mm, and so forth until the resin became so soft that no hardness could be recorded. The hardness at each 0.5-mm level was recorded in relation to irradiation distance. 1) The cavity depth was most often 4-5 mm in lower premolars, 5 6 mm in upper premolars and lower molars, and 5-7 mm in upper molars. In the latter teeth, 15% of the cavities were > or =8 mm deep. 2 and 3) The depth of cure decreased moderately and in a linear manner with increasing irradiation distance. An irradiation distance of 12 mm reduced the depth of well-cured resin only by about 1 mm as compared with close contact between exit window and surface of resin.

Surface hardness and wear of glass ionomers and compomers

PURPOSE

To determine the surface hardness and in vitro wear of glass ionomer cements (GIC) and compomers in comparison with a resin composite and to test for a possible correlation between the determined hardness and wear values.

MATERIALS AND METHODS

Samples were made of conventional GIC (Ketac-Fil, Fuji II, Fuji IX, Ketac-Prototype = Ketac-Molar), resin-modified GIC (Photac-Fil, Fuji II LC, Vitremer), compomers (Dyract, Compoglass) and resin composite (Z100) for the hardness test (n = 10) and for the wear test (n = 3). The materials were handled according to the manufacturers' instructions. The samples were stored in deionized water for 48 hours at 37 degrees C prior to the determination of Rockwell hardness and three-body in vitro wear.

RESULTS

Hardness could not be measured on Ketac-Fil and Fuji II as the samples fractured before a reading was obtainable. Statistically significant differences were found in hardness and wear between brands and between the four types of material. Surface hardness: resin-modified GIC < conventional GIC = compomers < resin composite. In vitro wear: resin-modified GIC > compomers > conventional GIC > resin composite. A negative correlation between hardness and wear was recorded.

Bonding of resin cements to a metal substrate: influence of pretreatment on the adherence energy

The adherence of resin cements to restoration as well as tooth structure is of prime importance for the longevity of cemented restorations. It was the aim of the study to investigate the effect of an acid and a base primer on (i) surface polarity of a nonprecious alloy and on (ii) adherence energy of resin cements bonded to the alloy. The beams were pretreated with a 3% acetone solution of either maleic acid or N,N-diethanol-p-toluidine, and the solvent evaporated. The polarity was determined by means of measurements of contact angles. The adherence energy was measured by means of the wedge test, according to which 2 beams were glued together with the resin cement. A wedge was introduced between the joined beams to create a fissure, and on the basis of the length of the fissure, the adherence energy was calculated. It was found that the polar component of the surface free energy of the alloy increased as a consequence of the pretreatments. The results also showed that the pretreatments gave rise to an increase in adherence energy of 11-15 J/m2, equivalent to relative increases of 22-54%. The observed increases in adherence energy may be due to an increase in polar interactions at the interface between adhesive and substrate.

Effect of propanal and diacetyl on quantity of remaining double bonds of chemically cured BisGMA/TEGDMA resins

The aim of the present study was to determine the effect of propanal and diacetyl addition on the quantity of remaining double bonds of chemically cured dental resins. Propanal (propionaldehyde) or diacetyl (2,3-butanedione) was added to monomer mixtures, which were then made chemically curable. The monomer mixtures were varied with respect to content of propanal or diacetyl. Addition of propanal or diacetyl to chemically curable resins resulted in a decrease in the quantity of remaining double bonds from 19.6% to 1.9% and from 19.6% to 11.4%, respectively. A negative correlation of statistical significance was found between content of propanal and quantity of remaining double bonds, while the relationship between content of diacetyl and quantity of remaining double bonds was found not to be linear. Propanal was equally effective in reducing the quantity of remaining double bonds in chemically cured and in the light cured resins studied previously. As regards diacetyl, a more pronounced effect on quantity of remaining double bonds was noted for light cured resins as compared with chemically cured resins. The most likely common reaction mechanism of propanal and diacetyl seemed to be that of chain transfer reactions. Furthermore, analysis of the data indicated a possible additional photoinitiating function of diacetyl.

