A method of improving marginal adaptation by elimination of singular stress point in composite restorations during resin photo-polymerization

Impact of optical fiber-based photo-activation on dental composite polymerization

OBJECTIVES

The study aimed to introduce a novel two-step optical fiber-based photo-activation of dental resin-based composites (RBCs) for reducing polymerization shrinkage stress (PSS).

METHODS

Proposed protocol design - in the first step, two flexible plastic optical fibers connected to a dental light curing unit (LCU), were used as light guides inserted into the filling to initiate low-irradiance polymerization from within; in the second step, fibers were extracted and remaining voids were filled with RBC, followed by conventional high-irradiance curing to finalize polymerization. Three bulk-fill RBCs were tested (Beautifil-Bulk Restorative, Filtek Bulk-fill Posterior, Tetric PowerFill) using tooth cavity models. Three non-invasive examination techniques were employed: Digital Holographic Interferometry, Infrared Thermography, and Raman spectroscopy for monitoring model deformation, RBC temperature change, and degree of conversion (DC), respectively. A control group (for each examined RBC) underwent conventional photo-activation.

RESULTS

The experimental protocol significantly reduced model deformation by 15 - 35 %, accompanied by an 18 - 54 % reduction in RBC temperature change, emphasizing the impact of thermal shrinkage on PSS. Real-time measurements of deformation and temperature provided indirect insights into reaction dynamics and illuminated potential mechanisms underlying PSS reduction. After a 24-hour dark-storage period, DC outcomes comparable to conventional curing were observed, affirming the clinical applicability of the method.

CONCLUSIONS

Protocol involving the use of two 1.5 mm fibers in the first step (300 mW/cm2 x 10 s), followed by a second conventional curing step (1000 mW/cm2 x 10 s), is recommended to achieve the desired PSS reduction, while maintaining adequate DC and ensuring efficient clinical application.

CLINICAL SIGNIFICANCE

Obtained PSS reduction offers promise in potentially improving the performance of composite restorations. Additionally, leveraging the flexibility of optical fibers improves light guide approach for restorations on posterior teeth. Meanwhile, implementation in clinical practice is easily achievable by coupling the fibers with commercial dental LCUs using the provided plastic adapter.

Dental Resin-Based Luting Materials-Review

As cementation represents the last stage of the work involved in making various indirect restorations (metal ceramic crowns and bridges, full ceramic crowns and bridges, inlays, onlays, and fiber posts), its quality significantly contributes to the clinical success of the therapy performed. In the last two decades, the demand for ceramic indirect restorations in everyday dental practice has considerably increased primarily due to the growing significance of esthetics among patients, but also as a result of hypersensitivity reactions to dental alloys in some individuals. In this context, it is essential to ensure a permanent and reliable adhesive bond between the indirect restoration and the tooth structure, as this is the key to the success of aesthetic restorations. Resin-based luting materials benefit from excellent optical (aesthetic) and mechanical properties, as well as from providing a strong and durable adhesive bond between the restoration and the tooth. For this reason, resin cements are a reliable choice of material for cementing polycrystalline ceramic restorations. The current dental material market offers a wide range of resin cement with diverse and continually advancing properties. In response, we wish to note that the interest in the properties of resin-based cements among clinicians has existed for many years. Yet, despite extensive research on the subject and the resulting continued improvements in the quality of these materials, there is still no ideal resin-based cement on the market. The manuscript authors were guided by this fact when writing the article content, as the aim was to provide a concise overview of the composition, properties, and current trends, as well as some future guidelines for research in this field that would be beneficial for dental practitioners as well as the scientific community. It is extremely important to provide reliable and succinct information and guidelines for resin luting materials for dental dental practitioners.

Comparative Evaluation of Postoperative Sensitivity Using Three Different Tooth-Colored Restorative Materials in Non-carious Cervical Lesions: A Split-Mouth Design In Vivo Study

Introduction

Three different tooth-colored restorative materials were evaluated and compared for postoperative sensitivity using a split-mouth design. This is a double-blinded clinical trial assessed for a one-month follow-up period in patients with non-carious cervical lesions (NCCLs).

