Visible light-activated resins--depth of cure

Effect of high-irradiance light curing on exposure times and pulpal temperature of adequately polymerized composite

This study investigated the effect of high-irradiance light-curing on exposure time and pulpal temperature of adequately-cured composite. Composite placed in a molar preparation was cured using high-irradiance light-curing units (Flashmax P3, Valo, S.P.E.C. 3 LED, Cybird XD) and tested for hardness occlusal-gingivally. The first group had exposure times set according to manufacturer settings (recommended), second group to yield 80% of maximum hardness at the 2 mm depth (experimental), and third group was set at 20 s (extended). Exposure time necessary to adequately polymerize the composite at 2 mm depth was 9 s for the Cybird XD and Valo and 12 s for S.P.E.C. 3 LED and Flashmax P3. None of the high-irradiance light-curing units adequately polymerized the composite at the manufacturer-recommended minimum-exposure times of 1-3 s. Exposure times necessary to adequately polymerize composite at 2 mm resulted in a maximum pulpal-temperature increase well below the temperature associated with possible pulpal necrosis.

Shear Bond Strengths of Brackets Bonded with a New Self-Adhering Resin Composite

Purpose

To determine the shear bond strength (SBS) of orthodontic brackets bonded with a self-adhering resin composite.

Methods

45 freshly extracted mandibular incisors were randomly divided into 3 groups (n = 15 per group). Brackets were bonded with 3 bonding systems and cured with an LED light. Brackets were bonded with a new self-adhering resin cement in the study group. A conventional etch-and-rinse adhesive system and a self-etching adhesive system were used for comparison. After bonding, the SBS of the brackets were tested with a universal testing machine.

Results

Analysis of variance indicated a significant difference between groups (p<0.001). The highest values for SBS were measured in group I (21.80 ± 2.57 MPa). The SBS was significantly lower in groups II and III than in group I (P<.001). The lowest values for SBS were measured in group III (5.90 ± 0.90 MPa). SBS was significantly higher in group II than in group III (p<0.001).

Conclusions

The self-adhering adhesive system showed a clinically insufficient SBS, whereas the SBS of conventional etch-and-rinse adhesive system and the self-etching adhesive system were significantly higher and clinically sufficient.

A comparison of shear bond strength of orthodontic brackets bonded with four different orthodontic adhesives

Objectives:

The objective of this study is to compare the shear bond strength (SBS) of stainless steel (SS) orthodontic brackets bonded with four different orthodontic adhesives.

Materials and Methods:

Eighty newly extracted premolars were bonded to 0.022 SS brackets (Ormco, Scafati, Italy) and equally divided into four groups based on adhesive used: (1) Rely-a-Bond (self-cure adhesive, Reliance Orthodontic Product, Inc., Illinois, USA), (2) Transbond XT (light-cure adhesive, 3M Unitek, CA, USA), (3) Transbond Plus (sixth generation self-etch primer, 3M Unitek, CA, USA) with Transbond XT (4) Xeno V (seventh generation self-etch primer, Dentsply, Konstanz, Germany) with Xeno Ortho (light-cure adhesive, Dentsply, Konstanz, Germany) adhesive. Brackets were debonded with a universal testing machine (Model No. 3382 Instron Corp., Canton, Mass, USA). The adhesive remnant index (ARI) was recordedIn addition, the conditioned enamel surfaces were observed under a scanning electron microscope (SEM).

Results:

Transbond XT (15.49 MPa) attained the highest bond strength. Self-etching adhesives (Xeno V, 13.51 MPa; Transbond Plus, 11.57 MPa) showed clinically acceptable SBS values and almost clean enamel surface after debonding. The analysis of variance (F = 11.85, P < 0.0001) and Chi-square (χ² = 18.16, P < 0.05) tests revealed significant differences among groups. The ARI score of 3 (i.e., All adhesives left on the tooth) to be the most prevalent in Transbond XT (40%), followed by Rely-a-Bond (30%), Transbond Plus with Transbond XT (15%), and Xeno V with Xeno Ortho (10%). Under SEM, enamel surfaces after debonding of the brackets appeared porous when an acid-etching process was performed on the surfaces of Rely-a-Bond and Transbond XT, whereas with self-etching primers enamel presented smooth and almost clean surfaces (Transbond Plus and Xeno V group).

Conclusion:

All adhesives yielded SBS values higher than the recommended bond strength (5.9-7–8 MPa), Seventh generation self-etching primer Xeno V with Xeno Ortho showed clinically acceptable SBS and the least amount of residual adhesive left on the enamel surface after debonding.

Bond strength of orthodontic brackets with new self-adhesive resin cements

INTRODUCTION

In this investigation, we determined the shear bond strength (SBS) of metallic and ceramic orthodontic brackets with new self-adhesive cements.

