Temperature rise during photo-polymerization for onlay luting.

PURPOSES: (1) To measure the temperature rise during long-time irradiation needed to lute adhesive indirect restorations, with one halogen and five LED high-power lamps, in a simulated oral environment, and (2) to propose a technique to reduce heat. METHODS: Temperature within the pulp chamber of an extracted human molar in a 35 degrees C water bath, before and after restoration with a ceramic onlay, was continuously recorded during 3 x 120 seconds of irradiation for different curing regimens, with and without cooling by compressed air, water and water spray. Temperature rise in a thermo-insulated well was also measured during 120 seconds of irradiation with LEDemetron II and TuTu. RESULTS: Maximal temperature rise of 6.2 degrees C/7.7 degrees C on intact/onlay restored tooth was found after 3 x 120 seconds of irradiation. Compressed air application reduced the temperature by 4 degrees C for LEDemetron II, while water spray increased the temperature by 15.1 degrees C for TuTu. Temperature decreased with irradiation distance, except for LEDemetron II. Air was heated up to 65 degrees C after 120 seconds of irradiation in an insulated well with TuTu.

Influence of curing rate on softening in ethanol, degree of conversion, and wear of resin composite.

PURPOSE: To investigate the effect of curing rate on softening in ethanol, degree of conversion, and wear of resin composites. METHOD: With a given energy density and for each of two different light-curing units (QTH or LED), the curing rate was reduced by modulating the curing mode. Thus, the irradiation of resin composite specimens (Filtek Z250, Tetric Ceram, Esthet-X) was performed in a continuous curing mode and in a pulse-delay curing mode. Wallace hardness was used to determine the softening of resin composite after storage in ethanol. Degree of conversion was determined by infrared spectroscopy (FTIR). Wear was assessed by a three-body test. Data were submitted to Levene's test, one and three-way ANOVA, and Tukey HSD test (alpha = 0.05). RESULTS: Immersion in ethanol, curing mode, and material all had significant effects on Wallace hardness. After ethanol storage, resin composites exposed to the pulse-delay curing mode were softer than resin composites exposed to continuous cure (P< 0.0001). Tetric Ceram was the softest material followed by Esthet-X and Filtek Z250 (P< 0.001). Only the restorative material had a significant effect on degree of conversion (P< 0.001): Esthet-X had the lowest degree of conversion followed by Filtek Z250 and Tetric Ceram. Curing mode (P= 0.007) and material (P< 0.001) had significant effect on wear. Higher wear resulted from the pulse-delay curing mode when compared to continuous curing, and Filtek Z250 showed the lowest wear followed by Esthet-X and Tetric Ceram.

Influence of curing protocol on selected properties of light-curing polymers: degree of conversion, volume contraction, elastic modulus, and glass transition temperature.

OBJECTIVES: The purpose of this study was to investigate the effect of light-curing protocol on degree of conversion (DC), volume contraction (C), elastic modulus (E), and glass transition temperature (T(g)) as measured on a model polymer. It was a further aim to correlate the measured values with each other. METHODS: Different light-curing protocols were used in order to investigate the influence of energy density (ED), power density (PD), and mode of cure on the properties. The modes of cure were continuous, pulse-delay, and stepped irradiation. DC was measured by Raman micro-spectroscopy. C was determined by pycnometry and a density column. E was measured by a dynamic mechanical analyzer (DMA), and T(g) was measured by differential scanning calorimetry (DSC). Data were submitted to two- and three-way ANOVA, and linear regression analyses. RESULTS: ED, PD, and mode of cure influenced DC, C, E, and T(g) of the polymer. A significant positive correlation was found between ED and DC (r=0.58), ED and E (r=0.51), and ED and T(g) (r=0.44). Taken together, ED and PD were significantly related to DC and E. The regression coefficient was positive for ED and negative for PD. Significant positive correlations were detected between DC and C (r=0.54), DC and E (r=0.61), and DC and T(g) (r=0.53). Comparisons between continuous and pulse-delay modes of cure showed significant influence of mode of cure: pulse-delay curing resulted in decreased DC, decreased C, and decreased T(g). Influence of mode of cure, when comparing continuous and step modes of cure, was more ambiguous. SIGNIFICANCE: A complex relationship exists between curing protocol, microstructure of the resin and the investigated properties. The overall performance of a composite is thus indirectly affected by the curing protocol adopted, and the desired reduction of C may be in fact a consequence of the decrease in DC.

