Introduction: The use of a secondary titanium insert has a beneficial influence on the stability of ceramic abutments and appears to be clinically useful for premolar and molar single-tooth replacements. Available titanium inserts in the market produced by Sirona, have a single height of 4.6mm with various platform diameters for different implant systems. The aim of this in-vitro study was to determine the fracture strength of IPS-e.max CAD hybrid abutment crowns with various crowns heights by simulating cyclic masticatory loads in vitro. Materials and Methods: Forty conical AstraTech EV dental implants, 4.2 mm in diameter and 9.0 mm in length were embedded 30 degree off-axis in acrylic resin blocks. Five groups (n=8/group) were designed to simulate the following treatment modalities for an implant crown #3i; Group 1 (control): Atlantis abutment and milled cemented zirconia crown with the dimensions of 11mm height, 10mm mesiodistal, 11mm buccolingual. Group 2: Atlantis CustomBase solution with milled cemented zirconia crown with the exact same dimensions as group 1. Groups 3, 4, 5: CAD/CAM hybrid abutment crown (TiBase) with the same mesiodistal and buccolingual dimensions as group 1 and different heights of the crown which were 11mm for group 3, 13mm for group 4, and 15mm for group 5. Implant position and wax-up crown were scanned with the Omnicam intraoral scanner. Lithium disilicate glass ceramic blocks (IPS e.max) were used to mill the full contour crowns in a milling machine. 100N mechanical cyclic loading at 30 degree off-axis was applied to the palatal cusps of each specimen for 250,000 cycles. Then, all survived specimens were loaded at the same angle in the universal testing machine with dislocation speed of 0.5 mm/min which increased from 0 to1000N or when fracture or deformation occurred. Removal torque value was measured after mechanical cyclic loading as well as compressive loading. A mixed-effects general linear model was employed for comparison among five groups. R-Studio and R 3.2.2 were used for all statistical analysis, and significance was accepted at p \u3c 0.05. Results: All TiBase groups did not demonstrate any significant difference in the amount of reduction in removal torque values before and after compressive loading, compared to control group (CBS). Furthermore, in the two-by-two comparison, the difference between TiBase 11mm and 15 mm was statistically significant (p value: 0.048). The fracture resistance ranged from 673.24-759.77 N for group 5, and 809.1-994.28 N for group 4. Different pattern of failure occurred for TiBase groups after compressive load test including plastic deformation of TiBase, screw bending, screw fracture, and ceramic fracture. Conclusion: Under the limitations of this study, Lithium disilicate hybrid implant supported restorations (IPS e.max CAD hybrid-abutment-crowns) with various crown heights including 11,13,15 mm did not fail during the estimate of 1-year chewing simulation. Under 1000N compressive load, all of the 15mm and most of the 13mm crowns failed, demonstrating the significant effect of the TiBase height on final fracture strength and survival of the crowns. Additional laboratory and clinical studies are needed to study the effect of the various titanium base design and abutment characteristics on the fracture strength of the complete structure