METHODS

One hundred extracted premolars were used. They were sterilized and their roots embedded in stone bases, with the facial surfaces perpendicular to the bottom of the bases. The teeth were divided into 2 main groups, to receive metallic or ceramic brackets (Victory series 3M Unitek, Monrovia, Calif). In each group, the specimens were further divided into 5 subgroups according to the cement used: an etch-and-rinse control, Transbond-XT (3M Unitek); a resin cement with self-etch primer, Esthetic Cement system NC-100, (Kuraray, Okayama, Japan); and 3 self-adhesive resin cements: Rely-X Unicem (3M ESPE, Seefeld, Germany), Biscem DC (Bisco, Schaumburg, Ill), and Breeze (Pentron, Wallingford, Conn). Ten brackets were cemented in each subgroup. The specimens were stored in distilled water at 37 degrees C for 7 days and subjected to 3000 thermocycles between 5 degrees C and 55 degrees C. The brackets were then debonded in shear with a testing machine.

RESULTS

Mean values for the metallic brackets cemented with Transbond XT, Esthetic Cement system, Rely-X Unicem, Biscem DC, and Breeze were 18.6, 6.0, 6.0, 2.2, and 8.4 MPa, respectively. For the ceramic brackets, the values were 22.7, 17, 7.7, 1.6, and 9.5 MPa, respectively. Analysis of variance (ANOVA) showed significant differences among the subgroups (P <0.05) for both bracket types. For the ceramic brackets, the Tukey test showed no statistical difference in mean SBS between Transbond XT and Esthetic Cement system.

CONCLUSIONS

The SBS values of brackets cemented with etch-and-rinse cement were significantly higher than those of the 3 self-adhesive cements. However, when the self-etch adhesive, Esthetic Cement system, was used with ceramic brackets, no significant difference was found in the SBS compared with Transbond XT (P = 0.052).

Light transmission on dental resin composites

OBJECTIVES

The purposes of this study was: (1) to examine the light transmittance characteristics of two light-cured resin composites, for different thickness, (2) to correlate the light transmittance through the resin composites and the filler contents, and (3) to determine the penetration depth of the light as a function of the wavelength.

METHODS

Two resin composites (Filtek Z250, shade A2 and Filtek Supreme XT, shade A2E) were used. Specimens of six different thicknesses (0.15, 0.25, 0.30, 0.36, 0.47 and 0.75 mm) were prepared (n=3). The transmittance at wavelengths from 400 to 800 nm was measured using a UV-visible spectrophotometer, before and after light polymerization.

RESULTS AND SIGNIFICANCE

Significant differences were found in the wavelength dependence of transmittance between the two materials, and between the unpolymerized and polymerized stages of each resin composite. At lower wavelengths, the light transmittance of the Filtek Supreme XT resin composite was lower than the Filtek Z250. At the higher wavelengths, however, Filtek Supreme XT presented higher light transmittance. For both resin composites, the penetration depth was higher after polymerization. However, Filtek Supreme XT showed a higher gain in transmittance at the 0.15 mm thickness. The difference in light transmittance characteristics of the resin composites may affect their depth of polymerization.

Hardness comparison of bulk-filled/transtooth and incremental-filled/occlusally irradiated composite resins

STATEMENT OF PROBLEM

Use of a bulk-fill/transtooth composite resin insertion/irradiation technique may not provide as well polymerized a restoration as when using a conventional incremental placement/irradiation technique. Little information exists as to how the hardness of restorations produced by the 2 techniques compare.

PURPOSE

The purpose of this study was to determine the effect of composite resin placement and an irradiation technique on the axial hardness at various depths in a Class I composite resin to include the influence of composite resin filler classification and shade.

MATERIAL AND METHODS

Cylindrical Class I preparations were made in 70 recently extracted human molars and restored with either a light (A1) or dark shade (A4) of a microfill, microhybrid, or nanohybrid composite resin, or with a single shade of a translucent material. Half were placed using a conventional 2-mm-thick incremental-fill/occlusal irradiation technique, and half using a bulk-fill/transtooth irradiation method (n=5). Specimens were sectioned occluso-apically and axial Knoop hardness values were obtained at depths of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm. Hardness at 0.5 mm was used as a control to compare to deeper readings. Statistical analyses consisted of multiple ANOVAs and Dunnett's post-hoc tests performed at appropriately determined significance levels.

RESULTS

For 3 multishaded materials tested, axial hardness values were relatively unaffected by composite resin shade or filler classification for the incremental technique, but were significantly affected by these factors when using the bulk-fill/transtooth irradiation method. A single shade translucent material was not affected in either the bulk or incremental condition.

CONCLUSIONS

Use of a bulk-fill/transtooth irradiation technique for composite resin placement does not result in axial hardness values equivalent to that of an incremental-fill/occlusal irradiation technique.

Effect of composite shade, increment thickness and curing light on temperature rise during photocuring

OBJECTIVES

To examine the effect of composite shade, increment thickness and curing light characteristics on the temperature rise associated with composite photocuring.

METHODS

Four shades (C2, A4, B1 and B3), four sample thicknesses (2, 3, 4 and 5 mm) of a hybrid resin composite and two curing units, one with two modes of curing, were investigated. The composite samples were packed in polytetrafluoroethylene (PTFE) moulds and cured for 40 s. Samples cured with the ramp curing mode were irradiated for only 20 s. Temperature rises on the undersurface of the curing resin composite were measured using an infrared scanning system.

RESULTS

Shade C2 produced the highest maximum temperature of all shades (56.7 degrees C). Thinner samples produced greater temperature rises (2mm induced 60.9 degrees C, 5 mm induced 45.7 degrees C). Samples cured with Optilux 501 unit produced greater temperature rises (60.9 degrees C) than those cured with Dentsply unit (56.2 degrees C).

