Materials and methods

Study design, setting, and duration

This in vitro study was conducted in the Department of Conservative Dentistry and Endodontics, Guru Nanak Dev Dental College and Research Institute, Sunam, Punjab, India, over a period of seven months from June 2025 to December 2025. As the study involved only commercially available restorative materials and did not include human participants, extracted teeth, patient records, animal tissues, or biological specimens, institutional ethical approval was not required.

Sample size estimation

The sample size was calculated using G*Power software (version 3.1.9; Heinrich Heine University Düsseldorf, Düsseldorf, Germany). A priori power analysis for one-way analysis of variance (ANOVA) was performed using an effect size (f) of 0.28, which was derived from a previous in vitro investigation [7], with a statistical power of 80% and a two-sided significance level of 5%. The minimum sample size required was estimated to be 145. To compensate for possible specimen loss during fabrication or testing, the sample size was increased by approximately 10%, resulting in a final sample size of 160 specimens. These included 80 bar-shaped and 80 cylindrical specimens, with 40 specimens allocated to each composite group. Each group was further subdivided into four subgroups (n = 10) for tensile strength, flexural strength, compressive strength, and Barcol hardness tests.

Study materials and group allocation

A total of 160 specimens were prepared and allocated equally into four experimental groups (n = 40 each) according to the nanohybrid composite resin evaluated: Group I, Tetric N-Ceram (Ivoclar Vivadent AG, Schaan, Liechtenstein); Group II, Fusion Universal (Prevest DenPro Limited, Jammu, India); Group III, NanoCom Composite (DentGist, New Delhi, India), and Group IV, NanoFill Composite (Waldent Innovations Pvt. Ltd., New Delhi, India). Each group was comprised of 20 bar-shaped specimens (25 mm × 2 mm × 2 mm) and 20 cylindrical specimens (5 mm × 10 mm). The bar-shaped specimens were further subdivided into two subgroups (n = 10 each) for tensile strength and flexural strength testing, whereas the cylindrical specimens were similarly divided into two subgroups (n = 10 each) for compressive strength and Barcol hardness evaluation. Thus, each composite group consisted of four subgroups of ten specimens, corresponding to the four mechanical tests. All restorative materials were procured from authorized distributors, stored according to the manufacturers' recommendations, and used before their expiry date. Specimen fabrication and polymerization were performed in strict accordance with the manufacturer’s instructions to ensure uniformity and minimize experimental variability (Figure 1).

Figure 1: Group allocation of study samples.Image created with Canva Pty Ltd (Surry Hills, Sydney, Australia).

Operator calibration and standardization

To minimize procedural variability, all specimens were fabricated by a single trained investigator who underwent calibration before the commencement of the study. A pilot standardization exercise involving 20 trial specimens was performed to ensure the reproducibility of specimen fabrication, incremental placement, light-curing protocol, finishing procedures, and testing methodology. The dimensions of all specimens were verified using a digital Vernier caliper with an accuracy of 0.01 mm, and only specimens within ±0.1 mm of the prescribed dimensions were accepted. The same visible light-emitting diode (LED) curing unit (Woodpecker Mini S LED Curing Light, Guilin Woodpecker Medical Instrument Co., Ltd., Guilin, China) operating at an output intensity of approximately 1000 mW/cm² was used throughout the study. The curing light intensity was periodically verified using a radiometer before specimen preparation each day. Mechanical testing was performed by the same operator using identical testing parameters for all the specimens to eliminate inter-operator variability.

Specimen preparation

A total of 160 specimens were fabricated under standardized laboratory conditions. Eighty bar-shaped specimens (25 mm × 2 mm × 2 mm) were prepared for tensile and flexural strength testing using customized addition silicone molds fabricated from addition silicone putty (Tech-Sil® S25 Addition Silicone Putty, Techno-Sil, Bidford-on-Avon, United Kingdom). Equal proportions of the base and catalyst were hand-mixed until a homogeneous consistency was obtained, following which a stainless-steel template of the desired dimensions was embedded into the unset putty to create a standardized mold. After complete polymerization of the putty, the molds were used for specimen fabrication.

