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

Influence of irradiation distance on the mechanical performances of resin composites polymerized with high-irradiance light curing units

Background

The aim of this study was to evaluate the influence of increased irradiation distance on the flexural strength (FS), dentin micro-shear bond strength (μSBS), and the degree of conversion (DC) of bulk-fill flowable, conventional flowable, and packable resin composites.

Methods

The resin composites tested were Surefil® SDR™ (SDR), Filtek Z350 XT Flowable Restorative A2 shade (Z3F), and Filtek Z350 XT Universal Restorative A2 shade (Z3P). Specimens were cured at four irradiation distances (0, 2, 4, and 8 mm) with an Elipar DeepCure-S LED curing light for 20 s. FS tests were performed (n = 15) using bar-shaped specimens (8 mm × 2 mm × 2 mm) of the resin composites. μSBS tests were performed on the occlusal surfaces of extracted third molars from humans that were ground to expose dentin (n = 15). DC was measured by using Raman spectroscopy on the top and bottom surfaces of disk specimens (2-mm thick) (n = 3). To further investigate whether extended irradiation times could compensate for reduced irradiance, additional Z3P specimens were prepared, which were light-cured at 8-mm distances for 40 and 60 s and subjected to FS tests, μSBS tests, and Raman spectroscopy. Both two-way and one-way ANOVA were used for statistical analyses.

Results

Both FS and DC of Z3P specimens cured at an 8-mm distance were significantly lower than those cured at shorter distances (p < 0.05), whereas the FS and DC of the Z3F and SDR specimens were not significantly influenced by increasing distances. The μSBSs of the three types of resin composites reduced with increasing irradiation distances. The FS, μSBS, and DC of the Z3P specimen light-cured at 8 mm for 40 s were comparable to those of the Z3P specimen cured at 0 mm for 20 s.

Conclusions

Increasing the irradiation distance to 8 mm can have a deleterious influence on mechanical performances, including the FS, DC, and dentin μSBS, of the resin composites polymerized with high-irradiance light curing units.

Impact of light-curing distance on the effectiveness of cure of bulk-fill resin-based composites

Objective

To investigate the effect of light-curing distance on the effectiveness of cure (EC) of bulk-fill resin-based composites (RBCs).

Materials and methods

Two bulk-fill RBCs (a Tetric N-Ceram Bulk Fill (TN) and a Filtek Bulk Fill (FK)) are evaluated. Specimens (4 mm high) are cured for 20 s at different distances (0 mm (D0), 2 mm (D2), 4 mm (D4), 6 mm (D6) and 8 mm (D8)) and stored for 24 h in 100% relative humidity at 37 °C. The top and bottom surface hardness (SH) (n = 12) are assessed using a Knoop microhardness tester and the EC is calculated. The EC is characterized by the hardness ratio (HR) (mean bottom: top SH). An HR of 0.8 is used as the benchmark for an effective/adequate cure. Data are analyzed using one-way analysis of variance and Tukey’s post hoc test (α = 0.05). Correlations between the top and bottom surfaces are examined using the Pearson correlation (α = 0.05).

Results

For the TN, the HR at D8 is significantly lower than all other light-curing distances, while for the FK, it is significantly lower than D0 only.

Conclusion

The effect of light-curing distance on the EC of bulk-fill RBCs is material dependent. Notwithstanding the light-curing distance, the EC of the FK and TN is below the threshold HR value of 0.8 when photopolymerized for 20 s in 4 mm increments in black opaque molds.

Effect of light absence or attenuation on biaxial flexural strength of dual-polymerized resin cements after short- and long-term storage

OBJECTIVE

To evaluate whether biaxial flexural strength (BFS) of dual resin cements is affected by light absence or attenuation, storage time, or cements' chemical nature.

MATERIALS AND METHODS

One hundred and twenty disk-shaped specimens were made from each cement (non-self-adhesive cement and self-adhesive cement) using Teflon molds on a controlled temperature surface (35°C). Specimens were polymerized as follows (N = 30): self-cured, directly light-cured, light-cured at a distance of 6 mm between the light tip and the specimen, and through a 6-mm thick composite resin barrier (indirectly light-cured). Each group was divided (N = 10) for storage purposes (15 minutes, 24 hours, and 6 months). Specimens were placed into a biaxial-flexure jig and a vertical load was applied until failure. The BFS values were subjected to generalized linear models statistical analysis and Weibull distributions (α = 0.05).