Softening and elution of monomers in ethanol.

OBJECTIVES: The purpose of this study was to investigate the effect of light-curing protocol on softening and elution of monomers in ethanol as measured on a model polymer. It was a further aim to correlate the measured values with previously reported data on degree of conversion and glass transition temperature for the same polymer and curing protocols. METHODS: Different light-curing protocols were used in order to investigate the influence of energy density, power density, and mode of cure on the properties of a model polymer. The modes of cure were continuous, pulse-delay, and stepped irradiation of the specimens. Wallace hardness was used to determine the softening of the polymer after storage in ethanol for 24h. Elution of monomers from the polymer was assessed after 7 days in ethanol by means of high-pressure liquid chromatography (HPLC). Data were submitted to two- and three-way analysis of variance (ANOVA), Newman-Keuls' multiple comparison test, and linear regression analysis. RESULTS: Energy density, power density, and mode of cure of the polymer influenced the softening and elution of monomers in ethanol. As energy density increased, softening and elution in ethanol decreased. At same energy density, the influence of power density varied with the mode of cure. When compared to the continuous mode of cure, and at same energy density, pulse-delay irradiation resulted in polymers that in general were more susceptible to softening, but eluted monomers to a lower extent. Less elution was also found with step-cured polymers. Significant, negative correlations were detected between softening and elution in ethanol, respectively, and degree of conversion and between softening and elution in ethanol, respectively, and glass transition temperature. SIGNIFICANCE: A complex relationship exists between curing protocol and the properties selected for investigation. The effect of different combinations of exposure periods and power densities are important to understanding how the curing protocol affects the properties of polymer-based materials.

Effect of evaporation of solvents from one-step, self-etching adhesives.

PURPOSE: To investigate whether and to what extent the bonding capacity of one-step, self-etching adhesives is influenced by the degree to which solvent is evaporated. MATERIALS AND METHODS: Seven one-step, self-etching adhesives were tested (Adper Prompt L-Pop, Clearfil S3 Bond, Futurabond NR, G-Bond, Hybrid Bond, iBond, Xeno III). The variation in degree of evaporation was obtained by varying the duration of the air-blowing step. The duration required to immobilize the adhesive layer, as established in a pilot study, was used as control. Two experimental air-blowing durations, shorter (half the control duration) and longer (double the control duration) than the control duration, were chosen. The resin composite Herculite XRV was bonded to flat human dentin surfaces treated with one of the adhesives following manufacturer's instructions, except for the air-blowing duration after application. After being stored in water at 37 degrees C for 1 week, the bonded specimens were broken in shear. Failure modes were evaluated under stereomicroscope. RESULTS: Air-blowing duration and brand of adhesive both had an effect on shear bond strength. An interaction was found between adhesive and air-blowing duration. Some adhesives were insensitive to variations in air-drying duration, but in general, air-blowing durations shorter than the control duration produced lower shear bond strengths. Significant effects of adhesive and air-blowing duration were also detected in relation to failure mode. More adhesive failures were observed with shorter air-blowing durations. A significant negative correlation between number of adhesive failures and bond strength was found. CONCLUSION: On the basis of this in vitro study, it may be concluded that the one-step, self-etching adhesives evaluated were sensitive to degree of evaporation of the solvents.

Class I and Class II restorations of resin composite: an FE analysis of the influence of modulus of elasticity on stresses generated by occlusal loading.

