Wear and marginal breakdown of composites with various degrees of cure

Effect of experimental heat treatment on mechanical properties of resin composites

The aim of this study was to verify the influence of an experimental heat treatment (170 masculineC/10 min) using a casting furnace on the mechanical properties (hardness and flexural strength) of 2 commercial direct resin composites (TPH Spectrum and Filtek P60) compared to a commercial indirect resin system (BelleGlass). Heat treatment temperature was determined after thermal characterization by thermogravimetry (TG) and differential scanning calorimetry (DSC). Data was analyzed by ANOVA and Tukey's test at 5% significance level. There was statistical significance for the main factor heat treatment (p=0.03) and composite (p=0.02), for flexural strength. For Knoop hardness, only the main factor composite was statistically significant (p=0.00). P60 presented higher hardness than TPH. No statistically significant correlation between mechanical properties tested was detected. Based on these results, it was possible to conclude that heat treatment influenced flexural strength of direct composites, while it was not observed for hardness. The association of direct composites with a simple post-cure heat treatment may be an alternative for current indirect composite systems, although more studies are needed to verify other properties of the composites for this application.

Evaluation of the surface hardness of composite resins before and after polishing at different times

PURPOSE

The aim of this study was to evaluate the surface hardness of six composite resins: Revolution, Natural Flow, Fill Magic Flow, Flow-it! (flowables), Silux Plus (microfilled) and Z100 (minifilled) before and after polishing at different times.

MATERIALS AND METHODS

For this purpose, 240 specimens (5mm diameter, 1.4mm high) were prepared. Vickers hardness was determined before and after polishing at different times: immediately, 24h, 7 and 21 days after preparation of the samples. Statistical analysis was performed by ANOVA and Tukey test.

RESULTS

There was no difference in the hardness of flowable resins, which had lower hardness than the minifilled resin. The minifilled resin showed the highest surface hardness as compared to the other materials (p<0.01). All materials exhibited higher hardness after polishing, being more evident after 7 days.

CONCLUSION

It may be concluded that, regardless of the composite resin, surface hardness was considerably increased when polishing was delayed and performed 1 week after preparation of the samples.

Effect of light curing unit on resin-modified glass-ionomer cements: a microhardness assessment

Objective:

To evaluate the microhardness of resin-modified glass-ionomer cements (RMGICs) photoactivated with a blue light-emitting diode (LED) curing light.

Material and Methods:

Thirty specimens were distributed in 3 groups: Fuji II LC Improved/GC (RM1), Vitremer/3M ESPE (RM2) and Filtek Z250/ 3M ESPE (RM3). Two commercial light-curing units were used to polymerize the materials: LED/Ultrablue IS and a halogen light/XL3000 (QTH). After 24 h, Knoop microhardness test was performed. Data were submitted to three-way ANOVA and Tukey's test at a pre-set alpha of 0.05.

Results:

At the top surface, no statistically significant difference (p>0.05) in the microhardness was seen when the LED and QTH lights were used for all materials. At the bottom surface, microhardness mean value of RM2 was significantly higher when the QTH light was used (p<0.05). For RM1, statistically significant higher values (p<0.05) were seen when the LED light was used. No statistically significant difference (p>0.05) was seen at the bottom surface for RM3, irrespective of the light used. Top-to-bottom surface comparison showed no statistically significant difference (p>0.05) for both RMGICs, regardless of the light used. For RM3, microhardness mean value at the top was significantly higher (p<0.05) than bottom microhardness when both curing units were used.

Conclusion:

The microhardness values seen when a LED light was used varied depending on the restorative material tested.

The effect of three composite fiber insertion techniques on fracture resistance of root-filled teeth

The aim of this study was to evaluate the effect of three methods of fiber insertion on fracture resistance of endodontically treated maxillary premolars in vitro. Sixty extracted human maxillary premolars received endodontic treatment followed by mesioocclusodistal (MOD) cavities with 2.5 +/- 0.2 mm thicknesses of buccal and lingual cusps at height of contour. The gingival cavosurface margin was 1.5 mm coronal to the cementoenamel junction (CEJ). Subsequently, the teeth were randomly divided into four groups. In the no-fiber group, the cavities were restored with composite, and in other groups, composite impregnated glass fibers were placed in the gingival, middle, and occlusal third surfaces of the cavities, respectively, and were restored similar to the no-fiber group. Subsequent to thermocycling and exertion of compressive forces, statistical analysis showed that the fracture resistance in the occlusal fiber group was significantly higher than that in other groups (p < 0.0005). In addition, the highest favorable fracture and the lowest cusp detachment rates were observed in the occlusal fiber group.

Degree of conversion and color stability of the light curing resin with new photoinitiator systems

OBJECTIVES

This study investigated p-octyloxy-phenyl-phenyl iodonium hexafluoroantimonate (OPPI) as a photoinitiator, in combination with camphorquinone/amine photoinitiation systems, for use with di(meth)acrylate-based composite resins. The investigation determined if the inclusion of OPPI improved degree and rate of conversion, initial color and color stability of a representative composite resin dental material.