RESULTS

After 15 minutes aging, neither material achieved enough polymerization to perform the BFS test when polymerized using the self-curing mode. The self-adhesive product demonstrated much lower variation in strength with storage time than did the non-self-curing cement.

CONCLUSIONS

Attenuated/light-curing reduced BFS values only for 15-minutes storage period for both materials. Flexural strength of the self-adhesive cement was less affected by light absence/attenuation and storage time.

CLINICAL SIGNIFICANCE

Biaxial flexural strength of a self-adhesive resin cement is less sensitive to variation in light application and storage time than is a non-self-adhesive cement.

Class II composite resin restorations: faster, easier, predictable

Composite resin continues to displace amalgam as the preferred direct restorative material in developed countries. Even though composite materials have evolved to include nanoparticles with high physical properties and low shrinkage stress, dentists have been challenged to efficiently create quality, long lasting, predictable restorations. Unlike amalgam, composite resin cannot be condensed making the establishment of a predictable, proper contact more difficult. In addition, composite requires an understanding of adhesives and an appreciation for their exacting application. These facts combined with the precise adaptation and light-curing of multiple layers makes placement of quality Class II composite restorations tedious and time-consuming. For private practicing dentists, it can also have an effect on economic productivity. Clinicians have always wanted an easier, efficient placement technique for posterior composite restorations that rivals that for amalgam. It appears that advances in instrumentation, materials and technology have finally delivered it.

Influence of successive light-activation on degree of conversion and Knoop hardness of the first layered composite increment

PURPOSE

To evaluate the influence of light-activation of second, third and fourth increments on degree of conversion (DC) and microhardness (KHN) of the top (T) and bottom (B) surface of the first increment.

MATERIALS AND METHODS

Forty samples (n = 5) were prepared. In groups 1-4, after each increment light-activation (multiple irradiation), T and B of the first increment were measured in DC and KHN. In groups 5-8, only the first increment was made (single irradiation) and measurements of DC and KHN were taken at 15 min intervals. The light-activation modes were (XL) 500 mW/cm(2) × 38 s (G1/G5); (S) 1000 mW/cm(2) × 19 s (G2/G6), (HP) 1400 mW/cm(2) × 14 s (G3/G7); (PE) 3200 mW/cm(2) × 6 s (G4/G8). Data for DC and KHN were analyzed separately by using PROC MIXED for repeated measures and Tukey-Kramer test (α = 0.05).

RESULTS

For KHN, B showed lower values than T. PE resulted in lower values of KHN in B surface. For single and multiple irradiations, T and B of first measurement showed the lowest KHN and the fourth measurement showed the highest, with significant difference between them. For single irradiation, first and second increments presented similar KHN, different from the third and fourth increment, which did not differ between them. For multiple irradiations, the second light-activation resulted in KHN similar to first, third and fourth increments. For DC, except QTH, T presented higher DC than B.

CONCLUSION

The light-activation of successive increments was not able to influence the KHN and DC of the first increment.

Development of optical guiding forceps for a direct bonding system using lightcured resin adhesives

Multi-bracket systems are popular orthodontic appliances and are commonly bonded directly to enamel surfaces by resin adhesives. In light-cured bonding, the tip of the curing unit must be kept at a distance from the adhesive on the tooth, which can lead to low polymerization and insufficient bond strength. The curing lights also generate low-frequency electromagnetic fields, which can be harmful to patient health. Furthermore, bacterial contamination of the light-curing tips during use presents an infection risk for patients. In this study, we describe the development of optical guiding forceps (OGFs) for polymerizing light-cured resin as a solution to these problems. With OGFs, polymerization of adhesives was deeper than with lower magnetic fields and the bonds had the same shear strength as those formed by conventional procedures. These results suggest that OGFs may have practical use in the direct bonding of orthodontic appliances as well as in provisional bonding.

Effects of exposure time and exposure distance on the degree of cure in light-activated pit and fissure sealants

OBJECTIVES

The study aims to measure and compare the effect of different exposure times and exposure distances on the degree of cure (DC) of light hardening resin based pit and fissure sealants.

METHODS

A representative selection of 13 commercial sealants brands was chosen. DC of each material (n=6) was measured in real-time by Fourier transform infrared spectroscopy (FTIR) at three clinically relevant exposure times (10, 20, 40s) and two fixed exposure distances (4mm and 7 mm) between sample and light source. Data were analyzed by a multi-variant analysis and partial eta-squared statistic.