CONCLUSIONS

There was a quantifiable amount of heat generated during visible light curing of resin composite. The amount of heat generated was influenced by shade selected, thickness of material and characteristics of the light curing unit.

Composite Depth of Cure Obtained with QTH and LED Units Assessed by Microhardness and Micro-Raman Spectroscopy

Lower depth of cure with the LED unit, compared to the QTH unit, is associated with different light scattering due to differences in spectral emission.

Influence of shade and irradiation time on the hardness of composite resins

This study tested the following hypotheses: 1. increasing light irradiation time (IT) produces greater values of superficial hardness on different depths (0 and 3 mm); and 2. a dark shade composite (A3) needs longer IT than a light shade composite (A1) to produce similar hardness. Disk-shaped specimens (n=24 per shade) were fabricated using a 3-mm-thick increment of composite resin (Z100). Specimens were randomly assigned to 3 groups (n=8) according to the IT (400 mW/cm2) at the upper (U) surface: A1-10 and A3-10: 10 s; A1-20 and A3-20: 20 s; A1-40 and A3-40: 40 s. Specimens were stored in black lightproof containers at 37ºC for 24 h before indentation in a hardness tester. Three Vickers indentations were performed on the U and lower (L) surfaces of each specimen. The indent diagonals were measured and the hardness value calculated. The results were analyzed statistically by ANOVA and Tukey's test (alpha=0.05). Statistically significant differences were found between U and L surfaces of each composite shade-IT combination (p=0.0001) and among the ITs of same shade-surface combination (p=0.0001), except between groups A1-20U and A1-40U, confirming the study hypothesis 1 and partially rejecting the hypothesis 2.

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.

Residual monomer in a composite resin after light-curing with different sources, light intensities and spectra of radiation

The purpose of this study was to investigate the amount of residual monomer in a composite resin after light-curing with different sources, light intensities and spectra of radiation. The resin specimens (4 mm in diameter; 2 mm thick) (n=5) were inserted in Plexglass matrixes and light-cured with a halogen lamp, LED and PAC units for 40, 40 and 5 s, respectively. The polymerized specimens were ground and 25 mg of each specimen were immersed in 8 mL 96% ethanol for 24 h to extract the residual monomer. The amount of residual monomer was determined by the standard straight line method. The light spectrum of the different LCUs was shot and processed through researches of the absorbation of the standard solutions at a level of lambda= 275 nm and their extracts. Data were analyzed statistically by variational dispersion analysis and Tukey-Kramer test at 5% significance level. It was observed that, the halogen lamp produced the smallest amount of monomer under sufficient light intensity. The spectrum of light radiation of PAC was within the limits of 450- 490 nm and was of extremely high intensity. This imposes the use of short-limited light impulses, but also might lead to deterioration quality of polymerization. The LED unit had the best spectral radiation. An increase of light intensity was proved necessary.

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.

How light source and product shade influence cure depth for a contemporary composite

Directly placed light activated resin based composite restorations are becoming increasingly popular for restoring cavities in posterior teeth. Marketing and patient as factors well as operator preference are behind this trend. Achieving adequate depth of cure is critical to the success of these restorations. Recently a number of second generation light emitting diode (LED) light activation units have been marketed and the manufacturer of one of these claims that it is capable of curing in half the time of its predecessor. This study tested that claim using a companion composite from the same manufacturer. The relationship between cure depth, shade changes on curing and opacity were also assessed. Under the limitations of the current investigation the results indicated that the second generation LED unit in question met the manufacturer's claim for halving cure time. Depths of cure approached those of a control halogen unit in half the 40 s radiation time of the latter. For the product tested depth of cure is strongly linked to material opacity.

Comparison of bond strength between a conventional resin adhesive and a resin-modified glass ionomer adhesive: An in vitro and in vivo study

The objectives of this study were (1) to compare the in vivo survival rates of orthodontic brackets bonded with a resin-modified glass ionomer adhesive (Fuji Ortho LC; GC America, Alsip, Ill) after conditioning with 10% polyacrylic acid and a conventional resin adhesive (Light Bond; Reliance Orthodontic Products, Itasca, Ill) bonded with 37% phosphoric acid, (2) to compare the in vitro bond shear/peel bond strength between the 2 adhesives, (3) to determine the mode of bracket failure in the in vivo and in vitro tests according to the adhesive remnant index (ARI), and (4) to compare the changes in surface morphology of enamel surface after etching or conditioning with 10% polyacrylic acid, with scanning electron microscopy. In the in vitro study, 50 extracted premolars were randomly divided into 4 groups: brackets bonded with Fuji Ortho LC or Light Bond adhesive that were debonded after either 30 minutes or 24 hours. Bond strengths were determined with a testing machine at a crosshead speed of 1 mm/min. Data were analyzed with analysis of variance and a paired Student t test. The in vivo study consisted of 398 teeth that were randomly bonded with Fuji Ortho LC or Light Bond adhesive in 22 subjects with the split-mouth technique. Bracket survival rates and distribution were followed for 1.3 years. Data were analyzed with Kaplan-Meier product-limit estimates of survivorship function. The in vitro study results showed significant differences (P <.05) among the adhesives and the debond times. Light Bond had significantly greater bond strengths than Fuji Ortho LC at 24 hours (18.46 +/- 2.95 MPa vs 9.56 +/- 1.85 MPa) and 30 minutes (16.19 +/- 2.04 MPa vs 6.93 +/- 1.93 MPa). Mean ARI scores showed that Fuji Ortho LC had significantly greater incidences of enamel/adhesive failure than Light Bond adhesive (4.9 vs 4.1). For the in vivo study, no significant differences in failure rate, sex, or location in dental arch or ARI ratings were found between the 2 adhesives. These results suggest that, compared with conventional resin, brackets bonded with resin-modified glass ionomer adhesive had significantly less shear bond strength in vitro. However, similar survival rates of the 2 materials studied after 1.3 years indicate that resin-reinforced glass ionomers can provide adequate bond strengths clinically. The weaker chemical bonding between the adhesive and the enamel might make it easier for clinicians to clean up adhesives on the enamel surface after debonding.