METHODS

Camphorquinone (CQ) and OPPI were combined in various proportions with the amine co-initiator 2-dimethylaminoethyl methacrylate (DMAEMA) and used at two levels in which CQ + OPPI + DMAEMA = 1 wt.% or 3 wt.% to photoinitiate a BisGMA/BisEMA/TEGDMA (37.5:37.5:25 wt.%) monomer blend.A total of eight groups (four groups for each level of total photoinitiator, 1% and 3%) were tested according to the following proportion of components in the photoinitiator system: Group C: CQ only. Group CO: CQ + OPPI (1:2). Group CA: CQ + DMAEMA (1:2). Group COA: CQ + OPPI + DMAEMA (1:1:1). Each monomer was polymerized using a quartz-halogen curing unit (Demetron 400, Demetron Research Corp., Danbury, CT) with an intensity of 400 mW/cm2 for 5 s, 20 s, 40 s, 60 s, 300 s and their conversion levels (DC) were determined at each exposure time using a Fourier transform infrared spectrophotometer (FTIR).To examine color stability, experimental composite resins were made by mixing 3.2% silanated barium glass (78 wt.%, average filler size; 1 microm) with each monomer system, except both CQ only group and 1% CO group, which were found to cure insufficiently to be able to prepare useful specimens. Disk-shaped samples (10 mm in diameter and 1.5 mm in thickness) were made and stored under the conditions of dry or saline solution at room temperature (25 degrees C) or 60 degrees C water bath. Each CIELAB scale was determined with a colorimeter (CHROMA METER CR-400) at the time of baseline (day after curing), 1 week, 2 weeks, and 4 weeks later.

RESULTS

The high level (3%) photoinitiated groups exhibited greater DC than the low level (1%) groups. In the 3% group, the COA group showed the fastest and the highest DC, while in the 1% group the CA and COA groups showed the greatest DC.In the color stability test, both CA groups were darker and more yellow than the CO and COA groups. Color was more stable in composite resins containing OPPI than those containing only the CQ and amine components. The least color change (greatest color stability) was found using 25 degrees C saline solution aging, and the most change (least color stability) occurred using 60 degrees C dry air aging.

SIGNIFICANCE

This study suggests that OPPI can be used to replace the amine in a given CQ/amine photoinitiator system to accelerate cure rate, increase conversion, reduce initial color and increase color stability.

Determination of the optimal photoinitiator concentration in dental composites based on essential material properties

OBJECTIVES

The aim of this study was to determine the concentrations of the photosensitizer (camphoroquinone, CQ) and coinitiator (ethyl-4-dimethylaminobenzoate, EDMAB) that resulted in maximum conversion but generated minimum contraction stress in experimental composites.

METHODS

Experimental composites were prepared with an identical resin formulation [TEGDMA:UDMA:bis-GMA of 30.25:33.65:33.65]. Five groups of resin were prepared at varied CQ concentrations (0.1, 0.2, 0.4, 0.8 and 1.6wt% of the resin). Five subgroups of resin were prepared at each level of CQ concentration, by adding EDMAB at 0.05, 0.1, 0.2, 0.4 and 0.8wt% of the resin, resulting in 25 experimental resins. Finally, strontium glass ( approximately 3microm) and silica (0.04microm) were added at 71.5 and 12.6wt% of the composite, respectively. Samples (n=3) were then evaluated for Knoop hardness (KHN), degree of conversion (DC), depth of cure (DoC) and contraction stress (CS).

RESULTS

There was an optimal CQ and EDMAB concentration that resulted in maximum DC and KHN, beyond which increased concentration resulted in a decline in those properties. KHN testing identified two regions of maxima with best overlaps occurring at CQ:EDMAB ratio of 1.44:0.42 and 1.05:1.65mol%. DC evaluation showed one region of maximum, the best overlap occurring at CQ:EDMAB ratio of 2.40:0.83mol%. DoC was 4mm. Overall, maximum CS was attained before the system reached the maximum possible conversion and hardness.

SIGNIFICANCE

(1) Selection of optimal photoinitiator/amine concentration is critical to materials' formulation, for excessive amounts can compromise materials' properties. (2) There was no sufficient evidence to suggest that contraction stress can be reduced by lowering CQ/EDMAB concentration without compromising DC and KHN.

Impact of refrigeration on the surface hardness of hybrid and microfilled composite resins

This in vitro study evaluated the Knoop hardness of the composite resins Charisma® (C) and Durafill VS® (D) polymerized in 3 different conditions: at room temperature (A) (23 ± 1°C); refrigerated at 4 ± 1°C and immediately photo-activated after removal from the refrigerator (0); and, refrigerated at 4 ± 1°C and photo-activated after a bench time of 15 min at room temperature (15). One hundred and twenty specimens (4 mm diameter and 2 mm depth) were made using a stainless steel mold and following manufacturer's instructions. All specimens were tested immediately after polymerization (I) and after 7 days of water storage in the dark at room temperature (7d). The data were subjected to ANOVA and post-hoc Tukey's test (a=0.05). On the top surface, CAI was statistically similar to C15I and DAI to D15I (p>0.05). On the bottom surface, CAI presented higher hardness values when compared to COI and C15I (p<0.05). The D groups showed no significant differences (p>0.05) on the bottom surfaces for any tested polymerization condition. After 7 days of storage, the Knoop hardness decreased significantly (p<0.05) for groups C7d and D7d except for C07d, which was not different from COI at either surface (p>0.05). D07d showed higher Knoop hardness (p<0.05) values on the top surface when compared to the other groups.

Kinetics of light-cure bracket bonding: power density vs exposure duration

INTRODUCTION

Recent technologic advances make it possible to increase the light power density to reduce the necessary exposure duration. The kinetics of polymerization are complex. The special case of indirectly curing the thin layler of composite below the metallic bracket base further increases this complexity. It was hypothesized that the concept of "total energy,"--the reciprocity between power density and exposure duration--does not hold for orthodontic light-cure bracket bonding.

METHODS

Stainless steel brackets were bonded on deciduous bovine incisors with a standard light-cured composite. A calibrated, powerful halogen lamp allowed for modification of power density from 300 to 3000 mW/cm2. Metallic brackets were bonded in 8 groups of 20 incisors each with various combinations of power densities and exposure durations to obtain 3 levels of energy density (6000, 12000, and 24000 mJ/cm2). Another group of 20 incisors served as the positive control with a conventional powerful halogen lamp (1000 mW/cm2) for 40 seconds. After storage for 24 hours at 37 degrees C in water, the bracket shear bond strength (SBS) and the adhesive remnant index (ARI) were measured.

