Discussion 
fracture
The choice of a proper implant abutment for single unit restorations is dependent on many factors. The masking ability, marginal adaptation, and fracture resistance of the restoration are among the factors to be considered. In the present study, the null hypothesis was partially rejected as the marginal gap was only significantly different between the ti-base group and the preparable abutment group after cyclic loading and thermocycling. However, no statistically significant difference was found between the 2 groups regarding the masking ability and the fracture resistance of the restorations. Lithium disilicate restorations were selected in the present study as they have shown favorable esthetic and biomechanical properties in single unit implant replacements in previous studies and ZrO2 can be an alternative [The perception of color differences varies among individuals. The literature provides varying values for the perceptible and acceptable color difference thresholds. The perceptible threshold ΔE ranges from 1.0 to 3.7 and the acceptable ΔE threshold ranges from 1.7 to 6.8 to the human teeth and gingiva respectively [The results of the present study showed no significant difference regarding the fracture resistance between the crowns cemented to ti-bases in comparison to preparable abutments. The used preparable abutments in the current study had vertical heights similar to the ti-bases. The occlusal thickness of the crowns for both groups was therefore similar explaining the insignificant differences in fracture resistance. Those results were in accordance with multiple studies conducted to evaluate the fracture resistance of lithium disilicate crowns with screw access channels cemented to different types of abutments [Marginal fit is one of the most important technical factors for the long-term success of restorations [Limitations of the present study include the in vitro designstandardizing all the variables that is not possible in the clinical practice. An in vivo study should be conducted to verify the findings. Also the small sample size. Further studies should be conducted with larger sample size with more types of the cement used and different heights of the preparable abutments.