Color stability of resin matrix restorative materials as a function of the method of light activation

The purpose was to investigate the influence of curing devices and curing times on the yellow value (b-value) of composites, ormocers and compomers after performing a suntest (EN ISO 7491). Eight samples of Charisma (CH), Durafill (DU), Definite (DE), and Dyract AP (DY) each were light cured with Translux Energy (tungsten halogen light) for 20, 40 or 60 s and with Apollo 95-E (plasma light) for 3, 10 or 20 s. All samples were subjected to a suntest. Before and after the suntest the yellow values (b-values) were determined and the change (Delta b) was calculated. When cured with Translux Energy for 20 and 40 s DU, CH, and DY revealed significantly negative Delta b-values. The b-value of DE remained nearly constant. When cured for 60 s, DU and DE shifted to more yellow while CH and DY still bleached a little. When cured with Apollo 95-E, a dramatic bleaching process of all materials investigated occurred after the suntest (significant negative Delta b). It may be concluded that the bleaching of composites, ormocers and compomers depends on (i) the used light curing device and (ii) the chosen curing times. The tungsten halogen light provided highly significantly superior results.

Bending strength and depth of cure of light-cured composite resins irradiated using filters that simulate enamel

This study evaluates the light-attenuating effects of enamel on the properties of light-cured restorative resins using simple experimental filters. Three filters were designed to replicate the light transmittance characteristics of 0.5, 1.0 and 1.5 mm thick human enamel. The bending strength, depth of cure, and levels of residual monomer for 12 shades of three commercial light-cured composite resins were examined. These resins were cured either using direct irradiation from a light source or irradiation through one of the filters. For all materials, the bending strength and depth of cure of specimens irradiated through a filter were lower and the levels of residual monomer were higher than those found in specimens irradiated directly. The results indicate that the light-attenuating effect of enamel reduces the polymerization efficiency, resulting in poorer mechanical properties of light-cured composite resins.

Effect of energy density on properties and marginal integrity of posterior resin composite restorations

OBJECTIVES

The purpose of this study was to determine the minimal extent of cure required by the base of a Class 2 resin composite restoration (Z250, 3M ESPE, St Paul, MN, USA) that allows it to support the rest of the restoration and maintain its marginal seal under simulated clinical conditions.

METHODS

Resin composite (Z250, 3M ESPE, St Paul, MN, USA) was placed incrementally or in bulk into Class 2 preparations in extracted human molar teeth and exposed to various light-curing energy densities. The restorations were subjected to 1000 thermal cycles (5-55 degrees C) and 500,000 fatigue cycles from 18 to 85 N using a stainless-steel sphere. Marginal integrity was evaluated using visual rating (ridit analysis) and microleakage. Degree of conversion (DC) and Knoop hardness (KHN) were determined at the occlusal and gingival surfaces using a reusable tooth template with identical preparation dimensions. Percentage of maximum DC and KHN were determined. Mechanical properties were tested in resin composite bars having similar KHN values as the resin composite at the gingival margins.

RESULTS

Energy density had a significant effect on gingival marginal defects as determined by ridit analysis but not on microleakage. Water had a significant dissolving effect on gingival margin integrity at very low degrees of conversion and energy densities (4000 mJ/cm2). There was no overall significant effect of thermal-mechanical stressing on gingival marginal defects or microleakage.

SIGNIFICANCE

Based on ridit analysis, a recommended lower limit of gingival margin acceptability in the bulk-filled Z250 resin composite restoration was created by 80% of maximum conversion, 73% of maximum hardness and approximately 70% of maximum flexural strength and modulus in the gingival marginal area.

Polymerization of dental composite resins using plasma light

Five visible light-cured composite resins used as restoration or adhesive materials in dentistry, were irradiated with high energy plasma light (1300 mW/cm2), and contraction, rate of contraction, irradiation-induced temperature were analysed. A comparison was carried out with the same materials irradiated with a conventional halogen light (400 mW/cm2). The exposure to the photoactivating lights was either continuously or sequentially in three or more intervals with 10 min between intervals. Comparing the lengths of exposure of both lights, which induced the same contraction in a given material, it was found that the exposure length to the plasma light was greatly reduced, when compared with the exposure length of the halogen light (1:10). Frequently, the final contraction of plasma-irradiated materials was lower, whereas the rate of contraction, as indicated by the linear dimensional variation curves obtained by laser beam scanning method, did not show significant differences between the two lights. The temperature increase induced by plasma light on the material did not exceed the temperature induced by conventional light.