RESULTS

It was confirmed that bracket SBS mainly depends on the energy density of the light cure. All groups with an energy density of 6000 mJ/cm2 had significantly lower SBS than the groups with higher energy densities (P <0.01). The dependence of SBS on exposure duration for the same energy density followed an exponential model of nonlinear regression (r2 = 0.98).

CONCLUSIONS

The concept of "total energy" does not hold for orthodontic light-cure bracket bonding. An exposure time of less than 4 seconds, irrespective of the power density, cannot guarantee sufficient bracket bond strength. There seems to be an advantage of power density over exposure duration in the context of metallic bracket bonding. These results show that, for an efficient light-cure bracket bonding, there is an absolute lower limit of exposure duration (4 seconds) and an upper limit of useful power density (3000 mW/cm2).

Effect of three types of light-curing units on 5-year colour changes of light-cured composite

The purpose of this study was to determine colour changes in a composite cured with tungsten-halogen, light-emitting diode (LED) or a plasma arc after 5 years. Five specimens 10 mm in diameter and 2 mm in height were prepared using Hybrid (Clearfil AP-X) composite for each test group. The corresponding specimens were cured with a tungsten-halogen curing light, a LED unit or with a plasma arc. Specimens were stored in light-proof boxes for 5 years after the curing procedure to avoid further exposure to light and stored in 37 degrees C in 100% humidity. Colorimetric values of the specimens immediately after curing and after 5 years were measured using colorimeter. The DeltaE*( ab ) values varied significantly depending on the curing unit used (p < 0.001). Curing time did not affect the colour changes of the specimens (p = 0.4). The results of this study suggest that composite materials undergo measurable changes due to the curing unit exposure.

Effect of exposure time on curing efficiency of polymerizing units equipped with light-emitting diodes

A study was conducted to evaluate the top and bottom hardness of two composites cured using polymerizing units equipped with light-emitting diodes [LED] (LEDemetron; Elipar FreeLight, Coltolux LED) and one quartz-tungsten halogen device [QTH] (Optilux 501) under different exposure times (20, 40 and 60 sec). A matrix mold 5 mm in diameter and 2 mm in depth was made to obtain five disc-shaped specimens for each experimental group. The specimens were cured by one of the light-curing units (LCUs) for 20, 40 or 60 sec, and the hardness was measured with a Vickers hardness-measuring instrument (50 g/30 sec). Data were subjected to three-way ANOVA and Tukey's test (alpha = 0.05). LED LCUs were as effective as the QTH device for curing both composites. A significant increase in the microhardness values were observed for all light LCUs when the exposure time was changed from 20 sec to 40 sec. The Z250 composite showed hardness values that were usually higher than those of the Charisma composite under similar experimental conditions. LED LCUs are as efficient for curing composites as the QTH device as long as an exposure time of 40 sec or higher is employed. An exposure time of 40 sec is required to provide composites with a uniform and high Knoop hardness when LED light-curing units are employed.

Degree of conversion and mechanical properties of a BisGMA:TEGDMA composite as a function of the applied radiant exposure

OBJECTIVE

Verify the influence of radiant exposure (H) on composite degree of conversion (DC) and mechanical properties.

METHODS

Composite was photoactivated with 3, 6, 12, 24, or 48 J/cm(2). Properties were measured after 48-h dry storage at room temperature. DC was determined on the flat surfaces of 6 mm x 2 mm disk-shaped specimens using FTIR. Flexural strength (FS) and modulus (FM) were accessed by three-point bending. Knoop microhardness number (KHN) was measured on fragments of FS specimens. Data were analyzed by one-way ANOVA/Tukey test, Student's t-test, and regression analysis.

RESULTS

DC/top between 6 and 12 J/cm(2) and between 24 and 48 J/cm(2) were not statistically different. No differences between DC/top and bottom were detected. DC/bottom, FM, and KHN/top showed significant differences among all H levels. FS did not vary between 12 and 24 J/cm(2) and between 24 and 48 J/cm(2). KHN/bottom at 3 and 6 J/cm(2) was similar. KHN between top and bottom was different up to 12 J/cm(2). Regression analyses having H as independent variable showed a plateau region above 24 J/cm(2). KHN increased exponentially (top) or linearly (bottom) with DC. FS and FM increased almost linearly with DC/bottom up to 55% conversion.

CONCLUSIONS

DC and mechanical properties increased with radiant exposure. Variables leveled off at high H levels.

Influence of light polymerization modes on degree of conversion and crosslink density of dental composites

This study analyzed the influence of light polymerization modes on crosslink density (CD) and the degree of conversion (DC) of dental composites. A minifilled hybrid and a nanofilled dental composite were photoactivated with two light polymerization modes: Conventional-850 mW/cm2 for 20 s and Gradual-50 up to 1,000 mW/cm2 for 10 s+1,000 mW/cm2 for 10 s. DC was determined by the use of FT-Raman-spectrometer. A softening test, using Knoop diamond indentation, was carried out at the top and bottom of 2 mm thick dental composite disks, before and after storage in 100% ethanol for 24 h, in order to represent the amount of crosslink density. Data were analyzed by ANOVA and Student-Newman-Keuls' multiple range test (alpha=0.05). The DC was influenced by light polymerization modes, with Gradual mode presenting lower DC. On bottom surfaces, the nanofilled dental composite was more susceptible to softening by ethanol than minifilled hybrid, and gradual light polymerization of nanofilled dental composite resulted in more softening than when conventional light polymerization was used. The results suggest that nanofilled composites are capable undergoing more plasticization if applied in thick increments.

