Fracture resistance and fractographic analysis of lithium disilicate and zirconia overlays manufactured with different marginal preparation design

Aims: This thesis investigated how preparation design, restorative material, and cementation strategy influence the biomechanical behavior of indirect partial restorations (IPRs). The study focused on CAD/CAM lithium disilicate (LiSi) and zirconia (Zr) overlays, evaluating marginal and internal fit, seating accuracy using pre-heated composite versus resin cement, fracture resistance, and the bond performance of universal adhesive/dual-cure resin cement systems. Methods: Four in vitro studies were conducted. First, micro-computed tomography (µCT) analyzed the marginal and internal adaptation of LiSi overlays fabricated with different preparation designs, followed by fractographic analysis. The second study evaluated the effect of finishing line design and cementation strategy: resin cement (RC) or pre-heated highly filled composite (HC), on seating accuracy using three-dimensional software analysis. The third study investigated fracture resistance of LiSi and Zr overlays through static loading tests and fractographic examination to determine crack initiation and propagation. The fourth study evaluated a dual-cure universal resin cement combined with universal adhesives applied to dentin and composite substrates under light-cure and self-cure modes, with and without artificial aging. Bond strength was measured using the microshear bond strength (µSBS) test. Results: Preparation design significantly affected adaptation. Preparations above the survey line (SL) with chamfer finishing lines improved seating accuracy and reduced cement thickness. RC showed superior seating compared to HC, which increased the tooth–restoration gap and compromised marginal adaptation. Zr overlays demonstrated higher fracture resistance than LiSi, particularly with preparations above SL. LiSi showed more catastrophic failures but benefited from smoother finishing lines. Dual-cure resin cements with chemical activation ensured reliable polymerization and bonding; aging effects varied depending on substrate and curing mode. Conclusions: Optimized preparation design, appropriate material selection, and proper cementation protocols are essential for successful IPRs. Rounded preparations above SL, resin cements, zirconia for enhanced strength, and chemically activated dual-cure systems improve clinical reliability.