The application of magnetic resonance microimaging to the visible light curing of dental resins. 3. Stray-field nuclear magnetic resonance imaging (STRAFI)

OBJECTIVE

To investigate the application of stray-field nuclear magnetic resonance imaging (STRAFI) to the visible light curing of dental restorative materials. STRAFI can overcome peak broadening associated with the conventional magnetic resonance microimaging (MRM) of glassy polymers, and has the potential to image dental restorative resins at both low and high degrees of conversion.

METHODS

Cylindrical composite specimens were light-cured from one end to produce some that were fully cured throughout their length and others that were fully cured at one end and uncured at the other. A one-dimensional probe was used to measure the magnetisation in 40 microm thick slices at 100 microm intervals along the length of the specimen. A quadrature pulse sequence was applied and the magnetisation decay recorded in a train of eight echoes.

RESULTS

A value for T(2) could be obtained only for the polymer (59+/-16 microms), therefore the echoes were summed to give an approximate indication of the degree of conversion. The echo sum for the monomer was significantly higher than that for the polymer. Differences in composite shade and cure time produced changes in the cure profiles.

SIGNIFICANCE

STRAFI produced measurements for both monomer and polymer in all stages of conversion that allowed cure profiles to be produced. Summing the decay echoes produced a qualitative measure of the condition of the material in the selected slice. The same data can be used to calculate T(2), a quantitative parameter. This first investigation has demonstrated that STRAFI is well suited to polymerisation studies.

The application of magnetic resonance microimaging to the visible light curing of dental resins. Part 2. Dynamic imaging by the FLASH-MOVIE pulse sequence

OBJECTIVES

To investigate the application of a rapid NMR imaging pulse sequence, FLASH-MOVIE, to the visible light curing of dental restorative materials.

METHODS

The light guide was applied at one end of a cylindrical specimen of visible light curing unfilled resin and the light directed along the cylinder. During polymerisation an NMR imaging pulse sequence, FLASH-MOVIE, was run at 15s intervals with a 50 ms repetition time. The image of a 1mm thick vertical slice was recorded with a (125 microm)2 pixel size.

RESULTS

Images with good contrast were obtained from all resin monomers. The image intensity from the polymer was indistinguishable from the background intensity. Thus, the progress of light activated polymerisation in the material could be followed in real time through a series of up to 16 images. Initially the image intensity increased in the material closest to the light guide, then decreased over time to zero. Concomitant with this fall, a "cure-front" moved through the specimen.

SIGNIFICANCE

The FLASH-MOVIE NMR pulse sequence applied to microimaging of dental diacrylate resins can be used to obtain a dynamic record of visible light curing. A more refined experimental protocol will be required to apply this unique data to models proposed for this polymerisation mechanism.

Curing Dental Resins and Composites by Photopolymerization

The development and continued evolution of photopolymerizable dental materials, particularly dental composite restoratives, represent a significant, practical advance for dentistry. The highly successful integration of the light-activated curing process for dental applications is described in this review. The basic mechanisms by which the photoinitiators efficiently convert monomers into polymers are discussed along with the variety of factors that influence the photopolymerization process. The conventional camphorquinone-amine visible light photoinitiator system used in most dental restorative materials is illustrated in addition to some alternative initiator systems that have been studied for dental materials applications.

CLINICAL SIGNIFICANCE

Photopolymerization has become an integral component of the practice of dentistry. A better appreciation of the photopolymerization process as well as its potential and limitations may aid the dentist in the delivery of both esthetic and restorative dental care.

Do dental composites always shrink toward the light?

Many of the current light-curing composite restorative techniques are rationalized in compliance with the theory that composite shrinks toward the light. Shrinkage directed toward the margins is believed to be responsible for the observed improved marginal properties. However, the dental literature does not consistently support this theory. Experimental determination of contraction patterns is very difficult. In this study, a finite element technique is used to analyze the direction of composite shrinkage as it cures. The process of polymerization can be characterized by pre- and post-gel phases. The stress developed in a restoration can be relieved quickly by the flow of material still in the pre-gel phase. Residual stresses arise after gelation. Both auto- as well as photo-curing composites were analyzed. In photo-curing composites, the gel-point varies throughout the material with the intensity of the light. Experimentally determined light transmittance data for different materials were used in the simulation. Degree of cure and time-dependent shrinkage properties were also included from experimental measurements. The analysis showed that the shrinkage direction was not significantly affected by the orientation of the incoming curing light, but instead was mostly determined by the bonding of the restoration to the tooth and by the free surfaces. Consequently, differences between the contraction patterns of auto- and photo-cure were minimal. It was concluded that composite does not shrink toward the light, but that the direction is predominantly determined by cavity shape and bond quality. Improved marginal properties should be pursued by the optimization of other factors, such as the polymerization process, the curing procedure, and the bond quality. The direction of shrinkage vectors in response to light position does not seem to be an appropriate criterion for the optimization of marginal quality.