RESULTS

Factors "material", "exposure time" and "exposure distance" had a significant influence on the DC across all materials (ηp(2)=0.927,0.774 and 0.266 respectively) with "material" and "exposure time" showing the strongest effect (significance level α ≤ 0.05). In general, an increased exposure time and reduced exposure distance between sample and light source led to increased DC for all the materials.

CONCLUSIONS

Degree of cure is influenced significantly by the brand of sealant and by exposure time. In some cases it is found that DC is also affected significantly by the exposure distance.

CLINICAL SIGNIFICANCE

On the basis of this study, an exposure time of at least 20s and a maximum exposure distance of 4mm between curing unit and material surface is recommended.

Class II composite restorations: importance of cervical enamel in vitro

OBJECTIVE

This study evaluated the importance of enamel at the cervical margin for support and retention of a class II composite restoration in relation to fracture strength, fracture mode, and leakage.

METHODS

Sixty-five newly extracted teeth were randomly divided into five groups. Within each group, standardized class II preparations were made at the mesial surface of the tooth with four different preparation designs. Group D (n=15) had the cervical margin placed below the cemento-enamel junction (the dentin group), and in the other three groups (the enamel groups: E1, E2, and E3), the cervical margin was within the enamel (n=15 each). Group E3 had restorations with cuspal coverage, while groups E1 and E2 differed in vertical dimension. Intact teeth without preparation or restoration were tested as controls (n=5). The area of the horizontal part of enamel at the cervical margin of the preparation (available cervical enamel) was calculated. The teeth were restored with a nanofilled composite material and an etch-and-rinse adhesive system. The teeth were subjected to artificial aging consisting of thermocycling and mechanical cyclical loading. The restorations were subsequently loaded until fracture. The teeth were examined microscopically to assess fracture mode and leakage at the interface between the restoration and the tooth substance.

RESULTS

The fracture strength in group D (without cervical enamel) and E3 (with cuspal coverage and cervical enamel) was lower (p<0.01) than in the other two groups (with cervical enamel). There was a correlation between the area of available cervical enamel and fracture strength (p<0.01). The median fracture strength for the control teeth was not significantly different from groups E1 and E2. Group D exhibited a predominance of adhesive fractures, while the other groups revealed more cohesive fractures.

CONCLUSION

The results from this study indicate that available cervical enamel has an impact on the performance of class II composite restorations.

Bisphenol A release from an orthodontic adhesive and its correlation with the degree of conversion on varying light-curing tip distances

INTRODUCTION

The aims of this research were to use high-performance liquid chromatography to assess the bisphenol A (BPA) released from an orthodontic adhesive (Transbond XT; 3M Unitek, Monrovia, Calif) with various light-curing tip distances and to correlate the release to the degree of conversion.

METHODS

One hundred thirty-eight premolar brackets were divided into 3 groups of 40 each for the high-performance liquid chromatography analysis and 3 groups of 6 each for assessing the degree of conversion. Fourier transform infrared spectroscopy was used for this purpose. Each group was studied at light-curing tip distances of 0, 5, and 10 mm. Statistical analyses were performed by using 2-way analysis of variance (ANOVA), post-hoc multiple comparisons Tukey HSD tests, and paired t tests. Pearson correlation was used to assess the correlation between the degree of conversion and BPA release.

RESULTS

BPA release was greater in specimens cured with a greater light-curing tip distance. The degree of conversion decreased with increased light-curing tip distances. A negative correlation was found between BPA release and degree of conversion.

CONCLUSIONS

Clinicians should ensure that the adhesive is completely cured by keeping the light-curing tip as close to the adhesive as clinically possible.

Curing Efficiency of Three Different Curing Modes at Different Distances for Four Composites

Doubling the exposure time of a high-intensity light-emitting diode curing light with a turbo tip and autofocus capability does not predictably compensate for distance in deep cavities.

Effect of thickness of indirect restoration and distance from the light-curing unit tip on the hardness of a dual-cured resin cement

This study evaluated the Knoop hardness and polymerization depth of a dual-cured resin cement, light-activated at different distances through different thicknesses of composite resin. One bovine incisor was embedded in resin and its buccal surface was flattened. Dentin was covered with PVC film where a mold (0.8-mm-thick and 5 mm diameter) was filled with cement and covered with another PVC film. Light curing (40 s) was carried out through resin discs (2, 3, 4 or 5 mm) with a halogen light positioned 0, 1, 2 or 3 mm from the resin surface. After storage, specimens were sectioned for hardness measurements (top, center, and bottom). Data were subjected to split-plot ANOVA and Tukey's test (a=0.05). The increase in resin disc thickness decreased cement hardness. The increase in the distance of the light-curing tip decreased hardness at the top region. Specimens showed the lowest hardness values at the bottom, and the highest at the center. Resin cement hardness was influenced by the thickness of the indirect restoration and by the distance between the light-curing unit tip and the resin cement surface.