Materials and Methods

A total of 60 NCCLs in 20 participants were considered in this split-mouth design study and randomly divided into three different groups: Flowable composite (n = 20), resin-modified glass ionomer cement (RMGIC) (n = 20), and Zirconomer® (n = 20). The restorations were evaluated for postoperative sensitivity at baseline (BL-day 0), three, seven, and 21 days using the Schiff cold air sensitivity scale. Data were analyzed using IBM SPSS Statistics for Windows, Version 23.0 (Released 2015; IBM Corp., Armonk, New York, United States) using a post hoc test for postoperative sensitivity and one-way Anova to analyze all the groups together at the time interval of three, seven, and 21 days.

Results

In Group 1 (flowable composites) and Group 3 (Zirconomer), a statistically significant difference in terms of reduced postoperative sensitivity was seen after three and seven days. However, a significant reduction in postoperative sensitivity was seen after three, seven, and 21 days in Group 2 (RMGIC).

Conclusion

In this study, RMGIC showed reduced postoperative sensitivity after restoring NCCLs compared to Zirconomer and flowable composites. Compared with flowable composites and Zirconomer, RMGIC showed better clinical performance in terms of less postoperative sensitivity after restoring NCCLs.

The use of an elastomeric methacrylate monomer (Exothane 24) to reduce the polymerization shrinkage stress and improve the two-body wear resistance of bulk fill composites

OBJECTIVES

Characterize the chemical structure of an elastomeric monomer (Exothane 24) and evaluate the degree of conversion (DC), polymerization shrinkage stress (PSS), rate of polymerization (Rp), flexural strength (F_Strenght), flexural modulus (F_Modulus), Vickers hardness (V_Hardness) and two-body wear resistance of dental bulk fill composites (BFCs) containing Exothane 24.

METHODS

The Exothane 24 was characterized using mass spectroscopy, elemental analysis, ¹³C- and ¹H NMR. BFCs were formulated containing Exothane 24 (E10, E25, and E50). Similar BFCs containing regular UDMA (U10, U25, and U50), commercial conventional, and BFCs were used as control groups. ATR-FTIR spectroscopy was used to measure DC and the Rp of the composites. The PSS was measured using the universal testing machine method. Specimen bars were used to assess the F_Strenght, F_Modulus, and V_Hardness. RBCs were submitted to a two-body wear test using a chewing simulator machine; the rate and volumetric wear loss were evaluated using an optical scanner. Data were analyzed statistically with α = 0.05 and β = 0.2.

RESULTS

Exothane 24 is a urethane isophorone tetramethyl methacrylate monomer with polymerization stress-relieving properties. No differences were found in the DC up to 4 mm in depth for E25. All BFCs had similar F_Strenght, except for E50. E25 had the lowest volumetric wear loss and wear rate. E25 had approximately 30% lower PSS and slower Rp than commercial BFCs with similar wear resistance to conventional commercial composites.

SIGNIFICANCE

The Exothane 24 reduced the PSS and increased the wear resistance of BFCs; however, the formulation is important to optimize the properties of the BFCs.

Application of Scanning Electron Microscopy in the observation of dentin-adhesive interface

The objective of this study was observation of the adhesive interface on original tooth samples, as well as their epoxy replicas, under SEM. A light-cure flowable composite was incrementally placed and light-polymerized in previously prepared cylindrical dentinal cavities on the buccal surfaces of extracted human third molars. After finishing procedures, impressions of the composite/dentin margin were made using polyvinylsiloxane in order to obtain accurate epoxy replicas for SEM analysis. Ultrastructural morphology of the adhesive surface was observed at high magnifications (≥1,000×) on original tooth samples, which were previously prepared to expose the part of the dentin surface, which participates in the formation of adhesive bond. SEM micrographs showed that marginal adaptation was mostly of acceptable quality. In some of the SEM micrographs of original tooth samples, marginal gap formation, and resin tag breakdown were noted, which were ascribed to polymerization shrinkage. Profound understanding of ultrastructural morphology is necessary for achieving more predictable and durable margin between composite restorations and surrounding tooth structures, and SEM analysis can serve that purpose.

