3-D Printed Printed Resin Composite Versus Milled Resin Composite and Hybrid Ceramic Onlays: 2-Year Clinical Performance, Microtensile Bond Strength, Marginal and Internal Adaptation.

The study was conducted as a combined clinical and laboratory investigation to evaluate and compare the performance of three restorative materials: 3D printed CAD/CAM hybrid resin, milled CAD/CAM hybrid ceramic, and indirect resin composite onlays. The clinical component followed a randomized controlled split-mouth design in accordance with CONSORT guidelines, while the laboratory component was designed as an in vitro experimental study.

- Clinical Part Patient Selection Patients were recruited from the Outpatient Clinic of the Faculty of Dentistry, Mansoura University. Eligible patients were selected based on predefined inclusion and exclusion criteria. Inclusion criteria required the presence of at least three posterior teeth with weakened cusps indicated for indirect onlay or overlay restorations, a stable occlusal relationship, permanent dentition, age above 18 years, good oral hygiene, and vital asymptomatic teeth with no sensitivity to percussion. The presence of antagonist teeth was also required.

Patients were excluded if they exhibited severe bruxism, uncontrolled systemic conditions, pregnancy or lactation, known allergies to restorative materials, or inability to attend recall visits.

All patients were informed about the purpose, procedures, and potential risks of the study, and written informed consent was obtained prior to participation. Ethical approval was granted by the Research Ethics Committee of the Faculty of Dentistry, Mansoura University.

- Randomization and Study Groups

A split-mouth design was used, and teeth within each patient were randomly assigned to three experimental groups according to the restorative material used:

• CAD/CAM milled hybrid ceramic (Vita Enamic)
• 3D printed hybrid resin material
• Indirect resin composite Randomization was performed using a simple randomization method to ensure equal distribution of cases among groups.

Clinical Procedure All clinical procedures were performed by a single experienced operator under local anesthesia and rubber dam isolation. Standardized adhesive onlay preparations were carried out using high-speed diamond burs under water cooling. The preparations were designed with 6-12° axial wall convergence, rounded internal line angles, and butt-joint margins. Occlusal reduction was performed to ensure a minimum restorative thickness of 1.5-2.0 mm.

Full-arch impressions were taken after tooth preparation, and provisional restorations were fabricated and cemented using temporary resin material.

Definitive restorations were fabricated either by milling (CAD/CAM subtractive method) or 3D printing (additive manufacturing), depending on the assigned material group. All restorations were fabricated in accordance with the manufacturers' instructions and by a single dental technician to ensure standardization.

After fabrication, provisional restorations were removed, and the final onlays were tried intraorally to assess marginal fit and proximal contacts. The adhesive surfaces of the restorations were treated according to the manufacturer's protocols. Teeth were isolated using rubber dam, and restorations were cemented using resin cement. Final finishing and polishing procedures were performed using a standardized polishing system.

- Clinical Evaluation Restorations were evaluated at baseline (1 week after cementation), 1 month, 6 months, 12 months, 18 months, and 24 months. Two independent calibrated examiners performed the evaluations using modified FDI criteria, which assessed aesthetic properties, functional properties, and biological properties. In case of failure, restorations were repaired or replaced as necessary.

- Laboratory Part Teeth Collection and Preparation A total of 60 extracted human molars were collected from patients attending the Outpatient Dental Clinic of the Faculty of Dentistry, Mansoura University. Teeth were extracted for periodontal reasons from healthy individuals after obtaining informed consent.

Soft tissue remnants and calculus were removed using hand scalers, followed by cleaning with pumice slurry and a rubber cup. The teeth were disinfected in 0.5% chloramine-T solution at 4°C for seven days, then rinsed and stored in distilled water at 4°C until use. Each tooth was examined under a stereomicroscope to exclude cracks, caries, or structural defects.

- Tooth Preparation Standardized Class II onlay preparations were performed on all teeth using a high-speed handpiece with water cooling. A single operator performed all preparations using a consistent protocol and standardized diamond burs.

The preparations included 6° axial wall taper, rounded internal line angles, and butt-joint cavosurface margins. The preparation design was standardized to ensure uniform restorative space and bonding conditions across all specimens.

- Fabrication and Cementation Impressions were taken for all prepared teeth, and restorations were fabricated indirectly in a dental laboratory by a single experienced technician following manufacturers' guidelines for each material system.

After fabrication, the onlays were cemented to their corresponding teeth using self-adhesive resin cement after appropriate surface treatment of both tooth and restoration according to manufacturer instructions.

- Specimen Grouping The teeth were randomly allocated into three experimental groups (n = 10 per group) based on restorative material type for each laboratory test.

- Marginal and Internal Adaptation Evaluation Marginal adaptation was evaluated using epoxy resin replicas obtained from impression materials. The replicas were examined under scanning electron microscopy (SEM) to measure marginal gap distances.

For internal adaptation, specimens were sectioned longitudinally through the center of the restoration using a water-cooled precision saw. The sectioned surfaces were gold sputter-coated and examined under SEM to evaluate internal adaptation and gap distribution.

- Micro-tensile Bond Strength Testing Each restored tooth was sectioned into beams with a cross-sectional area of approximately 1 mm² using a low-speed diamond saw under water cooling. The exact dimensions of each specimen were measured using a digital caliper.

Micro-tensile bond strength testing was performed using a universal testing machine (Instron 5965, USA) at a crosshead speed of 0.5 mm/min with a 5 kN load cell. Tensile force was applied until failure occurred, and bond strength values (MPa) were calculated by dividing the failure load (N) by the cross-sectional area (mm²).

- Statistical Analysis All data were recorded, tabulated, and analyzed using Microsoft Excel 2016 and SPSS software (version 25). Data normality was assessed prior to statistical testing, and appropriate parametric or non-parametric tests were applied accordingly. The level of significance was set at p < 0.05.

- Summary The study was designed as a comprehensive clinical and laboratory evaluation of three restorative systems, integrating long-term clinical follow-up with in vitro assessments of marginal adaptation and adhesive performance. The methodology ensured standardization of procedures, controlled variables, and reproducibility to provide reliable comparative data on modern indirect restorative materials.