Distortion of alloy by sandblasting

PURPOSES

To measure distortion of alloy plates caused by sandblasting, and to determine the influence on degree of distortion of type of alloy, plate thickness, and sandblasting conditions i.e. particle size of alumina, air pressure, and duration of blasting period.

MATERIALS AND METHODS

Plates were cast in a noble alloy and in a Co-Cr alloy. One end of the plates was fastened in a device, and the projecting 20 mm of the plates was sandblasted. Distortion was determined as the deflection of the plates at a distance of 20 mm from the mounting device.

RESULTS

The mean deflections varied between 0.37 mm and 1.72 mm. Plates of Co-Cr alloy showed less deflection than did plates of noble alloy. Deflection was increased by an increase in duration of the blasting, pressure, particle size of the alumina, and by a decrease in plate thickness.

In vitro wear, hardness, and conversion of diacetyl-containing and propanal-containing resin materials

OBJECTIVES

This study was conducted to determine the effect of diacetyl or propanal activities: 1) on the in vitro wear of 22 experimental resin composites; and 2) on the Wallace indentation hardness of the unfilled resins. The objective was to examine the correlation between wear, hardness, and quantity of remaining double bonds (determined previously).

METHODS

Diacetyl or propanal agents were added in varying concentrations to monomer mixtures. The resins were made light-curing and those used for measurement of wear were loaded with filter. The results were analyzed by ANOVA and Newman-Keuls' multiple range tests.

RESULTS

In the composite systems, adding diacetyl or propanal resulted in decreased in vitro wear. The quantity of remaining double bonds in the dental polymers with these additives was also reduced. Their effect on Wallace indentation depth was less clear-cut. Low concentrations of additives decreased indentation depth of the unfilled materials, whereas high concentrations increased indentation depth. If it assumed that differences in the polymer have a major influence on the wear when the filter content and particle matrix interface are kept constant, then the hardness data on the unfilled resin can be used to correlate property changes of the polymer in the composite. A three-dimensional regression analysis found that in vitro wear decreased with decreasing Wallace indentation depth and decreasing quantity of remaining double bonds.

SIGNIFICANCE

Addition of diketone or monoaldehyde to resin monomers may provide a means of increasing wear resistance of composites and allow their use in stress-bearing areas.

Factors affecting the adherence energy of experimental resin cements bonded to a nickel-chromium alloy

Reliable adherence of resin-based cements is of prime importance for the longevity of cemented restorations. The present study investigated whether a relationship exists between adherence energy to a metal substrate and the degree of cross-linking and wetting characteristics of resin-based luting agents. The adherence energies between a sand-blasted metal surface and a series of experimental resin cements were measured by means of the wedge test. The degree of cross-linking was calculated from the monomer composition of the resin cements. The measured wetting characteristics were work of adhesion and surface tension, and their dispersive and polar components. Adherence energy varied between 22 and 81 J/m2 and was influenced by the nature of the resin cements: Those with a low degree of cross-linking resulted in high adherence values. Furthermore, resin cements whose monomers were relatively polar gave rise to high adherence values. Although other metals may not behave in exactly the same way, these results may help in the formulation of new, more retentive resin cements.

Effects of surface treatment on the free surface energy of dentin

OBJECTIVES

The purpose of this study was to evaluate the influence of six treatments on the dispersive, acid, and base components of the free surface energy of dentin.

METHODS

Occlusal dentin surfaces were polished with 4000 grit abrasive paper, washed and air dried. Characteristics of the surface energy were calculated by measuring contact angles of the four following liquids:alpha-bromonaphtalene, glycerol, ethylene glycol, and water. The dentin was then treated with aqueous solutions containing: (1) oxalic acid and glycine (OX/GLY),(2) oxalic acid, glycine, and HEMA (OX/GLY/HEMA), (3) phosphoric acid (PA), (4) maleic acid (MA),(5) EDTA, or (6) NaCIO. After treatment, washing and air drying, the energy characteristics of the samples were evaluated again.