OBJECTIVES: It was the aim of the study to analyze by the FE method stresses generated in tooth and restoration by occlusal loading of Class I and Class II restorations of resin composite. On the basis of available information on the influence of the modulus of elasticity, the research hypothesis was that the marginal stresses would decrease with increasing modulus of elasticity of the restoration. METHODS: A cylindrical tooth was modelled in enamel and dentin and fitted with a Class I or a Class II restoration of resin composite. In one scenario the restoration was bonded to the tooth, in another the restoration was left nonbonded. The resin composite was modelled with a modulus of elasticity of 5, 10, 15 or 20 GPa and loaded occlusally with 100 N. By means of the soft-ware program ABAQUS the von Mises stresses in enamel and dentin were calculated. RESULTS: In the bonded scenario, the maximum stresses in the enamel were located at the occlusal margins (range 7-11 MPa), and in the dentin centrally at the pulpal floor (range 3.4-5.5MPa). The stresses decreased with increasing modulus of elasticity of the resin composite. In the nonbonded scenario, the stresses were higher in the dentin and lower in the enamel than in the bonded cases, and the influence of the modulus of elasticity was less pronounced. The marginal stresses in the restoration were below 6 MPa in the bonded scenario and below 3 MPa in the nonbonded scenario. SIGNIFICANCE: Occlusal restorations of resin composite should have a high modulus of elasticity in order to reduce the risk of marginal deterioration.

Class II restorations: influence of a liner with rubbery qualities on the occurrence and size of cervical gaps.

The aim of this study was to evaluate the ability of new rubbery liners, used as a cervical increment, to relieve contraction stress and thereby reduce the formation of cervical gaps in class II composite restorations. The investigated liners were made of polyester-acrylate (PE(1), PE(2) or PE(3)) or silicone-acrylate (S), mixed with UDMA, without (A, B, C, D) or with HEMA (AH, BH, CH, DH). A silanized filler was added to the mixture, DH, to give composites with 20, 40, 60, and 70% (w/w) of filler (DHF20, DHF40, DHF60, DHF70, respectively). The presence and width of cervical gaps were determined using a light microscope. Statistical analysis showed that six of the 12 rubbery liners (AH-DH, DHF20-DHF40) significantly decreased gap formation in comparison with the control group. In addition, the polymerization shrinkage, flow, and strain capacity of these liners were measured and the influence of these factors on gap formation was examined. Two- and three-dimensional regression analyses showed significantly negative linear correlations between gap formation and strain capacity, and between gap formation and flow, and a significantly positive linear correlation between gap formation and shrinkage.

Bonding of dual-curing resin cements to dentin.

PURPOSE: The purpose of the study was three-fold: 1) to determine the strength of the bond between a number of dual-curing resin cements and dentin treated with corresponding adhesive systems, 2) to determine the effect on bond strength of not light curing the cements, and 3) to investigate whether application of a solution of sodium sulfinate or ascorbic acid would increase the bond strength in the cases where the manufacturer's version of an adhesive system resulted in low bond strength with chemically cured cement, ie, cement cured without light. MATERIALS AND METHODS: The adhesive systems comprised 5 simplified systems (Adper Scotchbond 1 XT, ED Primer II, Excite DSC, OptiBond Solo Plus, and Prime and Bond NT), and as controls, two three-step etch and rinse systems (Adper Scotchbond Multi-Purpose Plus and Gluma Solid Bond). The corresponding dual-curing resin cements were RelyX ARC, Panavia F 2.0, Variolink II, Nexus 2, Calibra, RelyX ARC, and 2Bond2. The cements were either light and chemically cured or only chemically cured. The adhesive systems were used as recommended by the manufacturers, which for some systems involved inclusion of a so-called activator or catalyst when used with chemically cured cement. Sodium sulfinate and ascorbic acid were applied as a 1% ethanol solution. The bond strengths were measured in shear after storing specimens for 24 h in 37 degree C water. RESULTS: When the dual-curing resin cements had been both light and chemically cured, the bond strengths increased in this order: Gluma Solid Bond < ED Primer II < Prime and Bond NT < Adper Scotchbond Multi-Purpose Plus < Excite DSC < Adper Scotchbond 1 XT < OptiBond Solo Plus. Omission of light curing of the cements decreased the bond strengths with OptiBond Solo Plus and Prime and Bond NT. The use of activator in conjunction with OptiBond Solo Plus and Prime and Bond NT increased the bond strength to chemically cured cement, but not to the level obtained when the cement was both light and chemically cured. The use of the catalyst of Adper Scotchbond Multi-Purpose Plus did not increase the bond strength with chemically cured cement. Pretreatment with a solution of sodium sulfinate or ascorbic acid increased the bond strength with chemically cured cement in the case of OptiBond Solo Plus, whereas only the solution of sodium sulfinate was effective with Prime and Bond NT. CONCLUSION: Clinicians should be aware that in situations where a dual-curing resin cement cannot be light cured, some adhesive systems suffer a loss of efficacy, even when respective activators are used. Pretreatment with a 1% ethanol solution of sodium sulfinate may restore much of the lost bond strength.

