Preparation Design Influence on Marginal Adaptation and Fracture-Resistance of Ceramic-Partial-Restorations

Objective: This study aimed to evaluate the influence of different finishing margin lines (FML) on marginal and internal adaptation (MID), fracture resistance (FR) and fractographic analysis (FA) of CAD/CAM lithium disilicate (LiSi) partial restorations (PR). Methods: Four FML at different levels of the dental equator (DE) were executed on 40 human molars (n=10/group)): Rounded shoulder above DE (SA), Chamfer above DE (CA), Rounded shoulder below DE (SB), Chamfer below DE (CB). CAD/CAM LiSi PR were produced and luted on the prepared teeth with a universal resin cement system. Thermomechanical aging (1.200.000 mechanical + 5000 thermal cycles 5°-55°C) was performed. A 3D microcomputed tomography (μCT) was used to evaluate marginal discrepancy (MD), absolute marginal discrepancy (AMD), axial gap (AG), occlusal gap (OG) cement volume and internal voids. After embedding the specimens in acrylic resin 2 mm below the cement-enamel-junction the FR (in N) was assessed with a universal testing machine (spherical rod tip, diameter: 6 mm, 0.5 mm/min cross-head speed) under axial static load until fracture). FA of the fracture surfaces was performed with a 3D digital microscope (Keyence VHC 5000) to identify the crack origin and direction allowing for a map creation. MID and FR data were statistically analyzed (p<0.05). Results: FMLs below the DE showed greater cement volume (p<0.05); internal void remained unaffected (p>0.05). FML and aging significantly affected AMD, MD, and AG (p<0.05), but not OG (p>0.05). CB and CA showed, respectively, the highest and the lowest MD among groups (p<0.05). FML did not influence FR (p>0.05). FA revealed that SB and CB were more prone to dissipate the forces in the overall preparation until reaching the margin. Conclusion: FML above the DE demonstrated superior MID, along with different fractography behaviors compared to FML below. All preparations resulted in comparable FR.