Influence of the distance of the curing light source and composite shade on hardness of two composites

This study evaluated the influence of curing tip distance, shade and filler particle size on Vickers microhardness (VHN) of composite resins. Two composites were tested: Filtek Z250 microhybrid (3M ESPE; shades A1 and A3.5) and Filtek Supreme nanofilled (3M ESPE; shades A1B and A3.5B). For each resin, 42 specimens (5 mm in diameter and 2 mm height) were prepared being 21 for each shade. The specimens were exposed using a 20-second exposure to a quartz-tungsten-halogen light source with an irradiance of approximately 560 mW/cm², at the following distances: 0 mm (surface contact), 6 mm and 12 mm from composite surface. Effectiveness of cure of different resins, shades and curing distances was determined by measuring the top and bottom hardness (VHN) of specimens using a digital microhardness tester (load: 50 g; dwell time: 45 seconds) 24 hours following curing. The hardness ratio was calculated by dividing VHN of the bottom surface by VHN of top surface. Three-way ANOVA and Tukey's post-hoc test (p<0.05) revealed statistically significant differences for all analyzed factors. As for top hardness, as microhardness ratio (bottom/top), the factors shade, distance and composite filler particle size exerted influence on resin curing. Lighter shade composites (A1 and A1B) showed higher hardness values. At 6 and 12 mm curing tip distances, hardness was lower when compared to 0 mm. The microhybrid composite resin presented higheer hardness, being its microhardness ratio satisfactory only at 0 mm for both shades and at 6 mm for the lighter shade. The nanofilled composite resin did not present satisfactory microhardness at the bottom while the microhybrid composite resin had higher hardness than the nanofilled. Composite's curing tip distance and shade can influence hardness.

Influence of Photoactivation Protocol and Light Guide Distance on Conversion and Microleakage of Composite Restorations

In this study, soft-start photoactivation methods were demonstrated to be able to reduce microleakage with only a discrete reduction in the degree of conversion. This was achieved when this protocol was associated with a 7 mm distance from the light guide, which is common in deep preparations in clinical practice.

The effect of different light-curing units on tensile strength and microhardness of a composite resin

The aim of this study was to evaluate the influence of different light-curing units on the tensile bond strength and microhardness of a composite resin (Filtek Z250 – 3M/ESPE). Conventional halogen (Curing Light 2500 – 3M/ESPE; CL) and two blue light emitting diode curing units (Ultraled – Dabi/Atlante; UL; Ultrablue IS – DMC; UB3 and UB6) were selected for this study. Different light intensities (670, 130, 300, and 600 mW/cm², respectively) and different curing times (20s, 40s and 60s) were evaluated. Knoop microhardness test was performed in the area corresponding to the fractured region of the specimen. A total of 12 groups (n=10) were established and the specimens were prepared using a stainless steel mold composed by two similar parts that contained a cone-shaped hole with two diameters (8.0 mm and 5.0 mm) and thickness of 1.0 mm. Next, the specimens were loaded in tensile strength until fracture in a universal testing machine at a crosshead speed of 0.5 mm/min and a 50 kg load cell. For the microhardness test, the same matrix was used to fabricate the specimens (12 groups; n=5). Microhardness was determined on the surfaces that were not exposed to the light source, using a Shimadzu HMV-2 Microhardness Tester at a static load of 50 g for 30 seconds. Data were analyzed statistically by two-way ANOVA and Tukey's test (p<0.05). Regarding the individual performance of the light-curing units, there was similarity in tensile strength with 20-s and 40-s exposure times and higher tensile strength when a 60-s light-activation time was used. Regarding microhardness, the halogen lamp had higher results when compared to the LED units. For all light-curing units, the variation of light-exposure time did not affect composite microhardness. However, lower irradiances needed longer light-activation times to produce similar effect as that obtained with high-irradiance light-curing sources.

Effect of Distance from Curing Light Tip to Restoration Surface on Depth of Cure of Composite Resin

Aims

While light-activating composite resins, the light tip may not always be close to the surface of the restoration. This may be intentional in an attempt to create a ramp cure. The aim of this study was to determine the effect of a range of separation distances between the light tip and the restoration surface on the depth of composite cure for different types of light-curing units with a broad range of outputs.