Short- and long-term in vitro study of the bonding of eight commercial adhesives to normal and deproteinized dentin.

OBJECTIVE: The aims of this study were to investigate the influence of deproteinization of dentin on the shear bond strength (SBS) mediated by eight dentin adhesives, and to evaluate the long-term durability of the SBSs. The hypotheses were that deproteinization of dentin would not affect the capacity for adherence, and that in contrast to the SBSs to collagen-rich surfaces, the SBSs to deproteinized surfaces would be stable during a 1-year period of storage in water. MATERIAL AND METHODS: Ground surfaces of human dentin were either rinsed with water (normal dentin) or treated with sodium hypochlorite (deproteinized dentin). The dentin surfaces were analyzed by Fourier transform-infrared spectroscopy (FT-IR) using horizontal attenuated total reflectance (HATR). In addition, the SBS to normal and deproteinized dentin treated with the adhesives was measured after 24 h or 1 year of storage in water. RESULTS: The IR absorption peaks at approximately 1,640, 1,560, and 1,240 cm(-1) were assigned to the collagen matrix and peaks at about 1,000 cm(-1) were assigned to the phosphate group in hydroxyapatite. From the relative magnitude of the peaks, it was determined that the utilized deproteinization method was effective. Furthermore, the normal dentin group showed SBS values ranging from 10 to 39 MPa and the deproteinized dentin group showed SBS values ranging from 13 to 30 MPa. CONCLUSIONS: According to the statistical analysis, the results only partly supported the hypotheses: it was found that the influence on bond strength of deproteinization of dentin surfaces and the effect of 1 year of storage in water depended on the composition of the dentin adhesive.

Influence of eugenol-containing temporary cement on bonding of self-etching adhesives to dentin.

PURPOSE: To investigate the influence of eugenol-containing temporary cement on bonding of resin composite to dentin mediated by self-etching adhesives. MATERIALS AND METHODS: Flat dentin surfaces were produced by grinding of extracted human molars. The dentin surfaces were either used directly or after a one-week exposure to a zinc oxide-eugenol cement (IRM). A resin composite (Herculite XRV) was bonded to the dentin surfaces following treatment with 1 of 6 self-etching adhesives (AdheSE, Adper Prompt L-Pop, Clearfil SE Bond, iBond, OptiBond Solo Plus - Self-Etch Adhesive System, and Xeno III). An adhesive system containing 0.5 M EDTA as dentin conditioner (Gluma Classic) was included as negative control, and an etch-and-rinse adhesive system (OptiBond FL) was included as positive control. After storage in water at 37 degrees C for 1 week, the bonded specimens were fractured in shear (n = 8 in each group). RESULTS: Previous contact of dentin with zinc oxide-eugenol cement significantly decreased the bond strength mediated by the negative control and did not affect the bond strength mediated by the positive control. For the self-etching adhesives, no effect was found of previous contact with zinc oxide-eugenol cement. With and without previous contact with zinc oxide-eugenol cement, the etch-and-rinse adhesive system yielded a significantly higher bond strength than all other adhesive systems. CONCLUSION: Previous contact with zinc oxide-eugenol cement did not decrease the bond strength of resin composite to dentin mediated by self-etching adhesives. This finding suggests that eugenol-containing temporary cements may be safe to use in combination with self-etching adhesives.

