A total of 100 intact human mandibular first molars extracted for periodontal reasons were used for this study. All teeth were free from caries, defects, restoration and cracks and were used in compliance with ethical guidelines of Gazi University Faculty of Dentistry Clinical Research Ethics Committee (ethical protocol no. 2021.19/6). Plaque, attached periodontal tissues and calculus deposits were removed by hand-scaling and then stored in 0.1% thymol solution until use. The bucco-lingual and mesio-distal dimensions of the teeth were measured using a digital caliper (Insize 1112 − 150, Insize, Jiangsu, China). The teeth were divided into five groups so that there was no difference in the oneway analysis of variance (ANOVA) test between buccolingual and mesio-distal widths (p > 0.05) [32, 33]. The materials used in the study and their contents are listed in Table 1. Before preparing the cavities in the teeth, the distance between the cusps was measured using the digital caliper. To prepare a conservative access cavity, digital radiography was used to determine the borders of the access cavity according to the dimensions of the teeth. Then, the canal entrances were confirmed by considering the length of the periodontal probe according to the notches of the periodontal probe on the radiograph. The gingival step of the cavity Table 1 The composition of the materials used in the study Material (CEJ). The conservative mesio-occlusal (MO) endodontic access cavity was applied to all teeth except the control group (intact teeth) using a straight fissure carbide bur (Hicare Medical Co. Ltd., Guangzhou, China) using a highspeed handpiece under water cooling and all Cavo surface margins prepared at 90° with internal line angles rounded. The bur was changed after every five cavity preparations, and cavity dimensions were measured during the preparation using a digital caliper to ensure standardization. Following the endodontic access cavity, the working length was determined using #10 K-files (Shenzhen Perfect Medical Instruments Co. Ltd., Shenzhen, China), and the root canals were instrumented using ProTaper rotary files (Endoart, İstanbul, Turkey) up to sizes F3 with the crowndown method. During instrumentation, the root canals were irrigated with 1 mL of 2.5% NaOCl before each file was introduced into the canal and finally with distilled water. The canals were dried with paper points (Diadent Group International, Almere, the Netherlands) and obturated with gutta-percha cones and an AH Plus sealer (Dentsply De Trey, Konstanz, Germany) using the single cone technique. Excess gutta-percha cones were cut 1 mm apically from the canal orifices with a gutta cutter (C-Blade, Coxo, Guangdong Province, China) and covered with resin-modified glass ionomer cement (Ketac Cem Plus, 3 M, St. Paul, MN, USA). All teeth were examined after root canal treatment using periapical radiography to ensure accurate root canal treatment.To simulate the clinical conditions, each specimen was embedded in a block of self-curing acrylic cylinders at a level of 2.0 mm below the CEJ. The periodontal ligaments were simulated using a base plate wax at a 0.3 mm (Efes Dental, Bursa, Turkey). The distal cavity was determined and standardized according to the distal marginal ridge ness and periapical radiography. On the mesial side, the distance between the gingival margin and the CEJ was prepared at 1 mm. Care was taken to ensure a thickness of approximately 2 mm between the buccal and lingual walls and the interproximal cavity walls using the digital caliper. The buccolingual size of the cavity was 4 mm, and this width was measured above the cavities [34]. Clearfil S3 Bond Universal (Kuraray, Osaka, Japan) was applied using total etch mode with 35% orthophosphoric acid gel (Scotchbond Universal Etchant, 3 M ESPE, St. Paul, MN, USA). Acid gel was applied and left on enamel for 30 s and on dentin for 15 s, before being rinsed with water and gently dried with air to create a moist dentin surface. Clearfil S3 Bond Universal (Kuraray, Osaka, Japan) was applied using a micro-brush for 20 s and light-cured for 10 s with an LED light curing unit (D-Light Pro, GC, Leuven, Belgium). Upon completion of root canal treatment, the restorative steps for each group were as follows (Figs. 1 and 2): ● Group 1 (positive control group): Intact teeth without any cavity preparation were used as the positive control. ● Group 2 (negative control group): The mesial endodontic access was prepared, and coronal restoration was not applied. ● Group 3: A 4 mm glass fiber post (Cytec Blanco, Hannerkratt, Germany) was cut with a diamond fissure bur. The post was then fixed to the buccal and lingual walls of the cavity with a flowable resin composite (Filtek Ultimate Flowable, 3 M Espe, St. Paul, MN, USA). For this purpose, a flowable resin composite was applied to both ends of the post and adapted to the buccal and lingual cavity walls in the middle one-third of the cavity. The overflowing flowable resin composite was removed using a sond. Then, the flowable resin composite was polymerized at the connection points of the post with the cavity using an LED light device for 20 s. Then, bulk-fill resin composite (Filtek One Bulk Fill Restorative, FOB; 3 M ESPE, St. Paul, MN, USA) was applied in 4-mm layers for the restoration of the entire cavity and polymerized on the occlusal, buccal and lingual sides for 10 s using an LED Light Device. ● Group 4: A 4-mm thick fiber-reinforced composite (EverX Posterior, EXP; GC, Tokyo, Japan) was applied to the first half of the cavity and polymerized with an LED light device for 20 s. Then, a 4-mm thick bulk-fill resin composite (Filtek One Bulk Fill Restorative, FOB; 3 M ESPE, St. Paul, MN, USA) was applied to the upper part of EverX Posterior, and the restoration was completed by polymerizing the occlusal, buccal and lingual sides for 10 s using an LED Light Device from. ● Group 5: Coronal restoration completed with FOB using the bulk technique. The preparation and restoration of all specimens were performed by the same operator. Teeth were stored in distilled water for 24 h at 37 °C to prevent dehydration of the teeth and complete the post polymerization. They were then thermocycled at 5 °C and 55 °C for 10,000 cycles with a 30-s dwell time (MTE-101, MOD Dental, Ankara, Turkey).To determine fracture resistance, a 5 mm stainless steel spherical tip mounted on a universal testing machine (Shimadzu IG-IS, Tokyo, Japan) and a 45° oblique compressive load were applied to the central fossa of the teeth at a crosshead speed of 1 mm/min until a fracture occurred. The maximum load before fracture was recorded in Newtons (N). The fractured specimens were examined by two different operators, indicating the fracture mode as restorable (fracture above the CEJ or within 1 mm apical to the CEJ

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