Time-dependent visco-elastic creep and recovery of flowable composites

Creep behaviour of flowable composites was evaluated in relation to their filler fraction and the postcure period. Solid cylindrical specimens were prepared (4 x 6 mm) using steel moulds, with thorough light curing from multiple directions at 600 mW cm(-2). The specimens were divided into two groups (n=3/material) as follows: group I, loaded 5 min after preparation; and group II, loaded after storage for 1 month in water at 37+/-1 degrees C. A constant compressive stress of 36 MPa was applied to each specimen for 2 h followed by removal of the weight for 2 h of strain recovery. Axial strain measurements were obtained continuously over the total analysis time of 4 h. The maximum creep strain ranged from 2.04 to 7.69% and from 1.03 to 6.12% for groups I and II, respectively. Flowables that had the highest percentage of filler produced the lowest creep strain. The creep response decreased with 1 month of preload storage. Clinically, the finding of this study suggests that flowable composites are unsuitable for stress-bearing areas.

Effect of chain modifications on the physicomechanical properties of silsesquioxane-based dental nanocomposites

The objective of this study was to evaluate the physicomechanical properties of a series of polyhedral silsesquioxane (SSQ) methacrylate monomers developed for dental applications. The effect of chain modifications on the properties of the SSQ-based monomers was also evaluated. Physicomechanical properties that are investigated include polymerization shrinkage, degree of conversion, hardness, and modulus. Results obtained were compared with unfilled 1:1 (control) bis-GMA/TEGDMA materials (typical monomers used in dental composites). All samples investigated were cured using 400-500 nm light at 500 mW/cm(2) for 40 s. Shrinkage associated with curing and post-gel reactions for all synthesized SSQ compounds were found to range from (0.04 +/- 0.01)% to (0.33 +/- 0.03)% with degree of conversion ranging from (56.68 +/- 2.81)% to (84.53 +/- 2.62)%. At all time intervals, post-gel shrinkage associated with control was found to be significantly greater than all SSQ compounds. No significant difference in degree of conversion was observed for control, and all SSQ compounds except for SSQ attached with eight equivalents of short-chain methacrylate. Mechanical properties associated with SSQ compounds were found to be significantly lower than control. However, through chain modifications, mechanical properties of SSQ compounds can be improved by approximately 50%.

Conventional and High Intensity Halogen Light Effects on Water Sorption and Microhardness of Orthodontic Adhesives

Objective: To test the null hypothesis that when the equivalent total light energy is irradiated to three orthodontic adhesive resins, there is no difference between the microhardness and water sorption values regardless of the curing light sources.

Materials and Methods: Samples were divided into six groups according to the combination of three orthodontic adhesives (Kurasper F, Light-Bond, Transbond XT) and two light intensities (quartz tungsten halogen [QTH] and high intensity quartz tungsten halogen [HQTH]). One half of each of the 40 samples of three adhesive pastes was polymerized for 20 seconds by a QTH light source, and the other half was polymerized for 10 seconds by a HQTH light source. Water sorption was determined and Vickers hardness was established with three measurements per sample at the top, center, and bottom. Statistical analysis was performed using two-way analysis of variance (ANOVA) with multiple comparisons (Tukey-HSD).

Results: Statistically significant differences were found among all adhesives for water sorption and hardness values cured with QTH and HQTH. The HQTH curing unit resulted in higher values than did the QTH. The highest water sorption values were observed for Kurasper F cured with HQTH and the lowest value was observed for Transbond XT cured with QTH. For microhardness Light-Bond cured with HQTH produced the highest values, and Transbond XT cured with QTH produced the lowest.

Conclusions: When the equivalent total light energy is irradiated to three orthodontic adhesive resins, there are significant differences between the microhardness and water sorption values cured with the QTH and HQTH light source. The null hypothesis is rejected.

Depth of Cure of Dental Resin Composites: ISO 4049 Depth and Microhardness of Types of Materials and Shades

Achieving a high degree of cure throughout a 2 mm thickness of light-activated resin composite did not occur for many types and shades of resin composite. Clinicians should check the depth of cure by using the scraping method.

Describing Adequacy of Cure with Maximum Hardness Ratios and Non-linear Regression

Redefined hardness ratios, based on extended cure intervals and maximum hardness when used in conjunction with non-linear regression, provide a readily available and accurate characterization of the curing performance of LCU-composite combinations, which is superior to the use of traditional per-specimen hardness ratios. It is recommended that the light curing guidelines provided to clinicians should be based on this more accurate description of curing behavior.

Hardness of a bleaching-shade resin composite polymerized with different light-curing sources

The microhardness of a bleaching-shade resin composite polymerized with different light-curing units was evaluated. Composite samples (3M ESPE Filtek Supreme) were applied to brass rings (2 mm in thickness, 5 mm in diameter). Three commercial LED lights were used to polymerize the specimens and the results were compared to those of a conventional halogen light. The light sources used in the present study were: Demetron Optilux 401 (QTH), 3M ESPE Elipar FreeLight (LED 1); Kerr L.E. Demetron I (LED 2), and ColtoluxLED lights (LED 3). The microhardness of the top and bottom surfaces was assessed with a digital Vickers hardness-measuring instrument, under load. At the bottom surface, no significant difference among the light sources was observed (two-way ANOVA). At the top surface, the QTH light source presented significantly higher hardness values compared to the values observed when LED 1 and LED 3 were used. There were no significant differences between the QTH and LED 2 light sources. Significantly higher hardness values were also found at the top surface when compared to the values observed at the bottom surface. The power density of the polymerization light sources seemed to be responsible for the observed resin composite hardness, not their irradiance.

Experimental composite brackets: influence of filler level on the mechanical properties

INTRODUCTION

Despite their popularity in satisfying esthetic demands, plastic brackets still have some disadvantages because of decreased wear resistance and hardness. Our aim in this study was to investigate the mechanical properties of an alternative bracket-polymer urethane-dimethacrylate (UDMA) reinforced by silicon dioxide fillers with several filler levels.