Finite element analysis of stresses in endodontically treated, dowel-restored teeth.

STATEMENT OF PROBLEM: Endodontically-treated, dowel-restored teeth may experience fracture, but investigations of variables related to fracture are often inconclusive and occasionally contradictory. PURPOSE: The finite element method was used to analyze the stresses in dowel-restored teeth. The variables studied were material, shape, bonding, modulus of elasticity, diameter, and length of the dowel. MATERIAL AND METHODS: The model of the dowel-restored tooth involved dentin, ligament, cortical and trabecular bone, gingiva, and gutta-percha. The dowels were made of glass fiber, titanium, or zirconia and modeled as an approximation of the brands ParaPost Fiber White, ParaPost XH, and Cerapost, respectively. The dowel was cemented with zinc-phosphate cement or with bonded or nonbonded resin luting agents, and an approximation of the material properties of these 2 materials were used in the modeling. The restoration included a composite resin core and a gold crown. Other variables included taper versus parallel-sided posts, modulus of elasticity, diameter, and length of post. The model was axisymmetrical in 3 dimensions. A load of 100 N was applied to the crown at an angle of 45 degrees, and tensile, shear, and von Mises stresses were calculated. RESULTS: The generated stresses decreased with respect to the dowel material in the following order: glass fiber, titanium, and zirconia. Stresses were in general higher with tapered than with parallel-sided dowels. Stresses were reduced by bonding and with an increasing modulus of elasticity, increasing diameter, and increasing length of the dowel. CONCLUSIONS: Within the limitations of this study, it was found that all investigated dowel-related factors influenced the stress field generated in dowel-restored teeth.

Polymerization contraction of resin composite vs. energy and power density of light-cure.

This study measured the polymerization contraction of a resin composite cured at three levels of energy density, each attained at six different levels of power density. The polymerization contraction of the composite was recorded by the method of the deflecting disc (n = 5) for 1 h following the start of irradiation. Power densities of 50, 100, 200, 400, 800 and 1,000 mW cm(-2), as measured on a dental radiometer, were obtained by variation of distance and supply voltage of a commercial light-curing unit. The spectral distribution at each power density was recorded using a spectrophotometer. The absorption spectrum of camphorquinone was also recorded, and the efficiency of the radiation at each power density was calculated as the integral over wavelength of the product of absorption and emission. From the slope of the contraction curves, an approximation to the initial rate of polymerization, Rp, was calculated and was taken as an alternative measure of power density. Statistical analyses showed that polymerization contraction increased significantly with increasing levels of energy density received by the resin composite, and, for each level of energy density, that the contraction decreased significantly with increasing power density.

Bonding of resin cements to post materials: influence of surface energy characteristics.

PURPOSE: 1) To determine the surface energy characteristics of three variously treated post materials and two resin cements, and 2) to investigate if previously measured bond strengths between cements and treated posts could be related to the surface energy characteristics of the posts and cements. MATERIALS AND METHODS: The post materials were those of ParaPost XH, ParaPost Fiber White, and Cerapost, and the resin cements were ParaPost Cement and Panavia F. The post materials were surface treated by grinding, sandblasting, CoJet, or with Alloy Primer. Droplets of four reference liquids were placed on the surfaces and the contact angles measured. On this basis the dispersive, acid, and base components of the surface free energy of the materials were calculated. Then the thermodynamic work of adhesion between resin cements and treated post surfaces was calculated, as well as its dispersive and polar components. RESULTS: It was found that the surface energy characteristics varied significantly between the investigated surfaces. The previously measured bond strengths correlated statistically significantly (p < 0.02) to the dispersive component of the work of adhesion between posts and cements (r = 0.53), to the dispersive component of the surface free energy of the posts (r = 0.54), and to the contact angle determined with ethylene glycol as reference liquid (r = -0.52), but not to other surface energy characteristics. CONCLUSION: In spite of the significant correlations, the "explained" part of the variation in bond strength did not surpass 30%. This means that factors other than surface energy characteristics of adherend and adhesive play a role in determining the strength of the bond.