The composite resin was incrementally packed into the molds in horizontal layers. A transparent polyester Mylar strip was placed over the composite surface to obtain a smooth, oxygen-inhibited-free surface and remove excess material. Each 2-mm increment was polymerized using a visible LED curing unit operated in the standard continuous curing mode at an output intensity of approximately 1000 mW/cm² for 20 seconds, in accordance with the manufacturer's recommendations. During curing, the light guide tip was positioned perpendicular to and in direct contact with the Mylar strip covering the specimen surface.

After specimen fabrication and 24-hour storage in distilled water, all specimens were visually inspected by a single calibrated investigator under standardized illumination for surface defects, including voids, air bubbles, cracks, incomplete polymerization, and surface irregularities. Specimen dimensions were verified using a digital Vernier caliper with an accuracy of 0.01 mm. Only specimens exhibiting complete polymerization, smooth surfaces, uniform dimensions (within ±0.1 mm of the prescribed dimensions), and absence of visible defects were included in the study. Specimens demonstrating dimensional inaccuracies, voids, air bubbles, cracks, incomplete curing, surface irregularities, or fractures during fabrication or storage were excluded and replaced with newly fabricated specimens.

For the evaluation of compressive strength and Barcol hardness, 80 cylindrical specimens measuring 5 mm in diameter and 10 mm in height were fabricated using standardized plastic cylindrical molds. The composite resin was inserted in 2-mm increments, with each increment polymerized for 20s. Following the placement of the final increment, an additional Mylar strip was applied over the surface before final light curing. After complete polymerization, the plastic molds were longitudinally sectioned using a Bard-Parker blade, and the specimens were carefully retrieved. All the fabricated specimens were stored individually in distilled water at 37°C for 24 h before mechanical testing to simulate oral conditions and ensure uniform post-polymerization.

Mechanical testing

Direct tensile strength (uniaxial tensile strength) was evaluated using a Universal Testing Machine (Instron® Universal Testing Machine, Instron Corporation, Norwood, Massachusetts, USA). Bar-shaped specimens were secured in the testing grips, and a uniaxial tensile load was applied at a crosshead speed of 1 mm/min until fracture. The maximum load at failure was recorded, and tensile strength was calculated and expressed in megapascals (MPa).

The flexural strength was determined by a three-point bending test using the same Universal Testing Machine. Each specimen was positioned on two supporting rollers, and the load was centrally applied at a crosshead speed of 3 mm/min until fracture. Flexural strength was calculated and recorded in MPa.

Compressive strength testing was performed using cylindrical specimens positioned centrally between the compression plates of the Universal Testing Machine. Compression was applied at a constant crosshead speed of 1 mm/min until specimen failure, and the compressive strength values were expressed in MPa.

The surface hardness was assessed using a Barcol Impressor (Model GYZJ-934-1 Barcol Hardness Tester, Barber-Colman Company, Rockford, Illinois, USA) following ASTM D2583 specifications. A diamond indenter was applied perpendicular to the specimen surface under a standardized load, and the indentation reading was recorded after 2 s to obtain the Barcol hardness number.

Outcome measures

The primary outcome variables included the mean tensile strength (MPa), flexural strength (MPa), compressive strength (MPa), and Barcol hardness of each nanohybrid composite resin. These parameters were compared to determine the differences in the physical and mechanical performance of the tested restorative materials.

Statistical analysis

Data were entered into Microsoft Excel (Microsoft Corporation, Redmond, Washington, USA) and analyzed using IBM SPSS Statistics for Windows, Version 25 (Released 2017; IBM Corp., Armonk, New York, USA). Data normality was assessed using the Shapiro-Wilk test. Continuous variables are expressed as mean ± standard deviation and 95% confidence intervals. Intergroup comparisons were performed using one-way ANOVA followed by Tukey's honest significant difference (HSD) post-hoc test for pairwise comparisons. Statistical significance was set at p < 0.05.