METHODS

Three bracket groups were created: (1) unfilled UDMA, (2) UDMA with a filler content of 40% by volume, and (3) UDMA with a filler content of 70% by volume. Flexural strength, fracture force, and Vickers hardness of the bracket groups were tested. In addition, a 3-medium wear test was performed.

RESULTS

The filler-reinforced brackets showed increased hardness, greater stiffness, and improved wear resistance compared with unfilled brackets. Higher filler concentrations in a polymer matrix result in greater stiffness of the polymer. Despite this, the filled composite brackets were not statistically more susceptible to fractures than the unfilled plastic brackets.

CONCLUSIONS

Experimental UDMA brackets reinforced with silicon dioxide fillers showed an obvious trend for improved mechanical properties compared with unfilled UDMA brackets.

Network structure of Bis-GMA- and UDMA-based resin systems

OBJECTIVES

The commonly used dental base monomers 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenylene]propane (Bis-GMA) and 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA) require the use of a diluent monomer, such as triethylene glycol dimethacrylate (TEGDMA). The aim of this study was to measure double bond conversion of UDMA/TEGDMA and Bis-GMA/TEGDMA polymeric systems, determine the leachable portion, and analyze network formation by evaluating crosslinking and pendant double bonds.

METHODS

UDMA or Bis-GMA was combined with TEGDMA in systematic increments and irradiated to form light cured polymers. Fourier transform infrared spectroscopy in the near-infrared region was used to measure double bond conversion. The leachable sol fraction was analyzed by 1H NMR. Resin composites were formulated. Flexural strength was measured by three-point bending and volumetric shrinkage was determined with a mercury dilatometer.

RESULTS

The amount of base monomer greatly influenced double bond conversion, sol fraction, and crosslinking. Increasing base monomer concentration decreased double bond conversion, increased the leachable fraction, and decreased crosslinking and network formation. At mole fractions higher than 0.125, the UDMA polymers had significantly higher conversion than the Bis-GMA polymers. Bis-GMA polymers had higher leachable amounts of unreacted monomer, while UDMA mixtures had more crosslinking than the Bis-GMA mixtures. In regards to the physical properties of resin composites, increasing the base monomer improved flexural strength and decreased volumetric shrinkage.

SIGNIFICANCE

This systematic study for the evaluation of conversion, leachability, crosslinking, and network structure along with physical properties, like volumetric shrinkage and flexural strength, are required for the optimization of competing desirable properties for the development of durable materials.

Effect of time and polymerization cycle on the degree of conversion of a resin composite

The aim of this investigation was to verify the influence of 3 light curing units on the degree of conversion, using different irradiation conditions: 1) manufacturers' recommended times of photo-activation, 2) standardizing total energy density among the units and 3) standardizing energy density at the 450-490 nm wavelength range among the units and the effect of these irradiation conditions on the post-cure. Three light curing units were used: halogen, light emitting diodes (LED) and xenon plasma. Seven groups were tested (n=6). Twenty-four hours after the photo-activation procedures, half of the composite specimens were submitted to Fourier Transformed Infrared Spectroscopy analysis. The other half was analyzed after 1 month. The results were submitted to 2-way ANOVA and Tukey's test (5%). Twenty-four hour analysis revealed that the second set of irradiation conditions produced a similar degree of conversion among the LCUs. After 1 month, the conversion values were statistically higher for 20 seconds of halogen exposure (increased from 46.78 to 49.66%), 20 seconds of LED exposure (from 46.20 to 51.15%), 30 seconds of LED exposure (from 48.29% to 50.68%) and 3 seconds of PAC exposure (from 42.57 to 51.39%). The initial degree of conversion and post-cure depended on the photo-activation condition applied.

Curing effectiveness of a light emitting diode on dentin bonding agents

This study tested the hypothesis that dentin bonding agents (DBAs) cured with a light emitting diode (LED) light curing unit (LCU) would not show a statistically significant different dentin shear bond strength (DSBS) relative to a halogen LCU, when the LED and halogen LCUs had a supposedly similar irradiance. Five commercial DBAs were tested: ScotchBond Multipurpose, Single Bond, One Step, Clearfil SE Bond, and Adper Prompt. The LCUs used in this study were a VIP (Bisco) for the halogen light and an Elipar FreeLight (3M ESPE) for the LED. First, the emission spectrum and spectral irradiance of both LCUs were investigated. Next, the DSBS of the DBAs cured with each LCU was measured. Two-way analysis of variance was used to analyze whether there were differences in DSBS resulting from the type of LCU and DBA used. The halogen LCU showed a higher total irradiance value relative to the LED LCU, but the difference of spectral irradiance was reduced in the efficient wavelength ranges for camphoroquinone activation. Although the DBAs cured with both types of LCUs showed similar DSBS values, Clearfil SE Bond showed the highest DSBS value when cured with a halogen light. Therefore, the null hypothesis of this study was rejected, indicating that the curing effectiveness of the LED LCU on some DBAs can be lower than that of a halogen LCU in terms of bond strength.