Methods

Three halogen light units, one plasma arc-curing (PAC) light unit and two light-emitting diode (LED) curing lights in clinical use were tested, and a total of 570 restorations cured in a two-part human tooth model at separations ranging from 0 to 15 mm. The tooth was disassembled and depth of cure determined using the scrape test ISO 4049. Light intensity was also measured at each separation distance for each light.

Results

The depth of cure was generally found to decrease as the separation distance increased for all lights at the various cure times. However, the effect of increasing the separation distance was less than anticipated. The depth of cure was also related to the light output.

Conclusions

Depth of composite cure was directly related to intensity and duration of light exposure and inversely related to distance of the light source from the surface for halogen and plasma lights. However, the effect of increasing the separation distance up to 15 mm was less than expected. Altering the separation distance in order to modify the polymerisation characteristics is unlikely to be effective.

Marginal Integrity of Partial Ceramic Crowns Within Dentin With Different Luting Techniques and Materials

Resin coating improves the marginal seal of partial ceramic crowns to dentin compared to conventional luting, especially at the dentin/composite interface. However, the self-adhesive material RelyX Unicem shows significantly better marginal integrity than the other materials tested.

Variation of depth of cure and intensity with distance using LED curing lights

OBJECTIVES

The purpose of the study was to determine the correlation between intensity of light-emitting diode (LED) and tungsten-halogen light sources, and depth of cure of a resin composite at different distances.

METHODS

Four LED curing lights (Flashlite 1001, Freelight 2, Smartlite IQ and Ultralume 5) and one tungsten halogen (Optilux 501, with 8 and 11 mm tips) were evaluated. Intensity was measured according a modified ISO Standard 10650 at distances of 0, 2, 4, 6, 8, 10 mm between the light tip and detector. Depth of cure (DOC) of TPH Spectrum shade A2 was measured according to the international standard ISO 4049 at the same distances.

RESULTS

For all lights, intensity decreased as distance increased. The authors documented a logarithmic correlation between intensity and distance for all lights except the Smartlite IQ, Ultralume 5 and the Optilux 501 with the 11 mm tip, which showed a linear relationship between intensity and distance. All lights demonstrated a logarithmic correlation between intensity and DOC, and a linear correlation between DOC and distance. Smartlite IQ and Optilux 501 (11 mm tip) also had the least reduction in intensity and DOC at 10 mm.

SIGNIFICANCE

Clinicians often an experience difficulty placing the light tip close to the resin surface when curing resin composites. While both intensity and DOC decrease with increasing distance, the relationship between these factors and distance may not be similar for all lights and may depend on the characteristics of individual lights.

Does Hybridization of Intraradicular Dentin Really Improve Fiber Post Retention in Endodontically Treated Teeth?

This study tested the hypothesis that hybridization of intraradicular dentin eliminates interfacial gaps, thereby improving the coronal seal and retention of teeth restored with fiber posts. Post spaces were bonded with two types of fiber posts, using the corresponding etch-and-rinse adhesives and dual-cured resin cements. Longitudinal sections of the interfaces were examined for dentin hybridization in the coronal- and middle-thirds of the root canals. Resin replicas of these sections were evaluated for interfacial gap formation. Although intraradicular dentin hybridization was not compromised irrespective of whether the adhesives were light-cured before cementation, the universal occurrence of interfacial gaps along the hybrid layer surface or the post-cement interface reflects the challenge in bonding to post spaces with low compliance and high C-factors. The clinical success associated with bonded fiber posts is probably due predominantly to frictional retention.

The effect of specimen dimensions on the flexural strength of a composite resin

PUORPOSE: The aim of this study was to evaluate the effect of specimen dimensions on the flexural strength of a composite resin (Heliomolar RO). METHODS: The different dimensions tested - length x width x height (mm) were: 25x2x4; 25x2x2 (ISO 4049); 15x2x2; 10x2x2 and 10x2x1. Light-curing was performed at 600 mW/cm² for 40s, three times along the 25mm specimens, twice along 15mm specimen and once for the 10mm specimens. Specimens of all dimensions were light-cured on both surfaces, or only on one side. In the latter, the load was applied on the irradiated side or on the opposite one. RESULTS: It was shown that the flexural strength was not affected by specimen length. When light-curing was performed on both surfaces, similar flexural strength values were obtained for any dimension. Despite the number of irradiated surfaces, specimens with a height of 1mm also obtained similar values. On the contrary, specimens with a height of 4mm, exposed only on one surface, reached the lowest strength. CONCLUSION: The use of specimens with lower dimensions can lead to flexural strength values similar to the ones obtained with standardized specimen (ISO 4049), with the advantage of demanding less amount of material and being less time consuming.