Properties of four composite veneering materials polymerized with different laboratory photo-curing units

This study examined properties of four composite veneering materials polymerized with two different photo-curing units for the purpose of evaluating curing performance of the combination of composite material and curing unit. Two microfilled (Dentacolor and Thermoresin LC II) and two hybrid (Cesead II and Solidex) composite materials designed for prosthetic veneer were selected. The respective light sources of the units were a xenon (Dentacolor XS) and a metal halide (Hyper L II) lamp. The composite materials were exposed with the photo-curing unit for 60 s on each side (i.e. from top and bottom). Knoop hardness, compressive strength, flexural strength, flexural modulus, water absorption and water solubility were determined according to standardized testing methods. The specimens exposed with the metal halide unit generally exhibited greater Knoop hardness number, and compressive strength and lower solubility than those exposed with the xenon unit. A microfilled material (Thermoresin LC II) cured with the metal halide unit exhibited significantly improved results for all tests as compared with the same material cured with the xenon unit.

A survey of the efficiency of visible light curing units

OBJECTIVES

The purpose of this study was to survey the efficiency of visible light curing units in dental practices across Australia.

METHODS

Survey forms were distributed to representatives of 3M Health Care to complete when visiting dentists in their working areas. The information collected included the type and age of the unit, curing times used, history of maintenance, replacement of components, and the light intensity reading.

RESULTS

Of the 214 light curing units surveyed, approximately 27% recorded a light intensity of 200 mW cm-2 or less, a level regarded as inadequate to cure a 2-mm thick increment of composite resin. An additional 26% registered an output of between 201 and 399 mW m-2. This level would be considered acceptable with additional curing time; however, 44% of practitioners were curing for 20 s or less. A negative correlation was found between the age of the unit and the intensity recorded. Nearly 50% of respondents had never checked the light output of their unit.

CONCLUSIONS

The results indicate that just over one-half of the light curing units surveyed were not functioning satisfactorily. An obvious reduction in intensity was noted with the older units. There is a substantial lack of awareness among dentists of the need for maintenance and regular checking of the light intensity of these units.

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.

A clinical trial comparing the failure rates of two orthodontic bonding systems

A clinical trial was carried out to compare the failure rates of a visible light-cured fluoride-releasing material with a chemically cured adhesive. Fifty patients were used for the trial and 762 brackets were bonded in a split-mouth experimental design. Bracket failures were reported over the full period of orthodontic treatment. Statistical data analysis was carried out by means of chi 2 tests. The results of this study indicate that brackets, bonded with a chemically cured material (Lee Insta-bond, Lee Pharmaceuticals, EI Monte, Calif.), showed significantly fewer failures than brackets bonded with the light-cured material (Orthon, Orthon Dental Inc., Victoria, Canada). When the site of bond failure was evaluated in an overall manner, there were significantly more lower than upper, and more posterior than anterior bond failures.

Light-activated restorative materials: a method of determining effective radiation times

The aim of this investigation was to develop a test method that allows an operator to determine the appropriate radiation time for any light-activated material, irrespective of the characteristics of the light-activation unit or material to be cured. A computer-based radiometer was used to monitor the radiation energy transmitted through a number of light-activated materials during irradiation, and a method was devised to determine the predicted radiation time for any given material/shade/light unit combination based upon the change in transmission against time that occurred as the material polymerized. Additional test samples were cured for the predicted radiation times and upper and lower surface microhardness measurements were taken to verify the validity of the test method. Predicted times varied from 16 to 44 s for 2 mm thick samples of the materials/shade/light-activation units used. This method may allow operators to determine appropriate radiation times for any new light-activated restorative they decide to use with their own light-activation unit.

Depth of cure of radiation-activated composite restoratives--influence of shade and opacity

Inadequate depth of cure may reduce the longevity of visible light-activated composite restorations. Radiation-activated composites, originally marketed for the restoration of anterior teeth, have been developed and refined for restoring occlusal and proximal cavities in posterior teeth. Clinical factors such as the accessibility of the light source, the direction of the light, intervening tooth tissue and cavity depth may limit depth of cure. Unfortunately, a hard top surface is no indication of adequate polymerization at the base of the cavity. The current investigation used a penetrometer to evaluate the depth of cure of a range of recently introduced 'universal' composite materials and assess the influence of shade and opacity on depth of cure. Depth of cure of the enamel opacity is always greater than that of the same shade of dentine for each manufacturer's product range.

Analysis of optical transmission by 400-500 nm visible light into aesthetic dental biomaterials

The penetration of visible light into dental biomaterials is an essential factor in photoinitiation of setting reactions and in the optical aspects of dental aesthetics. Light of visible blue wavelengths, 400-500 nm, has been applied at normal angles to 0.2-5.0 mm sections of human dentine and representative ceramic, polymerceramic composites and hybrid glass-polyalkenoate materials. The integrated optical transmission has been determined for each material section. The data have been converted to absorbance values and analysed to check for mathematical conformity to the Beer-Lambert Law. It is found that conformity (typically, P < 0.01) to the linear Beer-Lambert Law is only attained by making a substantial correction for the intensity of light reflected from the surface of aesthetic biomaterials. This is otherwise expressed by distinguishing between true and apparent absorbance. From linear regression of apparent absorbance with section thickness, the intercept depends upon the logarithm of the surface-reflection ratio. This factor ranges from 30% to 90% in the materials investigated. It follows that there is a high degree of inefficiency in the transmission of visible light into and through aesthetic biomaterials for the purposes of photoactivation using existing technology. Means by which this limitation and inefficiency may be reduced are discussed. While the reflectivity of aesthetic biomaterials has been perceived by dental practitioners, the magnitude of this effect and its implications in connection with light-cured materials have not been analysed and emphasized hitherto.

