Materials and methods
TML, orthodontic
APICAL PERIODONTITIS, CAVITY
Forty first and second human primary molars from the upper and lower jaw with one sound proximal surface were included in this study. The molars were extracted due to apical periodontitis or orthodontic reasons and stored in 0.5% chloramine-T solution at 4 °C for no longer than two months. Children and parents gave their written consent for the use of the molars for research purposes. After extraction, all molars were irreversibly anonymised. Therefore, this study was carried out in agreement 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-2022-00962). Roots of the extracted molars were embedded in acrylic resin (Paladur, Heraeus Kulzer, Hanau, Germany) 3 mm below the cemento-enamel junction and mounted on custom-made holders. One standardised mesial or distal proximal cavity (3 mm in width, 1.5 mm in axial depth, with cervical margins 1 mm below the cemento-enamel junction) was prepared in each primary molar. Proximal cavities were prepared with 80-µm cylindrical burs (Universal Prep Set, Intensiv, Grancia, Switzerland) rotating at 40,000 rpm in a high-speed contra-angle handpiece (Sirius, Micro-Mega, Besancon Cedex, France). The bur was exchanged after preparing four cavities. The molars were then randomised into four groups (n = 10, computer-generated randomisation table, Microsoft Excel) based on the RBC they would be restored with and the light-curing mode they would be subjected to. The sample size was determined based on recent similar research [The molars were mounted and restored in a custom-made adjacent-tooth simulator with two plastic molars on each side on a laboratory desk. A stainless-steel matrix band (Omni-Matrix sectional, extended, Ultradent Products, South Jordan, UT, USA) was inserted and fixed with a wooden wedge placed 1 mm below the gingival margin of the cavity. A universal adhesive (Adhese Universal, Ivoclar Vivadent) was scrubbed for 20 s on all cavity walls in self-etch mode, thinned with a gentle blow of air in order to evaporate the solvent, and then light-cured for 10 s at 1200 mW/cm
Study design
The tested RBCs and their compositions according to the manufacturerBis GMA: Bisphenol A-diglycidyl dimethacrylate, UDMA: Urethane dimethacrylate, Bis-EMA: Ethoxylated bisphenol A dimethacrylate, Bis-PMA: Propoxylated Bisphenol A dimethacrylate, DCP: tricyclodecane-dimethanol dimethacrylate, AFCT: Addition fragmentation chain transferAfter the storage time, impressions were taken for each restoration with an A-silicon material (President Light Body, Coltene Whaledent, Altstatten, Switzerland). The impressions were poured out (Epoxyharz L, R&G Faserverbundwerkstoffe, Waldenbuch, Germany), fixed on aluminium holders (Cementit universal, Merz&Benteli, Niederwangen, Switzerland) and the formed replicas were sputter-coated with gold (Sputter SCD 030, Balzers Union, Balzers, Liechtenstein). After sputter coating, each replica was examined under a stereo microscope (x 20; Zeiss Stemi 1000, Oberkochen, Germany). The cemento-enamel junction buccally and lingually from the restoration was accentuated inside the epoxy resin using a fine scalpel. The replicas were quantitatively analysed for marginal integrity using scanning electron microscopy (SEM) at 20 kV and 200 × magnification (Amray 1810/T, Amray, Bedford, MA, USA). The marginal integrity for each restoration was expressed as a percentage of continuous margins in relation to the entire length of assessable margins. After this initial marginal analysis, all restorations underwent a thermo-mechanical loading (TML) inside an electronic masticator (CoCoM 2, ZPZ, Centre of Dental Medicine, University of Zurich, Zurich, Switzerland). The restorations were loaded on their occlusal part using a metal ball (diameter = 3 mm) for 400 000 loading cycles at 49 N [A-silicon impressions were taken again after TML and a final quantitative margin analysis using the same aforementioned protocol was carried out. The margin analysis was conducted by one calibrated and experienced operator (MZ) who was blinded to the groups and had only access to the SEM images. Figure 
SEM images and the quantitative margin analysis for a restoration’s replica (Power Fill, 10 s). 