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