Marginal integrity of classical and bulk-fill composite restorations in permanent and primary molars

www.nature.com/scientificreports OPEN Marginal integrity of classical and bulk‑fill composite restorations in permanent and primary molars Blend Hamza1*, Marcus Zimmerman2, Thomas Attin2 & Tobias T. Tauböck2 Bulk-fill composites enable timesaving and less technical-sensitive application of restorations. This study investigated and compared the marginal integrity of classical and bulk-fill composite restorations in primary and permanent molars before and after thermo-mechanical loading (TML). Two Class II cavities were prepared in each of 20 primary and 20 permanent molars. The molars were randomised in four groups for each molar type. Groups 1 and 5 were restored with a high-viscous bulk-fill composite (Tetric PowerFill), groups 2 and 6 were restored with a flowable bulk-fill composite (Tetric PowerFlow), groups 3 and 7 were restored with a high-viscous classical composite (Tetric Prime), and groups 4 and 8 were restored with a flowable classical composite (Tetric EvoFlow). In permanent molars, the flowable composites were covered with a 2-mm layer of high-viscous composite (groups 6 and 8). The restorations were subjected to TML in a custom-made chewing machine (5–50 °C, 2 min dwelling time, × 1000; 400 ,000 loading cycles, 1.7 Hz, 49 N), and quantitative marginal analysis was conducted using scanning electron microscopy. Marginal integrity of each restoration was calculated as a percentage of continuous margins before and after TML. The tested high-viscous bulk-fill restoration showed similarly high marginal integrity in primary and permanent molars as the classical restoration. The tested flowable bulk-fill restoration showed the lowest marginal integrity compared to all other restorations after TML. In contrast to flowable bulk-fill restorations, high-viscous bulk-fill restorations show similar marginal integrity as classical hybrid composite restorations after TML, in both primary and permanent molars. During the last few decades, resin-based composite restorations have increasingly pushed amalgam aside as the preferable restorative material in both permanent and primary molars1,2. Composite restorations exhibit good mechanical and tribological properties, which lead to low annual failure rate (1.1%) in vivo3–5. Classically, composite restorations were inserted in approximately 2-mm-thick layers into the tooth cavity, following the so-called “incremental layering technique”. This technique is, however, regarded as time-consuming and a possible source of air entrapment between the consecutive composite layers. To overcome the aforementioned shortcomings, bulk-fill composites, which can be inserted in up to 4–6 mm layers into the cavity, were introduced2,6. Similar to classical composite, bulk-fill composite materials are available in low- and high-viscosity forms. While some studies documented the superiority of bulk-fill composites to classical composites with regard to lower polymerisation shrinkage s tress7–9 and higher marginal integrity10,11, other studies reported the opposite12,13. The fact that bulk-fill composite materials can be inserted in large material volumes raised the concern about the resulting polymerisation shrinkage stress at the tooth-restoration interface. In addition to this volumetric shrinkage, another contributing factor to shrinkage stress is the viscoelastic behaviour of the material during polymerisation. To reduce interfacial shrinkage stresses, prepolymer stress relievers and stress-relaxant polymerization modulators have been integrated in bulk-fill composite materials. Applying an intermediate layer of lowmodulus flowable composite material on the cavity walls has also been shown to reduce the shrinkage stress7,14. It is logical to assume that the timesaving benefit, offered by bulk-fill composites, is even more advantageous when treating children compared to adults. Shorter treatment times might help achieve a better patient compliance15,16. The differences in the enamel structure between primary and permanent teeth (lower calcium and phosphate concentration, thinner thickness and occlusal direction of the cervical enamel rods for primary teeth) also lead to different behaviour when treated with adhesive s ystems17,18. Nevertheless, the performance of 1 Clinic of Orthodontics and Pediatric Dentistry, Center of Dental Medicine, University of Zurich, Plattenstrasse 11, 8032 Zurich, Switzerland. 2Clinic of Conservative and Preventive Dentistry, Center of Dental Medicine, University of Zurich, Plattenstrasse 11, 8032 Zurich, Switzerland. *email: Scientific Reports | (2022) 12:13670 | https://doi.org/10.1038/s41598-022-18126-7 1 Vol.:(0123456789) www.nature.com/scientificreports/ bulk-fill restorations in terms of marginal integrity and how this behaves on primary molars, in comparison to permanent molars, has not yet been investigated. This in-vitro study was therefore carried out to investigate and compare the marginal integrity of classical and bulk-fill composite restorations in primary and permanent molars before and after artificial aging. The nullhypotheses of the study were that (1) there are no differences in the marginal integrity of the tested composite materials between primary and permanent teeth, and (2) there are no differences in the marginal integrity between the tested restorations in primary and permanent teeth. Materials and methods Twenty sound primary and twenty sound permanent human molars were used in this in-vitro study. Molars were extracted due to periodontitis or orthodontic reasons and were stored in 0.1% thymol solution at 4 °C until use. All patients and/or legal guardians gave their written informed consent for the use of their teeth for research purposes and all molars were irreversibly anonymised immediately after extraction. The study was therefore carried out in accordance with the Federal Act on Research involving Human Beings (Human Research Act; article 2, paragraph 2) and the authorisation from the ethics committee was waived (Zurich cantonal ethics commission, BASEC-2021-00,635). To facilitate their handling, the roots of all molars were embedded in acrylic resin (Paladur, Heraeus Kulzer, Hanau, Germany) and mounted on custom-made holders. Two standardised Class II cavities were prepared mesially and distally in each molar (mesio-occlusal and disto-occlusal cavity). The proximal cavities in primary molars were 4 mm in width, 2 mm in axial depth and 3 mm in occlusal-gingival depth. The proximal cavities in permanent molars were 5 mm in width, 3 mm in axial depth and 4 mm in occlusal-gingival depth. The width, axial depth and mesio-distal depth of the occlusal cavities were kept at 2 mm in both primary and permanent molars. Cavity preparation was carried out using 80-µm cylindrical diamond burs (Universal Prep Set, Intensiv, Grancia, Switzerland) mounted on a high-speed contra-angle handpiece (Sirius, Micro-Mega, Besançon Cedex, France) rotating at 40,000 rpm. A new bur was used after (...truncated)