RESULTS

Three kinds of wetting behavior were observed: an increase (OX/GLY/HEMA), a stabilization (PA, NaCIO) or a decrease (OX/GLY, MA, EDTA) of the wettability of the dentin surface. The calculations demonstrated that dentin surfaces are basic.

SIGNIFICANCE

This study of the evolution of the surface energy components gave information on adhesion mechanisms involving hydrophilic and hydrophobic interactions. The results may be helpful in the formulation of conditioners and primers.

Bonding of resin cements to an aluminous ceramic: a new surface treatment

OBJECTIVES

The purpose of the work reported here was to develop a surface treatment of an alumina-based ceramic (In Ceram) that would make reliable bonding to a resin-based luting agent possible.

METHODS

The surface treatment studied was the application of a suspension of a fine-grained, refractory powder, which after drying was sintered to the surface at 960 degrees C. The adherence potential of the surface was determined by measurement of bond energy.

RESULTS

It was found that the surface treatment, in conjunction with a heat-treated, silane coupling agent, resulted in mean bond energies of 47 (+/- 19), 56 (+/- 22), and 525 (+/- 116) J/m2 for the three resin cements studied.

SIGNIFICANCE

It was concluded that the new surface treatment makes reliable bonding possible, which may allow new indications for this material.

Effect of wettability of adhesive resins on bonding to dentin

The purpose of this study was to investigate on human dentin the relationships between, on the one hand, the wettability of five adhesive compounds, characterized by different solubility parameters delta, and on the other, the thickness of the resin impregnated dentin layer (RIL) and the shear bond strength (SBS). Cut dentin surfaces were treated with conditioners of pH 1.25, 3.6 or 7.4. Shear bond strength was measured on 20 specimens each. RIL was determined by scanning electron microscope on specimens fractured perpendicularly to the bonding interface. When resins with delta 32, 30 or 39 (MJ1/2/m3/2) were used RILs were consistently 4 microns and 1 micron on dentin pretreated with the two acidic conditioners. No clearly measurable RIL was shown after application of the conditioner with pH 7.4, and when resins with delta-values of 20 and 25 were applied. For the same adhesive no difference in SBS was found, when applied on the differently conditioned dentin sites. Resins with delta > 30 produced significantly higher SBS than the two adhesives with smaller delta.

Bonding of luting materials for resin-bonded bridges: clinical relevance of in vitro tests

A retrospective clinical study on 78 resin-bonded bridges placed between 1981 and 1992 was used to validate the wedge test in the assessment of the bonding of resin-based luting material. According to the correlation obtained between the findings of the in vivo and the in vitro studies, the wedge test may be considered to be predictive of the clinical performance of resin bonds. The life expectancy of resin-bonded bridges bonded with the 4 META containing material tested appears to be longer than that for bridges bonded with conventional resin cements.

Adherence of resin-based luting agents assessed by the energy of fracture

The present study assessed the adherence of nine commercial resin cements by means of the wedge test. The beams of the test were made of a Ni-Cr-Be alloy, and the adhering surfaces were sandblasted with 250 microns Al2O3. The energy of fracture of the investigated cements varied from less than 10 J/m2 to 121 J/m2. The fracture energies were not influenced by 1) the thickness of the joint, 2) the width of the beams, 3) the use of an intermediary noncomposite resin, or 4) storage in water beyond 24 h. In spite of a rather high variability within groups, it is concluded that the wedge test may give relevant data on the performance of adhesive joints.

