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. 2020 Oct 31;66(5):257–263. doi: 10.1111/idj.12227

Five second photoactivation? A microhardness and marginal adaptation in vitro study in composite resin restorations

Carlos Alberto Kenji Shimokawa 1, Paula Mendes Acatauassú Carneiro 1, Tamile Rocha da Silva Lobo 1, Victor Elias Arana-Chavez 2, Michel Nicolau Youssef 1, Míriam Lacalle Turbino 1,*
PMCID: PMC9376633  PMID: 27061395

Abstract

Introduction: Studies defining the characteristics of light curing units and photoactivation methods are necessary to allow the correct choices to be made in daily practice. This study aimed to determine whether different photoactivation protocols for composite resins [periodic level shifting (PLS) – 5 second and soft-start] are able to maintain or enhance the mechanical properties and marginal adaptation of restorations. Methods: Restorations were placed in bovine teeth using the following photoactivation methods: continuous light for 20 seconds (control group); PLS technology (PLS – 5 second group); and continuous light and a light guide tip distance of 6 mm after which the tip was placed at the surface of the restoration (soft-start group). The teeth were transversely sectioned in the incisal-cervical direction. Thirty halves were randomly selected for Knoop microhardness testing (n = 10). The other 30 halves were subjected to scanning electron microscopy analysis. The images obtained were measured to identify the highest marginal gap, and statistical tests for variance analysis were conducted. Results: Microhardness tests showed no statistically significant difference between the photoactivation methods analysed (P ≥ 0.01). The tests showed a difference among depths (P < 0.01), with the deeper layers being the hardest. In analysing marginal adaptation, no significant difference was identified between the higher marginal gap values in the continuous (mean = 10.36) and PLS – 5 second (mean = 10.62) groups, and the soft-start group (mean = 5.83) presented the lowest values (P < 0.01). Conclusions: The PLS – 5 second and soft-start protocols did not alter the hardness of the restorations. Moreover, the PLS – 5 second protocol did not alter the marginal adaptation, whereas the soft-start protocol improved marginal adaptation.

Key words: Curing methods, restorative composite, polymerization, contraction stress

INTRODUCTION

The great demand for increasingly less noticeable restorations has led to an emphasis on the use of composite resins, even in posterior teeth1., 2.. However, deficiencies in this material, especially regarding contraction during polymerization, continue to be observed. Shrinkage is a physical process inherent to the polymerization reaction; this process occurs because of the conversion of the organic matrix monomers into a polymer. Shrinkage causes tension at the tooth–restoration interface, which may cause a gap within the interface and lead to marginal microleakage of oral fluids and microorganisms. Tension at the interface can also be transmitted to the tooth structure, causing cusp deflection or even tooth fracture3., 4..

Methods to reduce the probability of failure caused by stress have been studied, including changes in the photoactivation protocol5., 6.. In particular, methods with slower polymerization have been proposed. These methods avoid failure during the restoration procedure because they reduce tension during the polymerization reaction7., 8., 9., 10., 11., 12.. A decrease in the light intensity used for photoactivation causes a decrease in the conversion rate of monomers into polymer, increasing the pre-gel phase. During this phase, resin will probably undergo deformation, and the flow can compensate for shrinkage, thereby relieving tension. The reduced tension improves the adaptation of resin and reduces the occurrence of marginal infiltration in restorations13., 14.. The effects of the soft-start technique, namely slower polymerization and an increased pre-gel phase, have been associated with decreased marginal gap size and stress contraction, which could cause microleakage. The soft-start technique does not affect the physical properties of composites because the final polymerization is performed using high-intensity light7., 9., 15.. However, the effects of this type of photoactivation lack consensus in the literature because studies showed no significant difference between the soft-start and continuous methods of polymerization in the microleakage of photoactivated composite resin restorations16., 17., 18..