Resin composites: strength of the bond to dentin versus surface energy parameters.

OBJECTIVES: The strength of the bond to dentin treated with two adhesive systems was investigated. The bond was tested with six resin composites. It was hypothesized that the surface energy parameters of composite and adhesive-treated dentin would influence the bond strength. METHODS: The investigated adhesive systems were Prompt L-Pop and OptiBond FL, which were used as recommended. The bond strengths were tested in shear. The dispersive, the acid, and the base component of the surface energy were determined from measurements of the contact angle of four test liquids placed on the composites and on the adhesive-treated dentin. Relationships were investigated by regression analysis. RESULTS: The strength of the bond to dentin varied with the resin composite and with the adhesive system. The bond strength with both adhesive systems increased with the base component of the surface energy of the resin composites. Dentin treated with OptiBond FL had higher surface energy and gave higher bond strengths than was the case with Prompt L-Pop. SIGNIFICANCE: It is advisable to use a resin composite that is compatible with the adhesive system.

Temperature rise induced by some light emitting diode and quartz-tungsten-halogen curing units.

Because of the risk of thermal damage to the pulp, the temperature rise induced by light-curing units should not be too high. LED (light emitting diode) curing units have the main part of their irradiation in the blue range and have been reported to generate less heat than QTH (quartz-tungsten-halogen) curing units. This study had two aims: first, to measure the temperature rise induced by ten LED and three QTH curing units; and, second, to relate the measured temperature rise to the power density of the curing units. The light-induced temperature rise was measured by means of a thermocouple embedded in a small cylinder of resin composite. The power density was measured by using a dental radiometer. For LED units, the temperature rise increased with increasing power density, in a statistically significant manner. Two of the three QTH curing units investigated resulted in a higher temperature rise than LED curing units of the same power density. Previous findings, that LED curing units induce less temperature rise than QTH units, does not hold true in general.

Retention and failure morphology of prefabricated posts.

PURPOSE: This study evaluated the effect of cement, post material, surface treatment, and shape (1) on the retention of posts luted in the root canals of extracted human teeth and (2) on the failure morphology. MATERIALS AND METHODS: Posts of titanium alloy (ParaPost XH), glass fiber (ParaPost Fiber White), and zirconia (Cerapost) received one of several surface treatments-sandblasting, CoJet treatment, application of Metalprimer II, or sandblasting followed by silane application-and were then luted in the prepared root canal of human incisors and canines (n = 10). Following water storage at 37 degrees C for 7 days, retention was determined by extraction of the posts. Failure morphology of extracted posts was analyzed and quantified stereomicroscopically. RESULTS: Type of luting cement, post material, and shape of post influenced the retention and failure morphology of the posts. Because of limited adherence of the cement to the root canal, surface treatments did not always have a positive effect on retention. CONCLUSION: Choice of luting cement was critical for all three types of posts. Parallel posts showed superior retention to tapered posts.

Flexural strength and modulus of a step-cured resin composite.

In an attempt to reduce marginal contraction gaps, a step-curing mode of light polymerization of resin composite has been proposed. It was hypothesized that such an exposure mode, having an initial reduced curing rate, would result in composite having a lower modulus of elasticity than composite exposed in one step. The composite was initially exposed to power density levels of 50, 100, 150, or 200 mW/cm2 for durations of 10, 20, or 40 s. The final exposure and the exposure of the control group were performed at 750 mW/cm2 for 20 s. It was found that certain modes of two-step exposure resulted in a lower flexural modulus than did a one-step exposure of constant, high power density. Regression analysis showed, with statistical significance, that flexural modulus was relatively high following a short initial exposure at low power density or using a long initial exposure at high power density. Conversely, flexural modulus was relatively low following a long initial exposure at low power density or using a short initial exposure at high power density. The results were explained by probable differences in degree of conversion and cross-link density of the polymer network.

Effect of surface treatment of prefabricated posts on bonding of resin cement.

