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

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.