Currently, the time required for the photoactivation of composite resins is decreasing, and some manufacturers claim that their equipment promotes an acceptable degree of conversion with only a 5-second exposure to light. However, it is widely known that the faster the polymerization process, the lower the possibility of flow of composite resins during the pre-gel phase, which may increase the stress from shrinkage during polymerization5. The 5-second exposure time is made possible by the DEMI light curing unit, which uses periodic level shifting (PLS) technology. This technology consists of a pulsed light that increases in intensity from approximately 1 100 mW/cm2 to 1,400 mW/cm2 (according to the manufacturer). The advantage of the above-mentioned photoactivation technology is that it provides a high intensity light but no excessive heat to the tooth or to the light curing unit. However, the disadvantage inherent in fast polymerization reactions is also associated with this technology because the stress generated during the reaction may increase. Although El-Shamy and El-Mowafy19 obtained higher values for hardness using a high-intensity photoactivator [compared with other light-emitting diode (LED) photoactivators], other studies have shown that 5 seconds, as indicated by the manufacturer, is not sufficient for a composite resin to undergo appropriate polymerization20., 21..

The soft-start method can also be used by varying the distance between the light guide tip and the external surface of a restoration because the intensity of light reaching the resin decreases as the distance between the tip and the surface increases22., 23., 24.. After the initial polymerization, high-intensity light must be applied to promote the final polymerization to obtain the required mechanical properties of the resin.

To evaluate the efficiency of different methods of photoactivation, analyses of both the degree of conversion of composite resin after polymerization, and the shrinkage generated, are required. Hardness tests and marginal adaptation observations are appropriate methods for analysing these variables25. Marginal adaptation can be assessed using scanning electron microscopy, and an epoxy-resin replica is used in this type of analysis to avoid possible artefacts generated during preparation for microscopy, such as cracks in the tooth as a result of the dehydration process26., 27., 28., 29.. The hardness of a material is a property often used to measure its degree of conversion indirectly, and for the same material, higher hardness values correspond to higher levels of conversion30., 31.. However, this relationship is found only for the same type of composite resin. Thus, a comparison among different materials is not possible30., 32..

Hardness and marginal adaptation data are complementary and can be used to evaluate the quality of polymerization of the composite resin used in the restoration25. Such complementarity is the basis for this study, in which photoactivation protocols of composite resin were evaluated. The continuous photoactivation method, which is most commonly used in the clinical environment, was compared with the soft-start method, which can promote less polymerization stress, and with PLS technology, a method that can reduce the polymerization time. The objective of this study was to observe whether different photoactivation protocols of composite resin (PLS – 5 second and soft-start) enhance the mechanical properties and marginal adaptation of restorations.

METHODS

The proposal for this study was submitted to the Ethics Committee for the use of animals of the School of Dentistry of the University of São Paulo with protocol number 08/2013, and a waiver was granted by this committee. This study used 30 bovine teeth of similar size, from which 60 specimens were produced. The teeth were cleaned with periodontal curettes and pumice using a Robinson brush at low speed, washed thoroughly with running water and stored in distilled water. The roots were removed with a diamond disc coupled to an IsoMet 1000 cutting machine (Buehler Ltd., Lake Bluff, IL, USA) under constant refrigeration. The buccal surface was ground flat to remove both the top enamel layer and irregularities. Cavities (2 mm × 4 mm ×2 mm) were prepared in the buccal surface 2 mm from the cemento–enamel junction, using a high-speed diamond abrasive instrument (IAD, n° 1090) (KG Sorensen, Cotia, SP, Brazil) under water cooling. The prepared cavities had approximately 1 mm of enamel and 1 mm of dentin.

The OptiBondS etch-and-rinse adhesive system (Kerr Co., Orange, CA, USA) was applied according to the manufacturer’s specifications, with acid etching for 15 seconds followed by a thorough rinse to remove the acid. The cavity was gently dried, and adhesive was applied for 15 seconds with a light brushing motion and then subjected to air blowing for 3 seconds. The adhesive was light cured for 20 seconds, and the Premise nanohybrid resin shade A2 (Kerr Co.) was used in the restoration following the incremental technique (three oblique increments per restoration standardized by a periodontal probe).