Contemporary dental practice in the UK: indirect restorations and fixed prosthodontics

OBJECTIVES

To investigate, by questionnaire, the use and selection of materials and techniques for indirect restorations and fixed prosthodontics by dental practitioners in the North West of England and Scotland.

METHODS

A questionnaire was sent to 1,000 general dental practitioners selected at random from dentists in Scotland and the North West of England. Non-responders were sent another questionnaire after a period of 4 weeks had elapsed.

RESULTS

A total of 701 usable questionnaires were returned, giving a response rate of 70%. When selecting a material for the core build-up of vital teeth, practitioners used the following materials (%): amalgam (60), dual and light-cured resin composite (54), glass-ionomer cements (47), compomer (29) and resin-modified glass-ionomer cements (24). Where the use of a post was indicated indirect posts of both precious (67) and non-precious (37) alloys were preferred to prefabricated posts by the majority of practitioners for the restoration of root filled teeth. Direct titanium (14) and stainless steel (14) posts were not used extensively. Impression materials used by the practitioners were as follows: addition-cured silicone (70), condensation-cured silicone (20), polyether (9) and polysulphide (2). Traditional glass-ionomer cements (59) were used for the luting of single and multiple porcelain fused to metal units, with zinc phosphate and polycarboxylate cements (33) being the preferred alternatives.

CONCLUSIONS

The majority of the practitioners surveyed in this study used: amalgam for core build-ups; indirect posts; addition-cured silicone for impressions; and glass-ionomer cements for luting procedures.

Effects of resin composite composition and irradiation distance on the performance of curing lights

This study determined the effect of using five resin composites and two irradiation distances to test the performance of dental curing lights. Three types of curing lights with similar spectral distributions, but each delivering a different power density, were used for irradiation times ranging from 3 to 60 s. Power densities were measured at 2 and 9 mm from the tip of the light guide. Five composites 1.6 mm thick and of the same shade were irradiated at 2 and 9 mm from the light guide with energy densities of 1.2-38.0 J/cm(2). The Knoop hardness at the top and bottom of the composite specimens was measured 15 min after irradiation and again after immersion in water at 37 degrees C for 24 h. There was a linear relationship between the hardness and the logarithm of the energy density received by the composite (r2 > 0.81). The analysis of variance showed that the composite, the side tested, the distance from the light guide, and the curing light/irradiation time combination all had a significant effect on the hardness (p < 0.01). Plots of the hardness at the bottom 15 min after irradiation by each light were generated for all the composites. These plots illustrated that the effects of the different curing light/irradiation time combinations on hardness were not the same for each composite. The effects of each curing light/time combination on hardness were also different at 2 and 9 mm from the light guide. In conclusion, when comparing the effects of different light sources on resin polymerization, several different composites should be irradiated at clinically relevant distances from the light guide. Using high-powered curing lights for 3 or 5 s did not deliver sufficient energy to cure the 1.6-mm thick specimens of composites used in this study.

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

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

Comparative efficiency of plasma and halogen light sources on composite micro-hardness in different curing conditions

OBJECTIVES

Recent developments have led to the introduction of high power curing lights, which are claimed to greatly reduce the total curing time. This study evaluated the effectiveness of a plasma-curing device (Apollo 95 E) and a halogen device (Heliolux DLX), in different curing conditions.

METHOD

Vicker's micro-hardness values were performed on 1 and 2 mm thick composite discs cured in a natural tooth mold by direct irradiation or indirect irradiation through composite material (2 or 4 mm) and dental tissues (1 mm enamel or 2 mm enamel-dentin). Measures were, respectively, performed after a 1, 3, 6 s (SC, step curing mode) or 18 s (3xSC) exposure to the plasma light, and a 5, 10, 20 or 40 s exposure to the halogen light.

RESULTS

With the PAC light used, a 3 s irradiation in the direct curing condition was necessary to reach hardness values similar to those obtained after a 40 s exposure to the halogen light. Using the indirect curing condition, hardness values reached after an 18 s exposure (3xSC mode) with the plasma light were either equivalent or inferior to those obtained with 40 s halogen irradiation.