Correlation between depth of cure and temperature rise of a light-activated resin

The temperature rise, caused by 10 different curing units, in a prepolymerized resin specimen was examined. For all units, the temperature increase in a 60-s cycle followed a logarithmic curve, with the most effective light sources giving the highest temperature rise. In the surface layer the change of temperature ranged between 3.6 and 29.2 degrees C, and 3.2 mm below the irradiated surface between 1.5 and 12.3 degrees C. The use of a 2-mm-thick isolating layer of glass ionomer resulted in a significant reduction in the temperature increase. The correlation between the depth of cure and the temperature rise was of an exponential or power nature; i.e., a small increase of the depth of cure was followed by a disproportionately high increase in temperature.

Cusp fracture of endodontically treated posterior teeth restored with amalgam. Teeth restored in Denmark before 1975 versus after 1979

The aim of the present study was to analyze the frequency and the severity of cuspal fracture for posterior teeth endodontically treated by 91 Danish dentists and restored with amalgam either before 1975 or after 1979. A database from a previous study was analyzed (1584 teeth with an MO, a DO, or an MOD amalgam restoration without cuspal overlays). Teeth restored before 1975 had a significantly lower frequency of cusp fracture than teeth restored after 1979. Moreover, when comparing the frequency of subcrestal fractures, that of the second group (after 1979) was more than twice as high as that of the first group (before 1975). Two factors with a bearing on these findings changed in the time period 1975-1979; the high-copper amalgams took over in Denmark, and the use of Gates-Glidden burs to achieve straight-line access to the root canal was introduced. It is suggested that the weakening of the coronal part of the root, caused by the use of Gates-Glidden burs, and the expansion and low creep of high-copper amalgams may be two of the reasons for the increased frequency and severity of cuspal fracture found in the past decade in Denmark.

Reliability of three dental radiometers

The light intensity from 80 different curing units was recorded on three dental radiometers. Even though the correlation between the recordings obtained with the three testers was rather high, some units were categorized as good units by one radiometer and poor ones by another. It was also found that, in some case, there was a pronounced difference between the output within the same type of new units. Cavities were made in extracted third molars, filled with a microfilled resin, irradiated for 40 s with 20 of the curing units, and the depth of cure was measured. The radiometers were not able to rank the 20 units in accordance with their depth of cure. We conclude that the three radiometers are not fully reliable. However, they may become very useful tools for monitoring the output from a curing unit if the radiometer and the unit are calibrated.

Solubility parameters, fractional polarities, and bond strengths of some intermediary resins used in dentin bonding

An effective bonding of resin composites to dentin is generally preceded by a conditioning of the surface of the dentin. Previous studies have indicated that the intermediary or adhesive resin should have specific wetting characteristics matching those of the conditioned dentin, in order that optimum bonding can be ensured. The wetting characteristics may be expressed in terms of solubility parameter (delta) and polarity (p) of the resin. The aims of the present study were to determine these variables for a number of compounds used in adhesive resins and to investigate the effects of delta and p on the shear bond strength to dentin. Solubility parameters were obtained according to the method of Small. Fractional polarities were calculated on the basis of measurements of refractive index and dielectric constant of the resins. In the measurements of bond strength, Scotchprep, EDTA + Gluma, or Al2Ox3/glycine were used as dentin conditioners in combination with intermediary resins having various delta and p. For each conditioner, the shear bond strength (BS) could be "explained" by an exponential expression of the form BS = e(a + bx), where x = (delta + cp + d)2, and where a-d are constants depending on the conditioned dentin. It may be concluded that solubility parameter and polarity of the intermediary resins are important variables in the process of bonding to dentin.

Correlation between depth of cure and surface hardness of a light-activated resin

Microfilled test specimens were polymerized for 40 s with 10 different curing units. The specimens were stored for 7 days at 36 +/- 0.5 degrees C, and then the hardness of the surface was measured. The depth of cure of the 10 units was also measured, but no correlation was found between surface hardness and depth of cure. Even inferior curing units were able to polymerize the surface just as effectively as good light sources. The conclusion is that one cannot use the surface hardness of an irradiated resin to assess the quality of a curing unit.