In the continuous and soft-start groups, the Elipar FreeLight 2 light curing unit (3M ESPE, St Paul, MN, USA), which provides an energy intensity of ≈700 mW/cm2, was used to photoactivate the restorations. A 20-second photoactivation period was used in the continuous group, and a 30-second period was used in the soft-start group. In the PLS – 5 second group, the DEMI light curing unit (Kerr Co.), which provides a pulsed light with an energy intensity that varies from ≈1,050 mW/cm2 to 1,200 mW/cm2 per second (PLS technology), was used to photoactivate the restorations, with a 5-second photoactivation period. Wavelengths and energy intensities were measured using a spectroradiometer compound and a USB-2000 spectrometer (Ocean Optics Inc., Dunedin, FL, USA) coupled to a probe via an optic fibre to collect radiation. The spectroradiometer was calibrated with an OL 200 calibrated radiation source (Optronic Laboratories, Orlando, FL, USA), the calibration of which is determined by the National Institute of Standards and Technology, USA (NIST).

In the soft-start group, the light guide tip was positioned 6 mm from the restoration during the first 20 seconds for the initial application of lower light intensity. The energy intensity measured at this distance was approximately 50% lower than the intensity measured with the light guide in contact with the radiometer. The time required for photoactivation in this group was calculated so that the energy density was equal to that used in the continuous group33. Based on the calculations, a period of 10 seconds was required for final photoactivation with the light guide tip in contact with the restoration surface. Restored teeth were cut transversely in the inciso-cervical direction to obtain two (mesial and distal) specimens per tooth, and each half was subjected to different tests (microhardness and scanning electron microscopy). The two halves were abraded on their cut surfaces to make them flat and polished, thus allowing scanning electron microscopy and microhardness readings.

One of the two specimens from each restored tooth was randomly selected, and 30 specimens (30 halves) were evaluated in the microhardness test. The HMV-2000 microhardness tester (Shimadzu Co., Tokyo, Japan) with a Knoop penetrator was used in the microhardness test (load = 50 gf (gram-force) 30 seconds), and readings were performed using CAMS-WIN software (Newage Testing Instruments Inc., Feasterville, PA, USA). Indentations were made on the cross-section of the composite resin at three different depths: the surface (100 μm from the contact surface to the light guide tip); medium (1 mm below the surface); and deep (100 μm from the bottom of the restoration). Five indentations were made at each depth, with a distance of 100 μm between them.

The other 30 halves were examined by scanning electron microscopy, and cuts were made to reduce the size of specimens until they reached 4 mm × 6 mm. Specimens were subject to demineralization [in 1 M hydrochloric acid (HCl) for 30 seconds], washed thoroughly and then subjected to a deproteinization process [in 2.5% sodium hypochlorite (NaClO) for 10 minutes] followed by thorough washing. Then, the cross-section of the specimens was moulded with a vinylpolysiloxane impression material and replicated with Epofix epoxy resin (Struers, Ballerup, Denmark) for scanning electron microscopy examination. The images obtained were examined using ImageJ software (National Institutes of Health, Bethesda, MD, USA) to measure the highest gap width found therein (Figure 1).

Figure 1.

Figure 1.

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Scanning electron microscopy, 300× magnification was used to measure the highest gap width found (*). CR, composite resin; D, dentin.

All data were statistically analysed and subjected to normality and homogeneity tests. Because the data distribution was normal, split-plot and one-way tests for analysis of variance (ANOVA) were applied to the microhardness and marginal adaptation data, respectively, with post hoc Tukey tests. A significance level of 1% (P < 0.01) was the criteria for a significant difference.

RESULTS

The microhardness tests provided 450 data points, and measurements of the highest gap width found provided 30 data points. Means and standard deviations for those values are shown in Tables 1 and 2.

Table 1.