Curing capability of halogen and LED light curing units in deep class II cavities in extracted human molars

Class II cavities were prepared in extracted lower molars filled and cured in three 2-mm increments using a metal matrix. Three composites (Spectrum TPH A4, Ceram X mono M7 and Tetric Ceram A4) were cured with both the SmartLite PS LED LCU and the Spectrum 800 continuous cure halogen LCU using curing cycles of 10, 20 and 40 seconds. Each increment was cured before adding the next. After a seven-day incubation period, the composite specimens were removed from the teeth, embedded in self-curing resin and ground to half the orofacial width. Knoop microhardness was determined 100, 200, 500, 1000, 1500, 2500, 3500, 4500 and 5500 microm from the occlusal surface at a distance of 150 microm and 1000 microm from the metal matrix. The total degree of polymerization of a composite specimen for any given curing time and curing light was determined by calculating the area under the hardness curve. Hardness values 150 microm from the metal matrix never reached maximum values and were generally lower than those 1000 microm from the matrix. The hardest composite was usually encountered between 200 microm and 1000 microm from the occlusal surface. For every composite-curing time combination, there was an increase in microhardness at the top of each increment (measurements at 500, 2500 and 4500 microm) and a decrease towards the bottom of each increment (measurements at 1500, 3500 and 5500 microm). Longer curing times were usually combined with harder composite samples. Spectrum TPH composite was the only composite showing a satisfactory degree of polymerization for all three curing times and both LCUs. Multiple linear regression showed that only the curing time (p < 0.001) and composite material (p < 0.001) had a significant association with the degree of polymerization. The degree of polymerization achieved by the LED LCU was not significantly different from that achieved by the halogen LCU (p = 0.54).

Volume contraction in photocured dental resins: The shrinkage-conversion relationship revisited

Polymerization shrinkage and degree of conversion (DC) of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as increased monomer conversion invariably leads to high polymerization shrinkage values.

OBJECTIVES

This paper aims at accurately determining the polymerization volume contraction of experimental neat resins and to link it to the number of actual vinyl double bonds converted in single ones instead of, as generally done, to the degree of conversion.

METHODS

Different mixtures of Bis-GMA/TEGDMA (traditionally used monomers) were analyzed. Contraction of the polymers was determined by pycnometry and the use of a density column. DC was determined by the use of Raman spectrometry.

RESULTS

An univocal relationship has been found between the volume contraction and the actual number of vinyl double bonds converted into single ones. A contraction value of 20.39 cm3/mole (of converted C=C) was deduced from 27 measurements.

SIGNIFICANCE

This relationship helps in finding solutions to the polymerization shrinkage problem. A reduction of the polymerization shrinkage due to the chemical reaction may obviously be expected from the addition of molecules allowing a decrease in the number of double bonds converted per unit volume of resin matrix, while maintaining the degree of conversion (of Bis-GMA and TEGDMA) and thus the mechanical properties. Further research will be directed at this objective.

Effect of the composite photoactivation mode on microtensile bond strength and Knoop microhardness

OBJECTIVE

This study evaluated the effect of the composite photoactivation mode on microtensile bond strength and Knoop microhardness.

METHODS

Standard class I cavities (3 x 4 x 3mm) were restored with two adhesives systems, Single Bond (SB) and Clearfil SE Bond (CE), and the TPH composite. The photoactivation of the composite was carried out using three modes: Conventional (CO: 400 mW/cm(2) x 40s), Soft-Start (SS: 100 mW/cm(2) x 10s+600 mW/cm(2) x 30s) and Pulse-Delay (PD: 100 mW/cm(2) x 3s+3 min wait+600 mW/cm(2) x 37s). For the microtensile test, beams obtained from the buccal wall bond interface were tested under tension at 0.5mm/min crosshead speed until failure. For the microhardness test, the restorations were sectioned in the mesio-distal direction and indentations were made on the internal composite surface of each half at three different depths. Data of two tests were analyzed using two-way ANOVA and LSMeans (alpha=0.05).

RESULTS

In the microtensile test, SS presented the highest values. PD presented intermediate values without differing significantly from the other modes. For adhesives, SB presented the highest values. In the microhardness test, PD presented the highest values, differing significantly from SS. CO presented intermediate values but without any statistical difference from the others. The SS-CE interaction presented the lowest values with statistical differences from all the others.

SIGNIFICANCE

By the SS technique, the highest bond strength was obtained. However, this technique made it possible for the adhesive system to intervene with the hardness of the composite.

Effect of irradiation mode on polymerization of dental composite resins

Many researchers have sought to reduce the polymerization shrinkage in composite resins. Controlling the irradiation light seems practical because the polymerization process initiates with light by activating the photoinitiators. The present study evaluated the effect of irradiation mode on microhardness and polymerization shrinkage of composite resins. Three different irradiation modes, STD, EXP, and MED, were taken under 40-s exposure condition. As results, the irradiation mode significantly affected the microhardness difference. However, in all products, the microhardness difference between the STD and MED modes was less than 10%. In all irradiation modes, microhardness was linearly correlated with the filler content of the tested specimens. The effect of mode on the difference of the polymerization shrinkage was not uniform even though the shrinkage values decreased following the order of the STD > EXP > MED mode. An inverse correlation was found between filler content and polymerization shrinkage in all irradiation modes and specimen thicknesses. In spite of the statistical significance of the irradiation modes on the difference of the measured values, their numeric difference was not great. Especially, compared with the STD and MED mode, the exponentially increasing light (EXP mode) produced no noticeable difference except for a delayed shrinkage process.

Relative curing degree of polyacid-modified and conventional resin composites determined by surface Knoop hardness

OBJECTIVES

The aim of the present study was to investigate the relative curing degree at a depth of 2 mm of several polyacid-modified composites (PAM-Cs) as a function of shade.

METHODS

The Knoop hardness of the irradiated top and non-irradiated bottom surfaces of 2 mm thick samples of the PAM-Cs Hytac, F2000, Glasiosite, Dyract, Dyract AP, and Compoglass F and of the resin composites Z100, Herculite Enamel XRV, and Durafill VS, were determined for shades A2 and A4.

RESULTS

The top and bottom hardness of F2000 and Glasiosite ranged between that of the two composites Herculite and Z100. Compoglass, Dyract and Dyract AP had a lower top and bottom hardness than the hybrid composites, but higher than that of the microfilled composite Durafill. The top hardness of Hytac compared with that of the first group, whereas the bottom hardness compared with the second group. The bottom-to-top KHN ratio reflecting the relative curing degree at a depth of 2 mm was less than 80% for the two shades of Hytac and Compoglass as well as for the A4 shade of Dyract AP and Herculite.

