Study Protocol Background The use of partial coverage restorations had increased significantly as a conservative alternative to full-coverage crowns, particularly in cases where the coronal tooth structure was extensively damaged and insufficient dentin remained to support direct restorations. These restorations were designed to preserve sound tooth structure while reinforcing weakened teeth affected by caries or fracture. Depending on the extent of cuspal coverage, partial coverage restorations were classified into inlays, onlays, and overlays.

With the advancement of digital dentistry, computer-aided design and computer-aided manufacturing (CAD/CAM) technologies had become widely adopted, enabling the fabrication of restorations with improved precision, mechanical performance, and esthetics. Various restorative materials were introduced, including glass ceramics, polycrystalline ceramics, and resin-matrix ceramics (hybrid ceramics). Hybrid ceramics, such as Vita Enamic, combined a ceramic network reinforced by a polymer phase, providing enhanced mechanical properties and favorable clinical behavior.

More recently, additive manufacturing technologies such as 3D printing had been introduced for the fabrication of definitive single-tooth restorations. However, limited evidence was available regarding their long-term performance, particularly in comparison with subtractive CAD/CAM materials. Additionally, marginal adaptation and bond strength were critical factors influencing clinical success, as failures were often initiated at the tooth-restoration interface.

Therefore, this study was designed to evaluate and compare the clinical and laboratory performance of 3D printed restorations, milled CAD/CAM hybrid ceramics, and indirect resin composite onlays.

- Aim of the Study

The study aimed to evaluate and compare the following:

1. The 2-year clinical performance of 3D printed, milled CAD/CAM hybrid ceramic, and indirect resin composite onlays.
2. The marginal and internal adaptation of the tested restorative materials.
3. The micro-tensile bond strength to dentin. - Clinical Part Summary A randomized controlled clinical trial was conducted following CONSORT guidelines. Patients were selected from the outpatient clinic of the Faculty of Dentistry, Mansoura University. Inclusion criteria required the presence of at least three posterior teeth with weakened cusps indicated for indirect onlay or overlay restorations, good oral hygiene, and healthy vital teeth with no symptoms. Patients with severe bruxism, systemic instability, or allergies to study materials were excluded.

Each patient provided informed consent prior to participation, and ethical approval was obtained from the institutional review board. A split-mouth design was used, and teeth were randomly assigned into three groups according to the restorative material.

Standardized tooth preparations were performed under local anesthesia using adhesive onlay preparation principles. Impressions were taken, and provisional restorations were placed. Final restorations were fabricated using either milling or 3D printing techniques depending on the material type. All restorations were cemented using resin cement following manufacturer instructions under rubber dam isolation.

Clinical evaluation was performed at baseline, 1 month, 6 months, 12 months, 18 months, and 24 months using modified FDI criteria, assessing aesthetic, functional, and biological properties.

- Laboratory Part Summary For the in vitro component, 60 freshly extracted human molars were collected from patients attending the Faculty of Dentistry, Mansoura University. Teeth were cleaned, disinfected, and stored appropriately until use. Teeth with cracks, caries, or structural defects were excluded under stereomicroscopic examination.

Standardized Class II onlay preparations were performed on all specimens using a uniform protocol. Impressions were taken, and restorations were fabricated indirectly in a dental laboratory according to manufacturers' guidelines. Specimens were randomly assigned into three groups corresponding to the tested materials.

Marginal and Internal Adaptation Marginal adaptation was evaluated using epoxy resin replicas examined under scanning electron microscopy (SEM). Internal adaptation was assessed after sectioning the specimens longitudinally through the restoration axis. Gold sputter-coated samples were analyzed under SEM to measure internal gaps and adaptation quality.

Micro-tensile Bond Strength Test Restored teeth were sectioned into small beams with a cross-sectional area of approximately 1 mm². Each specimen was tested using a universal testing machine at a crosshead speed of 0.5 mm/min until failure occurred. Micro-tensile bond strength values were calculated by dividing the failure load by the bonded surface area.

- Statistical Analysis Data were recorded and analyzed using statistical software.

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.

Inclusion Criteria:

• Inclusion Criteria:

Patients aged 18 years or older with permanent dentition, presenting with at least three posterior teeth indicated for indirect onlay or overlay restorations, with favorable and stable occlusion. Patients should have good oral hygiene and vital, asymptomatic teeth with no sensitivity to percussion. Presence of antagonist teeth is required.

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Exclusion Criteria:

• Patients with severe bruxism, unstable medical conditions, or known allergy to any study materials. Patients unable to attend follow-up visits, pregnant or lactating females, and cases with pulp exposure during caries excavation are excluded.

"Individual participant data may not be shared due to privacy and confidentiality concerns."

No masking was applied due to the nature of the restorative interventions. Both the operator and the participants were aware of the type of restorative material used (3D printed CAD/CAM hybrid resin, milled CAD/CAM hybrid ceramic, and indirect resin composite onlays). Outcome assessment was performed by independent examiners.