Microhardness (Knoop) values

Indentation depth Continuous PLS – 5 second Soft-start
Surface 53.7 ± 0.5a 53.7 ± 1.6a 53.0 ± 0.9a
Medium 56.5 ± 0.8b 55.6 ± 1.9b 56.0 ± 1.1b
Deep 59.5 ± 1.3c 58.9 ± 1.7c 58.0 ± 0.9c
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Values are given as mean ± standard deviation. Mean values with the same superscript letters are not significantly different for microhardness (P > 0.01).

Table 2.

Highest gap width (μm)

Continuous PLS – 5 second Soft-start
Width (μm) 10.36 ± 2.17a 10.62 ± 3.25a 5.83 ± 1.43b
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Values are given as mean ± standard deviation. Mean values with the same superscript letters are not significantly different for highest gap width (P > 0.01).

A statistically significant difference was observed between microhardness values obtained at different depths (P < 0.01) but not among different photoactivation protocols (P ≥ 0.01). Higher microhardness values were observed in the deep region followed by the middle region and then the superficial region, which showed the lowest values.

For the highest gap width found, a statistically significant difference was observed between the values for the continuous and soft-start groups, as well as between the PLS – 5 second and soft-start groups (P < 0.01), but not between the continuous and PLS – 5 second groups. The continuous and PLS – 5 second groups showed higher values for gap width than the soft-start group.

DISCUSSION

An absence of gap-free restorations, as observed in this study, is commonly seen in clinical and laboratory situations34. In vitro observation of failures at the tooth–restoration interface is a method of predicting the occurrence of failures in vivo because these conditions correlate. Before replicas were prepared for scanning electron microscopy examination, the specimens were treated with HCl and NaClO to remove any mineral or organic residue that could affect the visualization of gaps in the tooth–restoration interface35. The possibility that such treatment could induce gap formation was a concern. However, this possibility was discarded because some intact interface regions were found after treatment.

Analysis of the highest gap width has been successfully presented by other authors for the assessment of marginal adaptation29, and as the highest gap width has a strong relationship with contraction stress, it can be used to assess polymerization shrinkage and its effects27. Values for the highest gap width in the resin being studied did not differ significantly between the continuous and PLS – 5 second groups. In the PLS – 5 second group, this finding indicates that photoactivation with high-intensity LED for 5 seconds resulted in restorations comparable with those in the continuous group with regard to marginal adaptation. The soft-start group showed higher values for marginal adaptation, and this result has already been reported15. Previous studies have shown a decrease in the contraction of resin during polymerization, or a decrease in tension related to contraction, when using this protocol7., 9., 10., 11., 36.. Such a decrease can explain the current results because less tension will affect the marginal integrity of restorations if contraction during polymerization and related stress are diminished37.

Lu et al.36 observed a decrease in the degree of conversion when the soft-start method was used, and they used this fact to explain the decrease in shrinkage stress. A decrease in the degree of conversion of the composite resin impairs restoration integrity because a loss of mechanical properties is associated with a lower degree of conversion25. A reduction in the mechanical properties of the material may result in increased wear (including fracture) of restorations; this reduction is a major cause of replacement of restorations38. However, no statistically significant difference between the hardness values for composite resins photoactivated according to different protocols was found in the present study or in previous reports11., 39.. The exposure time (5 seconds) required for resin polymerization in the PLS – 5 second group is desirable under clinical conditions as it is more comfortable for both patients and clinicians. Although Rueggeberg et al.20 found inferior properties in restorations when they used 5 seconds to photoactivate the resin (as recommended by the manufacturer), significantly different hardness values were not observed in the present study with the use of continuous light. One possible explanation for the difference between the results of the two studies is that in the above-mentioned study, the light guide tip was kept at a distance 2 mm from the resin to simulate specific clinical situations, whereas, in the present study, the light guide tip maintained contact with the resin.

For the same intensity of light emitted, a greater distance between the light guide tip and the surface of the resin represents a decrease in the light intensity received by the resin21., 22., 23.. In this study, the initial distance (6 mm) used in the soft-start group decreased the emitted intensity by approximately 50%, a result similar to that found in the literature40. This increased distance generated an initial intensity of 350 mW/cm2. That intensity, associated with a time of 20 seconds (energy density of 7 J/cm2), was effective as an initial light exposure, improving the marginal adaptation of those specific restorations made in vitro in this study.