Polymerization shrinkage stress of resin-based dental materials: A systematic review and meta-analyses of composition strategies

PURPOSE

A systematic review was conducted to determine whether there were composition strategies available to reduce and control polymerization shrinkage stress development in resin-based restorative dental materials.

DATA SOURCES

This report was reported in accordance with the PRISMA Statement. Two reviewers performed a literature search up to December 2016, without restriction of the year of publication, in seven databases: PubMed, Web of Science, Scopus, SciELO, LILACS, IBECS, and BBO.

STUDY SELECTION

Only laboratory studies that evaluated polymerization shrinkage stress by direct testing were included. Pilot studies, reviews and in vitro studies that evaluated polymerization shrinkage stress by indirect methods (e.g., microleakage or cuspal deflection measurements), finite elemental analysis, or theoretical and mathematical models were excluded. Of the 6113 eligible articles, 62 studies were included in the qualitative analysis, and the meta-analysis was performed with 58 studies. The composition strategy was subdivided according to the modified part of the material: filler phase, coupling agent, or resin matrix. A global comparison was performed with random-effects models (α = 0.05). The only subgroup that did not show a statistical difference between the alternative strategy and the control was 'the use of alternative photo-initiators' (p = 0.29).

CONCLUSION

Modification of the resin matrix made the largest contribution to minimizing stress development. The technology used for decreasing stress in the formulation of low-shrinkage and bulk-fill materials was shown to be a promising application for reducing and controlling stress development.

Polymerization shrinkage stress of resin-based dental materials: A systematic review and meta-analyses of technique protocol and photo-activation strategies

PURPOSE

A systematic review was conducted to determine whether there were any alternative technique or additional step strategies available to reduce and control polymerization shrinkage stress development in dental resin-based restorative materials.

DATA SOURCES

This report followed the PRISMA Statement. A total of 36 studies were included in this review. Two reviewers performed a literature search up to December 2016, without restriction of the year of publication, in seven databases: PubMed, Web of Science, Scopus, SciELO, LILACS, IBECS, and BBO.

STUDY SELECTION

Only in vitro studies that evaluated polymerization shrinkage stress by direct testing were included. Pilot studies, reviews and in vitro studies that evaluated polymerization shrinkage stress by indirect methods (e.g., microleakage or cuspal deflection measurements), finite elemental analysis or mathematical models were excluded. Of the 6.113 eligible articles, 36 studies were included in the qualitative analysis, and the meta-analysis was performed with 25 studies. A global comparison was performed with random-effects models (α = 0.05). The strategies were subdivided as follows: the use of an alternative technique protocol of placing the material inside the tooth cavity; the modification of the irradiation intensity or total energy delivered to the material; the use of an alternative light-curing source; or the use of an alternative photo-activation mode. All alternative strategies showed statistically significant differences when compared with their respective controls (p < 0.05).

CONCLUSION

The use of alternative light-curing sources contributed more to minimizing stress development than placing the material by means of an alternative technique protocol or by modifying the irradiant intensity or total energy delivered to the material during photo-activation. Moreover, the use of an alternative photo-activation mode (intermittent light, exponential, soft-start or pulse delay modes) was shown to be an effective strategy for reducing and controlling stress development in resin-based dental materials.

In vitro fracture resistance of molar teeth restored with a short fibre-reinforced composite material

OBJECTIVES

The purpose of this in vitro study was to evaluate the efficiency of a short fibre-reinforced composite (SFRC) material compared to conventional composites when restoring class II. MOD cavities in molar teeth with different layering techniques.

METHODS

One hundred and thirty mandibular third molars were divided into 5 groups (n=26). Except for the control group (intact teeth), in all other groups MOD cavities were prepared. The cavities were restored by either conventional composite with horizontal and oblique layering or by SFRC with horizontal and oblique layering. The specimens were submitted to static fracture toughness test. Fracture thresholds and fracture patterns were evaluated.

RESULTS

In general, no statistically significant difference was found in fracture toughness between the study groups, except for horizontally layered conventional composite restorations, which turned out to be significantly weaker than controls. However, SFRC yielded noticeably higher fracture thresholds and only obliquely applied SFRC restorations exhibited favourable fracture patterns above chance level.