Depth of cure of radiation-activated materials--effect of mould material and cavity size

A method for determining depth of cure of radiation-activated restorative materials has been developed. The effect of mould material and cavity size on depth of cure was determined using a digital penetrometer method. Values were obtained for three selected shades of three dentine products and, in addition, the same three shades of an enamel variant were assessed. All these values were obtained using a 4 mm diameter x 6 mm deep stainless steel mould.

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.

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.

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.

Local and systemic responses to dental composites and glass ionomers

For many years, the dental profession worked mainly with rather inert restorative materials that had a limited contact with vital tissue, and the opportunity for local and systemic complications was minimal. However, conditions have changed in recent years where the two leading non-mercury-containing materials, resin composites and glass-ionomer cements, are chemically active compounds and can have detrimental effects on pulp tissue. With the advent of light-curing techniques with incremental layering, resin component formulae that were formerly found to be quite irritating to the pulp have become less so with the elimination of the need for matrices and pressure for good adaptation to be gained. As experience revealed the deficiencies and dangers of ultraviolet-light-curing techniques, visible-light-curing systems were developed that provided greater depth of cure, a higher degree of polymerization with less shrinkage with incremental layers, and less porosity. When glass-ionomer cements (GICs) were first introduced, with just one acid (polyacrylic), pulpal responses were classified as bland. With the addition of many more acids to enhance certain characteristics and reduce the setting time, GICs have become more irritating, especially when used as luting agents in areas where the remaining dentin thickness is 0.5 mm or less. Gold foil and amalgam are inert and innocuous restorative materials but require pressure for condensation which creates an exaggerated inflammatory response. This presentation emphasizes the pulpal responses and side-effects of these non-mercury-containing restorative materials and how to keep them within an acceptable range of biocompatibility. Despite the lack of any substantial appearance of soft tissue and systemic responses to resin composites and GICs, the results of a survey of recent literature are included.

Light-activated glass polyalkenoate (ionomer) cements: the setting reaction

The setting reaction of two light-activated glass polyalkenoate cements has been investigated using differential thermal analysis and surface hardness measurements. One material was found to have two distinct phases to its setting reaction. Light activation resulted in a rapid initial set with a large exotherm. A slower setting reaction was detectable when the cement was allowed to set in the absence of light. The surface hardness of this cement increased for some time after the cessation of light activation, indicating continuation of the chemical setting reaction within the material. The ultimate hardness of the cement was significantly higher than its hardness at the termination of light activation. Light activation of the second material increased the rate of set of the cement but no evidence of a dual setting reaction was observed. This material remained very soft and flexible for over 1 h after light activation. A 30 s exposure with a visible light source produced some immediate hardening up to 1.5-2.0 mm below the surface for both materials.

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.

A study of bond strength between light- and self-cured orthodontic resin

Light-cured orthodontic composite resin has been widely advertised recently for use in bonding. However, the curability of light-cured resin when light waves are diffused through metal, ceramic, or resin brackets is doubtful and questionable. This study evaluated the effectiveness of a visible light source in curing the resin under a solid metal bracket, compared the tensile bond strength at different exposures, and analyzed the broken interface distribution between light-cured resin with various light exposure times and self-cured resin. The bond strength results revealed that the difference between light-cured resin (Transbond) with 60, 40, and 20 seconds of light exposure, respectively, and self-cured resin (Concise) was 1.05, 0.92, 0.61, and 0.71 kg/mm2, respectively. The bond strength of Transbond with 60 and 40 seconds of light exposure was greater than both the bond strength of Transbond with 20 seconds of light exposure and the strength of the self-cured resin of Concise, with statistical significance (p less than 0.01). There were also no statistical differences between Transbond with 60 and 40 seconds of light exposure or Transbond with 20 seconds of light exposure and Concise. The bond failure interfaces were located between the bracket and the resin, within the resin itself, or between the resin and the enamel. Tooth fragmentation was rarely found. There were no statistical differences (p greater than 0.05) among broken interfaces. This indicates that visible light is powerful in curing the visible light-activated composite resin under solid metal brackets.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect on bonding of curing through dentin

The aim of this study was to investigate the effect of the thickness of dentin (1, 2, or 3 mm) placed between the light guide and the composite resin on the shear bond strength to dentin treated with a simplified Gluma system. The effects of the thickness of composite resin (2 or 3 mm), irradiation time (20, 40, or 60 sec), and shade (universal or brown) were also examined. The results showed that the thickness of dentin influenced bonding and interacted with the other three variables. It is suggested that dentin located between the light guide and composite resin may attenuate the light aimed at the bonding interface in the same manner as a layer of composite resin.