Regarding polymerization at different depths, no statistically significant difference was observed between the study groups. All groups presented acceptable values for hardness in the 2 mm depth of restorations studied because values lower than 80% hardness obtained in the hardest surface (bottom) were not observed in the body of the restoration (maximum hardness percentage), indicating that the composite resin achieved acceptable polymerization41. Although the energy density obtained in the PLS group was lower than in the other groups, comparable results among groups may be explained by the different spectral radiances of curing lights or by the thickness of increments. Increased hardness was observed at increasing depths. This observation can be explained by the incremental insertion technique used for the placement of composite resin because the deeper layers, up to 2 mm, received a higher energy density with three light cycles (three increments applied to the restoration), whereas the superficial layer received only one photoactivation cycle. Although the cavity depth was 2 mm and this thickness is recommended for increments by the manufacturer, three increments were used to decrease the ratio of bonded surfaces to unbonded surfaces (i.e. the C-factor) of the restoration, which would result in less contraction stress42. A high C-factor may overestimate the results because it could lead to an increase in the gap widths43. Although the C-factor was controlled, overexposure of the deeper layers of the restoration is a limitation of this study because the hardness of the PLS – 5 second group may have been overestimated.

In this study, the comparison of microhardness values is valid because the same type of resin was used throughout this study32. Values obtained at different depths in the same restoration could also be compared44. The microhardness test is a method used to evaluate the mechanical properties of a composite resin that are positively correlated to the degree of conversion30., 31.. However, a reduced initial intensity of photoactivation has been associated with a reduction of cross-linked density36, and this reduction could have negative consequences for the mechanical properties45. Therefore, despite knowing that the soft-start protocol used in this study was effective in maintaining the microhardness of the restorations, the degree of conversion of composite resins photoactivated with different protocols could not be considered as identical.

Given the complexity of the polymerization reaction and the factors affecting stress developed in the restorations, establishing a single photoactivation protocol for all circumstances and materials does not seem reasonable46. The Knoop hardness test and the measurement of marginal adaptation by scanning electron microscopy were efficient for analysing the quality of polymerization. In this study, the soft-start protocol showed advantages, such as higher values for marginal integrity and maintenance of transverse hardness of the resin, despite the fact that a higher total photoactivation time was required. Clinically, these advantages may indicate that restorations produced using this protocol have a lower risk of secondary caries and have mechanical properties similar to those obtained with other methods of photoactivation. The PLS technology was better in terms of working time (i.e. a shorter time period was required). However, the values for marginal adaptation obtained with this technology were lower than those observed in the soft-start group but still comparable with those in the continuous group. Although the PLS group had a hardness comparable to that of the continuous and soft-start groups, clinicians must be cautious with curing performed using the DEMI photoactivator. For light curing units with short photoactivation times, dentists should keep the tip of the curing unit stable over the resin so that no significant reduction in energy density occurs47. Thus, within the constraints of this study, dentists who opt for a faster procedure in their clinical practice in favour of convenience could be missing out on quality because restorations with better marginal adaptation were obtained with the application of the soft-start polymerization method.

Despite the limitations of this study, the PLS – 5 second protocol did not result in significant changes in restoration hardness compared with the continuous and soft-start groups. Additionally, the soft-start photoactivation protocol improved the marginal adaptation in composite resin restorations, and the PLS – 5 second photoactivation maintained the marginal adaptation compared with the continuous group. Thus, the 5-second photoactivation protocol can be used safely with regard to mechanical properties and marginal adaptation.

Acknowledgements

The authors thank Prof. Dr. Gesse Eduardo Calvo Nogueira for the measurement of spectral radiance of the light curing units used in the study.

The authors also declare that this study did not receive any financial support.

Conflict of interest

The authors certify that no conflicts of interest exist for this manuscript.

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