CONCLUSIONS

The application of SFRC did not lead to a statistically significant improvement of the fracture toughness of molar teeth with MOD cavities. Still, SFRC applied in oblique increments measurably reduces the chance of unrestorable fractures of molar teeth with class II MOD cavities.

CLINICAL SIGNIFICANCE

The restoration of severely weakened molar teeth with the use of SFRC combined with composite might have advantages over conventional composites alone. It was observed from the statistical data, that the application of SFRC with an oblique layering technique yielded not significantly but better fracture thresholds and more favourable fracture patterns than any other studied material/technique combination. Thus further investigations need to be carried out, to investigate the possible positive mechanical effects of SFRC. The application of the horizontal layering technique with conventional composite materials is inferior to the oblique technique and SFRC materials.

Vertical and horizontal polymerization shrinkage in composite restorations

OBJECTIVE

Polymerization shrinkage developed in vertical and horizontal directions after light activation of light-curing composite restorative materials. The purpose of this study was to examine the effects of vertical and horizontal polymerization shrinkage on: (a) dimensional changes of resin composites in tooth cavities; (b) shear bond strengths to enamel and dentin; and (c) marginal gap width in a non-reacting Teflon mold.

METHODS

Vertical and horizontal polymerization shrinkage in tooth cavities were measured immediately (3 min) after light activation. With the same time lapse, shear bond strengths to enamel and dentin and marginal gap widths in Teflon mold were also measured.

RESULTS

There was a significant correlation between vertical and horizontal polymerization shrinkage (r=0.647, p=0.043) in the tooth cavity. Composite materials which produced small vertical shrinkage also produced smaller horizontal shrinkage. Composite materials which produced small vertical shrinkage in the tooth cavity exhibited greater shear bond strengths to both enamel (r=-0.697, p=0.025) and dentin (r=-0.752, p=0.012). Composite materials which produced smaller horizontal shrinkage produced smaller marginal gap widths in the Teflon mold (r=0.829, p=0.003). No relationships were observed between horizontal shrinkage in the tooth cavity and shear bond strengths to both enamel and dentin (p>0.05).

SIGNIFICANCE

During the early stage of setting (<3 min) in tooth cavities, the vertical shrinkage of light-activated composite restorative materials was correlated with horizontal shrinkage.

Interfacial Nanoleakage and Bonding of Self-Adhesive Systems Cured with a Modified-Layering Technique to Dentin of Weakened Roots

Objective

The purpose of the study was to evaluate the nanoleakage and bond strength of different self adhesive systems cured with a modified-layering technique (MLT) to dentin of weakened roots.

Methods

Twenty-one maxillary incisors were decoronated and then root canals were instrumented and obturated with the cold lateral compaction technique. Weakened roots were simulated by flaring root canals until only 1 mm dentin thickness remained. Teeth were distributed into three groups. The canals were backfilled with Vertise Flow (VF group), a self-adhering system, following a modified-layering technique using two light-transmitting posts, sizes 6 and 3. DT Light Post size 2 was cemented using the same material. Remaining roots were prepared and cured in the same way as the VF group. However, in the TS/MF group, Clearfil Tri-S Bond (TS) adhesive and Clearfil Majesty Flow (MF) composite were used, while in the ED/PF group, ED primer II (ED)/Panavia F2.0 (PF) were used. After one week of storage, each root was sectioned to obtain six slices (two slices from each root third: coronal, middle and apical) of 0.9 ± 0.1 mm thickness. Interfacial nanoleakage expression was analyzed using a field emission scanning electron microscope (FEG-SEM), and the micro push-out bond strength (μPOBS) was measured at different root regions. Modes of failure were also determined using SEM. Data were statistically analyzed using two-way analysis of variance with repeated measures and Tukey post hoc test (p≤0.05).

Results

With MLT, all adhesive systems showed nanoleakage. For μPOBS, there was a statistically significant effect for adhesive systems (p&lt;0.001) but not for root region (p&lt;0.64) or for their interaction (p=0.99). Tukey post hoc test revealed that the bond strength of the VF group was significantly higher than the TS/MF and ED/PF groups for all root regions.

Conclusion

All of the tested self-adhesive systems cured using MLT had slight nanoleakage and were not sensitive to root regional differences. Self-adhering systems had higher bond strength than self-etch adhesives.