SIGNIFICANCE

Direct polymerization with the plasma light used requires longer exposure times than those initially proposed by the manufacturer. The effectiveness of plasma generated light was lowered by composite or natural tissues, and therefore requires an important increase in the irradiation time when applied to indirect polymerization. The practical advantage of this polymerization method is less than expected, when compared to traditional halogen curing.

Polymerization shrinkage evaluation of three packable composite resins using a gas pycnometer

Modern restorative dentistry has been playing an outstanding role lately since composite resins, allied to adhesive systems, have been widely applied on anterior and posterior teeth restorations. The evolution of composite resins has mostly been verified due to the improvement of their aesthetic behavior and the increase in their compressive and abrasive strengths. In spite of these developments, the polymerization shrinkage inherent to the material has been a major deficiency that, so far, has been impossible to avoid. Using a gas pycnometry, this research investigated the polymerization shrinkage of three packable composite resins: Filtek P60 (3M), Prodigy Condensable (Kerr), and SureFil (Dentsply/Caulk), varying the distance from the light source to the surface of the resins (2 mm or 10 mm). The pycnometer Accupyc 1330 (Micromeritics, USA) precisely records helium displacement, allowing fast and reliable measurements of the volume of composite resin immediately before and after polymerization, without interference of temperature or humidity. Results were not found to be statistically different for the three tested resins, either for 2 mm or 10 mm-distance from the light source to the composite surface.

Polymer structure of a light-cured resin composite in relation to distance from the surface

The aim of the investigation was to study the polymer structure of a light-cured resin composite in relation to the distance from the irradiated surface. Ten cylinders (4 x 8 mm) of composite were light-cured in a mold at 580 mW cm-2 for 40 s. The cylinders were expressed from the mold and, after 1 wk of dry storage at 37 degrees C, embedded in dental stone. Grinding parallel to the long axis of the cylinders on carborundum paper exposed the resin composite. The Wallace hardness in relation to distance from the irradiated surface was measured before and after 1 d of ethanol storage. Before ethanol storage and at distances from the surface of 0.5-3.5 mm, no difference in hardness was recorded. The ethanol storage gave rise to a softening of the resin composite. The softening recorded at 3.0-3.5 mm was significantly more pronounced than the softening taking place at 0.5-2.5 mm. This result was explained by a slower polymerization occurring at the greater distances from the irradiated surface, resulting in a polymer with reduced crosslink density.

A pilot investigation of operator variability during intra-oral light curing

OBJECTIVE

To test the hypothesis that operator experience influences the efficacy of light curing in a typical posterior intra-oral location. To investigate whether short cure cycles affect performance.

DESIGN

A cross-sectional single-centre study designed to assess the efficacy of experienced and inexperienced operators when undertaking simulated intra-oral curing.

SETTING

An in vitro laboratory based investigation conducted in a dental school during 2001.

MATERIALS AND METHODS

A computer-based technique was used to monitor light intensity in a clinical simulation. Dentists and student operators were tested for their ability to cure a posterior restoration effectively. Relative light intensity was assessed against time for each operator and test run.

RESULTS

Experienced (qualified) operators produced more effective and consistent cure results than less experienced undergraduate students. Operator performance was not affected by variations in irradiation time.

CONCLUSIONS

This cross-sectional pilot investigation demonstrates that operator experience is a factor in successful clinical photo-curing of posterior restorations. Stable and accurate light guide positioning are required throughout the entire irradiation cycle to optimise intra-oral cure of light-activated restorations. Further investigations are planned to assess the potential of this novel method of assessment for use as a routine teaching aid in clinical practice.

[Effect of techniques of composite resin insertion and polymerization on microleakage and microhardness]

The aim of this study was to evaluate the influence of techniques of composite resin polymerization and insertion on microleakage and microhardness. One hundred and eighty class II cavities were prepared in bovine teeth and assigned to six groups: G1 - bulk filling + conventional polymerization; G2 - bucco-lingual increments + conventional polymerization; G3 - bulk filling + soft-start polymerization; G4 - bucco-lingual increments + soft-start polymerization; G5 - bulk filling + progressive polymerization; G6 - bucco-lingual increments + progressive polymerization. All cavities were restored with the Z100/Single Bond system (3M). After thermocycling, the samples were immersed in 2% methylene blue dye solution for 4 hours. Half of the samples were embedded in polystyrene resin, and Knoop microhardness was measured. The Kruskal-Wallis test did not reveal statistical differences (p > 0.05) between the polymerization and insertion techniques as to microleakage. Regarding microhardness, the two-way ANOVA and the Tukey test did not reveal statistical differences between the restorative techniques (p > 0.05), but progressive polymerization (G5 and G6) was associated with smaller Knoop microhardness values (p < 0.05): G = 144.11; G2 = 143.89; G3 = 141.14; G4 = 142.79; G5 = 132.15; G6 = 131.67. It was concluded that the evaluated polymerization and insertion techniques did not affect marginal microleakage, but a decrease in microhardness occurred when progressive polymerization was carried out.