SIGNIFICANCE

A hard top surface of a PAM-C is not an indication of adequate in depth polymerization. Shade A2 results in significantly greater values for the curing degree compared to shade A4, the effect depending quantitatively on the formulation of the material. Some formulations of PAM-C do not reach an adequate curing degree at a depth of 2 mm so that it is recommended to apply the incremental technique even in box-only cavities with layers of maximum 2 mm.

Wear and fatigue behavior of nano-structured dental resin composites

Theoretically, nano-structured dental resin composites are purported to have increased wear and fatigue resistance compared with microfill composites and may favor the achievement of restoratives with better long-term performance. This study sought to assess the behavior of nano-structured composites resulting from either abrasion and fatigue loading. Ten specimens (12 x 5 x 2.5 mm) were prepared from each of five composites: Ceram-X mono, Filtek Supreme, Grandio, Premise, and Heliomolar (serving as the microfill control). A surface profile was recorded using a three-dimensional profiling system, and the specimens were subjected to 10(5) cycles of three-body abrasion in the new OHSU oral wear simulator. A second profile was generated and the before and after profiles were fit and analyzed. The volume loss and maximum depth of the wear facet on each specimen were calculated. Another 30 specimens (25 x 2 x 2 mm) were tested for flexural fatigue limit (FFL) in four-point bending via the staircase method. The test was carried out until 10(4) cycles were completed or until fracturing the specimen. One-way ANOVA and Tukey's test demonstrated greater volumetric loss for Grandio and Ceram-X than that observed for the remaining composites. Kruskal-Wallis and the least significant difference test ascertained that Heliomolar, Grandio, and Supreme showed significantly higher FFL than Ceram-X and Premise. In terms of wear and fatigue resistance, nano-structured composites may perform either similarly or comparatively worse than a microfilled composite.

Influence of light curing source on microhardness of composite resins of different shades

INTRODUCTION

The evolution of light curing units can be noticed by the different systems recently introduced. The technology of LED units promises longer lifetime, without heating and with production of specific light for activation of camphorquinone. However, further studies are still required to check the real curing effectiveness of these units.

PURPOSE

This study evaluated the microhardness of 4 shades (B-0.5, B-1, B-2 and B-3) of composite resin Filtek Z-250 (3M ESPE) after light curing with 4 light sources, being one halogen (Ultralux - Dabi Atlante) and three LED (Ultraled - Dabi Atlante, Ultrablue - DMC and Elipar Freelight - 3M ESPE).

METHODS

192 specimens were distributed into 16 groups, and materials were inserted in a single increment in cylindrical templates measuring 4mm x 4mm and light cured as recommended by the manufacturer. Then, they were submitted to microhardness test on the top and bottom aspects of the cylinders.

RESULTS

The hardness values achieved were submitted to analysis of variance and to Tukey test at 5% confidence level. It was observed that microhardness of specimens varied according to the shade of the material and light sources employed. The LED appliance emitting greater light intensity provided the highest hardness values with shade B-0.5, allowing the best curing. On the other hand, appliances with low light intensity were the least effective. It was also observed that the bottom of specimens was more sensitive to changes in shade.

CONCLUSION

Light intensity of LED light curing units is fundamental for their good functioning, especially when applied in resins with darker shades.

A comparison of fatigue resistance of three materials for cusp-replacing adhesive restorations

OBJECTIVES

To investigate the fatigue resistance and failure behaviour of cusp-replacing restorations in premolars using different types of adhesive restorative materials.

METHODS

A class 2 cavity was prepared and the buccal cusp was removed in an extracted sound human upper premolar. By using a copy-milling machine this preparation was copied to 60 human upper premolars. In groups of 20 premolars each, direct resin composite restorations, indirect resin composite restorations and ceramic restorations were made. All restorations were cusp replacements made in standardized shape and with adhesive techniques. Cyclic load (5 Hz) was applied starting with a load of 200 N (10,000 cycles) followed by stages of 400, 600, 800 and 1000 N at a maximum of 50,000 cycles each. Samples were loaded until fracture or to 2,10,000 cycles maximum. In case of fracture, the failure mode was recorded.

RESULTS

No differences were seen in fracture strength between the three groups (Wilcoxon P = 0.16). No differences were observed with regard to failure mode above or below the cemento enamel junctions (chi2 P = 0.63). The indirect resin composite and ceramic restorations showed significantly more combined cohesive and adhesive fractures than the direct resin composite restorations, which showed more adhesive fractures (chi2 P = 0.03 and 0.002).

CONCLUSIONS

The results of this study suggest that ceramic, indirect resin composite and direct resin composite restorations provide comparable fatigue resistance and exhibit comparable failure modes in case of fracture, although the indirect restorations tend to fracture more cohesively than the direct restorations.

Effects of resin formulation and nanofiller surface treatment onin vitro wear of experimental hybrid resin composite

While adding nonbonded nanofillers and lowering the viscosity of the resin matrix have shown success in reducing deleterious polymerization stresses in dental composites, their effects on wear resistance is unknown. This study evaluated abrasion and attrition wear of experimental composites with varied resin viscosities [inherent to varied ratios of TEGDMA:UDMA:bis-GMA (47:33:16 wt%; 30:33:33 wt%; 12:33:51 wt%)] and nanofiller surface treatment (12.6 wt% silanated or unsilanated silica: OX-50; 0.04 microm). Specimens (n = 6) were light cured, aged in water at 37 degrees C for 7 days, and evaluated in the new OHSU oral wear simulator (100,000 cycles). Nonbonded nanofiller increased abrasion and attrition in the low and medium viscosity composites. Increase in resin viscosity increased abrasion and attrition in composites containing silanated nanofiller, with equivocal effects in composites containing unsilanated nanofiller. Nonbonded nanofiller can lower the overall wear resistance of some composite formulations. Increasing resin viscosity generally lowers the wear resistance, but had minimal effect on composites containing nonbonded nanofiller.

