Shrinkage vectors in flowable bulk-fill and conventional composites: bulk versus incremental application

Clinical Oral Investigations https://doi.org/10.1007/s00784-020-03412-3 ORIGINAL ARTICLE Shrinkage vectors in flowable bulk-fill and conventional composites: bulk versus incremental application Dalia Kaisarly 1,2 3 1 4 & Moataz El Gezawi & Andreas Keßler & Peter Rösch & Karl-Heinz Kunzelmann 1 Received: 16 January 2020 / Accepted: 11 June 2020 # The Author(s) 2020 Abstract Objectives Sufficient depth of cure allows bulk-fill composites to be placed with a 4-mm thickness. This study investigated bulk versus incremental application methods by visualizing shrinkage vectors in flowable bulk-fill and conventional composites. Materials and methods Cylindrical cavities (diameter = 6 mm, depth = 4 mm) were prepared in 24 teeth and then etched and bonded with OptiBond FL (Kerr, Italy). The composites were mixed with 2 wt% radiolucent glass beads. In one group, smart dentin replacement (SDR, Dentsply) was applied in bulk “SDR-bulk” (n = 8). In two groups, SDR and Tetric EvoFlow (Ivoclar Vivadent) were applied in two 2-mm-thick increments: “SDR-incremental” and “EvoFlow-incremental.” Each material application was scanned with a micro-CT before and after light-curing (40 s, 1100 mW/cm2), and the shrinkage vectors were computed via image segmentation. Thereafter, linear polymerization shrinkage, shrinkage stress and gelation time were measured (n = 10). Results The greatest shrinkage vectors were found in “SDR-bulk” and “SDR-increment2,” and the smallest were found in “SDRincrement1-covered” and “EvoFlow-increment1-covered.” Shrinkage away from and toward the cavity floor was greatest in “SDR-bulk” and “EvoFlow-increment2,” respectively. The mean values of the shrinkage vectors were significantly different between groups (one-way ANOVA, Tamhane’s T2 test, p < 0.05). The linear polymerization shrinkage and shrinkage stress were greatest in Tetric EvoFlow, and the gelation time was greatest in “SDR-bulk.” Conclusions The bulk application method had greater values of shrinkage vectors and a higher debonding tendency at the cavity floor. Clinical relevance Incremental application remains the gold standard of composite insertion. Keywords Flowable bulk-fill composite . Bulk application . Incremental application . Shrinkage vectors . Total-etch technique . Medical image registration Introduction * Dalia Kaisarly 1 Department of Conservative Dentistry and Periodontology, University Hospital, LMU Munich, Goethestrasse 70, 80336 Munich, Germany 2 Biomaterials Department, Faculty of Oral and Dental Medicine, Cairo University, Cairo, Egypt 3 Department of Restorative Dental Sciences, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia 4 Faculty of Computer Science, University of Applied Sciences, Augsburg, Germany The main drawback of resin-based composites is their shrinkage upon polymerization, leading to shrinkage stresses with possible debonding from the cavity walls, interfacial gap formation, and microleakage [1, 2]. Possible complications include discolored restoration margins, recurrent caries, undesirable pulp affections, and enamel cracks. Furthermore, the patient might complain of postoperative hypersensitivity [3–8]. The consequences of polymerization shrinkage are usually evaluated only with indirect methods, such as volumetric or linear shrinkage measurements outside a cavity [1, 2] or linear cuspal deflection measurements evaluated inside a cavity [9]. Other indirect measurements of the adverse effects of polymerization shrinkage are interfacial adaptation assessments at the margin of a restoration and bond strength tests [10–12]. Unfortunately, neither of these tests provides Clin Oral Invest information on the real flow of material due to polymerization shrinkage. The nondestructive volumetric evaluation method of polymerization shrinkage using microcomputed tomography (micro-CT) scans displays areas of debonding and leakage around restorations but not internal displacement movements [13–20]. Bulk-fill composites were introduced with the intention of reducing application times due to thicker increments. The materials are optimized to ensure the higher increment thickness (<5 mm) is safely cured. The optimizations include photoinitiator systems that allow for greater depth of cure than conventional composites. Moreover, the shrinkage stresses are controlled via matrix and filler modifications. Smart dentin replacement (SDR) was the first clinically well-accepted bulkfill material, and its volumetric polymerization shrinkage is lower than that of hybrid composites. Furthermore, the shrinkage stresses in SDR are lower than those in other bulk-fill composites [21, 22]. Similar to conventional composites, bulk-fill composites are available in various viscosities to fulfill the requirements of various application techniques. Flowable bulk-fill composites, such as SDR, are intended for use as a base below a layer of hybrid composite [23]. Bulk-fill composites with higher viscosity can be used as a direct posterior restorative material without the need for a covering hybrid composite [24]. The different viscosities are related to the filler content, which directly affects the elastic modulus. In the case of low-viscosity composites, stress reduction is achieved by a low elastic modulus, which allows the polymerization shrinkage to be compensated for by deformation of the restorative material [21]. The polymerization shrinkage and shrinkage stress can be evaluated in vitro with a simple sample geometry [2, 25, 26]. However, the real effects of polymerization shrinkage of a composite restoration can be seen only when applied in a cavity with its associated boundary conditions [27–32]. Earlier studies investigated shrinkage vectors of a flowable composite (Tetric EvoFlow, Ivoclar Vivadent, Schaan, Liechtensetin) applied with a 3-mm thickness; these studies were conducted before bulk-fill composites were widely available [30–33]. Bulk-fill composites have been optimized to enable higher increment thicknesses and to produce flowable bulk-fill composites with improved adaptation to the cavity walls. The aim of this study is to investigate—via shrinkage vector evaluation—the shrinkage behavior of SDR applied in 2mm-thick and 4-mm-thick increments and compare the results to the polymerization shrinkage behavior of a conventional flowable composite, Tetric EvoFlow, applied in 2-mm-thick increments. Furthermore, the linear polymerization shrinkage, shrinkage stress, and gelation time of the composites are studied. The null hypothesis states that the application method— bulk versus incremental—does not influence the polymerization shrinkage behavior. Materials and methods Shrinkage vector evaluation Sample preparation A total of 24 intact human molars were gathered and deposited in sodium azide. The Ethics Committee of the Medical Faculty at the Ludwig-Maximilians—University of Munich, Germany approved the experimental procedures. The teeth were randomly divide (...truncated)