The effect of rapid high-intensity light-curing on micromechanical properties of bulk-fill and conventional resin composites

www.nature.com/scientificreports OPEN The effect of rapid high‑intensity light‑curing on micromechanical properties of bulk‑fill and conventional resin composites Matej Par1,2*, Danijela Marovic2, Thomas Attin1, Zrinka Tarle2 & Tobias T. Tauböck1 Rapid high-intensity light-curing of dental resin composites is attractive from a clinical standpoint due to the prospect of time-savings. This study compared the effect of high-intensity (3 s with 3,440 mW/cm2) and conventional (10 s with 1,340 mW/cm2) light-curing on micromechanical properties of conventional and bulk-fill resin composites, including two composites specifically designed for high-intensity curing. Composite specimens were prepared in clinically realistic layer thicknesses. Microhardness (MH) was measured on the top and bottom surfaces of composite specimens 24 h after light-curing (initial MH), and after subsequent immersion for 24 h in absolute ethanol (ethanol MH). Bottom/top ratio for initial MH was calculated as a measure of depth-dependent curing effectiveness, whereas ethanol/initial MH ratio was calculated as a measure of crosslinking density. High-intensity light-curing showed a complex material-dependent effect on micromechanical properties. Most of the sculptable composites showed no effect of the curing protocol on initial MH, whereas flowable composites showed 11–48% lower initial MH for high-intensity curing. Ethanol/initial MH ratios were improved by high-intensity curing in flowable composites (up to 30%) but diminished in sculptable composites (up to 15%). Due to its mixed effect on MH and crosslinking density in flowable composites, high-intensity curing should be used with caution in clinical work. The development of materials and techniques in adhesive dentistry follows a continuous trend toward simplification of restorative procedures, as highlighted by the evolution of bulk-fill resin c omposites1–3, universal adhesives4, and high-intensity light-curing u nits5. The benefits made possible by these advancements reach beyond mere improvements in the cost-effectiveness of the restorative treatment, as simplified procedures also reduce the risk of iatrogenic errors6. The evolution of light-curing protocols has been following the technological improvements of light-curing devices, which generally involved increasing radiant exitance and narrowing the emission spectrum to the useful wavelength range7. The approach of shortening exposure time by increasing radiant exitance has raised justified concerns related to polymerization shrinkage stress, which motivated investigations on various modulated light-curing protocols as a potential means for minimizing shrinkage stress and its detrimental consequences8. Although laboratory studies have demonstrated convincing evidence for shrinkage stress reduction attained by using various modulated light-curing protocols9–11, their benefits were less clear in the clinical setting12,13. Due to the lack of evidence for clinical benefits, modulated light-curing protocols could not become generally accepted, whereas clinical practices are dominated by continuous light-curing protocols with radiant exitances of about 1,000 mW/cm214. In the course of the development of light-curing units, the term “high-intensity” has acquired an ambiguous meaning. During the last two decades, radiant exitances of LED curing units have gradually increased for a whole order of magnitude14, leading to the corresponding adjustments to the meaning of “high-intensity” in the literature. In the 1990s, the radiant exitances of 100–200 mW/cm2 were common for early LED curing units, leading to light-curing protocols of 450 mW/cm2 being regarded as “high-intensity”15,16. During the 2000s, the range of radiant exitances associated with the term “high-intensity” shifted to 1,000–2,000 mW/cm217–20. As 1 Department of Conservative and Preventive Dentistry, Center for Dental Medicine, University of Zurich, Plattenstrasse 11, Zurich, Switzerland. 2Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Gunduliceva 5, Zagreb, Croatia. *email: Scientific Reports | (2020) 10:10560 | https://doi.org/10.1038/s41598-020-67641-y 1 Vol.:(0123456789) www.nature.com/scientificreports/ Composite viscosity Composite type Conventional Flowable Bulk-fill Conventional Sculptable Bulk-fill Composite name (abbreviation) Filler content (wt%/ vol%) Resin matrix Photoinitiator Manufacturer Shade/LOT No. Tetric EvoFlow (TEF) 58/31 Bis-GMA, UDMA, decandioldimethacrylate CQ/amine Ivoclar Vivadent, Schaan, Liechtenstein A2/Y15650 x-tra base (XB) 75/60 Bis-EMA, UDMA CQ/amine Voco, Cuxhaven, Germany, Universal/1,932,130 Tetric PowerFlow (PFW) 68/46 Bis-GMA, Bis-EMA, UDMA Ivoclar Vivadent, CQ/amine + Ivocerin Schaan, Liechtenstein Ceram.x (CER) 76/57 Methacrylate modified polysiloxane, dimethacrylate resin CQ/amine Dentsply Sirona, Konstanz, Germany A2/0,189 Filtek One Bulk Fill (FIL) 77/59 UDMA, aromatic UDMA, DDDMA, proprietary AFM CQ/amine 3 M Espe, St. Paul, MN, USA A2/NA60719 Tetric EvoCeram Bulk Fill (TECBF) 77/54 Bis-GMA, Bis-EMA, UDMA CQ/amine + Ivocerin + Lucirin TPO Ivoclar Vivadent, Schaan, Liechtenstein IVA/Y16932 77/54 Bis-GMA, Bis-EMA, UDMA, propoxyCQ/amine + Ivolated bisphenol A dimethacrylate, DCP, cerin + Lucirin TPO β-allyl sulfone AFCT agent Ivoclar Vivadent, Schaan, Liechtenstein IVA/X56571 Tetric PowerFill (PFL) IVA/Y15023 Table 1.  Resin composites investigated in this study. Bis-GMA bisphenol-A-glycidyldimethacrylate, UDMA urethane dimethacrylate, Bis-EMA ethoxylated bisphenol-A-dimethacrylate, DDDMA 1, 12-dodecanediol dimethacrylate, AFM addition fragmentation monomer, DCP tricyclodecanedimethanol dimethacrylate, AFCT addition-fragmentation chain transfer, CQ camphorquinone, TPO 2,4,6-trimethylbenzoyldiphenylphosphine oxide. radiant exitances of 1,000–2,000 mW/cm2 have nowadays become commonplace, the term “high-intensity” is currently being used to denote values over 2,000 mW/cm221,22. The described evolution of terminology refers mainly to LED curing units, which have dominated both the dental market and practice during the last decade. Another type of high-performance curing units, namely plasma-arc curing units, with radiant exitances reaching up to 7,500 mW/cm211,23,24 have also been present during that time, but never became widely accepted by dental practitioners. Changing the parameters of light-curing is related to two main concerns: (I) on curing effectiveness throughout the composite increment19, and (II) on possible differences in the crosslinking density of the polymeric network resulting from different radiant exitances18. The depth-dependent curing effectiveness is commonly evaluated by comparing microhardness (MH) or degree of conversion between the top and bottom specimen surfaces25, whereas crosslinking density is usually indirectly evaluated through ethanol softening, i.e. (...truncated)