Effect of light dispersion of LED curing lights on resin composite polymerization

PURPOSE

This study evaluated the effect of light dispersion of halogen and LED curing lights on resin composite polymerization.

MATERIALS AND METHODS

One halogen (Optilux 501, SDS/Kerr, Orange, CA, USA) and five light-emitting diode (LED) curing lights (SmartLite iQ, Dentsply Caulk, Milford, DE, USA; LEDemetron 1, SDS/Kerr; FLASHlite 1001, Discus Dental, Culver City, CA, USA; UltraLume LED 5, Ultradent Products, South Jordan, UT, USA; Allegro, Den-Mat, Santa Maria, CA, USA) were used in this study. Specimens (8 mm diameter by 2 mm thick) were made in polytetrafluoroethylene molds using hybrid (Z100, 3M ESPE, St. Paul, MN, USA) and microfill (A110, 3M ESPE) composite resins. The top surface was polymerized for 5 seconds with the curing light guide tip positioned at a distance of 1 and 5 mm. Degree of conversion (DC) of the composite specimens was analyzed on the bottom surface using micro-Fourier Transform Infrared (FTIR) spectroscopy (Perkin-Elmer FTIR Spectrometer, Wellesley, PA, USA) 10 minutes after light activation. DC at the bottom of the 2 mm specimen was expressed as a percentage of the mean maximum DC. Five specimens were created per curing light and composite type (n=5). Percent mean DC ratios and SDs were calculated for each light under each testing condition. Data were analyzed by analysis of variance (ANOVA)/Tukey's test (alpha = .05). A beam analyzer (LBA-700, Spiricon, Logan, UT, USA) was used to record the emitted light from the curing lights at 0 and 5 mm distances (n=5). A Top Hat factor was used to compare the quality of the emitted beam profile (LBA/PC, Spiricon). The divergence angle from vertical was also determined in the x- and y-axes (LBA/PC). Mean values and SDs were calculated for each light under each testing condition (0 and 5 mm, x- and y-axes) and analyzed by a two-way ANOVA/Tukey's test (alpha = .05).

RESULTS

For DC ratios, significant differences were found based on curing light and curing distance (p < .05). At 1 mm, Optilux 501 and FLASHlite 1001 produced significantly higher DC ratios with the hybrid resin composite. No differences were found among lights with the microfill at 1 mm. At 5 mm, SmartLite iQ, FLASHlite 1001, LEDemetron 1, and UltraLume LED 5 produced significantly higher DC ratios with the hybrid resin composite, whereas LEDemetron 1 and SmartLite iQ produced significantly higher DC ratios with the microfill resin composite. The UltraLume LED 5, Allegro, and Optilux 501 had significant reductions in mean DC ratios at curing distances of 1 and 5 mm with both resin composite types. For dispersion of light, significant differences were found in Top Hat factor and divergence angle (p < .001). SmartLite iQ had overall the highest Top Hat factor and lowest divergence angle of tested lights. A linear regression analysis relating pooled DC with pooled Top Hat factors and divergence angles found a very good correlation (r2 = .86) between dispersion of light over distance and the ability to polymerize resin composite.

CLINICAL SIGNIFICANCE

The latest generation of LED curing lights provides DC ratios similar to or better than the halogen curing light at a curing distance of 5 mm. Dispersion of light plays a significant role in the DC of resin composite. To maximize curing effectiveness, light guides should be maintained in close proximity to the surface of the light-activated restorative material.

A pilot study of a simple photon migration model for predicting depth of cure in dental composite

OBJECTIVES

The purpose of this study was to build a photo migration model to calculate the radiant exposure (irradiancextime) in dental composite and to relate the radiant exposure with extent of cure using polymer kinetics models.

METHODS

A composite (Z100, Shade A2) cylinder (21 mm diameter by 15 mm deep) was cured with a tungsten-halogen lamp emitting 600 mW/cm2, 1 mm above the composite for 60s. For each of the 2x1 mm grids along the longitudinal cross section (diameter versus depth), the degree of conversion (DC) and hardness (KHN) were measured to construct the curing extent distribution. The inverse adding-doubling method was used to characterize the optical properties of the composite for the Monte Carlo model simulating the photon propagation within the composite cylinder. The calculated radiant exposure (H) distribution along the cross section was related to the curing extent DC/DC(max) distribution and fitted with two polymer curing kinetics models, the exponential model DC=DC(max)[1-exp((ln0.5)H/H(dc)(50%))] and Racz's model [Formula: see text] , where H(dc)(50%) is a fitting parameter representing the threshold for 50% of the maximum curing level.

RESULTS AND SIGNIFICANCE

The absorption and scattering coefficients of uncured composite were higher than that of cured composite at wavelengths between 420 and 520 nm. A roughly hemi-spheric distribution of radiant exposure in the Monte Carlo simulation result was comparable with the curing profiles determined by both DC and KHN. The relationship between DC (or KHN) and H agreed with the Racz model (r2=0.95) and the exponential model (r2=0.93). The H(dc)(50%) was 1.5(0.1), equal for the two models (P<0.05). The estimated radiant exposure threshold for 80% of the maximum curing level was between 3.8 and 8.8 J/cm2. The simulation results verify that the radiant exposure extends to a greater depth and width for composite with lower absorption and scattering coefficients. Given the optical properties and the geometry of the composite, and the spectrum and the geometry of the light source, the Monte Carlo simulation can accurately describe the radiant exposure distribution in a composite material to predict the extent of cure.