Some factors influencing the depth of cure of visible light-activated composite resins

Some of the factors influencing the depth of cure of four composite resins of different composition were examined. Knoop hardness measurements were carried out at the surface and 1, 2, 3, 4 and 6 mm below the surface to which the light was applied. The hardness of the composites decreased with increasing depth and shorter exposure times. The composites continued to polymerize after removal of the light source. Composition of the composite resin has a major effect on the surface hardness and depth of polymerization.

Laser photopolymerization of dental materials with potential endodontic applications

Photopolymerizing resins were exposed to three different wavelengths of light emanating from the argon laser. It was determined that the most efficient wavelengths for photopolymerization of camphorquinone-activated resins were at 477 and 488 nm. The 514.5-nm wavelength was relatively ineffective in activating polymerization. Four camphorquinone-activated resins were placed in the root canals of teeth and tested for polymerization depth using a 488-nm wavelength laser beam coupled to an optical fiber 200 microns in diameter. In regard to polymerization depth, these materials ranked as follows: Genesis greater than Prisma-Fil greater than Prisma Microfine greater than Prisma VLC Dycal. Alterations in the positions of the optical fiber and the surface of the resin in the canal made only minor differences in polymerization depth of the samples. The results indicate that an argon laser coupled to an optical fiber could become a useful modality in endodontic therapy.

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

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

Output from visible-light activation units and depth of cure of light-activated composites

A simple and reproducible method for monitoring the intensity of radiation from composite light-activation units has been developed. The method depends upon the use of a cadmium sulfide photo-conductive cell, the electrical resistance of which varies with the amount of light falling upon its surface. Filters were used for selection of the wavelength of light that is thought to be most effective in activating polymerization. The use of broad-band wavelength filters failed to give results for light intensity that correlated with depth of cure. Narrow-band interference filters, having a band width of only 10 nm and being selective within the wavelength range of from 460 to 480 nm, produced results for light intensity that correlated with depth of cure. The depths of cure for various types of composite material were measured with use of a penetrometer that enabled the thickness of unpolymerized material at the base of the test mould to be determined. The depth of cure was inversely proportional to the attenuation of light caused by the composite resin at 470 nm. The relationship between depth of cure and light intensity at 470 nm was not a simple linear one over all intensity values. Above a certain critical value of intensity (about 550 lux for a 3.5-mm aperture in these experiments), the depth of cure appeared to be almost independent of intensity. Below this critical value, depth of cure fell markedly with decreasing intensity.

An in vitro evaluation of a visible light-cured resin as an alternative to conventional resin bonding systems

An in vitro study of 69 premolars was conducted to evaluate a visible light-cured resin system used in orthodontic bonding. The material was evaluated under various parameters to determine its relative value as an alternative to the conventional chemically activated resin systems. The 30-hour bond strength for the visible light-cured resin system was approximately one half of that found for a chemically cured resin system. Initial 1-hour bond strength of the visible light-cured resin system was found to be only 26% of the 30-hour bond strength. Enamel loss associated with debonding and subsequent cleanup of the visible light-cured resin was approximately one half of that found with the chemically cured, heavily filled resin. With the visible light-cured resin system, cleanup of remaining resin required the use of hand scalers only.

Effect of glass-ionomer base on composite resin hardness

The effect of the glass-mm, and 3 mm thicknesses were placed over glass-ionomer bases, and the microhardness of the external surface was compared with that of controls without a base. A significant reduction in surface hardness (p less than 0.05) was determined only for the 1-mm test specimens.

Analysis of photo-initiators in visible-light-cured dental composite resins

Seven commercial visible-light-cured (VL) dental composite resins were analytically studied for identification of the photo-initiator consisting of photo-sensitizer and reducing agent. Gas-liquid chromatography (GC) was used for the determination of the dilute components extracted from the composite resin. Mass spectroscopy (GC-MS) was used for confirmation of the qualitative data obtained by GC. The results showed that all composite resins examined included camphorquinone (CQ) as a photo-sensitizer. The concentration of CQ in the resin phase, however, ranged from 0.17 to 1.03% w/w. The composite resin with hybrid-sized filler tended to have a higher concentration of CQ than did the micro-filled composite resin. As for the reducing agent, two out of seven brands contained dimethylaminoethyl methacrylate (DMAEMA), and one included dimethyl-p-toluidine (DMPTI). The mixing ratio between CQ and the amine in these three composite resins also varied. Another four brands did not contain either DMAEMA or DMPTI, and would utilize different reducing agents.

Stiffness increase during the setting of dental composite resins

Changes in the dynamic Young's modulus under flexure of self-cured and light-cured composite resins during setting were demonstrated to reflect accurately the condition of the curing composite resins. The rate of stiffness increase during setting varied considerably with the product, and the light-cured composite resins generally showed a faster rate of increase of stiffness. In the initial stage, the Young's moduli were very low, especially for the self-cured composite resins. This accounts for the damage (adhesion disruption and cross-linking interruption) that can be inflicted on a freshly placed composite resin filling that is not yet mechanically stabilized. Therefore, such fillings are vulnerable to aggressive distortion for at least 10 to 15 minutes after placement.