A model of the viscoelastic behavior of flowable resin composites prior to setting

OBJECTIVE

The aim of this study is to develop fractional derivative models for the assessment of viscoelastic properties related to handling characteristics of dental resin composites belonging to two classes: flowable (Revolution Formula 2 and Filtek Ultimate) and posterior "bulk-fill" flowable base (Smart Dentin Replace).

METHODS

Rheological measurements on all materials tested in this study were performed using dynamic oscillatory rheometer at temperature of 23°C. A parallel plates module with a diameter of 20mm was used to measure the properties of the resin composites tested. We developed two models to describe the obtained data: the generalized Newton model and the generalized Zener model (the so-called three parameter model). Both models contain fractional derivatives of Riemann-Liouville type. By determining the parameters of the model we were able to fit experimental data with high accuracy.

RESULTS

Our results show that flowable "bulk-fill" resin-composite material (SDR) has distinct properties as compared to other two flowable resin composite materials (Revolution Formula 2 and Filtek Ultimate). Thus, previously found SDR properties as "bulk-fill" flowable base results in the fact that it is described by generalized Zener model (i.e., it has properties of solid like viscoelastic material).

SIGNIFICANCE

Our model may be used to predict behavior of tested composites in different flow conditions. The SDR has initially small almost constant complex viscosity showing that it has good self-leveling property.

Noncarious Cervical Lesions Restored with Three Different Tooth-Colored Materials: Two-Year Results

Introduction:

The aim of this two-year prospective clinical study was to evaluate and compare the clinical performance of three different adhesive esthetic materials in noncarious cervical lesions.

Material and Methods:

A total of 90 restorations (30 per material) were placed in 30 patients who ranged in age between 18 and 50 years and of both genders, by a single operator with no previous preparation. The restoration of noncarious cervical lesions was done with either a microfilled composite (Esthet.X/Dentsply/De Trey, Konstanz, Germany, and Prime&Bond NT/Dentsply/De Trey), a nanohybrid composite (TetricEvoCeram/Vivadent, Schaan, Liechtenstein, and AdheSE/Vivadent), or a compomer (Dyract eXtra/Dentsply/De Trey and Xeno III Dentsply/De Trey). All restorations were evaluated by independent examiners using a modified US Public Health Service criteria at baseline and after 12 and 24 months for six clinical categories. Data were analyzed statistically by Pearson's chi-square or the Fisher's exact test at 5% significance level (p&lt;0.05).

Results:

Results showed that most of the restorations were clinically satisfactory after 12 and 24 months, with no statistically significant differences among the three groups for all evaluated criteria.

Conclusion:

Treatment of noncarious cervical lesions using composite and compomer materials, combined with the appropriate adhesive systems and properly implemented restorative procedures, gives satisfactory results after a two-year evaluation period.

[One-year clinical evaluation of tooth-coloured materials in non-carious cervical lesions]

INTRODUCTION

The restoration of non-carious cervical lesions is specific, mostly because of the location of their margins, especially the cervical margin, which is determined in cement and/or dentine. This feature makes the cervical margin more susceptible to micro-leakage, causing marginal discoloration, postoperative sensitivity, development of the secondary caries and loss of the restoration.

MATERIAL AND METHODS

One of the criteria for inclusion of a patient in this study was the presence of at least two non-carious cervical lesions to be restored with the minimum depth of 1 mm, independently of their location in the dental arch. A total of 60 restorations were placed in 30 patients. and every patient received both tested materials (composite resins and compomer) on their non-carious cervical lesions. The clinical evaluation of the therapeutic success was performed six months and then one year after the day of the placement of restorations using the modified-United States Public Health Service criteria. The following was evaluated retention, marginal integrity, marginal discoloration, wear, postoperative sensitivity and secondary caries.

RESULTS

A statistically significant high percent of restorations/teeth with postoperative sensitivity was found in the group of resin composite restorations after six months. At the end of the evaluation period, that is after one year, there were no statistically significant differences between materials for all evaluated criteria.

CONCLUSION

The results of this study show the identical quality of both examined materials one year after the readjustment of non-carious cervical lesions.