This in vitro study evaluated the effect of various surface treatments of prefabricated posts of titanium alloy (ParaPost XH), glass fiber (ParaPost Fiber White) and zirconia (Cerapost) on the bonding of two resin cements: ParaPost Cement and Panavia F by a diametral tensile strength (DTS) test. The posts received surface treatments in three categories: 1) roughening by sandblasting and hydrofluoric acid etching; 2) application of primer by coating with Alloy Primer, Metalprimer II and Silane and 3) a combination treatment in the form of roughening (sandblasting or etching) supplemented by the application of a primer or in the form of the Cojet system. After surface treatment, the post was embedded in a cylinder of resin cement (diameter = 4.0 mm, height = 4.0 mm). The surface-treated post was centered in the resin cement-filled mold with the aid of fixation apparatus. Fifteen minutes from the start of mixing the resin cement, the specimen was freed from the mold and stored in water at 37 degrees C for seven days. Following water storage, the specimen was wet-ground to a final length of approximately 3 mm. The DTS of specimens was determined in a Universal Testing Machine. The bonding of resin cement to titanium alloy posts was increased by several surface treatments of the post. However, coating with primers as sole treatment had no effect on bonding. With the DTS method applied, none of the surface treatments had an effect on the bonding to glass fiber posts. The bonding of both resin cements to zirconia posts was improved by Cojet treatment, while sandblasting, followed by silane application, improved bonding of Panavia F.

Determinants of in vitro gap formation of resin composites.

OBJECTIVES: To investigate whether polymerization shrinkage, flow, modulus, and bond strength influence marginal gap formation of resin composite restorations in vitro. METHODS: Eleven proprietary resin composites were studied. Shrinkage was measured by the 'bonded-disk method' (n=3). Flow was measured as the diameter of a constant volume of resin composite to which a load of 20 N had been applied for 60 s (n=3). Modulus was measured in 3-point bending (n=6). Bond strength mediated to human dentin by an adhesive system was measured in shear (n=6). Gap formation was measured in a light microscope along the margins of all-dentin, butt-joint cavities restored with the adhesive system and each resin composite and expressed as the widest gap in percent of the cavity diameter (n=6). RESULTS: Significant differences were found between the resin composites regarding all determined properties. A significant linear correlation was found between flow and gap formation (r=-0.68, P<0.025). Three-dimensional regression analysis showed a significant correlation between polymerization shrinkage (X(1)), flow (X(2)), and gap formation (r=0.79, P(1)<0.05, P(2)<0.005). The highest coefficient of correlation was found when the first part of the polymerization shrinkage (from 0 to 10 s) was disregarded (r=0.90, P(1)<0.005, P(2)<0.0005). CONCLUSIONS: With the adhesive system used, polymerization shrinkage and flow were found to be significant determinants of gap formation around resin composite restorations in vitro.

Influence of specimen diameter on the relationship between subsurface depth and hardness of a light-cured resin composite.

In pilot studies of the relationship between subsurface depth and hardness of a light-cured resin composite, it was found that the resin composite was softer at a depth of 0.5 mm than at, for example, a depth of 1.0 mm. It is possible that the increase in hardness at intermediate subsurface depths compared with the hardness at small depths is due to the heat of polymerization, causing a greater increase in temperature at intermediate depths than at small depths. As the temperature rise increases with volume of the test specimen, it was hypothesized that the increase in hardness would increase with the diameter of the irradiated specimen. The hardness of a resin composite was measured as a function of subsurface depth for cylindrical specimens of 3, 4, and 6 mm diameter. It was found that the resin composite was softer at 0.5 mm than at 1.0-1.5 mm depth independent of specimen diameter. Possible explanations are oxygen inhibition of polymerization and high rate of cure of material at small subsurface depth. It was also found that, corresponding with increasing specimen diameters, the specimens became significantly softer at depths of 3.0 mm, 3.5 mm, and 4.0 mm, respectively. The heat production and reflection of light from the walls of the molds may explain the latter, but not the former finding.