Curing-light intensity and depth of cure of resin-based composites tested according to international standards

BACKGROUND

Several factors control the light curing of a resin-based composite: the composition of the composite, the shade of the composite, the wavelength and bandwidth of the curing light, the distance of the light from the composite, the intensity of the curing light and the irradiation time. The authors investigated the depth of cure of several shades of five brands of resin-based composites when irradiated via light in the 400- to 515-nanometer wavelength bandwidth at the International Organization for Standardization, or ISO, recommended intensity of 300 milliwatts per square centimeter. The resin-based composites were irradiated for the times recommended by the products' manufacturers.

METHODS

The authors used a curing light adjusted to emit 300 mW/cm2 in the 400-nm to 515-nm wavelength bandwidth to polymerize five samples of each composite brand type and shade. They measured depth of cure using a scraping method described in the ISO standard for resin-based composites. Depth of cure was defined as 50 percent of the length of the composite specimen after uncured material was removed by manual scraping. The authors determined a mean from the five samples of each composite brand and shade.

RESULTS

Thirteen (62 percent) of 21 composite materials met the ISO standard depth-of-cure requirement of 1.5 millimeters. Six of the eight remaining materials met the depth-of-cure requirement when the authors doubled the irradiation time recommended by the product manufacturers.

CONCLUSIONS AND CLINICAL IMPLICATIONS

Curing lights with an intensity of 300 mW/cm2 appear to effectively cure most resin-based composite materials when appropriate curing times are used, which, in some cases, are longer than those recommended by the manufacturers. Dentists should verify the depth of cure of a composite material as a baseline measure, and then check depth of cure periodically to confirm light and material performance. The ISO depth-of-cure measurement method can be used for this purpose.

Microdureza de resina composta: efeito de aparelhos e tempos de polimerização em diferentes profundidades

As propriedades das resinas compostas têm sido estudadas com freqüência, bem como os fatores que podem influenciar seu grau de polimerização. Diante da evolução desses materiais e da necessidade de buscarmos melhora do seu comportamento na cavidade bucal, objetivamos, por meio deste estudo avaliar a eficácia de dois aparelhos fotopolimerizadores do tipo pistola (de alta intensidade de luz), comparando com a de um aparelho a cabo (de baixa intensidade de luz), com tempos de exposição de 20 e de 40 segundos e em profundidades de 1 a 4 milímetros. Os testes avaliaram o grau de polimerização da resina por meio de testes de microdureza Knoop. Os resultados mostraram haver diferença estatisticamente significante entre os tempos, sendo que com 40 segundos a dureza foi maior que com 20 segundos para as 4 diferentes profundidades. Para o fator aparelhos, os dois aparelhos tipo pistola se comportaram superiores ao do tipo cabo Fibralux (Dabi Atlante), e entre eles, o XL 1500 (3M) promoveu dureza maior que o Optilight II (Gnatus) no tempo de polimerização de 40 segundos. As profundidades de 1, 2, 3 e 4 milímetros mostraram estatisticamente diferença entre si tendo sido encontrada maior dureza para as menores profundidades (p < 0,05).

Improved efficacy of dentin-bonding agents

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

Dental materials citations: Part A, January to June 1997

OBJECTIVE

A search was conducted in biomedical journals published from January 1997 to June 1997 to identify all dental materials publications and sort them into major categories.

METHODS

Tables of contents for 79 journals for the period of January to June, 1997 were inspected and divided into 17 categories. Citations were analyzed by both frequency in journals and in categories, as well as compared to frequencies for previous years.

RESULTS

A total of 445 citations were detected in 79 journals for the period January 1997 to June 1997. Certain journals (n = 19) demonstrated a higher citation frequency (> or = 10 citations for 6 months) and represented 77.8% of all citations. The greatest number of citations continued to involve bonding (n = 97), resin-based restorative materials (composites; glass ionomers) (n = 95), prosthodontic materials (n = 51), and pulp protection/luting materials (n = 48). Frequencies by category were very similar to those for the last four years.

SIGNIFICANCE

The compiled literature citations provide a supplement for researchers and academicians seeking information in existing electronic databases.