Influence of ketones on selected mechanical properties of resin composites

The present study investigated a concept for additional cross-linking of dental polymers, by which resistance to wear of resin composites might be increased. Bifunctional ketones were added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to type and ratio of monomer and ketone. For measurement of possible effects of the cross-linking agents added, four mechanical properties of the experimental resin composites were determined. Addition of the bifunctional ketone diacetyl resulted in the following increases in mechanical properties: diametral tensile strength, 11%; flexural strength, 29%; modulus of elasticity, 19%; and modulus of resilience, 50%.

Effect of temperature and duration of post-cure on selected mechanical properties of resin composites containing carboxylic anhydrides

Resin composites containing carboxylic anhydrides have recently been formulated for use as inlay materials. To optimize the post-cure, the present study measured the effect of post-cure temperature and duration on four mechanical properties of anhydride-containing resin composites. Temperatures were varied between 37 degrees C and 225 degrees C, and durations were varied between 0 and 24 h. An optimum in strength and stiffness was found when post-cure temperatures of 125-150 degrees C were used. Strength and stiffness were found to increase with increasing duration of post-cure. Due to discoloration of the resin composites when post-curing for 6 or 24 h, a post-cure period of 1 h was preferred.

Ketones in resin composites. Effect of ketone content and monomer composition on selected mechanical properties

To optimize the improvements by diketones of the mechanical properties of resin composites, diacetyl was added to two different monomer mixtures in different quantities. There was a positive correlation between content of diacetyl and tensile strength, flexural strength, and modulus of elasticity, respectively, of both the BISGMA/TEGDMA- and the UEDMA/HEMA-based materials. Addition of diacetyl did not influence the modulus of resilience significantly. Addition of diacetyl resulted in increases in mechanical properties which were of the same relative size for BISGMA/TEGMA-based materials as for UEDMA/HEMA-based materials. However, because of higher control values, except for modulus of elasticity, the properties of UEDMA/HEMA-based composites were superior to those of the BIGSMA/TEGDMA-based materials. A content of approximately 24 mole% diacetyl seemed to have optimum effect on mechanical properties, giving a mean increase of 25% in tensile strength, flexural strength, and modulus of elasticity.

Influence of aldehydes on selected mechanical properties of resin composites

The present study investigated whether propanol, a monofunctional aldehyde, was able to improve the mechanical properties of dental polymers. The underlying hypothesis was that a cross-linking reaction is possible between various functional groups of different polymers. Propanol was added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to monomer composition and content of aldehyde. Four mechanical properties of the experimental resin composites were determined. Addition of propanol gave rise to significant improvements in mechanical properties, which may be indicative of a cross-linking ability of monofunctional aldehydes. With the exception of modulus of elasticity, the mechanical properties of resin composites based on UEDMA/HEMA were superior to those of BISGMA/TEGDMA-based materials, even though the improvements in flexural strength and modulus of resilience were most pronounced for the BISGMA/TEGDMA-based resin composites.

Modulus of resilience as predictor for clinical wear of restorative resins

Eight posterior restorative resins were tested with respect to flexural strength, modulus of elasticity, and modulus of resilience. The mechanical properties were correlated to the two-year results of clinical wear tests. Linear relationships were found between flexural strength and clinical wear and between modulus of resilience and clinical wear. It was concluded that modulus of resilience be used in research and quality control for the prediction of clinical wear.

Selected variables in bonding to dentin

The aim of this study was to investigate the effect of three variables on the shear bond strength between a composite resin and dentin treated with a simplified Gluma system. Thickness of material (2 or 3 mm), distance between light guide and the surface of composite resin (0, 1, 3, or 5 mm) and irradiation time (10, 20, 40, or 60 s) were selected as variables. Thickness of material and distance between light guide and composite resin affected bond strength, but there was little influence of irradiation time under the conditions of the study. It is suggested that bond strength depends on the amount of light transmitted through the composite.