Sample size calculation
The power of the study was evaluated using post-hoc analysis. Sixty samples (n = 20 for each group) with a one-way ANOVA study achieves a power of 100%. The effect size f = 1.3 and the significant level was set at 0.05 (G*Power, v3.1.9.7 for windows).
Sample collection
Sixty intact single-rooted premolars that were freshly extracted for orthodontic purposes, using the luxation method, were collected from National Research Centre and used in this pre-clinical study. All teeth were examined under transillumination and magnification (2 × magnification lens) and only teeth that were free of cracks, caries, fractures, and without any previous restorations were included in the study. A hand scaler was used to remove any soft tissue and calculus deposits from the teeth, then all teeth were rinsed in distilled water and stored in a saline solution.
Sample preparation
Standardized endodontic access cavities were prepared in the teeth using 2# diamond round bur with coolant in a high-speed handpiece (Dentsply, Tulsa, USA) for penetration of the pulp chamber, and then tapering cylinder bur was used to complete the access cavity. For all specimens, MOD cavities were then prepared using cylindrical diamond burs size 2# (Diatech, Heerbrugg, Germany) by one operator with the following dimensions: The width of the occlusal isthmus was one-half of the inter-cuspal distance, the pulpal floor was 2.5 mm in depth, the axial wall depth was 1.5 mm, the width of the proximal box was half the buccolingual dimensions, the gingival floor was placed 1 mm above the cementoenamel junction. The bur was changed after each five prepared cavities.
A 10 k-file (Mani Inc, Japan) was used to establish the working lengths. The root canals were sequentially enlarged using a Pro-taper system from SX to F3 till the determined working length according to manufacturer guidelines using X Smart Endomotor (Dentsply-Maillefer, Ballaigues, Switzerland) Irrigation was performed using 3 mL of 2.5% NaOCl after every change of instrument. Following biomechanical preparation, 17% EDTA was used for 1 min, followed by distilled water for 1 min, and dried with medium paper points (Dentplus, Choonchong, Korea). After the complete preparation of the teeth, all the canals were obturated using Gutta-percha (Diadent, Group International, Korea) with the use of a cold lateral condensation technique and AD sealant (Meta Biomed, Cheongwon, Korea) (Moosavi et al. 2012). A hot instrument was used to remove excess gutta-percha from the coronal orifice of the canals, and all samples were stored for seven days in 100% humidity to allow for the setting of the sealer. After that, the pulp chamber was filled with resin-modified glass ionomer cement (GC Fuji II LC Capsule, GC Corporation) to a thickness that extended 1 mm occlusal to the cementoenamel junction (CEJ).
The Specimens were then divided randomly into three groups (n = 20 each) according to the type of resin composite used for restoration, Group 1: Alert fiber-reinforced resin composite [Alr], Group 2: Ever X flow fiber-reinforced bulk fill flowable composite [Exf], and Group 3: Filtek Z350 nano-filled resin composite [FZ3].
After applying auto-matrices in all spacemen, the application of 37% phosphoric acid (Ivoclar Vivadent) was done for 30 s for enamel and 15 s for dentine followed by water rinsing for 20 s then air dryness was done for 5 s. A single bond (3 M ESPE, St. Paul, MN, USA) was applied followed by light curing with Optilux 500 (Demetron-Kerr, Orange, CA, USA) with a light intensity of 1200 mW/cm2 for 10 s. In all cavities of Alert and Nano-filled composite, the first layer of resin composite (2 mm thick) was applied on the gingival seat of both proximal boxes in the MOD cavities and packed near the axial wall then light-cured for the 40 s. Subsequent layers of the same thickness were placed in a direction from the gingival floor to the occlusal surface to fill the preparation and each increment was then light-cured for the 40 s. Regarding Ever X flow resin composite application, it was applied into the cavity as one bulk layer to fill all the MOD cavities leaving only 1 mm from the occlusal surface, and then light-cured for the 40 s. A universal resin composite (GC Corporation Tokyo, Japan) was applied over the Ever X Flow for the thickness of 1 mm at the surface of the tooth and cured for 40 s. (Goda and Abogabal, 2020) (Fig. 1).
In all groups after the removal of matrix bands, post-curing was done from the mesial and distal surface for the 40 s according to manufacture instructions, then restorations were finished and polished using 12# finishing bur and rubber point at a low-speed handpiece. After that, all specimens were stored in distilled water for 24 h. The self-cure acrylic resin was used to fix all the spacemen in cylindrical tubes (5 mm × 5 mm) in a direction parallel to the long axis of the tube, leaving 1 mm from the cementoenamel junction.
Fracture resistance testing
For the fracture resistance test, each group have mounted individually on a computer-controlled testing machine (Model 3345; Instron Industrial Products, Norwood, MA, USA) with a load cell of 5 k, N, then the readings were registered using computer software (Instron® Bluehill Lite Software). The spacemen were fixed by securing screws to the lower fixed chamber of the testing machine. A compressive load was applied occlusally by using a metallic cylindrical rod with a 3.8 mm round tip connected to the upper movable chamber of the testing machine traveling at a crosshead speed of 1 mm/min. The tip was parallel to the long axis of the teeth and touch 3 points on the surface of the tooth: the occlusal surface, the buccal, and the lingual walls. The failure mode was manifested by a detectable crack and was established by a recorded sharp drop in the load–deflection curve detected on the computer software (Bluehill Lite Software Instron® Instruments). Fracture load was calculated in Newton.
Flexural strength testing
For testing flexural strength 10 specimens from each tested material were prepared according to the following methodology: Stainless steel mold with a dimension of 2.5 widths × 2 length × 2 height was used to fabricate the specimen according to ISO 4049 specifications. A glass slab was placed below the mold and then the restorative material was packed into the mold-covered over the celluloid strip to obtain a finished surface then another glass slab was placed over the mold with light pressure for the removal of excess material. Polymerization of each specimen was done using a light cure machine Optilux 500 (Demetron-Kerr, Orange, CA, USA) in the 40 s each. After curing, each specimen was stored in distilled water till tested within one week.
A three-point bending flexural strength test was done using a universal mechanical testing system (Instron Corp., Canton, MA). Each specimen was placed over a 2 parallel support (2 mm in diameter each and 20 mm apart). The load with a speed of 0.75 mm\min was then applied from a 2 mm rod placed in the center between the 2 supports. The maximum force (N) exerted on the specimen before being subjected to fracture was recorded. The following equation was used for the calculation of the flexural strength (σ):
$$\sigma = \frac{{3{\text{ps}}}}{{2{\text{bh}}^{2} }}$$
P: is the maximal force exerted on the specimens. S: is the distance between the supports (20 mm).
b and h are the widths in mm and the heights in mm of the specimen, respectively, measured immediately before testing.
Statistical analysis
Weibull analysis was used for statistically analyzing the Fracture Resistance (N) and Flexural Strength (MPa) data (R4, R Foundation for Statistical Computing, Vienna, Austria). Its parameters were calculated by Wald estimation, and pivotal confidence bounds were calculated with Monte Carlo simulation. The different groups were compared at the characteristic strength (63.2% probability of failure and 10% probability of failure) (α = 0.05).
